Creature Feature: Hippity Hoppity This Tree is My Property

As the weather gets warmer, more and more animals are beginning to come out of their hibernation, brumation, and migration, adding sound to the previously quiet landscape. These sounds come from a variety of animals, notably birds, insects, and amphibians. In my opinion, some of the most soothing calls of nature come during the evening, where a chorus of many different animals begins. One of these animals will be our Creature Feature for this month, so let's dive in and take a look!

Whether they are found on the edges of windows and houses, or at your local Rainforest Cafe, tree frogs are famous for their disk-shaped tips that stick to surfaces, and the calls that they make at night. Of the 800 different documented species of true tree frogs found throughout the world, Michigan has gray tree frogs. One would think that you could tell what a gray tree frog looks like by the way it is (it looks gray), but the truth is that these amphibians are able to camouflage to their surroundings, causing them to appear not just in gray, but also green and brown as well. Think of gray as being their default color.

Which image contains the gray tree frog from left to right: 1, 2, 3, or 4? 

Trick question, because they are all gray tree frogs.

There are actually two different species of gray tree frog in the state of Michigan: THE gray tree frog (Hyla versicolor), and Cope’s gray tree frog  (Hyla chrysoscelis). It is almost impossible to tell the two frog species apart with the exception of two methods of comparison. If you are a geneticist and looked at each frog's chromosome set under a fancy microscope, you would find that Cope’s tree frog has two chromosome sets (the usual amount we would expect to find in vertebrates, including humans). We call this arrangement diploid, “di” meaning two. The H. versicolor tree frogs, on the other hand, are different. They have four chromosome sets, making them tetraploid (“tetra” meaning 4). The other method of differentiating the two species is far less expensive and time-consuming. It involves listening to their mating call, which one may begin hearing in late April and run through to August. The mating call from Cope’s tree frog is reported to go at a faster pace and have a higher pitch than their cousin species.

Since they are nocturnal, hearing the evening call of a gray tree frog means that a male is either calling out for a mate, or a male is declaring territory around a tree, shrub, or structure. When not on the lookout for a mate, the daytime activity involves hiding out in places that do not get direct sunlight, and remaining in an inactive state. Feeding time takes place in the evening when the frogs will venture out into the trees or understory to feed on insects and other invertebrates.

Have you ever wondered how an animal such as a tree frog is able to survive the cold winter temperatures? Interestingly, they allow themselves to freeze while still being able to come back kicking (in this case, hopping) in the spring! Here is how they do it: tree frogs are able to make a chemical called glycerol within their bodies. This glycerol transforms into a basic sugar, glucose, that spreads across the frogs’ cells and tissues. Glucose is like antifreeze for the frog, and protects it from ice that would form through it. Any remaining water found inside the frog, such as the water found in blood, proceeds to freeze as the temperature drops, causing the frog to stop beating its heart. Despite not breathing, the tree frog itself is still intact and unaltered by the freezing temperatures due to the glucose. Once its internal blood thaws, the heart can begin beating again and it’s alive!!!!!

If you are interested in not just tree frogs but frogs altogether, come join a naturalist at the Johnson Nature Center on April 21st for a night hike themed on frog sounds from 9-10pm!

Click here to learn more about the Gray Tree Frog

Submitted by Zach Mork, naturalist, April 2023