Sperm Competition and its Evolutionary Consequences in the Insects

"Since Geoff Parker’s work on dung flies, the study of sperm competition in insects has gone from strength to strength. Leigh Simmons was one of Geoff’s students and wrote this book in 2001. The title is the same as Geoff’s 1970 paper on the subject. Though still very active in the field, Geoff hasn’t written his own book. This is a tribute to him for getting things started. The $64,000 question is why females are promiscuous. It’s easy to see why a male might be promiscuous from an evolutionary point of view: he might father more offspring. There is quite good evidence for this in birds. If a male stays with his partner and prevents her from mating with anyone else, he might produce, say, four offspring. If he was promiscuous, he might produce one or two extra offspring in other birds’ nests –but only if he’s not himself cuckolded. We use molecular techniques to look at parentage and the fertilisation success he has at home and elsewhere. In the species we looked at, males definitely increased their success by being promiscuous. It’s very difficult to see what females get out of being promiscuous. They are not going to have more offspring by mating with more males. If there is a cryptic female choice going on, in which females choose between sperm, the assumption has been that if they choose the right sperm they’ll have better offspring. So far, there isn’t much evidence for that. We are still left with this mystery as to why females are promiscuous. I think the majority of birds have some level of extra-pair paternity. Few are monogamous, though mute swans are. But look at sparrows, blue tits, robins and blackbirds: ten per cent of their chicks will be fathered by someone other than the male of the pair. A lot of those might be accidents. A female out foraging may bump into another male and think: ‘Let’s get it over with.’ But there are other birds, like the reed bunting, where extra-pair paternity levels are as high as 75 per cent. They can’t just be accidents. The most common view is that they are getting better-quality offspring. You find nests and do molecular parent analysis on the chicks. You need chicks from the pair and extra-pair chicks raised together. If the female has made a strategic decision who to have the extra-pair chicks with, you would expect those chicks to do better. And growing faster, and being better at fighting off parasites and diseases. Some studies show that; others do not. A similar idea is that by having offspring fathered by several different males you increase the genetic diversity. Another explanation, for which there is very little support, is that a female must ensure it has some offspring in case its partner is infertile. In birds, there is almost no evidence of infertility in the wild. In humans, there’s a lot to suggest that females not getting pregnant in the pair will go off and have an extra-pair copulation. To ensure pregnancy, women will take the problem into their own hands."