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  Unreported variation in Tree Frog Hyla geographica in Payamino

 

 

Background
During June, 2005, a number of students and staff from Manchester University visited the Payamino area for the first time. The 10 day trip was organised to assess the feasibility of conducting an annual tropical field course in the area and, as a first trial, the field course was an overwhelming success. During the time spent in the area, students were introduced to the vagaries of field working and some vivid new experiences. The trip facilitated only a mere snapshot assessment of the level of diversity in specific animal groups, including that of the amphibians of the region.   

Introduction
Approximately 4,000 species of frogs are recognized worldwide, with nearly 1,600 living in South America. The Amazon area is by far the most diverse place for frogs  in the world and more than 80 species have previously been found at single sites in the upper Amazon Basin of Ecuador.

Information relating to frogs found at and surrounding the newly established San Jose de Payamino Research Station (South 00° 28.965’ West 77° 17.053’) is presented. The specific site of the Research Station has been inhabited by local people for many generations and human disturbance may well have resulted in the low densities encountered.  However, several nearby locations provided a better indication of frog species richness and densities in the area, given that activity of many species can be seasonal and strongly affected by rainfall. During this short visit we could accurately  account for 22 species of frog that occur in this specific area.  Although the preliminary visit was very short, investigations of the frog fauna highlighted some extremely interesting and unusual results.

Results
One particular type of tree frog that was located living near a lagoon by the new research station could not accurately be identified. The specimens collected are most similar in appearance to Hyla geographica. These are slender brown frogs that have golden reticulations on their lowever eyelids and irregular markings on their dorsum. All the calling (adult) males located at the lagoon site are consistently smaller than the other H. geographica found during survey work over the river from the Research Station. They were considerably different in appearance, having orange/black striping to their flanks and thighs.

I am very familiar with hylid (treefrog) species that occur in Amazonian Ecuador and am confident that I have not confused these with another similar species. Similar species that were also located in the area included large numbers of Hyla calcarata, another comparable species, but one that has striking blue/black striped flanks, larger calcars, and a lower eyelid that is unpigmented.  Hyla fasciata, another similar-sized hylid frog that has an unpigmented eyelid was clearly identified as living sympatrically with these frogs.

It is very interesting to note the presence of two types of H.geographica in the survey area. Although adult male specimens of both shared features that characterize Hyla geographica, such as the golden reticulated lower eyelid (figure 1), irregular patterned dorsum (Figure 2), and small calcars, it was clear that they were visually very different (Figure 3b). They were also significantly different in size (fig 3a), having no overlap in snout-vent length (t=4.12 (5df), p=0.015. The larger, blueish-grey mottled flanked, H. geographica averaged a snout-vent length of 52.8 mm, and a weight of 5.57 grams, whereas the other orange and black barred specimens averaged 42.4 mm in length and an average weight of 30.0 grams.   

 

Reticulate lower eyelid

Figure 1. Reticulated lower eyelid.

Flank and thigh difference

Figure 3a: Flank and thigh differences between males of Hyla geographica (below) and the unidentified specimen (top) 

Dorsal markings

Figure 2. Dorsal markngs

Size difference

Figure 3b: Size differences between males of Hyla geographica (right) and the unidentified specimen (left) 

 
 

Discussion
It is clear that what has been discovered during this short visit is something quite unusual, and that the different specimens represent either a very distinct sub-species of Hyla geographica, or a possible new species. Although living very close to each other, the two populations are separated by the Payamino River. In the short term this could well act a definite barrier between the two, as it fairly wide at this point. However, the gradient is very low and the river forms oxbow lakes as it meanders. The lagoon is probably the remains of one of these. It is possible that the small form is an isolate that has diverged over many generations during which there has been accidental isolation due to the location of the river. We do not know how long patterns of river flow persist or how many generations would be required for the degree of divergence of the two forms, or the extent and boundaries of the territory of the small form.

This is an exciting example of possible evolutionary divergence and I intend to follow it further, in the first place by mapping the district of the small form and collecting material to test for genetic divergence.   

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