In the animal kingdom, we often don’t tend to think too much about male mate choice, the choice that the males have over the females they mate with. This is mostly because…well we basically think that males will hump anything that looks vaguely like a female. Or even that DOESN’T look vaguely like a female. Case in point:
Or
…maybe it’s just turtles?
But it turns out that males may in fact exercise a little more choice than we give them credit for. When the odds of finding a female are low (poor turtles), it’s every male for himself, but when there are plenty of females around, the females have very different reproductive capacities, or the males themselves have to give up a lot to mate successfully, choice will happen. Male animals discriminate between females based on things like body size and mass, previous fertility, and of course, ornamentation.
But the amount of choice the male makes can vary a lot depending on things like the social status of the males or environmental context. So the hypothesis is that males (especially males who have the opportunity to mate a LOT) will adjust, not who they mate with, but the quality of their ejaculate. By adjusting the quality of their ejaculate, males could maybe decide where to blow their wad, so to speak (yeah, I went there), and also put the best ejaculate where they need to compete against other males.
But how do we TEST the things which might make a male change the quality of his ejaculate? For this we need a REAL MALE. A titan. A marvel of virility and a masculine inspiration for men everywhere!
(What, you were expecting a tiger or something? Via Wikpedia)
Cornwallis and Birkhead. “Changes in Sperm Quality and Numbers in Response to Experimental Manipulation of Male Social Status and Female Attractiveness.” The American Naturalist, 2007.
Believe it or not, studies in various species have shown that males in more dominant roles often produce a LOWER quality and quantity of sperm than those in subordinate roles. This is presumably because the dominant males don’t have to compete as much as the subordinates, they get first pick of the females. But this hasn’t been tested before, because the animals being studied understandably get annoyed when you try to get between them and their chosen female to get a sample of the semen.
In this case they decided to try again, using chickens. But not your normal chickens, these were Swedish fowl that live in social groups of up to 16 animals. The males form a dominance hierarchy for access to the females. The most dominant males are obviously going to get first crack at the hens, but the hens will often have multiple matings, and sperm competition is intense. Not only do the females go multiple times, the males can ejaculate up to 40 times within a few hours, which often results in quantity over quality, as the sperm quality decreases over time.
So how to determine where male choice is taking place? Well, the females of this chicken breed have head ornamentation, in the form of a large, fleshy comb (like you usually see on a rooster). The larger the comb, the more likely the female is to be sexually receptive. Females with big combs also lay bigger eggs and eggs with heavier yolks, and tend to be socially dominant. So the authors took hens with varying comb sizes and…
All individuals were fully habituated to human presence,
which allowed natural ejaculates to be collected. Females
were fitted with small plastic harnesses and gently held on
the ground in front of the male in a soliciting position
(Pizzari et al. 2003; Cornwallis and Birkhead 2006). Males
readily mounted females presented in this way, and after
copulation, ejaculates were collected, their volume was
measured using a Gilson pipette, and ejaculates were stored
in 5{9f43b4361d9a125bc126dd2a2d1949be02545ec69880430bc4fed2272fd72da3} formalin for sperm counting, which was performed
using standardized methods (Bakst and Cecil 1997).
I love the idea of the harness and the “soliciting position”. I can picture it now. “LOWER THE HENS!!!”
They took males of high and low status, and put them through randomized mating trials over several females, ALSO of high and low status. They took the ejaculate and measured the number of sperm, as well as the velocity, or how good their little swimmers were doing.
You can see here that the dominant males (dark dots), and the subordinate males (light dots), both lost sperm numbers as the went down the line of females.
But here you can see the VELOCITY of the sperm. And this is where it gets interesting. You can see that the dominant chickens had a decrease in sperm velocity the more females they mated with, as they got tired and produced lower quality ejaculate. The SUBORDINATE males, on the other hand, stayed constant, slamming out fast swimmers over and over.
So now we know that the male’s status changes how he mates, with dominant males blowing the wad early with their high velocity sperm, while subordinate males try to get as many good matings in as they can.
But what about when you vary the status of the females?
Here you can see the dominant and subordinate roosters and their sperm numbers as correlated against the comb size of the hens (indicating hen status). You can see on the left that the dominant males saved the good stuff for the ladies with the big combs (you know what they say about girls with big combs…), as their sperm numbers increase the bigger the comb gets. The subordinate males, on the other hand, didn’t discriminate, putting out equal sperm numbers for any and all comb sizes. This held true regardless of the order in which the females were presented.
So what does this tell us? It tells us that the quality of male sperm in chickens is related not only to the male’s social status, but to the female’s as well. The males can choose (though “choose” is a rough term when it concerns the putative smarts of a chicken) whether to expend more or less sperm on a high or lower quality female, based on the status of the female and their own rank in the pecking order (what, you thought I would get through this post without using “pecking order”? Surely you jest). It also means that males have more ways of influencing how their mating goes than just picking a likely female. That’s a lot of processing to go from a chicken’s brain to his nether regions. Maybe they are smarter than we thought.
“LOWER THE HENS”
Cornwallis CK, & Birkhead TR (2007). Changes in sperm quality and numbers in response to experimental manipulation of male social status and female attractiveness. The American naturalist, 170 (5), 758-70 PMID: 17926297
