Trembling for sex: Biotremulation in Morelet’s frog

You’ve probably seen the Red-Eyed Tree Frog (Agalychnis callidryas) already, at least in pictures or documentaries about the Neotropics. They’re cute, I get it, but for the sake of “Loving the Unloved”, I will take you through some aspects of frog sex with their sister species: Agalychnis moreletii, the black-eyed tree frog.

Let’s get one thing out of the way: their eyes are actually DARK RED but seem black to human sight under normal light conditions. Moving on.

It is no secret that frogs sing. We’ve all read books with frogs going “ribit” or “quack”, you’ve probably heard some species already and maybe even confused them with insects (a common thing in the Neotropics at least!). Song diversity is way greater than species diversity itself. How can that be? Well, just as you wouldn’t talk to your partner the same way you’d talk to your pet, frogs also have different songs for different situations. However, the mating rituals of Agalychnis moreletii go beyond just “singing”!

Agalychnis moreletii live in tropical and subtropical moist lowland and montane forests, close to permanent or temporal lentic freshwater bodies, like ponds, throughout part of Mesoamerica. They are arboreal frogs, so they have adaptations to climb trees easily and they usually spend the days high above in the forest canopy.

This species presents a very clear sexual dimorphism, where the adult males are noticeably smaller than adult females (pictured above). Other than that and their inconspicuous genitals, the shape and coloration are pretty similar for both sexes. So how do the females choose which male is THE one?

It’s all about the Serenade

Yup, you guessed it, by singing! Or, to use the “technical” term: calling. Frog calls have different traits that the female frogs will judge, like call duration, call rate, and frequency. But calling is not cheap! It requires a lot of metabolic energy, which is why this is an indicator of how fit a male is, not just genetically, but also in how well he uses his available resources. So a good combination of the calls traits gives the females an idea of the males’ energy, motivation, and availability!

The calls males use to attract females are usually referred to as “zworp” calls.

So…are males just singing around and then females come and ask for it?

Not really. A. moreletii has a stereotyped calling behavior (which is one of the cutest mating rituals I know about so far). Most scientific literature on this species states that the males come down from the forest canopy shortly after sunset. They spread through the vegetation around a waterbody (usually temporally flooded areas during the onset of the rainy season), and sing in choruses! We’re talking around 1900H here. However, females don’t show up until around 2300H or past midnight. And you know what? They don’t even mate on the first night. Talk about playing hard to get!

Another important characteristic is the males’ motivation, so females will come every night to judge if this or that male is still showing up. The number of nights a male shows up to sing (meaning that he still has enough energy reserves) is an important factor too.

Females ovulate once, maybe twice per breeding season (not enough evidence for this yet!), so they have all the reasons to be so picky!

Once the Tenor has been chosen, the female will show herself receptive and not “fight” the male upon amplexus (fancy word for how the males hop on the females and hold on to them). She will then release the eggs while the male holds on to her (axillary amplexus in A. moreletii) and fertilizes the eggs as she lays them on palm leaves just above the water body. The egg clutches will hatch 4 to 6 days later and the tadpoles will fall directly in the water.

But not all males sing

There are some “satellite males” that will stand near a singing male so he can pretend that’s him calling when the female comes closer. Sneaky bastards, huh?

Sometimes a male can find itself hopping on top of another male, not for fighting, but because he confused him for a female. In these cases, males have a different call, “chucks”, which is another way of intraspecific intrasexual communication to let the other dude know that they’re not the egg-bearing females they’re after!

Another common question about this behavior is if all males sing the same. Briggs (2010) found some evidence that supports the idea that we can, in fact, distinguish male frogs to the individual level based on the call variability! How awesome is that?!

As in all the animal kingdom, mating systems can be driven by female choice alone or have some male-male competition involved as well. Here comes into play another type of intraspecific communication that A. moreletii exhibits: tremulation.

Biotremulation

Tremulation is basically a “vibrational display”. In the case of A. moreletii, the males exhibit this behavior when another male is too close to his calling territory.

Tremulations in Agalychnis callidryas (the red-eyed tree frog) had been reported before, but it was not reported for A. moreletii until last year by Serrano et al. (2018) in a population of A. moreletii in Guatemala.

The males basically shake their behinds to make the vegetation tremble. These are low vibrations that are thought to be receptive only by members of the same species as a “warning”, in order to not waste energy in engaging in a physical fight when they could invest that energy in their sexual calls. If this tremulation doesn’t work they’ll start using the “male calls” (“chucks”) to deter the “invaders” and even engage in pushing and kicking the other dude. Serrano et al. (2018) even recorded a tremulation display while amplexing!

This tremulation displays might be only a reproductive behavior since it has not yet been observed during the dry season.

Isn’t frog sex amazing? 

But they’re going down…

Sadly, like biodiversity worldwide, this species does not escape some threats. Mainly habitat destruction and urban encroachment are restricting this species’ populations. However, now that you know how important the acoustic communication is for them, you might’ve already figured out that the urban encroachment problem is not only a matter of reduced habitat but also of noise pollution!

This interference might force the males to change their calling behavior, which could imply the investment of more energy and maybe they will have to show up less for the chorus nights, disappointing the females. That is if the females can hear them at all with all this new noise! Actually, they might even have to stop calling before the females arrive at the choruses! Or maybe they will even have to change the calls, and no one likes a dude screaming right at you, am I right? More research needs to be done in this area of noise pollution for different amphibian species and their calling behavior.

However, there is one more threat this species is facing, one in which we can ALL do something about and that is pet trade. The species is under the CITES II appendix. According to some reports by CITES and IUCN, the main demand for tree frogs comes from the global North. So spread the word and tell your peeps to stop buying wildlife for pets and promoting the destruction of tropical habitats and everything that lives within!

References

1. Serrano, M., J.J. Cruz, M. Villatoro-Castañeda & D. Ariano-Sánchez. 2018. Tremulation Display in Male Agonistic Behavior of the Black-Eyed Leaf Frog (Agalychnis moreletii: Hylidae). Animal Behavior and Cognition. 5(2):222-228

2. Briggs, V.S. 2010. Call Trait Variation in  Morelett’s Tree Frog, Agalychnis moreletii, of Belize. Herpetologica. 3(66):241-249

3. Kaiser, K., D.G. Scofield, M. Alloush, R.M. Jones, S. Marczak, K. Martineau, M.A. Oliva & P.M. Narins. 2010. When Sounds Collide: The Effect of Anthropogenic Noise on a Breeding Assemblage of Frogs in Belize, Central America. Behavior. 148:215-232