Inbreeding avoidance mechanisms have evolved in response to selection against inbred offspring. Inbreeding avoidance occurs in nature by at least four mechanisms:
kin recognition, dispersal, extra-pair/extra-group copulations, and delayed maturation/reproductive suppression. For example, in a study by Simmons in field crickets, female crickets exhibited greater mating latency for paired siblings and half-siblings than with non-siblings. For example, in a study by Krackow et al., male wild house mice were set up in an arena with four separate openings leading to cages with bedding from conspecifics. The conspecifics exhibited a range of relatedness to the test subjects, and the males significantly preferred the bedding of non-siblings to the bedding of related females. For example, Holmes and Sherman conducted a comparative study in Arctic ground squirrels and Belding's ground squirrels. They manipulated the reared groups to include both siblings and
cross-fostered nestmates and found that in both species the individuals were equally aggressive toward their nestmates, regardless of kinship. In certain species where social groups are highly stable, relatedness and association between infants and other individuals are usually highly correlated. Therefore, degree of association can be used as a meter for kin recognition. Individuals can also use their own characteristics or phenotype as a template in kin recognition. For example, in one study, Mateo and Johnston had golden hamsters reared with only non-kin then later had them differentiate between odors of related and non-related individuals without any postnatal encounters with kin. The hamsters were able to discriminate between the odors, demonstrating the use of their own phenotype for the purpose of kin recognition. This study also provides an example of a species utilizing chemical cues for kin recognition. The
major histocompatibility complex genes, or MHC genes, have been implicated in kin recognition. One idea is that the MHC genes code for a specific pheromone profile for each individual, which are used to discriminate between kin and non-kin conspecifics. Several studies have demonstrated the involvement of the MHC genes in kin recognition. For example, Manning et al. conducted a study in house mice that looked at the species's behavior of communal nesting, or nursing one's own pups as well as the pups of other individuals. As Manning et al. state,
kin selection theory predicts that the house mice will selectively nurse the pups of their relatives in order to maximize inclusive fitness. Manning et al. demonstrate that the house mice utilize the MHC genes in the process of discriminating between kin by preferring individuals who share the same allelic forms the MHC genes.
Human kin recognition The possible use of
olfaction-biased mechanisms in human kin recognition and inbreeding avoidance was examined in three different types of study. The results indicated that olfaction may help mediate the development during childhood of incest avoidance (the
Westermarck effect).
Post-copulatory inbreeding avoidance in mice Experiments using
in vitro fertilization in the mouse, provided evidence of sperm selection at the
gametic level. When sperm of
sibling and non-sibling males were mixed, a fertilization bias towards the sperm of the non-sibling males was observed. The results were interpreted as egg-driven sperm selection against related sperm.
Inbreeding avoidance in plants Experiments were performed with the
dioecious plant Silene latifolia to test whether post-
pollination selection favors less related pollen donors and reduces
inbreeding. The results showed that in
S. latifolia, and presumably in other plant systems with
inbreeding depression, pollen or embryo selection after multiple-donor pollination may reduce inbreeding.
Dispersal Some species will adopt dispersal as a way to separate close relatives and prevent inbreeding. This may be attributed to the possibility of encountering kin within local ranges when dispersing. The extent to which an individual in a particular species will disperse depends on whether the benefits of dispersing can outweigh both the costs of inbreeding and the costs of dispersal. Long‐distance movements can bear mortality risks and energetic costs.
Sex-biased dispersal In many cases of dispersal, one sex shows a greater tendency to disperse from their natal area than the opposite sex. The extent of bias for a particular sex is dependent on numerous factors which include, but are not limited to: mating system, social organization, inbreeding and dispersal costs, and physiological factors. When the costs and benefits of dispersal are symmetric for both males and females, then no sex-biased dispersal is expected to be observed in species.
Male dispersal Male dispersal is more common in mammals with
cooperative breeding and
polygynous systems. Australian marsupial juvenile males have a greater tendency to disperse from their natal groups, while the females remain philopatric. In
Antechinus, this is due to the fact that males die immediately after mating; therefore when they disperse to mate, they often meet with female natal groups with zero males present. Furthermore, the Oedipus hypothesis also states that fathers in polygynous systems will evict sons with the potential to cuckold them.
Social cues from the surrounding environment often dictate when reproductive activity is suppressed and involves interactions between same-sex adults. If the current conditions for reproduction are unfavorable, such as when presented with only inbreeding as a means to reproduce, individuals may increase their lifetime
reproductive success by timing their reproductive attempts to occur during more favorable conditions. This can be achieved by individuals suppressing their reproductive activity in poor reproduction conditions. Inbreeding avoidance between philopatric offspring and their parents/siblings severely restricts breeding opportunities of subordinates living in their social groups. A study by O'Riain et al. (2000) examined
meerkats social groups and factors affecting reproductive suppression in subordinate females. They found that in family groups, the absence of a dominant individual of either sex led to reproductive
quiescence. Reproductive activity only resumed upon another sexually mature female obtaining dominance, and immigration of an unrelated male. Reproduction required both the presence of an unrelated opposite-sex partner, which acted as appropriate stimulus on reproductively suppressed subordinates that were quiescent in the presence of the original dominant individual. This improved quality in offspring is generated from either the intrinsic effects of
good genes, or from interactions between compatible genes from the parents. In inbreeding,
loss of heterozygosity contributes to the overall decreased reproductive success, but when individuals engage in extra-pair copulations, mating between genetically dissimilar individuals leads to increased heterozygosity. Extra-pair copulations involve a number of costs and benefits for both male and female animals. For males, extra-pair copulation involves spending more time away from the original pairing in search of other females. This risks the original female being fertilized by other males while the original male is searching for partners, leading to a loss of paternity. The tradeoff for this cost depends entirely on whether the male is able to fertilize the other females' eggs in the extra-pair copulation. For females, extra-pair copulations ensure
egg fertilization, and provide enhanced
genetic variety with compatible sperm that avoid expression of damaging recessive genes that come with inbreeding. Through extra-pair mating, females are able to maximize the genetic variability of their offspring, providing protection against environmental changes that may otherwise target more homozygous populations that inbreeding often produces. Whether a female engages in extra-pair copulations for the sake of inbreeding avoidance depends on whether the costs of extra-pair copulation outweigh the costs of inbreeding. In extra-pair copulations, both inbreeding costs and pair-bond male loss (leading to the loss of paternal care) must be considered with the benefits of reproductive success that extra-pair copulation provides. When paternal care is absent or has little influence on offspring survivability, it is generally favorable for females to engage in extra-pair mating to increase reproductive success and avoid inbreeding. ==Gaps==