American Society of Naturalists

“Females with Attractive Mates Gain Environmental Benefits That Increase Lifetime and Multigenerational Fitness”

Douglas G. Barron, Hubert Schwabl, Patrick A. Carter, Daniel T. Baldassarre, Willow R. Lindsay, Jordan Karubian, and Michael S. Webster: Read the article

A decade-long study by Barron et al. shows that female birds with attractive mates have lower breeding costs, so make more young across their long lives. Their attractive sons also produce more grand-offspring. Male environment, not genes, drives these multigenerational benefits.

As I child, I remember walking around a zoo and being delighted when a brilliantly blue peacock meandered across the path in front of me, feathers glimmering as they caught patches of sun. I was even more surprised when I learned that only the male peacocks displayed these stunning feathers and that in most animal species, unlike most humans, it is the male that is adorned with beautiful embellishments. These ornamentations are driven by sexual selection, a form of evolution driven by competition within the same sex for mating with the opposite sex, because females often prefer to mate with ornamented males. Although this phenomenon has been studied for decades, scientists still do not fully understand the reasons why females exhibit these mating preferences. Doug Barron and a group of other researchers set out to elucidate the reasons behind these mating preferences.

Ornamented males could potentially provide benefits to females in two main ways: environmental and genetic. Primary environmental benefits would mean that ornamented males provide an environment that allows their female partners to produce more offspring than females with non-ornamented males, or that helps the offspring survive better. There is also the possibility of secondary environmental benefits, which means that the ornamented male’s environment helps their female partner to birth higher quality offspring that would produce more offspring of their own. Finally, genetic benefits would be heritable traits passed down by the ornamented males that would allow their offspring to survive and reproduce better.

To resolve the relative contributions of environmental and genetic benefits in female mate choice, Barron and his team studied Red-backed Fairywrens, a useful model organism because the females are socially monogamous but sexually promiscuous. Thus, the environmental benefits afforded by a female’s social partner can be disentangled from the genetic benefits of the male that sired her offspring. In a given population of Red-backed Fairywrens, some males have ornamental red/black nuptial plumage, and other males are brown like their female counterparts. While it is evident that the females prefer the ornamented males, the reasons were not fully understood.

After continuously observing two populations of fairywrens in Australia for a decade, Barron and the team were able to evaluate the role of ornamentation in several different aspects of the female’s life and her offspring’s life. They identified a few important trends regarding the female’s life: (1) Females mating with red/black males laid a greater number of eggs and had higher annual reproductive effort because they began nesting earlier in the season. (2) Females that paired with red/black males improved their body condition throughout the span of the breeding season, while females paired with brown males did not. (3) Females paired with red/black males lived longer and had more offspring. (4) Females that began breeding earlier in a season had more grandchildren the following year.

Barron also obtained data regarding the fitness of the female’s offspring: (1) Sons born early in a season were more likely to develop red/black plumage and produce offspring the following year. (2) Sons raised by red/black social fathers were more likely to develop red/black plumage than those raised by brown social fathers. (3) The sons’ plumage color and reproductive success were not inherited from their genetic fathers.

Together, these findings can be used to clarify female mate preference. The benefits of mating with an ornamented male appear to be primarily environmental, not genetic: females paired socially with red/black mates – regardless of the genetic father – produced more offspring, had higher survival, and were more likely to produce sons with ornamented plumage. According to Barron’s model, the largest contributing factor to these observed benefits is that females paired with red/black males breed earlier – the earlier breeding time allows for more broods and more available resources for the offspring. Though the reason why ornamentation affects breeding time is not fully elucidated, Barron hypothesizes that females with red/black mates benefit from better resources on the superior territories of their mates, preparing them to breed earlier in the season.

Barron’s decade-long study has provided valuable insight into the mechanisms of ornamental breeding in Red-backed Fairywrens. Future studies into these birds and other ornamental breeders can further elucidate the environmental benefits of female mating preferences in different species and contexts. So, next time you pass by a dazzlingly colored bird or see the proud protruding antlers of a buck, take a moment to appreciate how these beautiful features also play a crucial role in the survival and reproduction of the species.

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Kaleigh Remick is currently a PhD student in the Department of Molecular Biology at Princeton University, where she studies replication of influenza A virus in the te Velthuis Lab. She graduated from Cornell University with a B.A. in biological sciences and a minor in English. When she’s not in lab, you can find her bartending, salsa dancing, tutoring at prisons, conferencing at the Writing Center, eating ice cream, or reading a book!