If you’ve ever tried to chase frogs, you’ve probably noticed that they can breathe on air and seemingly don’t need to resurface for air if they’re underwater. But is that really true? How do amphibians breathe – and can they breathe underwater?
Let’s start out with the basics of amphibian biology, then dig into how their bodies support their unusual lifestyles.
The term “amphibian” actually describes a class of animals. They’re in the same domain (Animalia) and phylum (Chordata, meaning they have a backbone) as you and me, but that’s about where the taxonomic similarities end.
The Class Amphibia includes three extant orders (which are groups of families, which are in turn groups of genera, which are groups of species):
There are probably about 7,000 species of amphibians currently alive. 85-90% of these species are frogs. That number is, sadly, dropping quickly because amphibians are so vulnerable to pollution. Because amphibians breathe partially through their skin (and their eggs don’t have shells), they are very susceptible to pollutants. Amphibians are often considered an “indicator” group, as they often drop in numbers before other species start to struggle to survive.
Amphibians require water to reproduce because, unlike reptiles and birds, their eggs lack an impervious shell. Some species of amphibians have adaptations that allow them to lay their eggs elsewhere, but the vast majority of amphibians require freshwater for their eggs. Amphibians are hatched into a larval stage that is quite different from their adult form. They must go through metamorphosis to reach adulthood.
Adult amphibians need to keep their skin moist even when they’re on land, so they’re limited to moist environments. Likewise, amphibians generally don’t survive in saltwater. A few species can live in brackish water, but otherwise, amphibians are creatures of fresh water.
Because amphibians are cold-blooded (ectothermic), their metabolic rates are low and they don’t need to eat much. Many amphibian species have ears that can detect vibrations in the air or water, helping them find prey and avoid being eaten.
Many amphibians have mucous glands on their skin – some of which are poisonous. This special skin is our main topic for the day because amphibians can also breathe through their skin. Let’s talk about the lungs and skin of amphibians and how they work together.
Most adult amphibians can breathe both through cutaneous respiration (through their skin) and buccal pumping – though some also retain gills as adults. Amphibians have primitive lungs compared to reptiles, birds, or mammals. This means that they deal with slow diffusion of oxygen through their blood.
Buccal pumping works by having the amphibian draw in air through its nostrils. The animal then closes its nostrils and pushes air through the lungs by contracting their throat (not using a diaphragm).
Amphibians like the Titicaca water frog and hellbender salamander that live in cold, fast-moving streams may be able to breathe through mainly cutaneous respiration. Cutaneous respiration allows the animal to absorb water through their skin directly into their bloodstream. Wrinkles in the skin help some species absorb more oxygen because they simply have more skin to use.
Famously, the lungless salamanders have neither lungs nor gills – they just breathe through their skin!
Some aquatic salamanders (and all tadpoles) have gills and can breathe underwater thanks to them. The Mexican axolotl, for example, never loses its gills.
As larvae (tadpoles), all species of amphibian can breathe underwater. As they go through metamorphosis, though, some species of amphibian lose their ability to breathe entirely underwater.
Breathing underwater isn’t easy for many species of amphibians, so they might need to surface for air if they’re working hard (escaping a predator, for example) whereas they can meet their oxygen needs underwater if they’re at rest. Many frogs and toads can even breathe through thick mud during hibernation.
Some species of terrestrial amphibian cannot breathe underwater, though they can hold their breath for hours as needed.