Wednesday, August 31, 2011

Fuliginous Frogs and Baffling Batrachians

Fuliginous means sooty, and is being used in the sense of obscure, as in seeing through a glass, darkly. In my case it certainly applies to Mink Frogs (Lithobates septentrionalis), since for the last more than half a century of my life I've been unaware that these small amphibians even existed. They also share the same habitat as the Bullfrog (Lithobates catesbeianus) and the Green Frog (Lithobates clamitans), all are similarly colored and can be confused with one another.

One thing that's easy to determine in all three species is the frog's sex: the male's tympanum or eardrum is significantly larger than the eye, in the female it's smaller. Useful information if one intends to kiss a frog and wants to know in advance if the result will be a handsome prince or a beautiful princess.

The key distinguishing feature of the Bullfrog is that it lacks dorsolateral ridges, the folds of skin that start behind the frog's eyes and run down its back. Of course, if the Bullfrog in question is fully grown there's no possibility of confusion, at over 15 cm in length it's much larger and more robust than its cousins.

So, looking at the Bullfrog: this is a female ...

... another a female ...

... and this frog with the large tympanum is a male.

Note the dorsolateral ridges along the backs of these of Green Frogs (for anyone interested, a grouping of frogs is called an "army" of frogs). "Green" is a bit of a misnomer, the color in all three species can vary from grungy brown, as in the individuals below, to green, with or without darker spots.

This male Green Frog is about 10 cm long, fully mature and sporting his breeding colors. Note the bright yellow throat – male Bullfrogs and Mink Frogs throats will also turn this color when they are ready to mate. A more important distinguishing characteristic of Green Frogs are the markings on their back legs: they are distinctly barred, with the stripes running transverse to the length of the legs.

A female Green Frog ... note the smaller tympanum.

Although this female was much smaller Mink Frogs can grow to a length of almost 8 cm. Like the Green Frog they have dorsolateral ridges, but Mink Frogs tend to be mottled, and their back legs have spots rather than transverse stripes. Males and often females also tend to show a tinge of yellow on their throats. However, none of these characteristics can reliably separate Green and Mink frogs in the field.

As a rule it's preferable to photograph wild things in a natural setting, but Mink Frogs do have one unique feature that sets them apart from their relatives and that's why this frog was captured. This is one froggie you definitely don't want to kiss because Mink Frogs come by their name honestly, and, as advertised, this little frog smelled exactly like rotting onions.