Juicy-Lipped Male Frogs Give Females Pheromone-Laced "Love Bites"

Love bites aren't usually delivered to the back. Image courtesy of Lisa M Schulte

Love can quite literally be in the air when it comes to pheromones, but some species take matters into their own hands – or mouths – when it comes to ensuring the chemical message meets their target. Direct sex pheromone transmission, as it’s romantically known, is well-documented among invertebrates, but is a less common strategy employed in animals with a backbone – that was, until pouty frogs came onto the scene.

New research published in the journal Frontiers in Zoology details the more direct approach taken by frog species from the Plectrohyla genus. It turns out the males of these species engage in “traumatic mating” to improve their chances of reproductive success, using specialized teeth to effectively scratch their love perfume into females. And they say chivalry is dead.

As it turns out, traumatic mating isn’t all that uncommon among amphibious folk.

“Traumatic mating can be observed in many animal species,” lead author on the paper Lisa M Schulte told IFLScience. “Many frog species pierce the females with spines on their thumbs during amplexus. It is likely that this also serves the transfer of pheromones.”

Amplexus is a fancy word for the sex position frogs and toads assume when mating, which sees the male clasp the female from behind. In this study, the researchers investigated whether the elongated teeth and swollen lips of male Plectrohyla frogs were also used to transfer pheromones during traumatic mating.

frog swollen lips pheromones
The plump pout of Plectrohyla male frogs is best admired from below. Image courtesy of Lisa M Schulte

To find out, they turned to experts on the topic: mating pairs of frogs. Watching the animals revealed that the males were using their bodacious lips to push onto the females’ back during amplexus. The action left small scratches behind, made by the frogs’ special teeth.

Samples from the frogs’ plump pouts revealed that they contained specialized mucus glands that looked similar to those known to excrete pheromones in amphibians. Their purpose was confirmed with whole-transcriptome sequencing, which showed sodefrin precursor-like factor proteins, a known amphibian pheromone.

However, pheromones perform many different behaviors (such as locust swarming) and there’s currently insufficient evidence as to what these “love bites” actually achieve in frogs. It’s possible that they may speed up a females’ egg-laying time, Schulte says, but what makes this study stand out is the species’ unique approach.

“We already found before that several frog species have glands that contain potential pheromones,” said Schulte. “However, all these species seem to transfer these molecules over the female's nose. The most intriguing result of this study is that there seems to be a different pathway, where pheromones are transferred [via skin].”

The researchers hope next to gain a better understanding of the effect male pheromones have on females during breeding among frogs and toads.

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