Gray treefrog

Species of amphibian

The gray treefrog (Dryophytes versicolor) is a species of small arboreal holarctic tree frog native to much of the eastern United States and southeastern Canada. It is sometimes referred to as the eastern gray treefrog, northern gray treefrog, common gray treefrog, or tetraploid gray treefrog to distinguish it from its more southern, genetically distinct relative, Cope's gray treefrog.

As the scientific name implies, gray treefrogs are variable in color. This ability to vary their color provides them with the ability to camouflage themselves from gray to green or brown, depending on the environment around them. D. versicolor can change from nearly black to nearly white. They change color at a slower rate than a chameleon. A unique aspect of the appearance of gray treefrogs is that its legs feature a dark band like pattern which then contrast sharply with the black marked bright yellow or orange under the sides of its legs and arms. Dead gray treefrogs and ones in unnatural surroundings are predominantly gray. The female does not call; however, the male does call. Female gray treefrogs are usually larger than their male counterparts. They are relatively small compared to other North American frog species, typically attaining no more than in length. Their skin has a lumpy texture to it, giving them a warty appearance. This species is virtually indistinguishable from Cope's gray treefrog, the only readily noticeable difference being that Cope's Gray treefrog has a shorter, faster call. This varies depending on the temperature, however, as the call rates of both gray treefrogs are temperature dependent. At lower temperatures, Cope's gray treefrog can have a call rate approximating that of the gray treefrog. This difference in calling can be heard, but it is best quantified by counting the number of pulses per second in their whistled trills. At usual temperatures, the gray treefrog has a pulse rate of 16 to 34 pulses per second, while Cope's gray treefrog has a pulse rate of 34 to 60 pulses per second. Even though there is potential for overlap, because of the temperature dependence of the pulse frequency the two species are easily distinguished where they occur together. At a given temperature, the pulse frequency for the gray treefrog is approximately one half that of Cope's gray treefrog. The gray treefrog also has 48 chromosomes (4n), and is sometimes referred to as the tetraploid gray treefrog in scientific literature. Cope's gray treefrog, or diploid gray treefrog, retained its 2n (24) original chromosome count. Hybridization between these species results in early mortality of many larvae, but some individuals survive to adulthood, but these individuals suffer from reduced fertility. Both of these similar species have bright yellow patches on their hind legs, which distinguishes them from other treefrogs, such as the bird voiced tree frog. The bright patches are normally only visible while the frog is jumping. Both species of gray treefrogs are slightly sexually dimorphic. Males have black or gray throats, while the throats of the females are lighter. Tadpoles have rounded bodies (as opposed to the more elongated bodies of stream species) with high, wide tails that can be colored red if predators are in the system. Metamorphosis can occur as quickly as two months with optimal conditions. During metamorphosis, the new froglets will almost always turn green for a day or two before changing to the more common gray. Young frogs will also sometimes maintain a light green color, only turning gray or darker green once adulthood is reached.

Gray treefrogs inhabit a wide geographic range, and can be found in most of the eastern half of the United States and as far west as central Texas and Oklahoma. They also range into Canada in the provinces of Quebec, Ontario, and Manitoba, with an isolated population in New Brunswick. The gray treefrog is capable of surviving freezing of its internal body fluids to temperatures as low as 8 °C (18 °F). The gray treefrog is most common in forested areas, as it is highly arboreal. Its calls are often heard in rural residential areas of the East Coast and the Midwest. It prefers to breed in semipermanent woodland ponds without fish, but it also lays eggs in swamps, vernal pools, man made fountains and water gardens, and even in rainwater filled swimming pool covers.

Male gray treefrogs rarely have large choruses, as they are mostly solitary animals, but might vocalize competitively at the height of breeding periods. Gray treefrogs have been observed to congregate around windows and porch lights to eat insects that are attracted to the light. Insect larvae, mites, spiders, plant lice, harvestmen, and snails also contribute towards the diet of the gray treefrog. Some populations have a diet high in ants and beetles. However, like most frogs, D. versicolor is opportunistic and may also eat smaller frogs, including other treefrogs. Aggressive calls are usually much shorter in length and have lower frequencies than advertisement calls. Aggressive calls specifically in D. versicolor males also do not show much variation in amplitude throughout the call, unlike advertisement calls which contain many pulses. This is very unique to the D. versicolor species since most species with graded aggressive calls have advertisement and aggressive calls with very similar structures. They are similar in that they both have two peak frequencies, but the aggressive call peak frequencies are usually lower.

Since females do not prefer call overlap between males in a close range of each other, this can cause a change in call timing as well as a change in the characteristics of the calls these males produce. When there are other male frogs calling, H. versicolor males will adjust the timing of their calls; however, this is done in a much less strict fashion than most frog species. Compared to other species, H. versicolor does not exhibit selective attention. Selective attention is the phenomenon observed in many chorusing male frog species to change the timing of their calls to reduce overlap based on their loudest one or two neighboring male competitors, while ignoring the timing of other calls farther away. Instead, H. versicolor males will avoid call overlap when paired with only one other male, but will not actively avoid overlap with adjacent frogs in a group nearly as much as other frog species do.

Male frogs will change their vocalizations when female frogs move closer to them. They do this in order to increase the likelihood that their advertisement call is received by a female over the other noise and vocalizations that could obscure it. D. versicolor males specifically do this by increasing the length of their calls to several lengths of a normal advertisement call. The social environment can change as male callers move around and as females arrive to assess their potential mates producing different levels of perceived male competition heard by D. versicolor males. This appears to be an example of reproductive character displacement to keep the species separate. In addition, to enforce speciation there may be unknown mechanisms of reinforcement deployed between these species and further research may be fruitful. Dryophytes versicolor and Dryophytes chrysoscelis call next to each other ponds resulting in interference of their vocalizations because their calls are so similar acoustically. In response to male advertisement calls, D. versicolor male answers with the same level of aggressiveness to males of the same species and to D. chrysoscelis males producing the initial call. D. versicolor male interactions with D. chrysoscelis males increase in aggressive intensity more quickly than with male interactions with their own species. Once the aggression levels intensified between these species, the weaker frog was more likely to retreat from the winner. In general, D. versicolor males initiate physical attacks during intense vocal conflict between the two species more often than D. chrysoscelis. In previous studies, D. versicolor mate attractiveness decreases when there is call overlap with D. chrysoscelis. The D. versicolor mate attractiveness decreases even more so than D. hrysoscelis when there is call overlap, which can explain why the D. versicolor male tends to initiate aggressive physical contact more often: the D. versicolor has more to lose from the call overlap continuing to take place. While the advertisement calls of D. versicolor and D. chrysoscelis are distinguishable, the aggressive calls between these two species are similar.