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.
Highlights
Females often choose to mate with multiple males despite being able to obtain sufficient sperm to fertilize their eggs from a single male
Pups that were the product of extra-group paternity were on average more genetically heterozygous than pups that are the product of within-group matings (LMM: 2(1) = 5.69, p = 0.017, Table 1, Figure 1a)
Pups fathered by extra-group males were significantly heavier at emergence from the natal den (30-40 days old) than pups fathered by within-group males (LMM: 2(1) = 5.28, p = 0.022, Table 2, Figure 1b)
Summary
Females often choose to mate with multiple males despite being able to obtain sufficient sperm to fertilize their eggs from a single male. Why they do so is not immediately obvious and has been a topic of much debate (Akçay & Roughgarden, 2007; Forstmeier et al, 2014). Among animals that live in stable groups, females often copulate with males outside their social unit (Griffith et al, 2002). Females can benefit from seeking multiple mates in two main ways. Females may obtain direct benefits from mating multiply. The paternity uncertainty created through polyandrous mating can lead to an increase paternal care (Goldizen, 1987; Santos & Nakagawa, 2013)
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