When Not to Avoid Inbreeding

Abstract

Avoidance of incestuous matings is widely reported across many animal taxa, and the adaptive value of such behavior is explained through inbreeding depression. However, an old and somewhat neglected theoretical result predicts that inbred matings offer another, positive effect on the inclusive fitness of parents: an individual who mates with a relative will help that relative to spread genes identical by descent. This benefit can be substantial, if the additional mating achieved by the relative does not harm his mating success otherwise, and in the context of selfing in plants the phenomenon is well known. Here, we develop a model that derives expected values of inbreeding tolerance, that is, the magnitude of inbreeding depression that is required to make individuals avoid inbreeding, for different animal life histories and parental investment patterns. We also distinguish between simultaneous and sequential mate choice, and show that inbreeding tolerance should often be remarkably high in the latter scenario in particular, although egalitarian parental care will lead to lower tolerance. There is a mismatch between theory and data: the almost complete lack of cases where individuals prefer to mate incestuously is at odds with a large overlap between the predicted range of inbreeding tolerance and estimates of inbreeding depression found in nature. We discuss four different solutions to this enigma, and suggest that inbreeding tolerance, where it is found, should not always be attributed to a simple constraint that has prevented finding any other mate.

Maternal effects alter the severity of inbreeding depression in the offspring

“…Depending on the parents' capacity to moderate the deleterious effects of inbreeding in the offspring, there might be selection for inbreeding avoidance, tolerance or even preference. Existing theory [46][47][48] has overlooked the possibility that maternal effects might influence animal inbreeding strategies. Until now, theoretical models have focused on how the costs of dispersal, mating system, mate encounter rate, and kin recognition might shape the balance between inbreeding tolerance and avoidance [46 -48].…”

Maintaining functional major histocompatibility complex diversity under inbreeding: the case of a selfing vertebrate

“…Although inbreeding avoidance is considered the norm, it can vary depending on the relative costs of inbreeding in relation to, for example, outbreeding [11] or dispersal [12]. In fact, inbreeding tolerance depends on the strength of inbreeding depression but also on the mating system and mate availability [13]. Thus, mating with close relatives could be beneficial, for example, in species with low inbreeding depression and limited mate availability [13] or when there is a conflict over parental care and related parents display increased cooperation in comparison with unrelated parents [14].…”

No postcopulatory response to inbreeding by male crickets

“…Fewer studies have documented preferences for related individuals as mates (Cohen & Dearborn 2004), although this may be due to the overwhelming preconception that mating with close relatives is bad (Kokko & Ots 2006). Consistent with the theoretical expectation, recent work with red jungle fowl has revealed counteracting sex-specific responses to inbreeding (Pizzari et al 2004).…”

“…For example, a male can increase his mating success, and therefore inclusive fitness, by mating with his sister if there are no opportunity costs associated with the mating. Similarly, a female will gain fitness benefits via mating with her brother, directly through her own offspring and indirectly by improving her brother's mating success (Parker 1979;Lehmann & Perrin 2003;Kokko & Ots 2006;Parker 2006). Whether selection should in fact favour inbreeding or its avoidance will depend on both the costs associated with inbreeding depression and the inclusive benefits of mating with kin.…”