Male Lau's Leaf Litter Toads Give Females A Piggyback In Novel Mating Method

When it comes to mating, Lau's Leaf Litter Toad likes to shake things up a bit. Image courtesy of Lee Wing Ho

We’re only a quarter of the way through 2021 and already it’s been a big year for frog news. From using their lungs as noise-canceling headphones to forming spiritual bonds with recreational frog lickers (please stop) and leaping out of bromeliad sex pools as just a wee tadpole, there’s seemingly no limit to these amazing amphibians’ talents. Now, toads can add piggybacks to their roster, as a new means of mating has been discovered in Lau’s leaf litter toad.

Published in the journal Ecosphere, the discovery comes from Lingnan University in Hong Kong, which happens to be the native homeland of the toad. As a rule of thumb in toad reproduction, it’s usually the males who grab onto the females’ backs when it’s time to turf out some spawn. Evidently, Leptobrachella laui, as they’re also known, chose to divert from tradition – as, for these amorous toads, it’s the lady who hops on top.

The unusual mating arrangement was observed during field surveys which were carried out between 2010 and 2017 with a goal to gain insights into L. laui interactions. Over the seven years, the researchers gathered evidence in the form of videos and photographs and reviewed what they’d seen back in the lab.

Frogs and toads are thought to exclusively mate through external fertilization, with males releasing sperm directly onto the eggs that have been released by a female. Often, this is achieved by the male climbing onto the female’s back to align his reproductive organs with hers so that no precious tadpole ingredients are lost in the process. For Lau’s Leaf Litter Toads, it’s a very different story. Seductively termed “sex-reversed inguinal amplexus”, L. laui lovemaking instead sees the male giving the female a piggyback as he whisks her away to a remote location.

piggyback toad sex
Males may be whisking their partners off to a secret spot that will protect their spawn. Image courtesy of Sung Yik-Hei

“This behaviour has not been observed in other species of frogs and toads, but the frogs conceal themselves soon after pairing, so that it is hard to determine if this is simply a ride on the male’s back or how the eggs are fertilised,” said Professor Sung Yik-hei, Assistant Professor of the Science Unit of Lingnan University, in a press release emailed to IFLScience. “In fact, despite the endeavours of local herpetologists, Lau’s Leaf Litter Toad eggs have never been documented before.”

One theory as to how the unusual mating strategy came about is that it was driven by the very limited options these toads have when choosing a place to lay and fertilize their eggs. With adequate options representing a rarity, the male takes it upon himself to find and defend an ideal egg-laying location. Popular choices include rock crevices where developing embryos are hidden from would-be predators.

If this is indeed the case, then males who piggyback their partners to preferential places glean reproductive success, being more likely to give rise to young who survive to adulthood compared to L. laui who get it on in any old place. Seeking and protecting prime real estate in this way has been observed in other frog and toad species, so while piggybacking females is a new string to the amphibian bow, there exists some precedent for the behavior.

“The discovery of this new mating behaviour demonstrates the variety of natural wonders even in a small city like Hong Kong,” continued Professor Sung Yik-hei. “Through keen observation and persistent hard work, we can begin to understand more about nature. This gives us the ability to conserve the amazing wildlife in Hong Kong.”

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