As a field researcher of primarily mammals, I often encounter highly enthusiastic amateur and expert herpetologists at the same sites at which I work. As you would expect, they all conduct their investigations into ‘herps’ in different ways, but one unifying methodology always stands out to me – not for it’s distinctiveness, but because it would be anathema when applied to mammals.

Herpetologists often extract their study subjects from the wild – bringing them back to a field laboratory or station for photographs, experiments, or closer examination – but they rarely return their subjects to the exact spot from which they were found.

Now, if I attempted to do that with the tamarins I work with, there would be no end of trouble from Animal Studies Committees, my colleagues, and my conscience – and for good reason too. Tamarins are highly territorial – if you mix them around in the wild and don’t pay attention to their spatial orientation in their habitat, the repercussions will be life-altering for them. Group membership, dominance hierarchies, reproductive fitness, and even their survival could be adversely affected.

It then comes as no surprise that I’ve always wondered how it is at all ethical to treat ‘herps’ differently – surely they must know their surroundings and exhibit some amount of site fidelity?

A recent paper in the Journal of Zoology helps answer some of these questions, using one of the most remarkable examples of reproductive biology known to science – the Darwin’s Frog (Rhinoderma darwinii). Scientists from the Facultad de Ecología y Recursos Naturales, Universidad Andres Bello, in Santiago, Chile, spent months at the southern Chilean site of Tantauco Park on Chiloé Island studying an area of 300 square metres with an estimated 56 Darwin’s frogs living within it. Their findings are not only remarkable, but give rise to some important questions that I sincerely hope some busy researcher is working hard to answer – or else the suspense will really kill me!

Rhinoderma darwinii, Darwin's frog, a classic example of neomalia. Photo credit Timothy Paine of amphibios.org
Rhinoderma darwinii, Darwin’s frog, a classic example of neomalia. Photo credit Timothy Paine of amphibios.org

Rhinoderma darwinii is a terrestrial anuran that was recently proposed as the unfortunate recipient of the “Endangered” category by the IUCN’s Red List of Threatened Species. Its sister species, the northern Darwin’s frog, was last observed in 1980 and is thought to be extinct today. What has brought scientists repeatedly back to R. darwinii is its remarkable reproductive biology – a phenomenon termed neomelia. The larvae of these frogs begin to move around 19-25 days after fertilisation within their egg sacs, in a clutch of eggs deposited at a carefully selected oviposition site. At this stage, a male arrives and incorporates the tadpoles into his vocal sac for a period of six to eight weeks during which he provides them with a nourishment fluid. He will care for them until metamorphosis has taken place, earning himself the title of a “brooding male.” When the transformation of larvae from tadpole to adult is complete, he regurgitates the juveniles into their brand new habitat, completing the last stage of neomalia.

ARKive video - Darwin's frog carrying tadpole in mouth
Rhinoderma darwinii carrying a tadpole in its mouth

Despite numerous investigations into the intricacies of this process, there is very little information on whether eggs are attended to or defended by males while they remain at the site of oviposition for the first 3 weeks after fertilisation.

Andrés Valanzuela-Sanchéz, lead author on the study, discovered that the average home range size of each frog was ~16 square metres, within which they displayed a characteristic sit-and-wait foraging behavior. He hypothesised that the males would be territorial if they could demonstrate site fidelity, resource limitation and resource defense.

“We found no evidence of territorial behaviour in Darwin’s frog males based on home range, movement, and social network analyses,” note the authors in their report. In fact, they saw no agonistic visual displays or physical confrontations of any kind among the males. The males instead exhibited high degrees of home range overlap, a complete contrast to the females whose territories never even touched each other. This significant lack of territorial behavior led the authors to affirm that territoriality is likely to be linked to defense of oviposition sites than it is to increased paternal care among these anurans.

A particularly interesting off-shoot of this discovery is the withdrawal of males subsequent to egg fertilization, prompting the all-important question of how males then identify where the eggs they fertilised are deposited for during initial larval development. There must be some kind of external trigger that signals that the eggs are ready for ingestion, but if the males remain unaware of oviposition sites, and there is significant overlap of male home ranges, how do males locate eggs specifically fertilised by them?

“Could it be possible,” wonder the authors, “that cross-fostering occurs in R. darwinii (i.e. males brooding another male’s eggs)?”

A fascinating concept indeed, that can probably be explained only by detailed paternity analyses through genetics.

Darwin’s frog as a juvenile – a size comparison. Photo credit: © Danté B Fenolio

In the meanwhile, the paper has contributed significantly to my original question on whether relocating anurans causes them subsequent harm. It reveals that site fidelity has been verified for many anurans, but that, in the end, it is even possible that oviposition sites are just stumbled upon in the wild by males. Some males, the authors note, have even been found with differently aged larvae in their vocal sacs, implying polygynous mating or possibly the ingestion of larvae from multiple sites – for reasons only the male in question knows!

This study does suggest to me that species differences in territoriality should definitely be taken into account when conducting investigations into tropical herps – particularly if the individual captured is not easily identifiable in the field, and its level of site fidelity is unknown. One suggestion would be to carry a hand-held GPS and log a point at every capture site, so as to release at the same location, just in case site fidelity is an important factor for the individual in question. However, for quite a few anurans, this might not pose as much of a threat as I’d originally surmised. The key is in knowing the difference.

References:

Valenzuela‐Sánchez, A., et al. “Home range and social analyses in a mouth brooding frog: testing the coexistence of paternal care and male territoriality.”Journal of Zoology (2014).

 

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