When strolling by a pond, river or large puddle at night, you may see two frogs clinging onto each other. This is a behavior called amplexus: it allows the male frog to place his cloaca near the female's in order to fertilize her eggs.
Frog Life Cycle
Starting life as an aquatic, fertilized egg, frogs then emerge from their eggs as tadpoles. The tadpoles slowly develop rear legs and then front legs as they grow. Tadpoles consume algae, small invertebrates, organic debris and other tadpoles. For the final step in the process, the tadpoles will absorb their tails and transition to the land—or trees, if they are a terrestrial or arboreal species.
The vast majority of frogs travel to aquatic sites for mating and egg deposition. Once there, male frogs will emit loud advertisement calls; sometimes they have to compete with other males for prime locations. Females are attracted to these sounds, and they approach the males for breeding. Egg fertilization is external in all but a few species: the male clasps onto the back of the female and releases sperm as the female deposits the eggs. This fertilizes the eggs. Some species form “breeding balls” in which one female is surrounded by numerous males; in some cases, female frogs drown during this process.
Adaptations for Amplexus
Many amphibians are slippery animals, which can make gripping them difficult. In response to this, some male frogs have enlarged thumbs, swollen forearms or structures on the hands that assist in gripping females. Among other species, leopard frogs (Lithobates pipiens), pickerel frogs (Lithobates palustris) and American toads (Bufo americanus) display these structures. In many species, these adaptations grow during the breeding season and are the most reliable means of determining a frog’s sex in the field. The highly unusual tailed frog (Ascaphus truei) takes amplexus a step further: the tail of these frogs is a copulatory organ that allows these frogs to achieve internal fertilization—the only species known to do so.
Frogs don’t have very good eyesight, and male frogs are very aggressive breeders. Because of this, male frogs often grab and attempt to achieve amplexus with a variety of things that aren't live females of their species. The most common result is that a male grabs another male of the same species, which results in the bottom frog's emitting a “release call.” This signals the offending male to let go, which he usually does so that he can instead find a female. Sometimes, this lack of discrimination causes other, more costly problems: in the Western United States, introduced bullfrogs have been documented to mate with two native species in a 2005 study. The study, by Christopher A. Pearl et al. and published in “American Midland Naturalist,” showed that in many locations, mixed-species breeding opportunities were more common, and produced a great deal of hybrid offspring. In more unusual cases, researchers have documented frogs clasping onto different frog species, decomposing female frogs, and inanimate objects like plastic cups.
- American Midland Naturalist: Observations of Interspeciﬁc Amplexus Between Western North American Ranid Frogs and the Introduced American Bullfrog (Rana catesbeiana) and an Hypothesis Concerning Breeding Interference [PDF]
- Herpetology Notes: Unusual Amplexus in Dendropsophus columbianus (Anura: Hylidae) [PDF]
- Biharean Biologist: Cases of Abnormal Amplexus in Anurans (Amphibia: Anura) from Bulgaria and Greece [PDF]
- Animal Diversity Web: Lithobates pipiens
- Savannah River Ecology Laboratory: Rana [Lithobates] Palustris
- The University of Rhode Island: Eastern American Toad