There are many ways to live your love life - you can be faithful to one partner alone, have a bit on the side, or be promiscuous, and have a lot on the side. The same can be seen throughout the world - and as animal mating systems evolved just like any other characteristic, we can investigate why things are the way they are. Some arrangements, though, have been harder to fathom than others.
When one male mates with many females, the benefit to him is obvious - he has more offspring, simultaneously with different mothers. Females can’t benefit like this - they can’t be ever increasingly pregnant, so the role of females in a promiscuous system was thought to be relatively passive.
Enter the yellow-toothed cavy, or cuis, a highly promiscuous rodent found in South America. Female cuis are far from passive - actively seeking matings from numerous males, and never letting one male monopolise them. But with so much female effort, we would predict a corresponding benefit to promiscuity - and Norbert Sachser and Anja Keil from Westfaelische Wilhelms-Universitaet, Germany, have been investigating what that might be.
They arranged experiments where females mated either with just one male, or with four males (in which case sperm would mix). The two groups were then left to raise their young - and the number raised successfully was compared between them.
Females from the two treatments gave birth to the same number of offspring, but fewer young from the single-male group survived to weaning than in the promiscuous group. How does promiscuity lead to this benefit - a litter of better babies?
By promoting sperm competition, it is thought that a female can ‘weed out’ poorer male genes, improving the quality of her litter overall. Although the mechanisms of such ‘cryptic choice’ tactics are a subject of debate, some suggest that females of several species can actually manipulate ejaculates or fertilised eggs inside them. Promiscuity lets females choose from a wider range of males - they can find a mate, but if a better one comes along, they can let his sperm jump the queue.
Background
Polygyny (one male mating with many females) is the most common mating system in mammals - one male has enough sperm to fertilise lots of females, and it will normally benefit a male to have as many offspring as possible.
Polyandry (one female with many males) and monogamy (where one male and one female mate together only) are relatively less common.
Sometimes it benefits the male more to help look after just one litter, rather than making more litters, each of which would be worse off - this is where monogamy would develop.