Natural History of Vertebrates
Lecture Notes
Chapter 9 - Salamanders, Anurans, and Caecilians

These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; they are a supplement to the discussion in class and the textbook. If you were to study only these notes, you would not learn enough to do well in the course. These notes are also linked with the notes from Vertebrate Structure and Development (ZO 515).

List of Terms

Extant Amphibians

The extant amphibians include three lineages (salamanders, frogs, and caecilians). They are probably a monophyletic group separate from the other tetrapods. Generally, they have smooth skin and lack scales. The skin is kept moist and all exchange gases through the skin (cutaneous respiration). All are carnivorous and will eat whatever comes within range that is not too big. The oldest fossils are from the Permian and clear fossils from all three modern lineages are known from the Jurassic.

The plethodontids are one of the best known of the salamander families and this groups has some rather interesting specializations.

This group of salamanders has lost its lungs. Respiration is wholly through the skin. The loss of lungs has allowed this groups of salamanders to develop a tongue which can be projected at prey items (figure 9-2). All salamanders lack ribs and thus to get air into the lungs must use a buccal pump, where air is held in the mouth and then compressed to move it into the lungs. This buccal pump uses several muscles that are attached to the tongue. Thus plethodontids had to give up breathing through their lungs so that the tongue muscles could be used to project the tongue instead of pumping air. The bolitoglossines (a group of plethodontids) can project their tongue the length of their trunk and can hit moving targets. Their eyes are moved more forward, giving them stereoscopic vision and the nerves from each eye project to both hemispheres of the brain as opposed to eyes more on the sides of the head and the nerves from each eye only projecting to the opposite hemisphere of the brain.

The social system of plethodontids has been studied by a number of investigators. Their small size and corresponding small home range makes behavioral studies relatively easier. Plethodontids have a nasolabial groove that runs from the external nares to the upper lip (figure 9-3). When it presses its snout on the substrate, it draws fluid into the nasolabial groove, which then passes into the nares and then to the vomeronasal organ, which is used for chemoreception (odors).

Males defend territories for feeding and mating purposes (figure 9-4). These territories are marked by pheromones and these marks are unique from one individual to another. Males are able distinguish the scent of a familiar male (his neighbor) from that of an unfamiliar male (an intruder). Males respond strongly and aggressively to the scent of intruders while ignoring the scent of neighboring males. This behavior of responding less to the scent of adjacent males has been called the "dear enemy" response.

Reproduction
Most use internal fertilization though some do not. There is no intromittent organ and thus the sperm can not be deposited directly into the reproductive tract of the female, rather the male deposits a packet of sperm (a spermatophore) and the female picks up the spermatophore into her cloaca (figure 9-11). For the transfer of sperm to take place, most species engage in a rather complicated courtship ritual, while in some the courtship is rather simple. During courtship a variety of secondary sexual characters (large tails, dorsal fans, coloration) are used as species recognition signals (figure 9-13). In addition, courtship involves the touching of the females by the males in order to deposit pheromones onto the skin of the female (figure 11-12). The female must also give positive feedback to the male for him to deposit the spermatophore. The female then walks over the spermatophore and picks it up into the cloaca.

If a species mates in the water, it will lay its eggs in the water. The young hatch as a gilled larval form that will eventually metamorphose into an adult. Paedomorphosis often occurs in which the larvae become sexually mature without metamorphosising into the adult form. This can vary from population to population within a species (Ambystoma).

The plethodontids, the most terrestrial of families, mates on land and some species lay their eggs on land. The larval stage occurs within the egg and the larvae metamorphose before they hatch. The young look like miniature adults.

There are only four species (genus Salamandra)that give birth to live young. The eggs may be retained until hatching and get all of their nutrients from the yolk and the young are born as larvae. In one species, the alpine salamander, the embryos get nutrients from oviductal secretion and are born much larger and more fully developed.

Frogs and Toads (Anura, figures 9-5 and 9-6) In general, frogs have longer limbs and make long jumps (figures 9-6 and 9-7), while toads have shorter limbs and tend to make short jumps (hops). Some species, especially arboreal frogs, have longer limbs and move by walking and climbing using all four feet. Some of these have enlarged toe disks for clinging to vertical surfaces (figure 9-8). Because of the mechanism by which the toes can cling or not to a surface, a frog must orient with its head facing up for its toes to cling to a surface.

Calls
Frogs and toads are very vocal and much research has gone into studying these calls. Calls are species specific and serve as pre-mating isolating mechanism for many species. Mostly the calls are used for mating or territorial defense; these are advertisement calls. In most species, only the males call. Those species with explosive breeding (for example toads) will have most of the males in the population calling at once, while those species (for example bullfrogs) with prolonged breeding seasons the males establish territories to attract females over many weeks. Calling has a cost. Calling increases energy use by a frog and calling frogs that are out of the water tend to dehydrate much faster than non-calling frogs (box 9-1). The more males that are calling at one time, the more energy a male frogs puts into calling (figure 9-15) Calling also increases the rate of predation. Many predators are attracted to the calls of the frogs (for example, humans or bats).

Reproduction
In most anurans, fertilization is external (tailed frogs are the exception). Males and females engage in amplexus, in which the male is on the back of the female and he grasps the female with his forelegs. Amplexus is usually maintained for several hours but may be maintained for several days. In toads that have explosive breeding, the chaos of breeding often brings males in contact with males. To avoid male-male amplexus the males have a release call so that one male can recognize another male. Parental care and/or investment in the eggs and young varies greatly among the anurans (figure 9-18).
Larval Stage
Tadpoles are aquatic (figure 9-19). Generally, filter feeders or algae gleaners (figure 9-20). Algae gleaners have small keratinized beaks for scraping algae. Only a few are predaceous and these have much larger horny beaks for attacking their prey (often other tadpoles). Because of the difference in habitat between tadpoles and adults, these two life stages do not compete with each other for food. Thus the selective advantage of having a tadpole larval stage is this reduction in competition.

Eventually, the tadpole becomes inefficient at gathering food and will metamorphosis into an adult. This involves a complete reorganization of the body plan (table 9-5); development and growth of legs; loss of the tail; loss of gills and the development of lungs; and reorganization of the skull as modes of feeding change. The process is controlled by thyroxine from the thyroid, which is controlled by thyroid stimulating hormone (TSH) from the pituitary. The effect of thyroxine is very tissue specific and region specific. For example, muscle cells in the legs are stimulated to grow, while muscle cells in the tail are stimulated to atrophy.

Metamorphosis is generally divided into three stages
premetamorphosis is the growth phase prometamorphosis is when the hind legs appear
metamorphic climax is the period from when the front legs appear to when the tail disappears. This stage occurs very quickly as this is the stage when the animal is most vulnerable to predation.

Caecilians Skin
The skin of all amphibians is highly permeable to water and gases. To exchange gases the skin must be moist, but the skin also represents a potential area for extensive water loss, thus amphibians (especially anurans) have developed a number of physiological and behavioral adaptations to reduce water loss (figure 9-12). For example, the skin is covered with mucus glands to aid in keeping the skin moist and in dehydrating conditions, an anuran will position its body to reduce the surface area that is exposed to the atmosphere. If an amphibian loses water, it can gain the water back once it is water. Many anurans have a pelvic patch, which is an area on the underside of the body that is highly vascularized for the uptake of water. An anuran can rehydrate by just having its belly in the water or on a wet substrate. Amphibians have several adaptation that allow them to survive periods of dehydration Amphibians have a lower osmotic pressure than other vertebrates. As they dehydrate, the osmotic pressure of the body fluid increases, but because it is so low to start with, the osmotic pressure can increase dramatically before it becomes life threatening. Anurans produce a very dilute urine, which is stored in the bladder. Unlike other vertebrates, anurans can get the water back. The bladder then serves as a canteen for an anuran out of water.

Defense in frogs and toads is often by poison, though this is also used by some salamanders (for example, the parotid gland on a toad or the skin glands in poison arrow frogs (Dendrobatidae)). Poisin arrow frogs use aposematic coloration as a warning to predators. A wide variety of compounds have been isolated from the skin and/or mucus of frogs and toads. These compounds have a variety of effects on predators that range from being noxious to lethal. Predators that have been captured where these animals naturally occur avoid attacking these animals, while naive predators quickly learn to avoid these animals.

Last updated on 24 February 2003
Provide comments to Dwight Moore at mooredwi@emporia.edu
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