Reptiles, including snakes and lizards, separated from their water-dwelling amphibian ancestors in the Paleozoic era. They reproduce sexually with internal fertilization and lay their eggs on land.
Like all ectotherms, reptiles get most of their body heat from the environment. They are able to regulate their temperature by basking in sunny spots and hiding in shady places.
Land Adaptations
Reptiles separated from their water-dwelling ancestors to live on land during the Paleozoic era more than 280 million years ago. They dominated the Earth’s ecosystems until a mass planetary extinction, but many reptile species still exist today. These include snakes, lizards and crocodiles.
The tough, scaly skin of reptiles helps protect them from damage and injury. They also lose moisture more slowly through their skin, because a layer of waxy lipids and the protein keratin reduce the rate of water loss. Reptiles’ scaly skin also helps regulate body temperature by reflecting or absorbing heat from the environment depending on the time of day and season.
Some reptiles, such as sea turtles, require temporary wetlands or aquatic sites for breeding and laying eggs, while others, like crocodiles, can spend long periods of time in saltwater habitats. These aquatic adaptations allow reptiles to feed on marine algae along rocky shores or dive into the ocean to forage in brackish and coastal waters. Marine reptiles have evolved mechanisms to control their bodies’ water and salt balance, including osmoregulation, lung capacity and oxygen storage, and the ability to enter brumation during the cold season.
Water Adaptations
Reptiles that live primarily in water have evolved a number of unique adaptations. These include a special kidney salt gland that allows them to excrete excess salt, which is crucial to surviving in marine habitats. Marine reptiles also have lungs with increased capacity for oxygen intake and the ability to store oxygen for extended dives.
In addition, marine reptiles have limbs that are adapted to swimming. Sea turtles, for example, have streamlined bodies and flipper-like limbs that function like paddles for strong propulsion in the water. Marine lizards and crocodiles use lateral undulations of their backs and tails to move through the water.
Reptiles are ectothermic, which means they rely on the environment to regulate body temperature rather than internal metabolic energy to heat their bodies. In hot environments, such as deserts, reptiles lose a lot of water through their skin. To prevent this, reptiles have a tough outer layer of overlapping scales made of a substance called keratin. This layer helps protect against loss of moisture and sunburns. It also helps reflect sunlight to keep reptiles cooler.
Osmoregulation
The process of maintaining an internal balance of water and salts in the body despite the concentration of these substances in the environment is called osmoregulation. Organisms that are osmoregulators, or tightly regulate their internal osmolarity, are more common in the animal kingdom.
Marine Fish
Most marine fish live in hypertonic environments (environments that are more salty than the fish). To survive in these conditions, marine fish employ sophisticated osmoregulation mechanisms that allow them to maintain homeostasis without drinking any water.
They do this by absorbing salts from the seawater through gills and excreting salts from their skin and urine. They also have specialized cells in their gills that actively uptake water by osmosis.
Amphibians and reptiles must osmoregulate to survive in desert environments, too, because these are hypotonic (lower than the animal’s osmotic concentration). Desert mammals like camels and kangaroo rats conserve water by reducing osmotic loss through their skin and having kidneys that concentrate urine. They also have specialized salt glands that excrete excess salt.
Lung Capacity and Oxygen Storage
Reptiles have evolved many adaptations for living on dry land, including tough keratin scales and efficient lungs that allow them to use the oxygen in the air. Reptiles such as the Savannah monitor lizard have lungs with one main tube that runs all the way to the back of the lung and opens into a big sac. Smaller tubes branch off from this central tube and distribute air into tiny chambers. The walls of these chambers have holes that let air flow not only from the lung into a central sac but also between them.
A specialized lung design allows crocodiles to store oxygen in their lungs during breath-outs, preventing it from competing with fresh air entering the trachea during a breath in. In addition, crocodiles have secondary ventral bronchi with large diameters that connect to each of their two lungs. From these, bronchi branch into smaller airways called dorsobronchi, which open into parabronchi that carry oxygen into blood capillaries.
Marine reptiles have also adapted to their aquatic lifestyles with streamlined bodies and flipper-like limbs that act as efficient paddles for swimming and flattened tails that serve as effective anchors for swimming. They also have a unique ability to retain salt by secreting it in their tears, which helps them maintain proper balance of salinity in their bodies.
Salt Glands
The saline landscape of the Salt Plains National Wildlife Refuge attracts a unique and specialized group of creatures. This habitat combines wetlands and prairie, with about 12,000 acres of salt flats where visitors can dig for hourglass selenite crystals. Thousands of Sandhill Cranes, ducks and shorebirds spend the winter here. These birds are adapted to the harsh salt flats, using special glands that allow them to control their internal body temperature.
These animals also rely on the ability to rehydrate in their salty surroundings. The salt glands of marine reptiles (such as the world’s only sea-going lizard, the marine iguana) can be found in their nares or on their tongue. They function to excrete excess salts accumulated during feeding (marine iguanas), or to conserve water in desert species. Excess salt is sneezed out of the gland in a fine spray of clear fluid. Pet Green Iguanas sometimes exhibit this behavior, sneezing small amounts of white deposits from their nares.
This is a unique feature of marine reptiles that challenges evolutionary theory, as it indicates that the genetic instructions for these glands evolved independently in different groups of reptiles. This demonstrates that their existence was designed by the Creator.