Dear Readers, have you ever read something and thought ‘why did I never think of that?’ My Open University course has moved on to looking at natural selection and survival, and in particular what it is about the behaviour of different species that adapts them to pass on their genes as efficiently as possible. There is a lot to unpack here, but one thing that had never occurred to me was why birds tend to stick with one partner, while mammals usually don’t.
Now, first let’s define terms. Monogamy is where individuals have one partner over a time period – sometimes just for one breeding season, and sometimes for life. Polygamy is where an individual has more than one partner, so a female might mate with many males (polyandry – unusual but not unknown) or a male might mate with many females (polygyny – extremely common). In birds, many, many species are monogamous for at least one season, with a male and a female working together to raise their young, though of course there are many exceptions, including that little brown job the dunnock, where everyone seems to mate with everyone. However, I am hard pushed to think of many examples of mammals where a pair of animals look after their young – red foxes do this and some primates (and you might be able to think of lots of other examples), but the normal pattern seems to be for the male to mate with as many females as possible, and for the females to raise most of the young.
The answer for why monogamy works better for birds than for mammals is all down to basic biology. In birds, the eggs are outside the body of the female, and so can be incubated by both partners. If it’s done by the female alone, it’s in the male’s interest to provide her with food, otherwise their offspring won’t survive to hatch because she’ll have to leave them at risk of chilling or being eaten by predators. Once the chicks are hatched, both partners can provide food. If you are a blue tit with an average brood size of ten nestlings, it’s clearly advantageous to have both parents looking for caterpillars.
The male bird is invested in the offspring, and they can only survive if he participates in their care. The story is different with mammals. Once a male has mated with a female, she has to carry the offspring until they’re born, and then she has to feed them for the first part of their lives, whereas he is free to go off and mate with as many females as possible, in the hope that at least some will survive to pass on his genes. The energy of many male animals goes into fighting off other males and trying to monopolise as many females as possible. You won’t see a male elephant seal providing his harem or offspring with fish, and you won’t see a male squirrel providing his babies with nuts, however cute they are.
Of course there are exceptions to the rule in both birds and mammals. The female jacana (an American bird of the moorhen family) for example, practices polyandry – she rules a large territory and entices as many males as possible to mate with her, whereupon she lays her eggs in a nest that the male has made, and leaves him to get on with the chick rearing. She spends most of her time beating up other females and, in a change to the usual rule, she is 60 percent larger than her male equivalent. However, in these watery habitats nesting sites are extremely rare and are often highly predated (by crocodiles and turtles amongst other animals) and so it’s in the interests of the species for the female to lay as many eggs as possible, and to have an opportunity to replace those eggs if they’re destroyed. The males will call to the females if they’re threatened and the females will race over to protect them, like feathery versions of Wonder Woman.
In mammals, the beaver is an interesting example of an animal with flexible behaviour. In Europe, beavers are largely monogamous, with a pair of adults raising their kits and often staying together for life. In the American beaver however, although beavers pair up and raise kits together like the European beavers, genetic analysis has shown that roughly 50% of beaver kits are not related to the male who is looking after them. This might be because the population of the North American beaver is much higher than that of its European counterpart, and so the females are coming into contact with more males. Combine this with the lower aggression of the North American animal and you have a recipe for a bit of genetic mixing that’s probably not unhealthy for the population as a whole.
And so, in short, in birds monogamy, at least for a season, is fairly common as it needs both parents to provision the young and ensure that they survive – the female has no milk to feed the youngsters and so she can’t do it alone. In mammals, the female carries the young inside her body, and can then provide them with their sole food, sometimes for months, so they have a good chance of surviving in many species without the involvement of the male, who can sow his ‘wild oats’ and improve his chance of passing on his genes that way. What’s interesting is all the many, many variations on this theme that have developed over the millenia, and how different species adapt to different situations.
Another interesting question concerns who gets to mate with who, and why, but that is definitely the subject of another post.