Review sheet 8             Biology 4900 – Behavior          James Adams and Kristen Sanders

Vertebrates:  Amphibians

Three orders: Anura (frogs and toads), Caudata (salamanders and relatives), and Gymnophiona (caecilians) – virtually all are predators

Gymophiona: We’ll start with Caecilians
            Caecilians are largely subterranean in the tropics of the world, and feed on a number of small, soil-dwelling organisms such as earthworms and termites.  Some are completely aquatic and eel-like. All have significantly reduced eyes and no legs, not surprising considering the subterranean lifestyle.

            Reproduction is exclusively through internal insemination in this group, unique among amphibians (in other words, they actually mate).  Egg laying species (approx.. 25%) will guard the eggs after laid. The majority (75%) give live birth, with the larvae maturing inside mom and actually feeding by scraping off pieces inside mom’s oviducts (larvae have teeth).  Once outside mom (or hatched), young may feed on the lipid-rich epidermis of mom as well, which she is able to replace on a frequent basis (see pg. 416, Fig. 11.11; see video on website as well).

Caudata (or Urodela):
            Salamanders, newts and others in this group are opportunistic predators on almost anything of appropriate size – the larger the salamander, the larger the prey items can be. Some species may eat other salamanders, and some may even have non-cannibalistic and cannibalistic forms, such as the Tiger Salamander (see pgs. 96-97, Fig. 3.29).

            There are three primitive salamanders that are all very large: Chinese (see video on website) and Japanese Giant Salamanders, and the eastern U.S. Hellbender (see image on website).  These reproduce externally, similar to many frogs, with females laying eggs and males releasing sperm over them in a nest built by the male.

            In most the rest of the Salamanders, males deposit a spermatophore on the ground that the female picks up with her cloaca, and, as such, the eggs are fertilized inside. This is not always the way sperm are transferred to females in aquatic species.

Sirens (see image on website) include the only herbivorous amphibians, with some eating algae and aquatic plants. Most also supplement their diet with gastropods and bivalves, as some also have a strong “beak” which may be used to open shells of the aforementioned animals. For sirens that have been studied, males in the lab will build a nest and participate in external fertilization (like the Giant Salamanders) and then guard the nest with the eggs in it.  Other studies have suggested that females lay eggs singly over a longer period of time, which suggests internal fertilization.  Stay tuned.

Tadpoles typically hatch from laid eggs, though some females retain eggs until they hatch or even until they turn into juvenile salamanders. Not all salamanders fully develop, retaining larval characters, such as gills, for a significant period of time or even throughout life (neoteny or paedomorphosis). For instance, sirens are all neotentic. All salamander families have species that exhibit this, and it may be a mechanism that allows terrestrial species to survive in the water when conditions on land are too harsh.

Like many frogs, many salamanders are distasteful, and indicate this aposematically. And many species use the color, as well as crests (see image on website of Great Crested Newt) and tails, for courtship.

Anura:
            Frogs and toads are also predatory. They, like terrestrial salamanders, have sticky tongues which can be extended very quickly to catch prey. They are ambush predators (also like the Salamanders).

            Frogs and toads are notoriously vocal, using the calls to attract mates. There are some species of Leopard Frogs that can produce viable offspring in the lab, but remain separate in nature because females only respond to the calls of appropriate males. Calls do indeed indicate fitness of the males to the females, and females may pay attention to different components of the calls in different species.  For example, in Gray Tree Frogs, speed and length of the call both are important, with faster paced trills and longer time calling are both preferred by females.  For this species, both indicate that the males have obtained enough energy to produce both fast trills and call for a long time – this means the males are good at getting food and avoiding predators. In Bullfrogs, it is the depth of the pitch of the call that matters, as deeper call means bigger, older frog. This does provide for the opportunity for cheating satellite males in Bullfrog mating groups (I will explain). Several species have males that will lek in large groups to attract females, though, as the study on Barking Treefrogs on pages 392-393 (Fig. 10.37) suggests, you need to be careful in assuming cause and effect. Both males and females of these treefrogs group together in similar places because of similar environmental tolerances and preferences.

            Of course, making sound also opens frogs up to predation; some snakes, other frogs and some  bats (see pages 296-297, Fig. 8.34) are known to prey on frogs.

            Females may reject male advances (see pages 345-346, Fig. 9.33), but if males are successful at attracting mates, the males grab the females in a hold called amplexus (see image on website).  They stay firmly attached to the females until they can convince them to lay eggs in a watery environment, and then the males deposit sperm on the eggs. Although bodies of water are the typical location for the eggs to be laid, they may be laid in other wet locations such as tank bromeliads in the trees (the location for most poison dart frogs (pages 423-424, Fig. 11.15; also see image on website) or in froth produced by the frogs (see pages 368-369, Fig. 10.13; also see image on website).  

Parental care is seen in slightly less than 20% of frogs. In those that do, there are examples of maternal, paternal, and both parental care. In cases of paternal care, eggs may be protected in the above-mentioned bromeliads, carried on the back of the male (pg. 310, Fig. 9.5) or in the mouth of the male, and brooded in the male’s stomach/mouth (see image on website). Indeed, in the case of poison dart frogs, paternal care may have evolved BECAUSE they are the ones guarding the habitat that attracts the females to lay eggs (pgs. 423-424, Fig. 11.15).

To avoid predation, many, many frogs have mildly to extremely poisonous skin, and again advertise the fact with bright color, as is the case with the poison dart frogs. I will explain why they are called poison dart frogs. Some toads, which are more adapted to dry conditions as adults (but still must return to water to reproduce), have glands that secrete additional toxins. The epitome of this is shown by the largest toad, the Cane Toad (see image on website), which is remarkably poisonous. There is also at least one frog that has expandable webbing between the toes that can hop from a tree and glide to safety (see video on website).