1. Amphibians are the only group of animals with legs like us (bones, organs etc – the tetrapods) that are not adapted to have young on dry land. Although this breaks down a little because there are some who have live young, such as stomach-brooding frogs or some caecilians. It’s nice and simple for us to think of eggs or frogs as ‘primitive’ and live births as ‘advanced,’ but the world doesn’t obligingly do what we want. Even some of the egg layers provide parental care, moving their tadpoles and laying unfertilised eggs for them to feed on.
There are three types of amphibians, frogs and toads, newts and salamanders (which look like lizards), and caecilians. I’d never heard of caecilians before, but they are completely legless and look like worms or slippery snakes. Some of the features of amphibians are:
- They go through a larval stage with gills (although this might be inside a parent).
- Their eggs have a simple membrane and need to be in water or wet soil.
- Their skin doesn’t have scales or keratin to protect it, so it needs to stay moist. They can breathe through it, although they generally have lungs as well.
- They cannot maintain their body temperature above their surroundings, so their activity depends on high temperatures. The advantage is that they need less food because they don’t have to generate their own heat.
- Adults are carnivores.
2. The thin skin is great to allow them to hibernate at the bottom of ponds, but it doesn’t give them protection from predators in the way scales or fur does. So all amphibians have developed toxins of some sort. This does not mean they are all poisonous to humans, but are aimed at their main predators. Some, such as the Golden Poison Frog are highly poisonous to humans.
Many of the poisonous amphibians are brightly coloured to warn off predators, the same way butterflies and caterpillars are. Have a look at the video for some interesting colours.
Cane toads are the most commonly known poison amphibian in Australia. The eggs and tadpoles are extemely toxic, but young toadlets are not. It isn’t until they are mature that they start to produce their own toxin. Some crows have discovered that the belly skin does not have poison glands, and have been known to flip the toads over and eat them from the belly side.
3. Newts are the only vertebrates capable of regenerating complex structures such as limbs. We have very limited regeneration ability, only being able to heal things like skin, the liver and some bones. Interestingly, children do appear to be able to regrow fingertips, and there have been isolated cases in adults as well.
4. Modern amphibians tend to be small, with the largest up to around 50cm for toads with stretched legs. But in the past some were much bigger, even several metres long. There is nothing inherently ‘works better when small’ about amphibians, although certain shapes are needed for large, cold-blooded animals. During the Carboniferous they were the top land predators and filled the same niches as crocodiles today, with roughly the same size and shape. But during the Triassic reptiles and mammals evolved.
These two aren’t tied to the water in the same way as amphibians are, who need it for their young. Mammals and reptiles have eggs that take the water with them or hold it inside themselves and thick skin, which allowed them to move away from rivers and lakes. With so many more places to live, reptiles took over many of the larger niches and pushed amphibians into the smaller ecological zones.
5. Up the top is an axolotl, or Mexican Walking Fish. They are a type of salamander that stays in the water their whole life, becoming sexually mature while they still looking and living like larvae. The frills behind its head are its gills, which are usually lost in adult amphibians. It’s a bit like a caterpillar that doesn’t bother turning into a butterfly. With great difficulty some can be forced to ‘grow up,’ but most die and the ones who change die far, far more quickly.
Staying like a juvenile while being sexually mature is quite common and is called neoteny (nee-O-tenee). It demonstrates that a small tweak in a genetic or developmental pathway can cause very large changes in the final adults, as you can see when you compare the Axolotl to the closely related Tiger Salamander adult.
A neotenous animal we all know well are humans. So many of our features, from bones in our feet, to relative hairlessness, to teeth, to rapid brain growth are like juvenile apes rather than adults. I’m certainly not saying we’re just baby apes, but a general stretching of development and staying more like a juvenile has been very important in human evolution.
Brightly coloured poisonous frogs, rather than hiding they warn predators off.
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