Tyler Cowen's interview with ornithologist Richard Prim is interesting throughout. A sample:
COWEN: If you were trying to give us the most fundamental explanation of why sexual dimorphism is different in birds compared to mammals, what would that be?
PRUM: Well, that’s actually a really big question. [laughs]
COWEN: Of course, but the most fundamental factor — what is it?
PRUM: The most fundamental factor is that most birds don’t have a penis.
COWEN: Talk me through the equilibrium there.
. . . .
PRUM: Birds have a very rapid period of rapid development. That means that they grow up and leave the nest, and you need two parents to do that efficiently in most diets or most kinds of ecologies. That means the dad’s got to be at the nest.
We usually thought that you have social monogamy, at least two birds helping raise the young, because the young are so needy and they have to grow up quickly. But there’s another possibility, which is that they could evolve to be so needy and grow up quickly because they managed to get males at the nest.
One of the things that happened in the phylogeny of birds — you’ve got ostriches and their relatives, and you’ve got chickens and ducks, and then you’ve got the rest of birds, and that’s a bunch. That’s the vast majority of them, and in that lineage leading to the rest of birds, the penis evolved away, and the question is why. My own theory is that female birds preferred mates that did not have a penis.
One of the ancillary benefits of that, one of the correlated benefits of that is that they were no longer subject to sexual coercion or sexual violence. They could be coerced behaviorally, but they couldn’t be forcibly fertilized. That means that they have freedom of choice, and what do they do with their freedom of choice? They choose beauty. One of the reasons why birds are so beautiful is that males don’t have a penis. They have to be subject to choice in order to effect reproduction, and also they have to invest if females require it.
I suppose a question is, why didn't mammals, for example, evolve the same way?
The explanation isn't much of an explanation, at least not to the layman. Another hypothesis might be the penis is extra weight during flight, so get rid of it. Look at how long it takes a duck to reach lift off :)
Fundamentally, evolution is just random mutations + random chance = favorable traits.
There are lots of possible factors at play, and it would require an expert to really begin to unpack what is truly likely, but let's take a quick stab at it anyway.
The most obvious difference is flight, which perhaps go a long way to explaining things - it's a bit hard to subject a female to sexual coercion or violence when they can just fly away from you. Of the bird species mentioned which retained penises, it's worth noting that ostriches and their relatives are all flightless, as are chickens. The one notable exception is ducks - presumably there is some factor (or factors) which make them notably different.
One interesting factor to note is that overall, wild birds have a notably higher mortality rate of females, causing adult populations to skew male despite offspring being born at a roughly equal ratio of male to female. This is in stark contrast to mammals, where adult sex ratios skew female.
Waterfowl in particular seem to skew very heavily male, and ducks even moreso. There is also a marked disparity among different kinds of waterfowl and ducks that correlates with mating pair bonding habits - in species that delay bonding, or which bond only briefly, populations skew much more heavily male. This is seemingly because the females do not receive as much support from their mates - females that nest, brood, rear, and molt without help from a bonded male seemingly face greater stresses and suffer severely increased mortality rates, thus causing adult males to drastically outnumber females within those species, even above the overall rate of male dominance in birds.
Seemingly, once male dominance in adult populations within a species reaches a certain tipping point, it becomes more evolutionarily beneficial for males to retain their penises than to lose them, because rape becomes a much more necessary method of outcompeting other males in passing on genes.
This seems to match up strongly with the fact that the species of birds who do retain penises overall do not practice monogamy and long-term pair bonding. Roughly 90% of bird species are monogamous, with similarly high percentages both lacking penises and being able to fly. The evidence for a possible connection seems strong.
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