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).