Archive for August, 2010

What’s Buzzin’?

Wednesday, August 11th, 2010

During July and August, near ear-splitting calls of cicadas fill the air during the day only to be replaced by the incessant buzzing of katydids at night. People in the south call cicadas “July flies,” with the accent on the “u” of July. Male July flies and katydids call to attract mates, and they all call at once in their frenzy to attract females carrying the most eggs. A few snakes are also “buzzers.” Rattlesnakes “buzz” for a very different reason, and they are more likely to rattle when it is hot, simply because their body temperatures are high and thus they are very active.

Adult Western Diamondback Rattlesnake coiled

Adult Western Diamondback Rattlesnake coiled

Rattlesnakes rattle when they are disturbed, and the loud sound has several defensive functions. One of course is simply a warning—“stay away because I am dangerous.” This function is similar to bright coloration of poison frogs and some mushrooms, warning of impending danger. From the perspective of the snake, if it wards off a potential attack, then it has saved the snake some energy defending itself. The sound also directs the attention of a potential predator away from the vulnerable parts of the snake (head and body) so that if an attack does ensue, the snake may be partly down a hole or under a rock by the time a predator figures out that the sound-producing end is the tail. The rattle is composed of overlapping rings of dead skin. Each time a rattlesnake sheds its skin, it adds a rattle. Rattles sometimes break off, so a large snake might have a short rattle. Some wives’ tales (perhaps more correctly, old drunk husband’s tales!) are that rattlesnakes get a new rattle each year and that rattlesnakes have to rattle before striking. Neither is true.

Rattlesnake rattle, showing the overlapping rings of skin that rub together to make the sound

Rattlesnake rattle, showing the overlapping rings of skin that rub together to make the sound

Science often goes through wives’-tale stages as well. Rattlesnake rattles were first believed to attract cicadas, which they supposedly ate. However, they don’t eat cicadas, so that was false. They were thought to attract mates, but snakes do not have ears, and we don’t hear choruses of male rattlesnakes during the breeding season, so that too was proven false. The nice thing about scientific theories, no matter how ridiculous they may seem, is that once proven false, they are usually discarded. Many non-scientific myths are not only passed on from generation to generation, they are usually embellished with time, often to the point that people believe them no matter how ridiculous they might be.

So, if you should have the fortune of hearing a rattlesnake while out hiking around, first move away, keeping in mind that the snake is letting you know that you are too close. Once you are a safe distance, see if you can locate the snake and add it to your life list of interesting sightings.

The Heat is On!

Wednesday, August 11th, 2010

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.

The dark area on the skin of the underside of this Red-spotted toad is the pelvic patch, where rapid water uptake can occur.

The dark area on the skin of the underside of this Red-spotted toad is the pelvic patch, where rapid water uptake can occur.

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.

This Northern cricket frog is only about one inch in total length.

This Northern cricket frog is only about one inch in total length.

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!