Almost a third of Laysan albatross couples are female-female pairs that build nests and rear young together. They are more reproductively successful than unpaired females. Photograph: Eric VanderWerf/Trends in Ecology & Evolution
Birds do it. Bees probably do it. No one's sure whether educated fleas do it. What they do is have same-sex relationships and, in a new review of published research on the subject, biologists have started to consider what it might mean for the evolution of the animals in question.
Nathan Bailey and Marlene Zuk, biologists at the University of California, Riverside, found that same-sex relationships were a universal phenomenon in the animal kingdom, seen in everything from worms to frogs to birds. "It's clear that same-sex sexual behavior extends far beyond the well-known examples that dominate both the scientific and popular literature: for example bonobos, dolphins, penguins and fruit flies," said Bailey.
Penguins have been known to form long-term same-sex bonds where males will engage in sexual activity. Toads generally don't discriminate between sexes while marine snails all start out male and, when they mate with another male, one of them helpfully changes sex. Dolphins will often touch their genitals together or one male might even mount another and penetrate its blowhole. Bonobos go the furthest in same-sex bonding with regular copulation among males.
But not all relationships should be considered the same. A male fruit fly, for example, may court other males because it lacks a gene that allows it to tell the difference between the sexes. "But that is very different from male bottlenose dolphins, who engage in same-sex interactions to facilitate group bonding, or female Laysan albatross that can remain pair-bonded for life and cooperatively rear young," said Bailey.