UF Research Ties Homosexual Behavior In Beetle To Evolution

October 20, 1999

GAINESVILLE — In a new hypothesis for a behavior observed in a number of species, two researchers say the process of natural selection may explain homosexual behavior in a beetle that preys on citrus in South Florida.

An article about the research co-authored by an Israeli researcher and a University of Florida professor is scheduled to appear in the Oct. 21 edition of the journal Nature.

Ally R. Harari, a researcher at the Volcani Center at Ben-Gurion University in Israel, and Jane Brockmann, professor and chair of UF’s department of zoology, studied the behavior of Diaprepes abbreviatus, an inch-long black beetle commonly known as the sugar cane rootstalk borer weevil. The research began in 1996 at UF when Harari was a post-doctoral researcher in UF’s department of entomology.

Both male and female beetles mount each other, Brockmann said. When she and Harari studied the females’ behavior in laboratory experiments, they discovered the sight of a pair of mounted females attracts large males, who are equally likely to mate with either of the two females. Small males, by contrast, stay away, apparently dissuaded by the size of the top female.

“We are hypothesizing that by mounting each other, the females are able to attract more attention from larger males than if they were seeking males alone,” Brockmann said, adding that bottom females are capable of pushing top ones off but do not do so.

Homosexual behavior is observed in a number of insects and other animal species, Brockmann said. The standard explanation for the behavior in domesticated animals such as cows is that mounting is a display of dominance, she said. The beetle’s behavior, by contrast, appears to suggest a different explanation.

“By mounting other females, females are improving their reproductive success because they are able to mate with larger males,” Brockmann said. “Larger males are advantageous because their large size may indicate their ability to find food, or large males may transfer valuable resources to the female.”

. Some insects attract mates by emitting certain chemicals, but Harari found that in the rootstalk borer weevil, these cues are unreliable, Brockmann said. Instead, males appear to rely on sight.

“Males are attracted by the sight of heterosexual couples,” she said. “The males and females are almost indistinguishable, so when the males see the mounted females they mistake them for a male mounting a female.”

The female beetles are challenging to study in nature because they mount each other for an average of 17 minutes, which makes it difficult to examine the phenomenon experimentally, Brockmann said. To get around this problem, she and Harari glued dead female beetles to the backs of other females, then observed the males’ behavior.

“We created natural situations, but we created them more often so we could examine the effect,” Brockmann said.

Brockmann said she and Harari are still studying male-male mounting in the species.