Estimating a Key Parameter of Mammalian Mating Systems: The Chance of Siring Success for a Mated Male

Abstract

Studies of multiple paternity in mammals and other animal species generally report proportion of multiple paternity among litters, mean litter sizes, and mean number of sires per litter. It is shown how these variables can be used to produce an estimate of the probability of reproductive success for a male that has mated with a female. This estimate of male success is more informative about the mating system that alternative measures, like the proportion of litters with multiple paternity or the mean number of sires per litter. The probability of success for a mated male can be measured both theoretically and empirically, and gives an estimate of the intensity of sperm competition and of a male's "confidence of paternity" upon mating. The probability of success for mated males for ten "exemplar" species of mammals is estimated and they are compared for insights into the functioning of their mating systems.

Multiple paternity and the number of offspring: A model reveals two major groups of species

“…[ 15 ] For viviparous (live‐bearing) species, the observed probability of multiple paternity should be influenced by at least two factors: the number of mates for females and the number of offspring in clutches. [ 16,17 ] As the number of mates increases, multiple paternity should increase; and multiple paternity should also increase as clutch size increases. Examination of genetic parentage of clutches in different groups of vertebrates and invertebrates using analyses of microsatellite DNA are yielding an increased number of species for which rates of multiple paternity and numbers of sires per clutch have been estimated.…”

“…However, the probability of multiple paternity in a population depends on the number of mates for the limiting sex and the number of offspring that this choosy sex produces. [ 16,17 ] For example, in a study of Soay sheep that produced twins, the rate of multiple paternity was about 74%. [ 20 ] Under the assumption of an equal chance of paternity among an average of two mating males, the expected probability of multiple paternity for a litter would be 50%.…”

“…Likewise, the probability of multiple paternity in any given clutch is also a function of clutch size and the probability of siring success given mating. [ 16,17 ] The primary assumption of our null model is an equal likelihood of siring success for every mated male (or mated female, for fish species with role‐reversed mating systems [ 25 ] ). Given values for the probability of multiple paternity and clutch size, the probability of success for possible sires can be derived numerically, as there is no closed‐form solution.…”

How much multiple paternity should we expect? A study of birds and contrast with mammals

“…For 92% of the bird populations, the observed probability of multiple paternity fell below the predicted probabilities under our null model. Only 11 of 138 populations (8.0%) exhibited observed values of multiple paternity above the mean model predictions, the latter being conservative estimates of predicted probability of multiple paternity under the assumption of equal likelihood of siring success for mating males (Abebe et al., 2019; Correia et al., 2021; Dobson et al., 2018). For deviations in multiple paternity from the null model, credible intervals of 21 populations overlapped zero, indicating that at most, only 15.2% of the populations were well‐described by the null model (Figure 3).…”

“…We used our previously developed null model to obtain the expected probability of multiple sires under random fertilization with an equal chance of siring success for mating males (Abebe et al., 2019; Correia et al., 2021; Dobson et al., 2018). We used an equation for multiple paternity defined as the probability of more than one sire occurring in a brood or litter, then estimated the probability of successful fertilization (i.e., siring success) for all mating males of a species given observed proportions of multiple paternity, litter sizes, and numbers of sires reported in the literature (Appendix 1: Section S1; Correia et al., 2021; Dobson et al., 2018).…”