Natural History

By starting at the beginning, looking at the natural history, we can start to understand unique amphibian adaptations, and how they fit into the ecosystems we can see today.

Based on fossil evidence, we know that amphibians evolved from fish with lungs and bony-limbed fins, slowly, over time developing the ability to move on land from water. This evolution likely occurred sometime in the Devonian period- before all other land vertebrates including reptiles, mammals and birds. The ability to use resources from both water and land gave early amphibians a huge survival advantage, as there were more food sources available, and more area for habitat.

Eggs were laid without a shell in water, as we see today. For more than 100 million years, amphibians likely remained the dominant land vertebrates, before some branched off and evolved into what we now know as reptiles, as animal adaptations improved for living on land.

Reptiles emerge as the shells of amphibian eggs become thicker over time, thus more protective, and self-containing. We know this because there are some amphibian species (such as salamanders) which have eggs which contain the larvae at their aquatic stage (as we see in reptiles) but are not quite hard-shelled. This shows an intermediate phase and gives us evidence for this evolution path.

True reptile fossils can be traced back to the Carboniferous period, after amphibians. They were likely better able to survive on land due to their amniotic eggs, eggs with liquid inside a protective shell, as well as the fact that they were able to move further away from water, as they did not need to lay their eggs in it. They also developed scales, which helped them live on land, and led to their eventual true reptilian form. 

By starting at the beginning, looking at the natural history, we can start to understand unique amphibian adaptations, and how they fit into the ecosystems we can see today.

Based on fossil evidence, we know that amphibians evolved from fish with lungs and bony-limbed fins. Slowly, over time they developed the ability to move on land from water. This evolution likely occurred sometime in the Devonian period- before all other land vertebrates including reptiles, mammals and birds. The ability to use resources both water and land gave early amphibians a huge survival advantage, as there were more food sources available, and more area for habitat.

Eggs were laid without a shell in water, as we see today. For more than 100 million years, amphibians likely remained the dominant land vertebrates, before some branched off and evolved into what we now know as reptiles, as animal adaptations improved for living on land.

Reptiles emerge as the shells of amphibian eggs become thicker over time, thus more protective, and self-containing. We know this because there are some amphibian species (such as salamanders) which have eggs which contain the larvae at their aquatic stage (as we see in reptiles) but are not quite hard-shelled. This shows an intermediate phase and gives us evidence for this evolution path.

True reptile fossils can be traced back to the Carboniferous period, after amphibians. Reptiles were likely better suited to survive on land because of their amniotic eggs.  Amniotic eggs have a protective shell to protect the embryonic liquid inside. This protective shell meant that reptiles did not need to lay their eggs in water, which provided the oppotunity for them to occupy more inland habitats. The development of scales also helped reptiles live on land, and led to their eventual replacement of amphibians as the dominant land vertebrates.

See Evolution Timeline

Amphibians are key members of their ecosystems

Ecology refers to the relationships in an ecosystem i.e. the different interactions between organisms that fit together to maintain harmony in the area.

Like all species, amphibians and reptiles are unique, and specialized to their environments. These two groups are especially unique in that they stay close to their habitats. They do not move as freely as birds or mammals, as they are bound to their environments for food, temperature control, a place to lay eggs, and relative safety from predators. For example, a red-back salamander, if brought to an open field would suffer greatly without the moist, dark protection of a rotten log in the forest.

Their ectothermic skin means they are more dependent on their environment than other groups of animals for temperature control. A garter snake warms its body by sunning itself in the open, but also requires protection from predators, and it is thus dependent on open areas with debris such as fallen logs, rocks, or brush piles.

Laying eggs for amphibians happens in water (or nearby if you are a red-backed salamander). Healthy ponds, rivers and vernal pools are critical for their survival so that they can breed, lay eggs, and live out their aquatic stage. Many amphibians continue to live near the water, spending much time swimming and cooling off even as they grow out of their tails and gills.

Amphibians and reptiles are a critical part of our ecosystem. They add biodiversity, especially in critical ecological niches within our system, like ponds and river edges. They move energy through these spaces; for example, tadpoles eat insects and algae, passing it along to predators as they are eaten. Additionally, as amphibians and reptiles eat insects and other invertebrates like worms and slugs, they keep populations in check, which might otherwise cause harm to forests and farms by causing damage and spreading disease if left unchecked. This group of animals also provides a food source for many species of birds on PEI, such as the Great Blue Heron or the Red-tailed Hawk, and many species of mammals, like Raccoons and Red foxes.

Amphibians and reptiles are champions when it comes to eating huge amounts of insects, including decomposer insects. This is a vital ecological service, because it slows down rates of decomposition in organic matter, and allows plants time to take up more of that energy. For this reason, amphibians and reptiles are great indicators of healthy forests.

Key Characteristics 

All Amphibians and Reptiles are ectothermic, meaning they are 'Cold-blooded', incapable of creating their body heat, like mammals, and therefore depend on their outside environment for body temperature regulation. Amphibian means 'living two lives' referring to the water and land stage.

Key distinctions between types of amphibians:

• Frog - A short-bodied tailless amphibian, with thin, moist, glandular skin.
  

• Toad - A short-bodied tailless amphibian, with dry, tough, bumpy skin.
  

• Salamander - A long slender-bodied amphibian with a tail, characterized by their lizard-like appearance. Newt - a sub-family of salamanders.
  

• Newts- Looks similar to a salamander, but is bright orange, and has a juvenile terrestrial phase called an eft. Adult newts have a flattened tail that aids their swimming.
  

What's the difference between Reptiles and Amphibians? 

Amphibians have a water stage and a land stage, and they have glandular skin, that is typically moist. They could not survive without water. Reptiles have scales, and they are born looking the same as their parents, rather than having a second life stage.

For example, the reptile species we have on PEI are all snakes, which all have scales, lay eggs with a more distinct shell and do not have two phases.

Other characteristics of Reptiles and Amphibians

Brumation: Amphibians and reptiles cannot make their body heat and do not have fur, so they brumate in the winter. This is like hibernation in mammals, where they hunker down alone or in groups to keep warm during the coldest months in a state of dormancy. This conserves energy in a time when there is a limited food supply. Brumation can be in areas like soil, woody debris, or leaf litter on the forest floor.

Burrowing: Digging slightly under the ground surface or among debris, such as leaf litter for protection and warmth. Toads burrow underground during brumation, to keep warm and save energy.

Life Cycles

Amphibians: The life cycle of each amphibian is slightly different, but all have: 1) a Water Stage: where eggs are laid in the water, in large clumps, so that at least some survive. Tadpoles breathe underwater with gills and hunt and swim in the water for this first stage of life. Following this, is the 2) Land Stage: Once legs can be used, and lungs

Here is an example of a more in-depth life cycle of a frog

Eggs: are laid in water- this can be in ponds or ditches, and typically happens in the spring on PEI. This allows for eggs to be able to stay in one place, get warm, and have a food source in the form of insects, and insect larvae. Eggs are laid in large clumps of egg masses ( typically hundreds or thousands of eggs in a mass to maximize survival rates) in the water, oftentimes clinging to water vegetation or the sides of the area for added protection. The embryo is surrounded by a protective sac that is clear and can be seen moving as it grows a tail. 

Tadpoles: typically hatch after about two weeks, depending on temperature, and species. They have tails to hunt and swim in the water for this first stage of life. They feed on anything they can in the water such as insects, insect larvae, other tadpoles, etc. They have gills to breathe underwater. 

Froglets: an early-stage frog. They are in the process of growing legs and have lungs. For this reason, you’ll often see froglets floating above water so that they can breathe. The tail slowly gets shorter and shorter as it gets reabsorbed into the body, as they prepare for the land stage.

Frog: Once legs can be used, an amphibian can move and fully live on land, although they will still live near water, and can still swim. They use water as a source to cool off, as they are not able to regulate their body temperature, and as a food source; feeding on insects etc. It offers protection from many predators, as they can hide in mud, surrounding vegetation, and fallen logs or woody debris.

See Frog Life Cycle Diagram

Reptiles: The life cycle of reptiles; in PEI’s case snakes, is very different. Instead of two life stages, like amphibians, they hatch out of their eggs looking like miniature versions of their parents. 

For example, in the Red-bellied snake, there is still an egg stage, but they hatch inside the female and emerge as live snakes! They can do this by incubating fertilized eggs internally. 

The Smooth Greensnake, named for its small smooth scales, can slither through vegetation well. It lays its eggs in protected woody areas, such as brush piles, logs etc.