UNDER THE WING OF ANOTHER DAD
Birds are way more promiscuous than previously suspected. In 2012, researchers have indeed shown for the first time that free-living songbirds may be able to increase the overall fitness of their offspring via “extra-pair copulation”, a scientific term for “infidelity”.From a logical point of view, this whole cheating business might make no sense. But picture a population with an equal sex ratio: despite everyone probably finding a breeding partner, not all may be lucky enough to find and pair-up with “the one”. In that case, why not broaden one’s horizon and breed outside the bond with several potential genetically superior mates.
Non-exclusive couples
With the advent of genetic testing in the late 1980's, many avian species which were once considered monogamous, because of their seeming devotion to their mates, lost some credibility in the eye of fidelity advocates. It is now known that, despite about 90 per cent of bird species being socially monogamous during the breeding season, pair bonding and eventual co-operative hatchlings nurturing do not necessarily guarantee either partners being sexually faithful. Even among socially monogamous birds there is a lot of extra-pair copulation going on and with DNA analysis; scientists have been able to flag cuckold males’ nests.Evolutionary success story
In the animal kingdom, the sine qua non to being an evolutionary success story is making many fit babies, which will ultimately be reproductively successful when they reach adulthood. The study revealed that there are major differences in legitimate and illegitimate offspring’s reproductive abilities. When they grow up, progeny born from promiscuous affairs had higher reproductive success, and male extra-pair offspring were more likely to be sexually unfaithful themselves.Quantity vs. quality
Since there are only a limited number of chicks that can be born from their respective female partners, males resort to infidelity. For the latter to pass down their genes to as many offspring as possible, they will seek to inseminate additional females. This approach is not flawless, however. Producing lots of babies that die before they reproduce defy the whole purpose of evolution and reproductive fitness. Nevertheless, if multiple mating does increase the probability of having genetically superior offspring, cheating might indeed favour evolutionary success.The brighter, the bigger, the better
A good breadwinner may not necessarily be the best sperm donor. But for our unsatisfied female juncos, not pairing up with the ideal guy appears not to be the end of the world. In their quest for males with ‘good’ or ‘compatible’ genes, females commonly hook up outside their bond. It has been argued that in many bird species, females would sometimes sneak behind their partner’s back to mate with bigger, more brightly coloured males or with the better singers (as may be the case with juncos). Thus, in the hope of producing babies with superior genetic qualities, our little ladies would follow these good health indicators when looking for mating partners.Thirty years of data have been used by Gerlach to not only look at nestling and juvenile quality (e.g. size, survival and disease immunity), but also at lifetime reproductive success in the juncos. During early life stages all offspring, whether born within a socially monogamous pair or outside, do not differ in a variety of traits, but extra-pair chicks of both sexes tend to be more fertile and better caregivers as adults.
Study implications
Many scientists have suspected that multiple mating could have genetic benefits in precarious environments whereby mothers would try to ensure that at least some of their progeny survive. Indeed, not having similar genetic background to their siblings, some chicks may by chance be more adapted to changing environments and survive harsher conditions.Because sexual promiscuity is prevalent among most living organisms, Gerlach’s study gives us an insight into the breeding system of monogamous creatures and may have implications for breeding of managed populations, such as captive animals or endangered species. Fertility issues (e.g. both partners in a couple being non-sterile but producing no offspring together due to genetic incompatibility) in these creatures may perhaps depend on specific individuals’ genetic contribution or behavior. Hence, looking into altering the social environment in which the animals interact and reproduce can help resolve reproduction puzzles.
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