Mammals tend to avoid incest as a rule, mainly due to the resulting health issues in offspring. There are exceptions, however, and a team of researchers has discovered—for the banded mongoose, at least—incest isn't always such a bad idea. The animals have an oddly high degree of incest as they tend to live in close-knit groups of about 18 throughout their lives, according to a study in Biology Letters. Researchers studied 14 such groups of the animals in a Uganda park over 16 years and found about 64% of newborns were born to members of the same natal group who mated with each other. Why? Researchers say a mongoose would actually be risking its survival if it tried to mate with a member of another group, Phys.org reports.
Loners who attempt to join a new group can be violently rejected. Plus, new groups have three times the mortality rate of established units. In observing the groups, researchers reported 27% of offspring came from mating between females and relatives like a half-sibling or grandparent, while more than 7% of pups came from parent-child or sibling relationships, Science reports. The conclusion? "The costs of inbreeding avoidance may sometimes outweigh the benefits," researchers say. As David Shultz at Science points out, "if you find yourself looking around the table over the holiday and thinking, 'I can’t believe I'm related to these people,' just be glad you’re not a mongoose." (Seals were recently spotted having sex with penguins.)