Why do females mate with multiple males ? The sexually selected sperm hypothesis

One debated issues in evolutionary biology is, why in many species females mate with multiple males. Several hypotheses have been put forward, yet the benefits of multiple mating (here defined as mating with several males) remain unclear in many cases. The sperm sexual selection (SSS) hypothesis has been developed to account for the widespread occurrence of multiple mating in females. It argues that multiple mating by females may rapidly spread, when initially a small fraction of the females mate multiply, and if there is a heritable difference among males in one or several of the four characteristics: (1) the quantity of sperm they produce; (2) the success of their sperm in reaching and fertilizing an egg; (3) their ability to displace the sperm that females stored during previous mating; and (4) their ability to prevent any other male from subsequently introducing sperm (e.g., differential efficiency of mating plugs).

The timing and interval of mate encounter affects investment during mating

The benefits obtained from mating are usually condition-dependent, favouring the evolution of flexible investment during copulation, for instance, in terms of invested time, energy, or sperm. Flexible investment strategies are predicted to depend on the likelihood of acquiring alternative mates and therefore they should depend on the timing of mate encounter. However, scarce experimental evidence for this hypothesis exists. Here we manipulated the time delay until first mating and the interval between first and second mating in the polygynandrous common lizard, Zootoca vivipara. We determined treatment effects on fertilisation success and copulation duration, the latter being a proxy for investment in mating and for quantity of transferred sperm. The duration of the second copulation decreased with increasing inter-mating interval and depended on the fertilisation success of first mates. The former provides evidence for time-dependent investment strategies, most likely resulting from the progression of the female's reproductive cycle. Fertilisation success of first mates increased with increasing inter-mating interval and was higher when females were closer to ovulation, showing that flexible investment strategies significantly affected male reproductive success. This points to fertilisation assurance, which may mitigate negative effects of low population density on reproductive success, e.g. Allee effects.

Female plumage spottiness and parasite resistance in the barn owl (Tyto alba)

The hypothesis that extravagant ornaments signal parasite resistance has received support in several species for ornamented males but more rarely for ornamented females. However, recent theories have proposed that females should often be under sexual selection, and therefore females may signal the heritable capacity to resist parasites. We investigated this hypothesis in the socially monogamous barn owl, Tyto alba, in which females exhibit on average more and larger black spots on the plumage than males, and in which males were suggested to choose a mate with respect to female plumage spottiness. We hypothesized that the proportion of the plumage surface covered by black spots signals parasite resistance. In line with this hypothesis, we found that the ectoparasitic fly, Carnus hemapterus, was less abundant on young raised by more heavily spotted females and those flies were less fecund. In an experiment, where entire clutches were cross-fostered between nests, we found that the fecundity of the flies collected on nestlings was negatively correlated with the genetic mother's plumage spottiness. These results suggest that the ability to resist parasites covaries with the extent of female plumage spottiness. Among females collected dead along roads, those with a lot of black spots had a small bursa of Fabricius. Given that parasites bigger the development of this immune organ, this observation further suggests that more spotted females are usually less parasitized. The same analyses performed on male plumage spottiness all provided non-significant results. To our knowledge, this study is the first one showing that a heritable secondary sexual characteristics displayed by females reflects parasite resistance.

Inter- and intra-individual variation of oxytocin secretion in oestrous mares exposed to stallions

The neuro-peptide hormone oxytocin regulates several reproductive mechanisms in mammals, such as uterine contractions during parturition and milk ejection in the lactating mammary gland. Oxytocin may also influence behavior and behavioral strategies, e.g. pair bonding, social recognition, maternal behavior, trust building, or anxiety. Teasing oestrous mares by a stallion provokes the release of oxytocin. We therefore tested whether such elevated oxytocin levels reveal possible mate preferences as determined in typical preference tests.