Frogs: The Animal Answer Guide

6. Reproduction and Development

59 How do frogs reproduce? All frogs have two sexes, females that produce eggs and males that produce sperm. Thus, all species reproduce sexually, with the male’s sperm fertilizing the female’s eggs. In most species, the female deposits her eggs outside of her body before fertilization. In the simplest situation, eggs are released from the female’s body into the water, and the male releases sperm on the eggs at which time fertilization occurs. During mating, the male in nearly all frogs grasps the female around the waist or under the arms, a process called amplexus, so that their cloacas are in close juxtaposition when the eggs and sperm are released. Because an egg and a sperm each has half the number of chromosomes as each animal, the resulting embryo has the full chromosome complement from the egg and sperm. Most frogs have somewhat elaborate mating procedures that begin with male frogs calling to attract females and then males and females congregating at a breeding site. Although males establish territories to compete for mates and sometimes engage one another in physical combat, the female generally chooses her mate before she releases her unfertilized eggs. Eggs usually hatch within a short time and, in many species, have free-swimming tadpoles that eventually emerge as the adult form. Although the common perception in North America and Europe is that the tadpole stage is typical of frogs, a high proportion of the world’s frog species undergo complete larval development in the eggs and hatch as froglets. Direct development of this sort is the norm for many tropical species of frogs. For example, none of the more than 180 species in the genus Eleutherodactylus have tadpoles. Chapter 6 Reproduction and Development 60 Frogs: The Animal Answer Guide Do all frogs lay eggs? Females of all frog species produce eggs, and most, but not all, lay them in an unfertilized state in the presence of a male that fertilizes eggs outside the female’s body. Most people are familiar with frogs that lay their eggs in water, where they hatch and develop into tadpoles. However, many species of frogs, especially in the Tropics worldwide, lay their eggs in moist places on land, and juvenile frogs emerge directly from the eggs without a freeswimming tadpole stage. At least five species of frogs are live-bearers, meaning that they produce eggs fertilized inside of the female’s body where they develop without being laid. Several species are in the genus Nectophrynoides (family Bufonidae) of Tanzania and a single species, the golden coqui (Eleutherodactylus jasperi) of Puerto Rico, is in the family Leptodactylidae. Depending on the species, the gestation can last from 1 to up to 9 months. The developing embryos are nourished inside the body of the female, who eventually gives birth to fully developed little froglets. Marvalee Wake of the University of California at Berkeley studied the golden coqui and determined that egg yolk was the only evidence of provisions by the female to the embryos. Egg yolk was still present when the baby frogs were born, and no direct maternal nourishment was given by the female. Additional species of frogs with internal development of eggs and embryos probably exist but have not yet been discovered and described. How many eggs do frogs lay? As with most other groups of animals with numerous species, the number of eggs produced by a female varies and depends on the species. Some frogs and toads are known to lay enormous numbers of eggs that may be in strings or clusters that number in the hundreds or thousands. In contrast, some frogs lay fewer than 10 eggs at a time. Water-breeding species lay the A male wood frog (Rana sylvatica) clasps the larger female as he waits to fertilize the eggs when she releases them into the water. Courtesy Victor Lamoureux 61 Reproduction and Development largest numbers of eggs. American bullfrogs (Rana catesbeiana) can lay up to 20,000 eggs at a time. The cane toad (Bufo marinus) has been known to lay more than 30,000 eggs at a time. Of course, offspring from most eggs laid by any species of frog do not survive to adulthood, and, in many situations, none survive during a particular year or at a particular location. For example, a drought may cause wetlands to dry, thus killing all eggs of a particular species in a particular year...