How Amphibians Behave
R.D. and Patti Bartlett
Amphibians are rather quiet animals. They don't have the derring-do we
equate with rattlesnakes or cobras. They can't rise up on their hind legs like the basilisk, and they certainly can't run across water, as do the basilisks. They can't even hide within themselves, the way a box turtle can. But amphibians have some behavioral peculiarities that make them interesting.
Amphibians can change color. These color changes can be affected by light levels, temperatures, color of background (for some, not all species) and stress. The mechanics of color changes are determined by color-containing cells within the dermis and epidermis called chromotaphores.
The rapid and reversible color changes, called physiological changes, involve the intracellular movement of pigment-containing organelles within the chromatophores. An example of this sort of quick change is demonstrated by Hyla punctata, a small treefrog found in the neotropics. By day, the frog is pale green with a yellow side stripe and yellow spots on the back. At night, the yellow fades and a red suffusion turns the frog's back to a reddish tan.
A slower color change is a morphological change, and it is caused by either pigment synthesis or destruction within the chromatophores. This is the process by which an amphibian becomes increasingly dark with age.
Amphibians know what's going on around them. Their skin contains a nerve network that sends messages about the environment to the brain. Part of this network is fairly straightforward and understandable. The dermis contains many uncoated nerve fibers, some of which extend into the epidermis. Here they serve as receptors for heat, cold, pain and pressure.
But some amphibians have an even better skin sensory system. This is a lateral line system, and it's found in aquatic amphibians, such as amphiuma, siren and hellbenders. The lateral line system contains both mechanoreceptors (the neuromasts) and electroreceptors or ampullary organs. These cells enable the animal not only to detect movement in the water but may indicate what's making the water move.
If an animal causes the movement, the lateral line ampullae sense the presence of an animal and may identity it by its electrical field. Even if the ID is only that an unidentified animal is approaching, at least the amphibian won't be caught flat-footed, as it were.
Amphibians can be aggressive. Perhaps the most aggressive amphibian is the horned or Pac-Man frog, a fist-sized ivory-with-brown blotched frog that conceals its plump body under leaf litter in its South American jungle habitat. When an appealing food item (an insect, mouse-sized mammal, bird, or another frog) wanders by, the horned frog roars out of its cover and grabs the food item. With a huge mouth, jaws strong enough to crush the skull of a mouse, and a pair of bony processes in the front of the jaw to make sure prey doesn't wriggle free, the horned frog packs an impressive bite. For humans, having a large frog enthusiastically biting one's hand can pack a distinct startle factor.
Other toads, like Cameroon giant toads and Roccoco toads, defend themselves against a perceived enemy by a combination of toxic skin secretions and head butting. They lower their head and launch themselves against the enemy. This presses the parotoid glands, which are laden with toxin, against the enemy, an effective deterrent.
Defense systems are well developed in amphibians. Salamanders can use defensive posturing to warn off potential predators. They stand straddle-legged, with the body and tail strongly arched. This is called the "unken" position, which is German for "boat."
Tiger salamanders and Pacific newts secrete a milky substance from the pores on their back, and in the unken position may wave their tail to fling the secretions toward the predator. Most predators recoil and retreat, which is what the amphibian wants, of course. But raccoons are smart enough to roll the newt or salamander around in the dirt/forest duff until the toxin supply is both exhausted and rubbed off, and at that point the animal becomes part of the food chain.
Amphibians and Their Eggs
Some amphibians take care of their eggs. In the case of the Pipa toads, the fertilized eggs are placed on the back of the female. The skin on her back swells and expands to form a pouch around each egg. Depending on the species, the tadpoles wriggle out and swim away or undergo what is called direct metamorphosis inside these pouches and emerge as tiny adults.
In some hylid frogs, the female carries the eggs in a brooding pouch on her back. The young emerge as small adults or as tadpoles, depending on the kind of frog.
Perhaps the most unusual are the gastric brooding frogs, Rheobatrachus silus and R. vitellinus. The female swallows the fertilized eggs and the tadpoles develop inside her stomach. The young are propelled out of her stomach when she opens her mouth, dilates her esophagus and essentially belches the young out. Only then can the female resume eating.
Some salamanders are extraordinary in their orientation and migration ability. They not only can orient themselves by the position of the moon and stars (celestial cues), but also have a sense of time of day so they know when to seek those celestial cues.
When taken out of their breeding stream and released up to eight kilometers away, Pacific newts can find their way back, even when blinded. Olfaction, celestial clues, and what is described as kinesthetic orientation (a sensation/recognition of an animal's position in relation to a site) play roles. It is also likely that amphibians can clue into magnetic waves, just as migrating birds do.
The pineal gland appears to be a major factor in gathering celestial cues. But, interestingly, most migrations back to the breeding pond or stream occur on rainy or overcast nights, when visual celestial orientation would not be possible.