During Fall, when temporary ponds fill with water in southeastern Oklahoma, thousands of Southern leopard frogs come in to breed. Males enter the ponds at night, usually when it is raining or within several nights following the rain. Males call to attract females, and, after a female selects the male with which she will mate, the frogs amplex. Although amplexus varies among frog species, Southern leopard frogs amplex with the male on top of the female with his front legs wrapped on the body of the female just behind her front legs. This is called inguinal amplexus because of the position of the male’s front legs. As you probably know, fertilization is external in almost all frogs, so the male releases sperm over the eggs as the eggs are being laid, fertilizing the eggs. The eggs are laid in a mass attached to vegetation in the water. If the pond doesn’t dry up, the eggs hatch into tiny tadpoles. Temporary ponds are usually rich in nutrients in Fall, and as a result, most surfaces in the ponds are covered with algae, and huge populations of various insects and other invertebrates build up as time goes on. The tadpoles feed on algae, scraping it off of the surfaces with tiny teeth surrounding the mouth. Because tadpoles of Southern leopard frogs are vegetarians (herbivores), nearly all of their digestive system is intestine. If you closely examine a tadpole, you can see the intestine through the skin. Once tadpoles hatch from eggs, the real race begins. Individual tadpoles face an uncertain future. The pond can dry up before they transform into a terrestrial frog. Also, aquatic birds and many insects, including larvae of dragonflies, damselflies, and predaceous diving beetles, eat tadpoles. Tadpoles eat and grow as fast as they can so that they can get out of the pond before it dries and so that they can get bigger than some of their predators (aquatic insects). One of the particularly fascinating things about frogs is the remarkable change in lifestyle that goes with transformation from a tadpole to a young frog. This will be a topic for a future blog, so keep posted.
Archive for December, 2009
Usually during this time of year (December), we don’t think about amphibians or reptiles because most are spending the winter hidden to avoid freezing temperatures. However, Southern leopard frogs (Lithobates [Rana] sphenocephala) breed both in Spring and Fall in temporary ponds. As a result, ponds that retain water in late Fall and early Winter in eastern Oklahoma contain tadpoles of Southern leopard frogs. As the ponds shrink in size, the tadpoles become easy prey for herons and other water birds. In addition, as temperatures go down and the ponds shrink, the possibility exists that the ponds might dry up or freeze solid, killing the tadpoles. One such pond at our research site in Le Flore County is now filled with tadpoles and the water is dangerously low. Although it might seem that producing tadpoles when the possibility exists that the ponds might dry up or freeze is not in the best interest of the individual frogs that deposited the eggs, that is actually not the case. These frogs cannot predict whether ponds will last long enough or be refilled. However, one thing is for certain—if they do not breed at all, then there is no possibility that they will produce offspring in the event that ponds do last long enough in a particular year. Also, they can try again in Spring, and if they survive the Summer, yet again the following Fall. So, in effect, they are hedging their bets. Individual frogs that breed in Fall have some chance that they will produce offspring that will survive, whereas individual frogs that do not reproduce in Fall absolutely produce no offspring.