Summer in southeastern Oklahoma can be hot and dry, hot and humid, or hot and wet, with “hot” being the constant variable. This year it has been mostly hot and humid. As you know, amphibians can lose a lot of water through their skin, often in a very short amount of time. Of course they gain water through their skin as well, as long as water is available. Toads and many other frogs gain water through a patch of skin on their ventral surface known as the pelvic patch. They simply press this part of their body against wet ground and absorb water.
Most reptiles do not gain or lose as much water through their skin as amphibians, and as a result, they can better tolerate drier conditions. To avoid the increasing temperatures during summer, a lot of amphibians, such as Pickerel frogs and Spotted salamanders, simply become inactive, seeking refuge underground, inside of trees, or under leaf litter. Others, such as Southern leopard frogs and Bullfrogs, spend most of their time in water. If enough rain has fallen to fill temporary ponds, amphibians such as Gray treefrogs and Eastern narrow-mouth toads breed through the first half of the summer. One of the most common frogs, the Northern cricket frog (Acris crepitans) seems to defy the “rules” of nature, often hanging out along the shores of ponds where surface temperatures exceed 100° F during much of the day. We will come back to this, but first, why would Northern cricket frogs hang out on the hot shoreline when they could simply lie in the cooler water all day? Ponds are filled with things that eat small frogs, including snakes, fish, and predaceous insects (dragonfly larvae, giant waterbugs, and predaceous diving beetle larvae). Consequently, it appears that they minimize risk of being eaten by staying out of the water.
Closer examination of Cricket frog behavior reveals that they are not defying the rules of nature, but rather, they are using a combination of behaviors and physics to maintain rather constant body temperatures while spending some of their time on hot shorelines. In a sense, what they do is not a lot different from what you might do if you were to visit a Florida beach during summer, especially if the beach that you chose happened to have a lot of sharks just offshore! You would lie on the beach and lose water through your skin and by breathing out moisture-laden air. Evaporation of water (sweat) from your skin would help to cool the surface of your skin. Evaporation causes a cooling effect because the change of state of water (liquid to gas) uses energy (technically called the latent heat of evaporation). This energy is taken from your body in the form of heat, thus causing a cooling effect. After you have used up some of your water keeping your body cool, you get thirsty and resupply your body by drinking water or Gatorade.
Cricket frogs fill themselves with water in the pond and then bask on the hot “beach” where they live, thus avoiding all of the predators waiting for them in the pond. They continually lose water by evaporation, resulting in loss of heat from their bodies, allowing them to maintain body temperatures lower than temperatures of the surrounding beach. When they run out of water, they jump back in the pond for a brief time period, refill, and climb back out on shore. One final point, Cricket frogs are very small, about an inch or so in total length. Again, from basic physics, remember that the amount of body surface relative to volume (or weight) increases as size decreases. In other words, a very small frog has a lot of surface relative to its weight compared to a huge frog like a Bullfrog. This means that the relative amount of skin (surface) available for water loss is large. As a result, the cooling effect by evaporation must be quite good, but at the same time, when small frogs run out of body water, they risk drying rapidly for the same reason. This is why you rarely see Cricket frogs very far from water when summer temperatures are at their hottest.
Cricket frogs are not the only frogs that do this. For example, the Canyon treefrog of the desert southwest lives on rocks in small streams. The rocks can be very hot during the day, but the streams can be dangerous because of the presence of gartersnakes, which eat the frogs. Canyon treefrogs jump into the streams for short time periods taking up water. They then crawl up on the hot rocks and cool themselves by evaporating water off of their skin until they run out of water. The cycle is then repeated.
Amphibians and reptiles have many other ways to deal with water loss, some of which we will discuss in future blogs. The next time that you see a frog sitting in the sun, visualize water evaporating from its skin helping to cool its body. If you watch long enough, it will jump back into the pond for a fill up!