State of the Planet

News from the Columbia Climate School

Evolution Supports a Rainbow of Biodiversity

Bonbobo mom and baby Pan paniscus at Cincinnati Zoo – Photo by Ltshears
Same-sex-relationships among animals seem to be in opposition to our understanding of Darwinian evolution—an organism who fails to secure a counterpart to mate with will not pass on its genes to the next generation. One could then infer that such costly behaviors would slowly be removed from the population through natural selection. However, same-sex bonds are far too common in the natural world to support such reasoning.

Previous studies had considered how same-sex relationships might have emerged, but they were not placed in an evolutionary context. Though counter intuitive on the surface, a wealth of research on same-sex relations indicates that homosexual behavior is not only prevalent, but often adaptive, even driving the evolution of animal physiology and socialization.

Scientific American recently reported that researchers had collectively observed over 1,500 vertebrate species engaging in homosexual activity. Though bonobos, giraffes, killer whales, and big horn sheep are among the most publicized species to engage in same-sex sex, the list extends far beyond the popular examples that dominate the literature. For example, male dolphins pursue sex with others males, female Japanese macaques happily mount one another, and bonobos engage in genital massages.

The deep-sea dwelling squid that ominously swims in the dark waters of the Pacific Ocean recently joined the countless other species that are known to engage in same-sex relations. Using remotely operated vehicles in the Monterey Submarine Canyon, researchers from the Monterey Bay Aquarium Research Institute and University of Rhode Island found evidence of mating on similar body locations in males and females of the squid, Octopoteuthis deletron. Among the 108 individual squid captured on video, the sex of 39 of them were discernible — 19 females and 20 males. Upon review images of these individuals, researchers concluded that 9 males and 10 females had embedded sperm, indicating that the males were trying to mate equally with other males and females.

According to Marlene Zuk (reported by the NY Times), author of the newly published “Sex on Six Legs: Lessons on Life, Love, and Language from the Insect World, “The animal is not making a mistake. It’s not mistaken to deposit sperm with another male,” because in the pitch black, unable to discern the sex of a fellow species member, it is far better to waste its sperm on other males than to miss out on an opportunity to reproduce. In other words, a male squid that has the programmed module, “shoot sperm at all organisms resembling a female squid,” will produce more offspring than a male that conserves his sperm. Ultimately, the behavior will increase in frequency in the population because it is an adaptive trait.

Such is the story for many homosexual behaviors that were once thought to be maladaptive.

Marlene Zuk and her collaborator Nathan Bailey have found that a single Laysan albatross is far worse at raising a chick than a female who pairs up with another female. There is strength in numbers after all.

However, in instances where such a strategy is costly, defense mechanisms may surface. For example, according to Bailey, “male-male copulations in locusts can be costly for the mounted male, and this cost may in turn increase selection pressure for males’ tendency to release a chemical called panacetylnitrile, which dissuades other males from mounting them.”

Evolutionary biology is itself continually evolving—the view that gender roles and behaviors are fixed in a binary state is no longer substantiated in the academic record. Nature supports a much richer existence, allowing for an array of diverse relationships.

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