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Mating and reproductive strategies of salamanders and anurans (frogs and toads)
TA: Maya Mukhida
BIOL 200 Lab (Wednesday 6:30pm)
Queen’s University, Kingston, Ontario.
March 26, 2003
Salamanders and anurans are amphibians, ectoderms that spend part of their lives in water, and part on land. As adults, many amphibians live on land, but most return to the water to breed. The word “salamander” comes from a Greek word meaning “fire animal”, for salamanders are often found in the logs that people set afire (Winner, 1993). Salamanders can be found all over the world except in Australia, the North and the South Poles (Winner, 1993). Frogs and toads are anurans, which are the tailless amphibians (Mattison, 1987)
To reproduce successfully, some species of salamanders, such as the California newt, identify their potential mates through the sense of smell. The male salamanders of other species, such as the European crested newt, signal their readiness for mating when their skin undergoes colour or pattern changes (Winner, 1993). Normally solitary, male and female salamanders search for one another during the breeding season, which occurs with the first warm rains of spring to avoid harsh environmental conditions. In order to mate, terrestrial salamanders must migrate nocturnally to the breeding pond where they hatched, which they find with their sense of smell (Winner, 1993). The migration can be exhausting, so some species of salamander breed only every other year (Winner, 1993).
Male salamanders are polygamous, and, in most species, mating involves a dance, the release of sperm, and the fertilization of eggs inside the female salamander (Winner, 1993). The courtship begins when a male finds a potential mate. The male sends his pherome towards the female salamander by moving in front of her and waving his tail (Winner, 1993). If the female wishes to mate, she joins him in a dance in which the pair swims or walks around each other; they often engage in head-butting as well (Winner, 1993). In most species, when the female is ready to mate, the male deposits a bundle of sperm – a spermatophore – on a rock or stick and guides the female to the bundle (Winner, 1993). The sperm enters the female’s body through an opening in her tail as she rubs herself over the spermatophore and the sperm fertilize her eggs (Winner, 1993). In other species, the male clasps the female’s body with his front or hind legs, which have rough patches of skin called “nuptial pads” and “claspers” to help the male secure the female, so that he can deposit the spermatophore into her (Winner, 1993). After the breeding season, the “nuptial pads” and “claspers” disappear. In some aquatic salamander species, fertilization occurs in the water as the male releases his sperm on top of the eggs that the female first releases into the water (Winner, 1993).
To keep her fertilized eggs safe and moist, the female salamander anchors them onto a stick or a rock that is in the water or on moist land. Some terrestrial salamanders, such as the female Dusky salamander, bury themselves with their eggs under logs or rocks to keep the eggs safe and moist (Winner, 1993). Other species of salamander, such as the black salamander, give live birth to their young (Winner, 1993). While some salamanders lay one or two eggs, others lay several hundreds in one breeding season. This may be related to the amount of time different species of salamanders invest in protecting their young. For example, the female Dusky salamander mentioned above lays only ten to thirty eggs; thus, she must remain with her eggs until they hatch to ensure the survival of her young. Salamander eggs are protected by a jelly coating, and the embryo is nourished by the yolk. Typically, embryos take one month to several months to hatch, so that in most species of aquatic salamanders, the female guards her embryos until they hatch, while the terrestrial species which breed in water do not (Winner, 1993). The aquatic Japanese giant salamander is the only known exception in which the male – rather than the female – tends the eggs (Winner, 1993).
Anurans (Frogs and Toads)
Triggered by the changes in light, temperature, and water levels, anurans are ready to breed after hibernation (Mara, 1994). “Seasonal” or “cyclic” breeders are anurans that have seasonal breeding cycles, while “opportunistic” breeders are anurans that breed whenever the environment is favourable – when the weather is warm and wet (Mattison, 1987). In the humid tropics, some anuran species may breed throughout the year, and unusually long periods of warm wet weather will trigger multiple breeding in most anuran species (Long, 1999). By responding differently to their surroundings and by having different breeding cycles, anurans are able to lessen the competition for resources and for mating partners during breeding season. Anurans that often breed biannually include gray treefrogs, western chorus frogs, green frogs, and bullfrogs (Long, 1999). Florida leopard frogs, living in a warm environment, typically breed three times a year (Long, 1999). Other species, such as the Oregon spotted frog and the tailed frog, sometimes skip a year between reproduction cycles (Long, 1999).
Anurans also reduce competition by reproducing at different ages. Some species of South American frogs can begin breeding when they are only six months old, while most North American frogs do not begin breeding until they are at least one year old (Long, 1999). Females in some species – such as bullfrogs – may be able to breed early in life but do not reach their reproductive peaks until later, because they breed only once a year when they first begin reproducing, but, in later years, they have more than one brood each year (Long, 1999).
To ensure that more of their eggs and tadpoles survive predation, many species of anurans in a local population will synchronize their breeding, which may be triggered by a bout of warm weather (Long, 1999). To attract mates, male anurans have distinct mating calls which helps identify the species and territory of the vocalist, as well as attract potential mates. During prolonged breeding seasons, the males are more choosy in selecting their mates and their territories are often well established (Long, 1999). Although these anurans are less likely to face violent competition for mates, they must often compete with opportunistic “satellite males” – solitary frogs of the same species that do not call for mates but that lurks in another male’s breeding territory to mate stealthily with the responding female (Long, 1999). Opportunistic behaviour has been noted with bullfrogs, pacific treefrogs, northern cricket frogs, striped chorus frogs, and spring peepers (Long, 1999).
Anurans are not monogamous: the males of most species will mate repeatedly as long as females respond to their mating calls. During mating, the male frog mounts and grasps the female from the back with his front legs. This mating position is known as “amplexus”; it ensures that the male’s sperm ejaculation will be synchronized with the female’s eggs ejaculation, and it also ensures that the sperm will have a higher chance of meeting the eggs (Mattison, 1987). There are several variations to amplexus, and one of these is “inguinal amplexus” in which the male anuran grasps the female in front of her hind limbs so that the cloaca is close to the female (Mattison, 1987). “Axillary amplexus” occurs when the male grasps the female behind the fore limbs so that their cloacae are close to each other without extra maneuvers from the male (Mattison, 1987). Poison dart frogs use “cephalic amplexus” in which the male presses the back of his hands against the female’s chin or throat (Mattison, 1987). Anurans belonging to the genus Mantidatcylus use “straddle amplexus”, in which the female forces her head beneath the male’s hind legs while he is clinging to a vertical leaf. As this always occurs in the rain, the sperm slides down the back of the female and over the eggs as they are laid (Mattison, 1987). Anurans with short limbs use “glued amplexus”, in which the male attaches himself to the back of the female with a sticky secretion from his abdomen (Mattison, 1987). “Prolonged amplexus” occurs in frogs of the genus Atelopus: the couple stays together for several days to several weeks, and the male becomes thinner as he stays on top of the female. It is not known if “prolonged amplexus” serves any advantageous function to the reproductive cycle of the anurans (Mattison, 1987).
Fertilization of the eggs occurs externally in all anuran species except for the tailed frog, probably because it lives and mates in cold, fast-running mountain streams, so that fertilization must occur quickly and efficiently (Long, 1999). Typically, anurans lay a high number of eggs to ensure that a number of tadpoles will survive and mature. Some species of anurans may lay up to one thousand eggs each time, but usually the clutches number around two hundred to five hundred eggs, especially for those anurans that create “bubble nests” (Mara, 1994). Perhaps the eggs of the anurans that create “bubble nests” are more vulnerable to predators and environmental dangers than those of the anurans that create nests hidden under leaves or pits. Many poison frogs of the family Dendrobatidae guard egg clutches, so that they do not have to lay as many eggs as the anurans that create “bubble nests” (Mara, 1994).
Works Cited (Primary References)
Long, Kim. 1999. Frogs: A Wildlife Handbook. Johnson Books. Boulder, Colorado.
Mara, W.P. 1994. Breeding and Keeping Frogs and Toads. T.F.H. Publications, Inc.
Neptune City, NJ.
Mattison, Chris. 1987. Frogs and Toads of the World. Facts on File, Inc. New York.
Winner, Cherie. 1993. Salamanders. Carolrhoda Books, Inc. Minneapolis.