Can a bird be a cougar? This is not a trick question. By the metrics of popular culture, a cougar is an older female who prefers younger males as mates. Now, this term can either be viewed in a derogatory fashion, wherein females are demeaned for choosing younger partners, and males suffer no such term in their pursuits for younger females. However, this term can also be empowering: it gives females agency over their sexual preferences, not subject to societal constraints, powerful in name and in invoked image. If applied to flocks of birds hailing from North Africa, females have the very same agency — although not subject to human standards of judgement — and they seem to prefer males younger in age for siring, or fathering, their young.
In their September 2019 report from the journal Proceedings of the Royal Society B entitled “Post-copulatory sexual selection allows females to alleviate the fitness costs incurred when mating with senescing males,” Pauline Vuarin and eight other researchers from across the French-speaking world as well as Abu Dhabi provide evidence that female Chlamydotis undulata undulata, or the North African houbara bustard, take advantage of certain mechanisms after mating with their male counterparts that allow them to secure offspring fathered by younger males. North African houbara bustards are polygamous, meaning females mate with multiple males of all ages during mating season, and this gives way to the mechanisms previously mentioned: firstly, because sperm from many different mates are trying to fertilize the same eggs, only the superior swimmers will avail. This is known as sperm competition, and it allows for females to be inseminated by those males with the best parental donations. Secondly, female houbara bustards can possibly choose which sperm they prefer to fertilize their eggs internally. This is known as cryptic female choice, and it allows for females to literally decide which sperm will best benefit the coming generation.
One might ask, why would female houbara bustards prefer the sperm of younger males? Would the obvious success in later life, and the experience in living itself, not be persuasive enough for females to prefer older mates? The authors of this report address this exact qualm, and they point out that as these North African male birds age, their fitness — or the ability to reproduce successfully — decreases, and this has a direct effect on female fitness, because their offspring will be affected by this reproductive ability as well as the aging traits donated by these males.
In designing their experiment, the researchers made sure to eliminate the possibility of female mate choice before the occurrence of insemination, as each female in the study was artificially inseminated. In this manner, the female houbara bustards could not reveal their mating preferences before fertilization, which would have skewed the results of the research. The authors of the report divided the females amongst three groups; the first group was inseminated with a balanced, combined mix of sperm from young and old males; the second group was inseminated with sperm from one age group, and finally, the third group was inseminated with sperm from the other age group. The eggs were then incubated, and when they hatched, they were monitored for 90 days, after which they were released into the wild. Survival rate, through GPS tracking, was checked 28 months following release.
Regarding the first group, results showed that, out of 118 fertilized eggs to which a father could be assigned (86 had hatched), older males had success 33.7% of the time while younger males had success 66.3% of the time. Because every fertilized egg did not survive until hatching, the researchers restricted the same analysis to the first egg laid by each female in this group in order to secure a higher rate of paternal assignment. These further results increased the difference in success between the male age groups, bumping younger males up to 75% success and relegating older males to 25% success.
Furthermore, evidence showed throughout all three female groups that eggs fertilized by older males were likely to hatch 58% of the time, while eggs fertilized by younger males were likely to hatch 70% of the time. After 90 days, those offspring fathered by younger males had higher body masses relative to their size compared with those offspring fathered by older males, and after 28 months in the wild, female houbara bustards fathered by younger males had the highest survival probability out of each sex fathered by each age range.
Although the authors note that more variables could have been accounted for in their study, such as varying sperm traits, as well as the acknowledgement that effects of so-called “sexual selection” between the sexes are sometimes hard to differentiate, they assert that these points do not take anything away from their results. It is a powerful notion that, even in the animal kingdom, females have agency over their sexual choices; that they are the ultimate judges of their future, and the future of their offspring. They may not look like cougars, in a literal or in a metaphorical sense, but they too have the ball in their court when it comes to sexual preference, and it is evident that there are obvious benefits to choosing the best for their young.
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