In the jungles of Borneo, there’s a small frog that waves his back leg to ward off rivals. After extending his leg over his head, the male rotates it backwards in an arc to expose the vivid white of his foot webbing before retracting it. This is called foot flagging, and it came about after their thigh muscles became more sensitive to testosterone over evolutionary time. The findings were published in Proceedings of the National Academy of Sciences this week.
The males of many frog species croak to court females and compete with other males over breeding sites. But Bornean rock frogs (Staurois parvus) live near loud waterfalls, so in addition to vocalizations, these males also use gestures to deter competitors. You can watch a cool little video here. Many species use limb gestures, but how natural selection incorporated them into display routines is still largely unknown.
A team led by Matthew Fuxjager of Wake Forest University wanted to see if the evolution of foot flags was brought about by a change in how hormones influence muscles. Testosterone, for instance, is known to regulate various sexual traits. "It seems a natural hypothesis that this steroid might also influence waving by affecting the motor systems that control physical movement," Fuxjager said in a statement.
The team captured wild Bornean rock frogs near fast-flowing streams in Ulu Temburong National Park in Brunei Darussalam, and their offspring were reared in a large terrarium in the Vienna Zoo that resembles the rainforests where they live. For 10 days, the team captured adult males seen foot flagging in the morning and injected them with either a saline control or with a dose of testosterone. Afterwards, two males who received the same treatment were placed into a small arena with an adult female. Turns out, testosterone increases foot flagging behavior.
Additionally, the team also compared tissue and RNA sequences from the rock frog with that of two other frogs who don’t produce foot flags: Rana pipiens and Xenopus laevis. They found that the evolution of foot flagging was marked by a huge increase in thigh muscle sensitivity to testosterone. Hormone receptors in the muscles that control hind limb movement in the rock frog is expressed at levels 10 times higher than in the leg muscles of the other two frogs.
"The evolution of this new display type is marked by a change in the way that hormones act on the muscles that control the waving movements," Fuxjager explained. "Moreover, we find that this change is similar to the one that happened to the voice box in frogs when they evolved the ability to croak." Changes to frog hormone systems helped refine their motor skills – which led to the evolution of novel sexual displays.