The Amazing Housefly Part 1: Stabilizers, Feet and Gender

You ever see some guy drive up into a parking lot, park his car, but then not get out for a bit? I see guys do that sometimes*, and I always assume it’s because they’re listening to something really good on the radio.**

*But never women. Let’s face it- guys are just weirder.

**Like one of those one-hit-wonder songs from groups that never put out another half-decent song- like Norman Greenbaum or Harvey Danger***- so you never bought the album, and you hang out till the end of the song because you don’t know when you’re going to hear it next. Or maybe they’re listening to a really funny comedy bit. Or maybe they’re just listening to one of those right-wing talk-radio jerks. Whatever.

***Confession: In the mid-90’s I actually bought a (the?) Harvey Danger album because I liked the song “Flagpole Sitta.” Terrible album.

But Friday morning after I pulled into my office parking lot, I sat in the parked car for a full 5 minutes, not to listen to “Spirit In The Sky”- the radio wasn’t even on- but to take close-up photos of the Housefly trapped inside the car, perched on the driver’s-side window. (Nice shot, eh?)

Tangent: I always imagine that for a fly- or any bug really- getting trapped in a car is like an alien abduction. 2 weeks ago we had a fly stuck in the car from Fruita, CO to Price, UT. What a weird trip it must’ve been for that fly.

As I’ve mentioned before, I have no idea how long I’m going to continue this project*. But before I end it- whenever that day comes- I have a list of things I really, really want to blog about, and I need to make sure I’ve covered all of the things on that list before I wrap it up. But here’s the thing: I haven’t actually written the list down, and if you asked me to recite the full list, I couldn’t. But every once in a while I’ll come across something, and just know it’s on the list. Houseflies are on the list.

*Though it is going to end one day, if only because I just hate things- 10 year-old sitcoms, “Rocky” movies, geriatric rock bands on yet another reunion tour- that never end. All things- especially good things- should have a beginning and an end.

All About Flies

Flies, true flies, of the order Diptera, are distinguished by their wing architecture. Dipteran have 2 wings, and 2 halteres behind those wings. Halteres are knobbed structures located behind the wings that are flapped rapidly during flight and act as flight stabilizers (maybe very roughly analogous to the tailfins of an aircraft, or the rear rotor of a helicopter.) (See diagram left from HowStuffWorks.) They almost certainly evolved from wings, as primitive insects had 2 pairs of wings, as many (most) species still do today. Dragonflies, damselflies, caddis flies, fireflies and butterflies are not true flies. Houseflies, horseflies, deerflies, botflies, gnats, midges and mosquitoes(!) are.

And, to start this series out with a bang, and show you just how awesome my little camera is, check this out: I got a photo of a Housefly haltere. Wow.

The fly in my car was a Housefly, Musca domestica. You can tell by the red eyes and the distinctive black and grey striped markings on the abdomen (pic below).

Those blue, metallic-looking flies people refer to as houseflies aren’t houseflies; they’re Bluebottle Flies, Calliphora vomitoria, which is a kind of blow-fly.

All About Houseflies

Houseflies occur all over the world, though they’re thought to have originated in Central Asia. They thrive in both tropical and temperate environments, and in both rural and urban settings. The Housefly lifecycle includes a complete metamorphosis from egg to larva (maggot) to pupa to adult. (pic below, not mine. BTW, adult at lower right = male, upper right = female, as we’ll see in just a bit.)

Females lay up to 500 eggs over a few days in batches of 75 to 150. Maggots hatch in a day or less, and immediately start feeding on whatever substrate they hatched in, ideally manure, though soils containing old manure also work. Horse manure is best for maggots, followed by human excrement, followed by cow manure.

After between 4 and 13 days the maggot pupates, transforming into an encased pupa while it develops into its adult phase. The pupal case is formed from the maggot’s last larval exoskeleton; maggots, like other invertebrates, need to regularly shed their old exoskeletons as they grow. The pupal stage lasts between 2 and 27 days, varying primarily due to temperature; in general the warmer it is, the faster the fly develops. The fly breaks out of the pupal case by hammering with a specialized organ on its head, the ptilinum.

Adult Houseflies can live as long as 2 months, but 2 to 4 weeks is more typical. They live longer at cooler temperatures, and lifespan appears to be extended by access to- get this- sugar. Once emerged from their pupal cases, their first order of business is eating; they need food before mating. The female requires protein (manure alone won’t cut it) to produce eggs, and so this is a time when she’s particularly likely to land on your hamburger. After mating, the female starts laying eggs in 4 to 20 days.

Speaking of eating, Houseflies- like Spiders- can eat only liquid food. They liquefy solid morsels by regurgitating digestive juices onto them (as of course you already know if you saw Jeff Goldblum in the remake of The Fly.)

This fast, productive lifestyle is extremely prolific. Flies in most of the US complete ~12 generations/ year, and closer to 20 down in the tropics. It’s been estimated that if an “Adam & Eve” pair of flies started in April, and every single one of their eggs survived and reproduced, and so on and so on, throughout the summer, by August they would have produced close to 200 quintillion (200 followed by 30 zeros) flies.

OK, but let’s get back to the pics I shot on my driver’s-side window, and the obvious question they raise: Why are my car windows so dirty*? Haha, no really, the question is: How do flies stick to glass? There are all kinds of answers floating around, from sticky feet to wet feet (capillary action) to apparently-smooth surfaces being actually pretty bumpy at a microscopic level to Van der Waal forces (which actually may well be partly at play as well) but the real answer is that a fly’s foot is pretty darn sophisticated.

*My car always needs washing. When people ask why my car is dirty, I tryy and spin it as a positive, attributing it to me go-getter-outdoorhead lifestyle. But really I’m just kind of lazy.

Fly legs- like most insect legs- are tipped with a pair of very small claws, called the tarsal claws. These claws can hang on to nearly any protuberance on a landing surface, but on glass or super-smooth plant surfaces there’s sometimes nothing to claw onto.

In between the tarsal claws, a Housefly has an organ called a pulvillus, which is a retractable floppy little sack, sort of like a deflated balloon. The pulvillus is coated with hundreds of teeny oily little hairs, the adhesion between which and the surface holds the fly in place.

Side Note: Hymenopterans- Bees, Ants, Wasps- have a similar but different structure, called the arolium.

So here’s the cool thing. In this photo you can just barely make out the extended, yellowish pulvillus holding the fly to the glass.

These pulvilli stick to all kinds of surfaces and things, including bacteria, and since Houseflies regularly land on manure, rotting flesh and human food, that’s how* they spread disease.

*One of the ways, anyway. The vomiting onto food-bits, and of course their feces “help” as well.

Back to names. As longtime readers go, I’ve thrown out about a thousand Latin names for different creatures in this blog, and most go in one ear and out the other. But I love the Housefly’s Latin name: Musca domestica. “Domestic” = “House” obviously, and “Musca” is practically the same as “Mosca”, which, as every Spanish-speaker or gringo who took high school Spanish knows, is the Spanish word for “Fly”. And that reminds me of a joke, which coincidentally, is a great setup for the 2 topics we’ll tackle about Houseflies: sex-determination and vision.

But here’s the thing: the joke only works in Spanish, because of the wordplay involved. But it’s easy Spanish, so surely if you live in pretty much anywhere in the US, you must know at least enough Spanish to follow along*.

El Chiste

Hay turista norteamericano en restaurante en Mexico.

Turista: ¡Oye camarero! ¡Hay un mosca en mi sopa!

El camarero mira la sopa…

Camarero: No señor, hay una mosca en su sopa.

Turista: ¡Ay caramba! ¡Que buena vista tienes!

*Really? You don’t speak any Spanish? OK, here’s the joke. An American tourist is in a restaurant in Mexico and he tells the waiter there’s a fly in his soup. But he uses the masculine version of the word “a” or “un” for the fly, or “mosca”, which is a feminine word in Spanish. The waiter corrects his Spanish with the feminine article, “una”, but the tourist misunderstands and thinks the waiter has identified the fly as female, as which point he exclaims, “Wow, what good vision you have!”

Tangent: If you don’t speak Spanish, you should. No, no, not because everyone in the US will speak Spanish in 10 years. That’s a bunch of xenophobic hokey. Recent immigrants speak Spanish, but their children and grandchildren overwhelmingly prefer English. No, the reasons are:

1- It’s a high ROI language to learn. You can travel to dozens of very different countries with some basic Spanish (as I highlighted in the Blue Piñon series.)

2- It’s a much better language than English. Seriously. It follows all the rules, each letter always makes one- and only one- sound only*, and a far lower proportion of the verbs are irregular than in English. Conjugation and syntax are simple and easy. At the same time though, it routinely conveys clearer meaning and specificity than English, through a complete subjunctive mode, a real 2^nd-person plural pronoun and conjugation (so you don’t have to say “y’all” or “you guys” all the time), a clear distinction between 2^nd person formal and informal conjugation (so you don’t talk to a Nobel prize-winner like you do to your toddler) and many, many examples of greater verb specificity, as highlighted in “ser” vs. “estar”, and “saber” vs. “concocer.”

*Well, except “h”, which makes no sound.

3- If there really is any kind of justice in the universe and an afterlife of any sort, it is an absolute certain given that in that next life, English-only speakers will be mowing the lawns, washing the dishes and cleaning the bathrooms.

Nested Tangent: I should confess that my own Spanish, though sufficient for travel, is pretty lousy. From time to time I endeavor to improve it, but I just don’t travel often enough. Lately I’ve been watching Univision in the morning while riding the trainer, but after following along a bit, I get lost, zone out and end up just spinning and staring at the weather-ladies*. Then I realize with a start that I’m essentially just watching soft-porn and switch the channel before Awesome Wife catches me.

*Seriously. Why are the meteorologists on Spanish-language TV so unbelievably-smoking hot?

But the joke brings up the whole fascinating topic of gender, and specifically gender-determination, in Houseflies.

All About Sex-Determination (In Flies)

With humans and other mammals sex is determined by sex-specific chromosomes: XX = female, XY male.

But insects are more complicated. Ants and Bees for example use a haploid-diploid system: females have 2 sets of chromosomes (like us) while males have just 1 (and arise from unfertilized eggs.) Flies are all over the place. Fruitflies for instance, which diverged from Houseflies ~120 million years ago, use an “X0” system, meaning that 2 “X”s make a female, and 1 “X” makes a male, and there is no “Y”.

Houseflies are a bit more like us. They have 12 chromosomes arranged in 6 pairs. One of those pairs is either “XY” or ”XX” and determines whether the fly is male or female, with the “Y” chromosome being passed through the male-line only, just like with us.

And that’s how it works. Sometimes.

Other Housefly populations have been discovered in which there is no “Y.” All of them- female and male- are XX. In these flies, another gene, called “M”, has been located that sits on one of the other non-sex-specific chromosomes, or autosome, (i.e. not the “X”) which creates a male Housefly when present.

Tangent: There’s a fascinating “Y”-less sex-determination system in the mammalian world. In recent years it’s been discovered that all Mole Voles, Ellobius lutescens, native to the Caucasus Mountains, are X-only- both females and males. There is no Y chromosome, nor does SRY, the gene on the mammalian Y chromosome responsible for “male-ness”, appear to exist on any other chromosome. The analogy isn’t perfect; Mole Voles are all (male and female) X0, rather than XX, with just a single X chromosome, and a diploid chromosome count of 17. So far, the sex-determining gene and its location are unknown.

The Mole Vole is of particular interest to biologists, because the mammalian Y chromosome is small, seems to be shrinking over time, and some researchers suspect that it may eventually become “extinct”, leading to the head-scratcher of how humans in the distant future might reproduce.

OK, so that’s interesting, but it gets even weirder. It turns out there are other Houseflies in which all the flies- male and female- are both XX and have M. In these flies, another gene- called FD, which is present in, and passed down by, female flies- deactivates M, and so sex is determined by the female, not the male. FD is a special variant of a gene, F, which is present in all Houseflies, and expresses “female-ness”, but is usually switched off by M. But the FD variant is dominant even when M is present, and flies carrying this (originally) mutant gene are always female.

Confused yet? There are even some other variants, but the point is that Houseflies show a wide range of- or a high degree of “plasticity” with respect to- methods of sex-determination, making them fascinating subjects for study.

Let’s return though to our waiter. Could the tourist have been right? Could the waiter have actually determined the sex of the fly by looking at it? It’s unlikely. But we can, and we can do it with my awesome driver’s-side window photos.

Male houseflies have larger eyes than females, set closer together. The bigger the separation between the eyes, the likelier that Housefly is female.

In this case the distance between the eyes is about as great- or greater than the diameter of the eye at its widest point, so we know it’s female.

Why are the male Housefly’s eyes so much bigger? To see better, one might reasonably (and correctly) assume, but that just raises another question: Why do males Houseflies need to see better than female Houseflies? To which the answer is: to find female Houseflies. Males that find females quickly and more often leave behind more descendants.

But this in turn raises a 3^rd question, which is how does a larger eye help the male Housefly see better? And this leads to the whole super-amazing architecture of Housefly eyes, which turns out to be way more complex, fascinating and all-around way cool than you ever knew.

Next Up: The Eye of the Fly

Jeezum! What Is The Flipping Deal With All These Fetching Box Elder Bugs?

My recent rant on Winter seems only to have angered her more (and isn’t that just like a stalker? I am telling you, you cannot reason with them!) But I’ve been teased by Spring, and I can’t get Spring-like thoughts out of my mind. I mentioned how lovely the weather was Saturday, and I went on about biking and Meadowlarks and such. But the sign of Spring that everyone in my family noticed was something else.

When I moved to Utah in the mid-nineties, there were 4 things I encountered during my first year that I did not expect. You probably think I’m going to list some things about religion, or the liquor laws or slot canyons some such, but you’re wrong. I knew about all those things long before I moved here. No, the first surprise was thermal inversions, which I’ve already blogged about.

All About Cursing In Utah

The second surprise was the whole, weird Alternative Cursing Lexicon here in Utah. Many people raised in Utah seem just a bit more averse than folks in other parts of the US to swearing or taking the Lord’s name in vain casually. That wasn’t a huge surprise, but what was a surprise was that in place of “regular” cursing”, a whole strange alternative lexicon of “pseudo-curses” has apparently evolved over time. Examples include “Fetch”, “Flip”, “Gol!”, “Jeezum”, “Oh My Heck!”, “Bullcrap”, and “The F-Bomb”, among others*. And native-Utahns will use-and even conjugate- these words just like we- er, pardon me- just like some people use “regular” curse words, such as in “Fetch! I broke my flippin’ derailleur!”

*Awesome Wife’s all-time favorite = “Oh For Tender!”

The third surprise was Fry Sauce, which I won’t get into, because if you live in Utah you already know all about it, and if you don’t, it’s not really worth explaining.

Personal Trivia Factoid: Over the past decade I have come to love fry sauce. I am the only non-native Utahn I know who puts fry sauce on burgers.

But the fourth surprise, and the point of this post, was/is Box Elder Bugs, Boisea rubrolineatus (pic left), which, as everyone in Utah knows, come out of the woodwork the first few warm days of Spring, as well as on scattered, warm “Indian Summer”-like days in the Fall. No really, I mean it- they actually come out of the woodwork, as in the woodwork of your house, as I’ll explain in a moment.

Box Elder Bugs occur across North America, coincident with the range of their favorite food source and home, Box Elder Trees. The Box Elder, Acer negundo, is one of Utah’s 3 native Maples, and though Box Elder Bugs favor that tree- specifically female A. negundo (Box Elder trees are dioecious, meaning trees are either male or female)- they’ll also make use of other Maples and even Ash species. There are 2 species of Box Elder Bug, which look (and behave) pretty similar: B. rubrolineatus from the Rockies West (and therefore here in Utah), and B. trivittata (pic right) East of the Rockies.

When I first encountered Box Elder Bugs I was highly disturbed. On the first (in a while) bright warm sunny day, they are everywhere, particularly on or near sunny windowsills, and when I first laid eyes on them, my first thoughts were of Cockroaches. I went to college in Philadelphia, during which time I resided in a succession of dorm rooms and apartments that were invariably roach-infested. My college years are a bit hazy now, but some of my clearest memories are of killing roaches with my bare hand and detonating chemical fog-bombs in my dorm room. So my first thoughts were, “Great. Utah Cockroaches.” But as I’ve learned about Box Elder Bugs I’ve found out that they’re far more benign than my collegiate nemeses.

The first thing worth knowing about Box Elder Bugs is that they are a True Bug. A lot of people- me most of all- throw the term “bug” around pretty loosey-goosey as sort of a catch-all phrase for all kinds of invertebrates, including insects, spiders, scorpions and woodlice. But to a real entomologist (like this guy- cool blog), the word “bug” has a very certain meaning, specifically an insect of the order Hemiptera, which includes 80,000 species of insects which all have similar characteristic mouthparts. In a True Bug, the mandibles (crush /biting /cutting mouthparts) have joined with the maxillae (steadying/ manipulating/ chewing mouthparts) to form a proboscis, which is a specialized beak-ish structure used to pierce tissues and suck out liquids.

What kinds of tissues and liquids? Usually plant tissues and sap, but not always, as we’ll see in a moment.

There are other defining features as well; Hempiterans often have partially-hardened forewings, as do Box Elder Bugs, which is different from Beetles (order = Coleoptera), which have fully-hardened forewings. But the deal with the mouthparts/proboscis is the key differentiator.

In my defense, Cockroaches look at first glance a lot like a True Bug. But to an entomologist the difference is obvious; the mouthparts are all wrong*; Cockroaches have distinct mandibles and maxillae. (diagram right)

*The wings are wrong, too.

In addition to Box Elder Bugs, you’re already familiar with lots of True Bugs. Aphids, Cicadas, Leafhoppers and Stink Bugs (Shield Bugs) are all True Bugs, as are- get ready for it- Bedbugs.

Geeky-But-Cool-Side-Note: The phylogeny, or family tree, or Hemiptera is in flux. Traditionally Hemiptera has been divided into 2 orders, Homoptera, which includes Aphids and Cicadas, and Heteroptera, which includes Box Elder and Stink Bugs. But it now appears that Cicadas are more closely-related to Box Elder and Stink Bugs than they are to Aphids, making Homoptera a paraphyletic grouping. This is exactly the same deal we saw last summer when we looked at Mules Ears and Balsamroots in the Wasatch: recent DNA evidence forcing biologists to reassess groupings of and relationships between different species, and it’s a great example of why now is such an exciting and revolutionary time in biology.

Tangent: Stink Bugs- also called Shield Bugs, for their shield-like shape- which are a worldwide group including over 5,500 species, are the second-most common insect we see inside of our house in the warmer months. I’m not sure, but I think the most common species we find indoors here in Utah is this guy, the Consperse Stink Bug, Euschistus conspersus.

Confusingly, there is another group of insects called “Stink Bugs” here in the Intermountain West, which are not actually Stink Bugs, or even True Bugs. You’ve undoubtedly seen- and probably run over- these guys if you mtn bike singletrack along the Wasatch. They’re Darkling Beetles, of the genus Eleodes, and are Beetles and therefore not True Bugs. They’re called “Stink Bugs” for their chemical defense: like true Stink Bugs, they spray foul-smelling chemicals from their rear ends when threatened, though they evolved this mechanism completely independently (yet another nice example of convergent evolution, like Vultures, as well as CAM and C4 photosynthesis.) They’re a favorite food of mice, who have figured out how to roll them over in a manner to avoid the spray.

Bedbugs (pic below, left) have put the proboscis architecture to a more dastardly (to us) purpose; they pierce animal tissues and suck blood. Bedbugs, by the way have one of the weirdest and creepiest sex lives/anatomy in the animal world. I won’t get into it here, but David Quammen’s book Flight Of The Iguana provides a riveting description. But in contrast to Bedbugs, as well as many of the other invertebrates we’ve looked at, the sex lives of Box Elder Bugs are pretty vanilla. There are no parthenogenetic females (Aphids), no deceptive, mate-stealing triploid-females (Weevils), no “throw-away” chromosomally haploid males (Bees, Ants, Wasps) and no mate-devouring females (Black Widows.) Genetically, Box Elder Bugs reproduce very much like us: gender is chromosomally determined by the male, who provides either an X or Y chromosome, while females are always XX.

Side Note: Not all Hempiterans use the XY system of sex determination. Many True Bugs, including Water Striders (pic right) and Water Bugs, use what is called the “X0” system, whereby females have 2 “X”’s, males have a single “X”, and there’s no such thing as a “Y”. An interesting corollary of an X0 system is that every male in the species is aneuploid, having an odd number of chromosomes.

Tangent: There’s actually a mammal that’s been found whose males have no “Y” chromosome, and when I learned of it I thought that it might be a mammalian example of an X0 system, but it turns out to be even weirder. The male Mole Vole, Ellobius lutescens, native to the foothills of the Caucasus Mountains has a single “X” and no “Y” chromosome. But females also have only a single “X”, and the sex determinant seems to be our old friend the SRY gene, which apparently sits on some “X”’s. So all Mole Voles- male and female- have an odd number of chromosomes. This critter is the subject of much interest to biologists because it hints at a possible evolutionary future for us mammals with our strangely shrinking (over the evolutionary time-scale) Y chromosome, but that’s something I’ll have to leave for another post…

And speaking of mating, that’ll happen soon enough. When you see Box Elder Bugs scurrying around on that first warm day, what’s going on is this: last Fall, as the weather cooled, they migrated toward any warm spot, in an attempt to conserve energy and remain alive*. They congregated on the sunny, South-facing walls of your house, and when the sun went down, they crawled inside, through cracks and windowsills, trying to stay warm. When Winter set in, they went into hibernation.

*2 quick clarifications here. First, they technically don’t “hibernate”; that’s something specific to mammals. But they go “dormant” in a very hibernation-like way. Second, I read somewhere that only females “hibernate”, but couldn’t confirm that in time for this post.

When bright sunlight again warms the South-facing walls, the bugs wake up and start scrambling. They want to get back outside, eat, mate, and lay eggs. They don’t want to stay indoors; a Box Elder Bug doesn’t lay eggs or reproduce indoors. When you see them scurrying around your windowsill, they are lost; they’re trying to get outside.

Side Note: When I learned this, a light bulb went on for me: We’ve lived in our current home for 7 years. In all that time I have never found a Box Elder Bug in a North-facing room. All the rooms we’ve ever seen/found them in- kitchen, dining room, family room, office, master bedroom- are all South-facing.

When and if they get outside they’ll start eating. Their favorite food is Box Elder seeds, which is why they’re drawn to the female trees. After a couple of weeks of eating and sunning, you’ll see them mating* everywhere, facing away from each other, joined at the rear (pic right). The female then lays her eggs (pic below, left) in crevices of the bark of female A. negundo. The nymphs (pic right, in left of photo), which look like little adults** but with bright red bodies and underdeveloped wings can be found come Summer, often congregating on their favored trees. And speaking of wings, Box Elder Bugs are lousy flyers; ~20 feet is usually the limit of a single flight segment, though a chance wind may catch one and drop it a couple of miles away.

*If, even after reading this post, you feel you simply must squash Box Elder Bugs, at least have the decency not to do so while they’re mating. That’s just wrong.

**Another distinguishing characteristic of all True Bugs is that the form of he young is the same basic structural form as the adults; they don’t undergo any kind of radical metamorphosis, as do moths or butterflies, for example.

Although many True Bugs are serious agricultural or garden pests, Box Elder Bugs really aren’t. In extreme cases they can damage or partially defoliate a female Box Elder or other Maple, but that’s really rare.

And that’s the most important thing to know about Box Elder Bugs; they’re not really pests. They don’t eat garbage, they don’t lay eggs in your cupboards, they don’t come out at night and crawl across your face. They’re just trying to get outside. They don’t bite or spread disease, so when you see one on the window sill, you might consider just picking it up and letting it go outside*.

*Or squash it if you want. They’re not endangered, and I’m not enough of bleeding heart to tell you to spare a bug or anything, but they really don’t do us any harm.

Postscript: I’ll be offline for a few. OC Rick, Clean Colin, Aurora Coryalis and I are responding to the Never-Ending Winter with pure, selfish cowardice: We’re abandoning our families and fleeing down to St. George/Hurricane for a weekend of desert mtn biking and camping.

Post-Postscript: Oh Fetch! This invertebrate-oriented post reminds me- I totally flipping forgot to do the Costa Rican Creepie-Crawlie Bonus Post! Crap. Well, I know it’s late, but I could probably squeeze it in next week sometime, if there’s interest. 3 or more “Yes, Do It” comments and I’ll do it.

Pigeon Week Part 2: Mating, Meiosis and Milk

The next way cool thing about Pigeons is how they raise their young. The next time you hear someone go on about “feathered rats”, consider this: on average, Pigeons are arguably more doting- and certainly more egalitarian- parents than we are. Pigeons are prolific breeders* and both sexes take the business of child-rearing quite seriously.

*Feral pigeons, especially, for reasons we’ll get into in Part 3.

Climate permitting, feral pigeons breed year-round. Here in North America that means they’ll breed year-round, or close to it, up to about latitude 40 degrees North*. In the UK, it’s clear up to 54 degrees North.

*Which is, by wonderful coincidence, the latitude of Salt Lake City. How cool is that? Pretty freaking cool, that’s how. I tell you, the longer I live here, the more I feel like it is the center of the universe.

Pigeon courtship involves a fair amount of cooing, bowing, playful running, and- apparently- foreplay, including something called “billing”, in which, in a weird analog of French-kissing, the female inserts her bill inside the male’s open bill, usually with both their eyes closed. Following mating, the male often makes a brief “display flight”, in which he claps his wings behind his back*. Pigeons mate for life.

*I love this. It’s like he’s strutting about, high-fiving himself.

About 10 days following mating, a female pigeon lays 2 eggs, roughly 40 hours apart. There are 2 really interesting things about these eggs. The first is that the eggs are almost always 1 male and 1 female, which seems neat but odd. In pigeons, like all birds, sex is determined chromosomally, in a manner similar to- and yet completely opposite from- that of mammals.

This Part You Already Know But I Included It Because On The Off-Chance You Don’t Know It You’ll Get Totally Lost

In mammals, like us, sex is determined by an XY chromosomal system. We have 46 chromosomes arranged in 23 pairs. Each pair represents 2 of the same type of chromosome, one of which received from our mother, the other from our father. The exception is our sex chromosomes. Females have 2 of the same type, called “X’, but males have 1 “X” and one “Y”. Only men carry a “Y” chromosome. When a human female produces an egg, it contains just 23 chromosomes*, one of which is always an “X”. When a human male produces a sperm cell, it contains just 23 chromosomes, one of which is either an “X” or a “Y”. So the chances of a human couple conceiving a boy or a girls are, generally speaking**, 50%.

*The obvious exception is the occurrence of trisomy caused by the production of a sex cell (by either parent) that contains 2- and not just 1- chromosome at a given location, resulting in a fertilized zygote containing 3- and not 2- chromosomes at that location. Trisomy can occur on any chromosome, but is only survivable (to birth) at a few locations, including the X/Y chromosomes, and chromosomes #21 (Down’s Syndrome), #13 or #18 (the latter 2 always resulting in severe birth defects.)

**It’s actually a bit more complicated than this. Slightly more human boys than girls are born. But that’s a topic way outside the scope of this post.

This Part You Already Know If You Are A Longtime Reader Of This Blog (Or Just Knowledgeable About Birds)

In birds, the exact opposite is going on. Pigeons have 18 chromosomes arranged in 9 pairs, 1 of which is the sex chromosomes. But it’s males who have the same chromosome- called “Z”- while females bear 1 “Z” and 1”W”. Only female pigeons carry a “W” chromosome. When a female pigeon produces an (unfertilized) egg*, it contains just 9 chromosomes, one of which is either a “W” or a “Z”. When a male pigeon produces a sperm cell, it contains just 9 chromosomes, one of which is always a “Z”. So the chances of a pigeon couple conceiving a male or female squab is, generally speaking, 50%.

*As opposed to a fertilized, make-an-omelette kind of egg. So I guess “egg” is loaded term here, and I probably should say “ovum.”

If you have a 50/50 chance for a boy or a girl, and you have 2 children, then out of the 4 possible combinations- boy-boy, girl-girl, boy-girl, and girl-boy- 50%, or 2, of your two-child pairs will include 1 boy and 1 girl, and the other 2 will be either both boys or both girls*.

*And of course this is the case with human fraternal twins, such as Twin A & Twin B. The reason most twins overall are one sex or the other is because all identical twins are always the same sex. BTW, the single dopiest question you can ask a parent of boy-girl twins is, “Are they identical?” Seriously, you might just as well tattoo “I HAVE NO CLUE” across your forehead.

Only with pigeons, that’s not what happens. The vast majority- far more than 50%- of pigeon squab nest-mates are male-female. How can this be? The mechanism is unknown.

It gets even weirder. 70% of the time, the first egg laid is male, and the second, ~40 hours later, is female. How can this be? Again, the mechanism is unknown. The male, laid earlier, generally hatches before the female, begins eating, and gains weight rapidly. If food is sparse, and only 1 squab makes it, it’s usually the slightly bigger, first-born male who survives, and the female who dies. The benefit of laying the male egg first seems to be a greater survival likelihood for the male, though the advantage of this to the parent is unclear.

Conjectural Tangent*: Whatever’s happening, it’s happening in the female. Remember, the “W” chromosome can only come from her, so it’s her ova that are determining the sex. Presumably there’s some benefit to pigeons, some slightly better statistical success-payoff in the long run, to bearing young in pairs of opposite sex. And presumably, if only one of your 2 squabs is going to make it, there’s some slightly better statistical success-payoff in the long run if that survivor is male ~2/3 of the time.

*Warning: All of my conjectural tangents are just that- totally conjectural, and almost certainly wrong. But they’re fun to think about.

I say “presumably” because of course I have no idea. But there are millions and millions of pigeons, with an average generation-time of just a couple of years. And across thousands/millions of years, and billions of pigeons, these weird statistics have worked out and hold steady worldwide. There has to be a long-term statistical driver behind it.

So somehow, female pigeons have evolved to either a) release 2 ova roughly concurrently, but slightly staggered, or b) release 2 ova concurrently, but, following fertilization, stagger development of the embryos by roughly 40 hours. And more remarkably, she’s evolved to produce these ova, in most cases, alternating between W and Z ova, which in turn implies that whatever mechanism is at work here is at the meiosis level. That’s incredible! How is it controlled? What is going on?

Meiosis- or the division of diploid cells into haploid gametes- is the process by which all sex cells are created, and occurs in 4 distinct phases, encompassing 2 stages of cell division. I won’t go through all the details, but in human females, the first phase- called Prophase 1- happens before birth. In Prophase 1 the chromosomes in the initial germ cell split, which, in a female bird, means that one half ends up with a “W”, and the other with a “Z.” The proto-eggs then remain “on ice” until puberty, when they complete the meiosis process. But the initial splitting of the germ cell’s chromosomes happens in Prophase 1.

Does the timing work this way in birds? I don’t know*. But if it does, it would mean that the sex determination of gender- which ultimately occurs in Prophase 1, specifically in the female- would happen before birth. If this is the case, it means that somehow the female pigeon is positioning or packaging eggs, by gender, for eventual sequenced release, before birth.

*Specifically, does Prophase 1 occur before birth? I spent- no kidding- an hour online trying to find out before I gave up.

NOTE 9/22/10: Anonymous commenter answered this footnote-question. In birds Prophase 1 occurs 24-48 hours before ovulation (very different from the timing in mammals). See comments for details, source..

This sounds crazy, but the other possibility sounds even crazier: the female pigeon, as an adult, actively and real-time recognizes and sorts eggs by gender internally. How could this be? What “signature” would an egg bear based on whether it contained a W or a Z chromosome?

Whatever is going on inside of female pigeons, it is some seriously complicated shit.

The couple shares incubation duties, the male incubating by day, the female by night. After about 18 days the 2 eggs hatch, usually about 24 hours apart. After the squabs hatch, the parents feed them, which is pretty normal for all sorts of birds. But what they feed them is not; they feed them milk.

It isn’t actually “milk”, in the strict, mammalian sense of a white emulsion of fatty globules in a water-based fluid. But it is a remarkable analog, and like so many avian-analogs we’ve looked at- color vision, foveal vision, thermoregulation, gender-determination, intelligence/self-awareness- it’s a fundamentally different way of solving the same problem.

Pigeon milk is produced by fluid-filled cells lining the crop, which is a specialized organ present in many birds, located at the bottom of the esophagus, and generally used for food storage. Crops are thought to have come about as a mechanism for a bird to quickly gather and accumulate food from a location more rapidly than it might consume it, thereby minimizing on-location exposure to potential predators. Pigeons and game birds tend to have larger crops. The evolution of development of “milk”-production in the crop is an apparent example of an exaptation, or the leveraging by natural selection of a component(s) originally evolved for a different function.

Pigeon milk, like mammalian milk, is a highly nutritious, easy-to-consume, digest and metabolize food source rich in fats and proteins. Only more so: the protein, fat and overall dry matter content of pigeon milk is higher than human or cow milk. “Milk” is also a misnomer in that the substance secreted is more like a yellowy cottage cheese in appearance and texture, consisting of 35% dry matter, compared with 8% dry matter for human, and 4% for cow milk.

For the first several days, the squabs are fed the milk by their parents, who regurgitate it up from their crops, and into their mouths. After a few days, the parents add regurgitated seeds- mixed in with the milk- to the squabs’ diet, and gradually wean them off the milk at around a week or so.

Tangent: To a human, regurgitating food into someone else’s mouth sounds like, well, the grossest thing imaginable. But it’s remarkable how common it is in the animal world. Birds do it all the time, as do all kinds insects, and wolves regurgitate partially-digested meat to their pups. Lots of critters do it, and it seems to work just fine. And besides, I suspect some of the things we do would seem pretty disgusting to other creatures. I sometimes think that to an egg-laying creature like a bird, a live birth must be about the grossest thing imaginable.

The evolution of “milk” in pigeons is cleverer than it seems. Pigeons are mainly vegetarians; they don’t generally eat bugs. One of the challenges for non insect-eating birds is getting enough protein to their chicks that they attain critical mass prior to fledging*. Some species alter their diets to include bugs when in need of additional protein, but milk-production represents another solution to the problem.

*And it was this aspect of House Finch diet that turned out to be the undoing of the Brown-Headed Cowbirds who parasitized their nests, as we saw in last year’s Bird Feeder Week. (Man, was that an great week or what?)

Side Note: Still another cool thing about milk-production in birds is that it has evolved multiple times independently. Flamingoes (pic right, not mine) feed “milk” to their young, which is produced, not in a crop, but rather in glands lining the upper digestive tract. In a creepy vampire-like twist, the milk contains large numbers of both red and white blood cells. Emperor Penguins (pic below, left, not mine) also feed their chicks “milk”, in this case produced directly in the esophagus.

Interestingly, though all 3- pigeons, flamingoes, and penguins- produce milk, the evolutionary drivers for doing so appear to have been different. In pigeons it was driven by the need to ingest maximum protein prior to fledging. In flamingoes, they’re only able to ingest liquid food while their specialized feeding apparatus develops. In Emperor Penguins, it serves as a critical supplement in a food-barren environment. (Penguins eat fish, but incubate/hatch eggs fairly far inland.)

If you didn’t notice already, I’ve been glossing over something in my description of pigeon milk. I’ve consistently said “parent” in place of “mother”, and the reason for this is that both sexes of pigeon produce milk, starting to do so a few days before eggs hatch. With the exception of a couple of species of bat, no male mammal is known to routinely produce milk.

I’ve already mentioned the admirable parenting habits of pigeon-fathers, and such behavior isn’t unusual in the world of birds. Although certainly many bird species are “bad dads”, the males of many, many species are exceptional caregivers, and in many species even assume the lion’s share of incubation and chick-rearing duties. In mammals, with a few notable exceptions, such as humans and wolves, the vast, vast majority of species- including bears, cats, deer, rats, and former presidential candidates from North Carolina- are completely uninvolved- if not outright absentee- fathers. 90% of bird species are primarily monogamous*; only 10% of mammal species are.

*Though virtually all will cheat, given circumstance and opportunity.

One reason why this might be so is eggs. A female mammal carries her young until birth, and there’s not all that much many male mammals can or need do to help things along. By the time the offspring is delivered, he’s long gone. But since so much of a bird’s prenatal development takes place outside the mother, it opens up all kinds of incubation and egg-protection scenarios, many, many of which involve ongoing paternal participation, in turn keeping the male around till hatching. Some number- a big number- of these high-paternal-involvement scenarios led to more chicks surviving to hatch, fledge, mature and mate, which in turn led to… well, you get it already.

The Phenomenal Unfairness of Being Born Female

Tangent: Thinking about birds and mammals and gender always leads me back to thinking about humans and gender, which leads in turn to what for me has always been one of the big, unfathomable mysteries of life: the unfairness of being female.

Any rational way you slice it for a human being, being born female seems like a bad deal. They’re physically weaker, and their entire bodies are, well, structurally compromised for child-bearing. To pass a human head during delivery, our females’ hips have become so wide that women basically walk wrong. Routine childbirth for humans is a far more traumatic, painful, and life-threatening event than it is for pretty much any other large mammal. Every month women are plagued with inconvenience and discomfort, and although all humans fall victim to cancer, breast cancer strikes down under-40 women in their prime far, far more often than prostate cancer afflicts similarly-aged men.

Women routinely live with threat (or reality) of sexual or spousal violence, and the common, unwanted attentions of people bigger and stronger than them. In love, the penalties of misjudgment, indiscretion, or plain bad luck (unplanned pregnancy) are always greater for women, in terms of health, time, missed opportunities and cultural stigma. In the majority of human societies throughout history, and arguably today, women have enjoyed far fewer legal and property rights. Even in modern America, women earn less, and are remarkably under-represented politically.

Obscenely-priced cosmetics and hair products, burkhas/ veils/ headscarves, pantyhose that endure only a handful of wearings, shoes that are both exorbitant and borderline un-wearable. How do they stand it?

Nested Tangent: There’s another mystery to me embedded in this one, and that is how theists- people of faith- regard gender. Theologians seem to spend endless cycles considering or debating specifics of the afterlife, the path to salvation, or the duality of Christ, and yet, with a few exceptions (immaculate conception, virgin birth, original sin), don’t seem to think much about the greatest of God’s mysteries: How’d he pick who gets/has to be male/female? Specifically, do souls have gender? Or are souls neuter, above/beyond gender and base sexuality, and we’re simply assigned a gender in this life. If gender is assigned, what did women do to deserve their female-ness*? And if they’re not assigned, how does God decide which souls to make female?

*Yes, I’ve read Genesis, and I say, “Give me a break.” Assigning punishment to the great (x-times) great granddaughter of a “criminal” is even more morally repugnant than the psychotic-feudal family labor camps of North Korea.

If souls don’t have gender, and you believe in an afterlife, are you and your spouse really going to want to hang together for eternity? Doesn’t anybody think about this stuff*?

*Yes, I know some people do. Mormons are a good example. In Mormon theology, soul is definitely linked to gender, and that’s important, because married couples remain together in the afterlife. For me though, that opens up a whole other can of worms. I love AW dearly, but I wonder if after 500, 10,000, or a million years or so, what would we talk about? Seriously, at some point, after a few millennia or so, you must wake up every day and be like, “What- you again?” Anyway, I don’t think there is an afterlife, but if there is I’m thinking maybe I’d like to play the field a bit.

But that- none of that- is the unfathomable part to me. No, the unfathomable part is that the vast majority of women with whom I’ve had this conversation don’t agree* with me. They’re glad they were born female! How can this be? Don’t they see what an awful, terrible hand life has dealt them? It seems sometimes that no matter how long you live around women, so much about them is forever a mystery.

*Of course, at some level I generally find it unfathomable that anybody ever disagrees with me about anything, but then I guess that’s part of the unfathomable mystery of being male.

After about 4 days, the parents begin to augment the squabs’ diet with regurgitated seeds mixed in with the milk. By day 8-10 they’re almost fully weaned, being fed just seeds. At around day 30-35 they leave the nest.

But usually around day 20 the female lays the next pair of eggs, initiating a somewhat crowded period in the nest called clutch overlap. Clutch overlap is a busy time, with both eggs to incubate and mouths to feed. Guess who picks up the slack? That’s right- Dad. During overlap the male assumes a majority of feeding duties for the squabs.

Clutch overlap, and the rapid, year-round reproductive cycle of pigeons leads to perhaps the most interesting questions about them: Where are they from, and how come there are so darn many of them?

Next Up*: Coloration, Defense, and the Remarkable Natural History of Feral Pigeons.

*Will bleed into next week. These theme “weeks” are tough to pull off on schedule. But aren’t pigeons cool? Jason? Hello? I am talking to you.

Bird Feeder Week Part 2: Avian Thermoregulation, And A Big Tangent About Sex (determination)

As regular readers of this blog know, I am a complete Cold Wuss. Although I make the best of our snowy Utah winters, I always have thoughts of hot, sunny summer days in the back of my mind as I shiver away the dark months. That’s why I’m totally in awe of birds that don’t fly South for the Winter. There they are, all winter long at the feeder, outside in sub 20F or colder temps, tiny little things, apparently doing just fine. How do they do it?

But before we answer that- why do they do it? Why don’t they fly South, like any sane bird does?

But that leads to an even bigger question, and that is why birds migrate in the first place? Seriously, long-distance migration is a monumental undertaking. Every Spring, millions and millions of birds fly thousands of miles North. They hang out, mate, hatch a few chicks, then come Fall turn around and fly thousands of miles South to stay ahead of the cold. The migrations are long, dangerous, and require tremendous preparation, risk and effort. Huge numbers of birds fail to complete these migrations (and failure here generally means death.) Any way you look at it, long-distance migration is a huge dick-dance. Why do it? Why not just stay in the tropics year-round, and instead of screwing around flying North, spend one’s time doing important bird-things, like foraging, nest-building, courting, mating and raising chicks?

Because the pay-off is a reproductive home run. The Northern climes may only be habitable for 4 or 6 months or the year, but during those months life explodes to make up for the short living year. And that explosion is a bonanza of seeds, fruit, bugs and worms for the birds who can reach it. The plentitude of food resources, combined with lots of open nesting/foraging space (something sorely lacking in the tropics, due largely to intense competition) gives migrating birds a set of caloric resources that enable successful chick-rearing on a level they could never dream of realizing if they stayed home in Columbia.

Migration, difficult as it is, has been a winning formula for countless thousands of bird species for millions of years. Migrating birds tend to funnel into common “flyways” that utilize favorable geography, winds and water resources.

The birds that stay behind- the Finches, Goldfinches, Chickadees, Juncos and many others- are the cold-climate specialists, those that followed an evolutionary path that allowed them to thrive in a climate at the margins, a climate that thins out competitors for resources. In this respect, they’re analogous to Polar Bears among the Ursids (bears), or even the Inuit among pre-technology humans. They figured out how to make a decent living where others couldn’t, and have thrived as a result.

OK, so now back to the original question- how do they do it?

In the last Bird Feed Week post we looked at the parallel rise of both birds and mammals. One of the interesting things about this rise is that birds and mammals have often developed different mechanisms for solving the same problems. Both have developed an insulating outer covering- hair for us, feathers for birds.

Tangent: And both make great coats. Though I’ve never owned a fur coat, here I am modeling my favorite Christmas gift: the Puffy Jacket, filled with goose down. This thing is so warm I break a sweat just thinking about it. I can’t wait for my next winter camping trip. My days of shivering are at an end. (And don’t I look sharp in it?)

Both have developed genes to enable and manage color vision, with birds building upon and enhancing their reptilian chromatic legacy, while we primates have re-evolved the ability via a completely different set of genes, following our long, evolutionary “night” of nocturnalcy (“Nocturnal-ness”? “Nocturnaltude”?), which we talked about in this post. Both birds and mammals have evolved distinct genetic mechanisms for sex determination, a trait lacking in many reptiles.

Big Tangent About Sex (determination)

Tangent: The whole subject of sex determination in mammals, birds, reptiles and insects is mind-blowingly fascinating. Here’s the short version:

In most mammals, including us, sex is determined chromosomally. If we have 2 “X” chromosomes , we grow up female. If we have 1 “X” and 1 “Y”, we grow up male. And of course since females have no “Y” chromosomes, it’s our father’s genetic contribution that determines whether we turn out male or female.

Nested Tangent: The specific gene on the “Y” chromosome that makes us male is called the SRY gene. If this gene is missing or damaged or otherwise non-functional, we turn out female anyway, even if we have an XY combination. In this sense, female is the “default” or “normal” state of a human being. Males, with their almost-freakish strength, size and aggression, are the result of that one gene’s effects upon the otherwise “normal”/female embryo.

In birds, 2 very different chromosomes, “W” and “Z”, determine sex. But here, the mixed version, WZ, is female, while the “same” or homogametic variant, ZZ, is male. And since males have no “W” chromosomes, it’s the mother’s contribution that determines sex.

Nested Tangent Revisited: Whether some gene on the “W” chromosome imposes “female-ness” on a default male state, or whether the double “Z” combo somehow imposes “males-ness” on a default female state is as yet unknown.

In reptiles, sex determination is all over the place. In many reptiles, alligators being a classic example, sex is determined by the temperature at which the eggs are incubated. In other cases, such as the Green Iguana, sex is determined genetically via the mammalian-style XY system. But in yet others, such as snakes, sex is determined by the WZ system.

With insects it’s different still- Hymenopteran insects (bees, ants, wasps) use the haploid-diploid chromosomal mechanism I explained when we looked at Honeybees, and yet other insects use an “XX/X0” system, where 2 “X”’s make a female, 1 “X” makes a male, and there’s no such thing as a “Y”. Got it?

This whole topic of course leads to the fascinating question of why all these animals have 2 sexes in the first place, but this tangent’s gone on long enough…

Anywho, back to birds in Winter, and the point I was trying to get to, which is that birds and mammals have also tackled the challenge of thermogenesis (heat-generation) differently.

2 things mammals do to get through the Winter is grow more hair and more fat. Birds do both of these things as well; the American Goldfinch, Carduelis tristis, (pic left) increases it’s feather mass by 50% in winter. Birds also add fat. The Dark-eyed Juncos, Junco hyemalis, (pic below, right) in my yard carry an average of 14% more fat right now than in summertime, but there’s a very practical limit to how much fat you can pack on when you’re in the business of flying.

But there’s a 3^rd trick we mammals have that birds lack. Way back in September, when we got that first early snow and I went on about how unbelievably cold I was, I explained the 2 mechanisms we mammals have for making heat. First is shivering, a strictly temporary/emergency measure designed to crank up our core heat quickly for a short period. But the more effective, longer-term method of thermogenesis we employ is brown fat, a specialized type of fat that is specially optimized for heat generation, the chemistry of which I explained in this post.

Birds never evolved brown fat. And what this means to a Winter bird such as a House Finch or a Crow or a Magpie is this: when they are not flying, they are shivering pretty much all the time. This probably isn’t as miserable for the birds as it sounds. Shivering for birds isn’t a quickie-emergency fix; it’s something they’ve evolved to do for extended periods, and they’re able to shiver far more efficiently than we can for much longer periods.

Tangent: Another thing birds can’t do as well as mammals is change posture. People, dogs, cats and squirrels can all curl up to conserve heat. Birds can tuck their bills down a bit, or maybe hide their head under a wing, but that’s pretty much it.

Birds have other tricks, the most obvious of which is the fluffing of feathers, which creates air pockets that help improve the insulating capacity of their feathers. And one bird at my feeder has developed a really neat trick: controlled hypothermia.

Unlike the other birds at my feeder right now, the Black-Capped Chickadee, Peocile atricapilla, doesn’t shiver nearly continuously. Instead it repeatedly allows its internal body temperature to decline a few degrees, and then at regular intervals, shivers vigorously to bring it back up a bit. Chickadees exercise this ability most often at night, lowering their body temps by as much as 20F, which provides estimated energy savings of up to 50%. (And in fact Chickadees lower their body temp nocturnally year-round, including on cool summer nights.)

By way of comparison, if a human’s body temp is lowered by just 9F, they’re in stage 3 hypothermia. That’s the stage just before death, from which it’s pretty much impossible to emerge without external heat/assistance. But the Chickadee pulls off this amazing trick routinely throughout the Winter, apparently without even thinking about it.

Next Up: The star of my winter feeder…