Reproduction comes at a cost: animals that reproduce a lot age faster than animals that don’t. There is also a cost associated with sexual interaction itself, not just the resulting reproduction. Promiscuous female fruit flies age faster than their monogamous or celibate friends. This is partly because some of the proteins in male fly semen are toxic to females. But just living with males and not even mating is enough to shorten life span and impair fertility of female flies. So the presence of males alone is enough to hasten ageing in females.
A new study shows that the sex life of lady flies doesn’t just affect them, it also affects their children. The offspring of flies that only mated once lived longer and were at a lower risk of dying than the offspring of the flies that mated multiple times.
This study compared three groups of female flies. The first group mated once, then were housed away from males. The second group mated once, then were housed with neutered males who could still woo them but couldn’t perform the act. The third group were housed with fertile males so mated many times. Offspring were then collected from all three groups.
The offspring from the first group of monogamous mothers tended to live longer than offspring from the other groups. Offspring from the second group, whose mothers were housed with neutered males, were 27% more likely to die than the offspring of the mothers who had only mated once. The offspring from the promiscuous mothers in the third group were 15% more likely to die than offspring in the first group. These results are interesting because the effect was more pronounced in the offspring of the females that were housed with neutered males. Perhaps this is because the males were trying harder to mate and so were stressing the females more with their romantic interludes. This also happens in guppies – if female guppies are sexually harrassed, they produce daughters with smaller bodies and sons with smaller gonads.
This study questions the cost-benefit ratio of promiscuity. Throughout the animal kingdom many females choose to mate with multiple males. This is thought to pass better genes to offspring; at the cost of causing the mother to have a shorter life span. Overall this would be a net positive effect for that population. But if promiscuity is stressing the mother, maybe it is actually causing a net negative effect?
In a separate study, female flies that mated with monogamous males didn’t suffer as much life span reduction as females who mated with promiscuous males. Males that were forced to be monogamous evolved over 47 generations to harm their mates less, and monogamous females evolved to be less resistant to harming. Monogamous flies also produced more offspring than promiscuous matings.
These two studies suggest that the promiscuity that is almost ubiquitous in the animal world may not have quite the evolutionary benefits we once thought.
The Scientific Papers
Dowling et al. Maternal sexual interactions affect offspring survival and ageing. J. Evol. Biol. 2014
Chapman et al. Cost of mating in Drosophila melanogaster females is mediated by male accessory gland products. Nature. 1995
Partridge and Fowler. Non-mating costs of exposure to males in female Drosophila melanogaster. J. Insect. Physiol. 1990
Holland and Rice. Experimental removal of sexual selection reverses intersexual antagonistic coevolution and removes a reproductivce load. PNAS. 1999