National Geographic: bi and gay dophins

Male Bottlenose Dolphins often form lifelong pairs. Adolescent and younger males typically live in all-male groups. Homosexual activity is common in these groups. Within these groups, a male develops a strong bond with a particular male partner for the rest of his life. The two male Dolphins become constant companions, often traveling widely. Homosexual sex is a constant part of these relationships. These same-sex couples sometimes take turns guarding while their partner rests. They also defend their mates against sharks and protect them while they are healing from wounds. (Sometimes three males form a trio.) If a partner dies, a male Dolphin may spend a long time searching for a new boyfriend. This is difficult because most other males of the same age already have a stable boyfriend and will stay faithful. If, however, he can find another "widower" whose male partner has died, the two may become a new same-sex couple.

The male Bottlenose Dolphins are best described as bisexuals. They tend to combine periods of extensive bisexuality with periods of exclusive homosexuality. As adolescents and young males, they have regular homosexual interactions in all-male groups, sometimes alternating with heterosexual activity. From age 10, most male Dolphins lives with a stable male partner, and because they do not usually reproduce till 20-25 years old. This "gay period" can be 10-15 years long. When they reach breeding age, they still retain their primary boyfriend while searching for female Dolphins for reproduction. In some populations the boyfriends would cooperate in herding females.

The above is adapted from "Biological Exuberance: Animal Homosexuality and Natural Diversity" by Bruce Bagemihl. I think that is what this National Geographic video is based on, too.