Dunbar says the bonobo's use of homosexual activity for social bonding is a possible example, adding, "One of the main arguments for human homosexual behavior is that it helps bond male groups together, particularly where a group of individuals are dependent on each other, as they might be in hunting or warfare."
For instance, the Spartans, in ancient Greece, encouraged homosexuality among their elite troops. "They had the not unreasonable belief that individuals would stick by and make all efforts to rescue other individuals if they had a lover relationship," Dunbar added.