Adjustment of costly extra-group paternity according to inbreeding risk in a cooperative mammal

Abstract

Females of many animal species seek mating opportunities with multiple males, despite being able to obtain sufficient sperm to father their offspring from a single male. In animals that live in stable social groups, females often choose to mate outside their group resulting in extra-group paternity (EGP). One reason proposed to explain female choice for extra-group males is to obtain compatible genes, for example, in order to avoid inbreeding depression in offspring. The benefits of such extra-group paternities could be substantial if they result in fitter, outbred offspring. However, avoiding inbreeding in this way could be costly for females, for example, through retaliation by cuckolded males or through receiving aggression while prospecting for extra-group mating opportunities. We investigate the costs and benefits of EGP in the banded mongoose Mungos mungo, a cooperatively breeding mammal in which within-group mates are sometimes close relatives. We find that pups born to females that mate with extra-group males are more genetically heterozygous are heavier and are more likely to survive to independence than pups born to females that mate within their group. However, extra-group matings also involve substantial costs as they occur during violent encounters that sometimes result in injury and death. This appears to lead femalebanded mongooses to adaptively adjust EGP levels according to the current risk of inbreeding associated with mating within the group. For group-living animals, the costs of intergroup interactions may help to explain variation in both inbreeding rates and EGP within and between species.

Keywords: extra-group paternity; extrapair paternity; intergroup interaction; mammal; mating system; warfare..

Figure 1

Effects of EGP on (a) offspring homozygosity, (b) offspring body mass at emergence (30–40 days), and (c) offspring survival to independence (90 days). Bars and confidence intervals show predicted means and standard errors, respectively (while controlling for a significant effect of rainfall on survival to emergence).

Figure 2

The effects of (a) an IGI occurring during group estrus and (b) pack age (years since the group was founded) on the probability of EGP occurring within a communal litter. Figures show predicted means and standard errors from 2 GLMMs.

Figure 3

The impact of pack age (years since the group was founded) on the number of IGIs occurring during estrus (60±5 days before birth). Points show raw data whereas the line and shaded area show the predicted trend with confidence intervals from a GLMM while controlling for zero-inflation and the number of IGIs observed in a comparative time period after estrus (40±5 days before birth).