Featured paper: cheating Common Yellowthroats, or, the world is more complex than you think

There is a thing that happens a lot in biology, especially in animal behavior: one set of researchers finds an interesting relationship, like, say, “Birds prefer to eat bugs off of cows with lots of spots, and don’t like to eat bugs off of cows with no spots.” (This is a made-up example.)

Starlings flying near a cow; Pt Reyes, CA.

Blackbirds flying near a cow, Pt Reyes, CA.

Then, some other researchers do a study and say, “Hey, our birds prefer to eat bugs off of cows with no spots! That’s the opposite!”

Then still different researchers do another study and say, “Our birds don’t care at all about the number of spots, they just care whether the spots make a shape like a smiley face. You guys must all have made a mistake. The Smiley Face Rule is the new Lek Paradox! #nobelplease”

To put it less ridiculously: scientists get different results sometimes, and it can be hard to figure out why. Did someone make a mistake? Who is right? Today’s featured paper takes an example of this confusing scientific disagreement and elegantly makes sense of everything, with the help of this handsome little bird:

Common Yellowthroat (male). Photo by Dan Pancamo*

Common Yellowthroat (male).
Photo by Dan Pancamo*

The puzzle that Taff et al. (2013) address has to do with attractiveness and cheating. We know that most birds, even socially monogamous birds, “cheat” on their partners: they may raise chicks with just one bird, their social partner, but they sneak off and mate with other birds on the sly. When those matings result in chicks, this is called extra-pair paternity—i.e. it’s paternity from outside the social pair.

This means that if you’re a male bird trying to have as many chicks as possible, you have two things to think about: first, siring some extra-pair chicks with other birds; and second, making sure that your social chicks—the ones you’re raising—are really yours, not some other male’s. To do this, you need to be appealing to female birds: you need to impress your own mate more than other males, and you need to impress other females more than their mates.

Go ahead, impress me. (Female Common Yellowthroat. Photo by Kelly Colgan Azar*)

Go ahead, impress me.
(Female Common Yellowthroat. Photo by Kelly Colgan Azar*)

Some male birds use plumage ornaments to be attractive to females—think of a peacock’s extravagant tail. The male with the most handsome feathers is the most attractive to females. Other male animals attract females with the best song, or the sexiest odor.

Plumage and song for me; I'll skip odor, thanks. Photo by Andy Morffew*

Plumage and song for me; I’ll skip odor, thanks.
Photo by Andy Morffew*

Bringing these ideas together, you would expect that the most attractive males—the ones with the best feathers/song/odor/etc.—would be the most successful at siring chicks both in their own nest and with birds other than their partner. It seems simple: females prefer attractive males and mate with them more, so attractive males have more chicks with everyone. But when you study it, you get decidedly mixed results. Sometimes the number of chicks sired lines up well with the attractiveness of a male. Sometimes it doesn’t.

What’s going on? Who’s wrong? Often the studies that don’t find a link between attractiveness and success conclude that they are wrong: maybe they measured a trait that the females really don’t care about. Maybe their sample size was too small. Maybe their reasoning was right but they’re being undermined by some other thing, like maybe attractive males get matings but then have weak sperm.

I fluff my feathers in indignation. I have EXCELLENT sperm. Photo by Henry T. McLin*

I fluff my feathers in indignation. I have EXCELLENT sperm.
Photo by Henry T. McLin*

Implicit in this doubt of the negative results is the idea that everyone has to find the same thing: everyone has to agree. It’s hard to shake this idea. Of course we all have to agree! This is science! There is The Truth, and everything else is Not The Truth. We have to find The Truth.

But the truth isn’t always: “This. No matter what.” Sometimes it’s more like, “This, but also, it depends on this other thing.” Maybe everyone is right, and they’re just missing an extra explanatory variable that makes it all make sense.

What could it be? Photo by Kelly Colgan Azar*

What could it be?
Photo by Kelly Colgan Azar*

Taff et al.’s idea was that male attractiveness does affect success at siring chicks—but only in the right conditions. In Common Yellowthroats, we know that males are more attractive if they have larger black face masks, larger yellow areas on the chest, and brighter yellow on the chest. The researchers had previously looked to see if male attractiveness, measured by these plumage traits, explained variation in their success at siring chicks. It didn’t.

Bummer. Photo by John Flannery*

Bummer.
Photo by John Flannery*

Then they realized that they were assuming that all of the males had the same opportunity to sire chicks—that they were all starting from the same place, flashing their masks and yellow chests at the same number of females. But when the researchers checked their data, they found that this wasn’t true. Some males had lots of females nesting nearby, waiting to be wooed, but others had no close neighbors at all. When they incorporated the males’ opportunity to sire chicks into their calculations, they found that it made a big difference. In males with lots of opportunity to sire chicks, attractiveness mattered a lot, just as you would expect. In males without much opportunity, though, attractiveness didn’t make a difference: you can be as hot as anything, but if there aren’t any neighboring females to impress, it won’t matter.

The moral is that females are more important than anything else. Photo by John Benson*

The moral is that females are more important than anything else.
Photo by John Benson*

The key message in this is that context matters. Something that is true in one environment may not hold in another, and that discordance isn’t an error: it’s genuine variation. It’s real.

So when we see results that seem to contradict each other, we shouldn’t just assume that someone messed up. That contradiction may be a clue that something even bigger and more complex is going on.

Photo by Kelly Colgan Azar*

Photo by Kelly Colgan Azar*

Featured paper:

Taff CC, Freeman-Gallant CR, Dunn PO, Whittingham LA. 2013. Spatial distribution of nests constrains the strength of sexual selection in a warbler. Journal of Evolutionary Biology 26(7):1392-1405.

*Photos obtained from Flickr and used via Creative Commons. Many thanks to these photographers for using Creative Commons!

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3 thoughts on “Featured paper: cheating Common Yellowthroats, or, the world is more complex than you think

  1. I have always been skeptical of the truth of the attractiveness theory. It must be a very difficult thing to study in birds but I would love to know where to read more from those who challenge the commonly held attractiveness rules belief. What about aggression, persistence and testosterone levels? I know there’s a link between testosterone and song in some avian species. I am intrigued by this population context approach.

    • Hi Dolores, glad you’re interested! I’ve simplified things a bit for this post; “attractiveness” can involve behavioral traits (like aggression and persistence), visual traits, song, etc. Hormones like testosterone definitely play a role, influencing behavior as well as, in some cases, possibly being involved in visual traits.

      I don’t think people challenge the existence of an effect of attractiveness so much as struggle to reconcile confusing results with generally-supported theory. The more we find out, the more nuanced our theories become; for example, we now know that females may have different preferences: it isn’t always that one male is The Best, but rather, different males may be The Best to different females. This is all on the general topic of sexual selection, so any kind of text on sexual selection will be a useful starting point if you want to explore further.

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