Why is it that less than 5 per cent of mammal species are monogamous, compared with about 90 per cent of bird species?
Hazel Russman
London, UK
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This is a consequence of what is known as Trivers’s cruel bind, named after the evolutionary biologist Robert Trivers.
A male animal that impregnates a female leaves her literally holding the baby. She then has to raise any infants to the best of her ability because they might represent her entire genetic investment for the year. The male is free to go off and impregnate other females, increasing his contribution to the next generation. But this will only work if the female is, in fact, capable of raising young on her own.
Most female mammals can do this because they produce milk to feed their young. Occasionally, this isn’t enough. Wolves, for instance, are endurance hunters, and a lactating female can’t follow prey for hours on end. So, to her while she is raising her cubs.
Among birds, only pigeons and a few other species produce any kind of milk. Chickens and game birds have young that can feed themselves almost immediately, so males can safely leave them in Trivers’s cruel bind. But most birds produce young that are naked, blind and helpless. They need to be brooded, kept warm and fed a high-protein diet, such as insects (songbirds) or meat (raptors). This takes two partners.
Most male birds are monogamous because it is the only way they can be sure of having any surviving offspring.
David Muir
Edinburgh, UK
Evolution has equipped animals that reproduce sexually to do so in the most efficient manner to pass on genes to their progeny. Most female mammals can successfully raise at least some of their young by themselves, so males go off to spread their genes elsewhere.
Birds need to leave the nest to find food, leaving defenceless eggs or hatchlings behind. For a male to maximise his genetic success, it is best for him to stay with the one female, share parenting and help protect his genetic investment. However, some birds vary their reproductive strategy.
Hedge sparrows, also known as dunnocks (Prunella modularis), adapt to . Less food means territory size has to be larger and more likely to overlap with neighbours’ territories, leading to greater opportunity to interact with more dunnocks. This socialisation leads to cooperation in defence of territory and much more besides.
Four reproductive strategies are recorded in P. modularis: monogamy; polyandry, between one female and more than one male; polygyny, between one male and more than one female; and polygynandry, a reproductive relationship between more than one male and more than one female. DNA fingerprinting has demonstrated that chicks in the same brood often have different fathers, and it isn’t that unusual to see multiple parenting at the same nest.
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