The social environment has little impact on inbreeding depression in a social mammal

King To Chan, Alexandre Courtiol, Leonie F. Walter, Oliver P. Höner, Larissa S. Arantes, Eve Davidian, Philemon Naman, Loeske E. B. Kruuk, Josephine M. Pemberton, Kasha Strickland

Inbreeding depression describes the decline in fitness caused by breeding between relatives and is now known to be widespread in natural populations. Yet, its relative strength across different fitness components and its sensitivity to social and demographic environments are poorly understood. Using nearly 30 years of life-history, behavioural, pedigree and genomic data from a wild population of spotted hyenas (Crocuta crocuta) in the Ngorongoro Crater, Tanzania, we tested for inbreeding depression in three key fitness components and evaluated whether it is moderated by socio-demographic factors. We estimated individual inbreeding using both pedigree-based and genomic-based approaches. We found evidence for inbreeding depression in two out of three fitness components: when inbreeding coefficients increased, both lifespan and lifetime reproductive success decreased. Juvenile survival also decreased, but not significantly so. We found little evidence that the strength of inbreeding depression varied systematically with sex, social rank, or clan size. Genomic and pedigree estimates of inbreeding yielded broadly comparable conclusions about inbreeding depression when the pedigree was restricted to individuals with at least three known grandparents. Together, these results demonstrate inbreeding depression may be present, and yet not strongly influenced by the social environment, in a socially structured wild population.