08. Chickadee Notes

The Gray Treefrog in New Brunswick


Gray Treefrog

New Brunswick is the northern limit of the range for the Gray Treefrog, Hyla versicolor. Not surprisingly, the species is uncommon here. Several naturalists of the last century noted the presence of the Gray Treefrog at widely scattered locations in the province. Nonetheless, until recently the Gray Treefrog was known with certainty in the Maritimes from a single marsh at Barker's Point, near Fredericton. For this reason, and because the marsh is threatened with development, the species has been considered a candidate for protection under the provincial Endangered Species Act. However, field studies carried out between 1980 and 1990 have revealed that the Gray Treefrog is present in at least 13 additional wetland sites in southwestern New Brunswick. Although the status of the Gray Treefrog here now appears secure, the population is scattered in New Brunswick and there remains concern for the future of this treefrog at individual sites.

Natural History

The Gray Treefrog is a thumb-sized amphibian of 4-5 cm body length. The back and legs are mottled green, black, and gray, and the inner thighs are a vivid orange. The species is certainly one of the most attractive of New Brunswick amphibians. In response to temperature the Gray Treefrog is capable of considerable colour change, hence the Latin name versicolor. Prominent sticky discs on the end of fingers and toes allow the species to easily clamber about vegetation in search of spider and insect prey. Males have a dark pigmented throat, the deflated vocal sac, while females have a light throat and are the larger sex. The call of the male is a distinctive pulsating trill.

Breeding sites in New Brunswick...are most frequently the shallow pools left in abandoned pits or along roadsides.

Hyla versicolor is one of a pair of closely related species that have a wide range over eastern and central North America. The two Gray Treefrogs in the pair are morphologically indistinguishable but can be separated based on the trilling rate in calling males, or at the cellular level by counting chromosomes. One species in the pair trills at a faster rate at any given temperature than the other; one Gray Treefrog has two sets of chromosomes, one has four. Both calling rates and chromosome numbers have been examined in New Brunswick and only Hyla versicolor, the slow- trilling Gray Treefrog with four chromosome sets, occurs here.

During the breeding season, which in New Brunswick extends from late May until the end of June, the Gray Treefrog may be found in the low vegetation surrounding breeding ponds. During the day a favorite resting spot is often the crotch of a pond-side willow or wild cherry. Here a mottled Gray Treefrog can easily be mistaken for a tuft of lichen.

Breeding sites in New Brunswick include beaver ponds but are most frequently the shallow pools left in abandoned pits or along roadsides where construction has interrupted drainage. Summer water temperatures are high in these shallow, slow moving waters, providing ideal circumstances for the rapid development of eggs and larvae. During the rest of the summer these amphibians live among the vegetation of trees and shrubs in the vicinity of breeding ponds, but they are so well camouflaged that they are rarely observed once the males stop calling.

Calling males are territorial and will readily fight with other males...

Individual males generally spend only two or three consecutive nights calling at a breeding pond each year. It is during this short interval that they must attract and mate with a female.

Calling males are territorial and will readily fight with other males who approach them. Some fights involve only a few brief shoves, but frequently male Gray Treefrogs will battle vigorously, pushing each other with their forelimbs, butting with their heads, kicking swiftly with their hind legs and jumping on each other. However, intruders into a territory rarely achieve victory. These subordinate males seldom stray far from the battle scene, preferring to remain silently in the vicinity of the perch of the dominant male. Such a strategy is not without its rewards. Occasionally a silent male will try to intercept a female as she moves towards a calling male, but she will usually repel his advances. However, should the dominant male be fortunate enough to lure a female he will then vacate his perch and move into the water to mate and fertilize the eggs. This process may occupy the rest of the evening. Calling or perch sites are usually in short supply but improve a male's chances of attracting a mate. The subordinate male will therefore immediately occupy the vacant perch and begin to vocalize, intent on the business of attracting a female in the few hours of the night that may remain.

...some populations of Gray Treefrog can survive temperatures as low as -9 C...

Gray Treefrogs reach sexual maturity at the age of about two years. Females lay a number of egg masses containing from 15 to 35 eggs, each attached to grass or leaves near the water's surface. Tadpoles emerge in a few days to a week and are easily distinguished by a scarlet or orange wash of colour on the tail. Those tadpoles able to avoid predators may reach a length of 50 mm, emerging as tiny emerald-coloured froglets at the end of several months.

Gray Treefrogs hibernate on land, burrowing beneath moist sod or soil. Although hibernation sites may freeze, research has shown that some populations of Gray Treefrog can survive temperatures as low as -9s C and the freezing of more than 50 % of total body water. Freeze tolerance is associated with the presence of high concentrations of glycerol, which acts as a cryoprotectant, in tissues and urine. Although internal organs do not freeze, blood and extracellular water are frozen and breathing and heart beat cease. These are remarkable adaptations that aid survival in a northern climate.

Helping Out

There are only about 14 wetlands in New Brunswick where the Gray Treefrog is known to breed. Some of these are little more than roadside ditches. Other sites in the province probably await discovery. The easiest way to find the Gray Treefrog is to learn to recognize the call, which once identified, is quite distinctive. There are several commercially available recordings of North American amphibians that naturalists can use to become familiar with frog calls. If you think you have discovered a Gray Treefrog breeding pond contact the New Brunswick Museum where experts can offer assistance and where information on the distribution of provincial amphibians can be found. A wide variety of wildlife depend on wetland habitat, including the Gray Treefrog. Such habitat is increasingly threatened and should always be treated with respect.

Further Reading

Cook, F.R. 1984. Introduction to Canadian amphibians and reptiles. National Museums of Canada. 200 pp.

Fellers, G.M. 1979. Mate selection in the Gray Treefrog, Hyla versicolor. Copeia (1979): 286-290.

Layne Jr., J.R. and R.E. Lee Jr. 1989. Seasonal variation in freeze tolerance and ice content of the treefrog Hyla versicolor. Journal of Experimental Zoology 249: 133-137.

McAlpine, D.F., T.J.Fletcher, S.W. Gorham and I.T. Gorham. 1991. Distribution and habitat of the Tetraploid Gray Treefrog, Hyla versicolor, in New Brunswick and eastern Maine. Canadian Field-Naturalist 105: 526-529.

McAlpine, D.F., S.W.Gorham and A.D.B. Heward. 1980. Distributional status and aspects of the biology of the Gray Treefrog, Hyla versicolor, in New Brunswick. Journal of the New Brunswick Museum (1980): 92-102.

Wright, A.H. and A.A. Wright. 1949. Handbook of Frogs and Toads. Comstock Publishing, Cornell University Press, Ithaca, New York. 640 pp.