is significantly smaller than that from non-inbred litters indicating inbreeding depression. In natural populations of the bird Parus major (great tit), inbreeding is likely avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative. Toads display breeding site fidelity, as do many amphibians. Individuals that return to natal ponds to breed will likely encounter siblings as potential mates. Although incest is possible, Bufo americanus siblings rarely mate. These toads likely recognise and actively avoid close kins as mates. Advertisement vocalisations by males appear to serve as cues by which females recognise their kin. See also Pre-copulatory isolation mechanisms in animals Biology and sexual orientation Green Porno, a series of short films about animal mating, enacted by humans, airing on the Sundance Channel List of animals displaying homosexual behaviour r/K selection theory Polygamy in house mouse Sexual behaviour of dogs Sexual behaviour of horses References Bibliography Further reading Ronald D. Nadler (1980) Reproductive physiology and behaviour of gorillas., in R. V. Short and Barbara J. Weir (eds) The Great Apes of Africa, Cambridge, 1980. Journals of Reproduction and Fertility Ltd. Sexual behavior of horses External links National Geographic San Francisco Zoo has run a "sex tour" covering animal sexuality, on Valentine's Day A wild, and gay, kingdom World Science Is it relevant to look at the animal kingdom to determine if human same-sex behaviour is "natural"? Ethology Reproduction in animals Mating Articles containing video clips