Grey treefrog rejected as food by a fisher

Fisher (Martes pennanti). Photo: Mark Andrew Conboy.

Fishers (Martes pennanti) have become a relatively common species in Eastern Ontario over the past three decades but for such a potentially important predatory mammal we know very little about them at QUBS. Very few observations of the foraging behaviour and diet of fishers have been made at the station. Fishers have been seen eating and caching carrion in winter and pursing small mammals along rocky outcrops in spring.

Observing fisher foraging in real time is difficult, but following fisher tracks is an alternative method of gathering behavioural information. On February 28, 2010 I followed fisher tracks for 1.7 km on the Pangman Tract. Over that distance I noted one occasion where the fisher stopped to investigate a large bone, one scat deposit and four places where the fisher dug into the snow and/or the exposed leaf litter. Two of those digs were in the duff at the base of eastern white pines (Pinus strobus). Another dig was in the leaf litter beside a log. In all three cases it was impossible to determine what the fisher was digging for, but I suspected it may have been trying to find hibernating frogs. On the fourth dig I found a hole 13 cm deep through snow and humus beside which was a tetraploid grey treefrog (Hyla versicolor); the frog appeared to be dead, though it was not at all damaged by the fisher. Treefrogs normally freeze solid during hibernation but this frog was completely thawed. I took it back to the station to see if it would revive, but it never did.

Tetralpoid grey treefrog (Hyla versicolor) that was dug out of a hole (right) by a fisher. Photo: Mark Andrew Conboy.

At first I was puzzled by the fisher’s rejection of the treefrog, but Fred Schueler reminded me that grey treefrogs are unpalatable to shrews and probably other mammals. The bright orange-yellow markings on the treefrog’s legs probably serve as an aposmatic warning to would-be predators. Not all of our frogs are so distasteful. Wood frogs (Rana sylvatica), for example, also hibernate in the leaf litter and may have been the intended prey of the fisher. – Posted by Mark Conboy

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