Biologists have long wondered why animals commonly engage in sexual behavior with others of the same sex despite the act not leading to actual offspring production and continuation of the species. It appears, however, that the gene encouraging this can actually offer evolutionarily benefit to the opposite sex.

Swedish researchers recently discovered that when male beetles were selectively bred for same-sex sexual behavior (SSB), their sisters laid more eggs and gave birth to more offspring than previously.

When they bred female beetles for SSB too, the resulting genes assisted male reproduction.

“[W]e noted that males that had been bred for increased same-sex mounting behavior were less discriminating when given a choice between courting a male or a female in later tests,” said study author David Berger, “while their sisters laid more eggs and produced more offspring than before.”

Study co-author Dr. Alexei Maklakov echoed that some SSB-bred males still mate with females, with the involved genes linked to the concept of “opportunity cost.” The more indiscriminate attempt at sex may tire them out, he added.

Since males and females share most genes, SSB is hypothesized to occur in one sex as its underlying genes provide benefits when expressed in the other.

The researchers tested their hypothesis in small seed beetles, where both male and female express low SSB levels. Using artificial breeding to produce genetic strains leading to increased SSB predisposition, they found that when a specific sex was bred for greater SSB, opposite-sex siblings enjoyed more optimal reproductive performance.

Berger explained that the genetic links shared by different male-female characteristics can clue in on how genetic conflicts between sexes shape the evolution of traits. Same-sex sexual behaviors are one example.

“The genetic mechanism explaining the occurrence of SSB that we demonstrate in these beetles could apply equally well in very different animals,” he added in an Independent report.

Seed beetles undergo a courtship ritual prior to mating. The males, for instance, mount their partner from behind before inserting their version of a penis.

There could be risks involved in sex between males, such as the penis getting trapped in the wing casing of the partner and leading to death of one or both. Same-sex practice in females, on the other hand, is deemed less risky.

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