By Rowan Etzel
Back
at Las Cruces Biological Station, several of us found some frogs exhibiting new
calling behavior for their species, which happens to be endangered. Because
these observations appeared to be novel, I decided to take on the task of
writing up our observations in greater detail, perhaps eventually for submission
to a scientific journal. Here are those notes:
Ptychohyla legleri,
Legler’s Stream Frog, is a moderately sized nocturnal red-eyed treefrog,
usually found near small streams in premontane wet forest of southern Costa
Rica and western Panama. Males are thought to call during February–July in
vegetation above flowing water, and have been found calling from rocks in
streams at Las Cruces Biological Station, Costa Rica. Amplexus and oviposition
behaviors are unknown for the species in the wild, but observations in
captivity observed males to be territorial and defend small egg-laying cavities
under an artificial waterfall where eggs were laid below the waterline.
Clearly, the naturally history is poorly described for this frog.
On 10 October 2017, we found six
male P. legleri making advertisement calls for females at a first-order
stream at Las Cruces. At about 4 PM, we heard two individuals calling from a
short seepage down a vertical bank of the stream. Upon closer examination, the
individuals were located one each within two separate burrows in the bank. The
burrows were elevated 30–50 cm above the stream, ca. 15 m apart, and were ca.
3–4 cm in diameter. Water coursed down the stream bank around the burrow
entrances and was also present within the burrows. The individuals were perched 3–4 cm within
the burrows and were half-exposed from the water. Upon shining a flashlight
into the burrows, the individuals both stopped calling temporarily and
retreated beneath the surface of the water. After a few minutes without
exposure to a flashlight, they resumed calling with the typical advertisement
call. The individuals chorused together, with one individual leading and the
second calling in response to the first. Shortly thereafter, we located a third
individual ca. 15 m upstream that was calling from within leaf litter adjacent
to the stream. A fourth individual was also calling here near the third, but it
eluded visual detection.
When
we returned after dark, we captured three more individuals. First, we located
who we presume was the undetected fourth individual from the afternoon; it was
a male and he was calling from among gravel beneath a rock. The final two
individuals were found ca. 15 m upstream from the third and fourth, and were
found calling together from underneath two rocks adjacent to the stream and
were situated within small puddles with leaf-litter and twig organic detritus.
We think our observations represent
undescribed calling behavior for P. legleri which may provide insight
into the breeding biology of the species. First, we observed individuals calling
from subterranean burrows. It’s unclear whether those individuals constructed
the burrows themselves, or used pre-existing burrows created by other
mechanisms. In addition to the more typical nocturnal calling behavior, we also
observed individuals that called diurnally in the late afternoon from within
hidden calling sites such as terrestrial burrows and retreat sites beneath
rocks. To us, terrestrial advertisement is uncommon among the hylid frogs of
Central America, so this is atypical. Calling from covered locations such as
terrestrial burrows may provide two functions. First, concealed calling sites
may afford individuals additional protection and concealment from potential
predators. In addition, these sites also resemble oviposition sites used by
individuals in captivity. So, we think that these observations of terrestrial/fossorial
calling may provide insight into where the frogs may lay eggs.
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