REPTILES: TAXONOMY, CHARACTERISTICS, THREATENED STATUS

REPTILES

monitor lizard

Reptiles are cold-blooded, hairless, egg-laying vertebrates. They are they divided into four orders: 1) snakes and lizards, 2) turtles, 3) crocodiles and alligators, and 4) the tuatara (a creature found in New Zealand that looks like a lizard). Most reptile species are either lizards or snakes.

The ancestors of modern reptile are though to have emerged around 240 million years ago. The oldest fossils date to between 170 million and 200 million years ago. The most famous reptiles from the past are dinosaurs who many scientists believe are more closely related to modern birds than they are to modern reptiles.

Age estimates for reptiles are made by counting bone layers. They have annual cyclic bone growth that can be measured using by staining methods.

As of 2022, 10,196 reptile species had been identified. Another source gave 12,162 as of November 2024. This is up from around 7,400 in the early 2000s. Reptiles live mostly on land. They are found everywhere in the world except in polar regions. They are most plentiful in tropical regions. A total of 31 species are already extinct

According to the BBC: Despite their low publicity profile, the cold-blooded vertebrates play an essential role in the balance of life. "Reptiles are good for people because they help control pests such as insects and rodents," said Prof Blair Hedges of Temple University in Philadelphia, US. If we were to lose all reptile species at risk of extinction, 15.6 billion years of phylogenetic biodiversity (a measure of genetic variation based on the tree of life) would be wiped from the planet. [Source: Helen Briggs, BBC, April 28, 2022]

Websites and Resources on Reptiles: Reptile Database reptile-database.org ; Reptileweb reptilesweb.com ; Reptile Phylogeny whozoo.org/herps/herpphylogeny ; Websites and Resources on Animals: Animal Diversity Web animaldiversity.org ; BBC Earth bbcearth.com; A-Z-Animals.com a-z-animals.com; Live Science Animals livescience.com; Animal Info animalinfo.org ; Encyclopedia of Life eol.org , a project to create an online reference source for every species; World Wildlife Fund (WWF) worldwildlife.org the world’s largest independent conservation body; National Geographic National Geographic ; ; Endangered Animals (IUCN Red List of Threatened Species) iucnredlist.org ; Biodiversity Heritage Library biodiversitylibrary.org

Reptilia, Including Birds

Phil Myers wrote in Animal Diversity Web: Reptilia, presented as a Class in our classification, includes turtles (Testudines), snakes and lizards (Lepidosauria), crocodiles and their relatives (Crocodilia), and birds (Aves), as well as a number of extinct groups. Reptiles (including birds!) are amniotes; that is, their eggs are protected from dessication and other environmental problems by an extra membrane, the amnion, not found in the first terrestrial vertebrates (amphibians). Mammals (Mammalia) are also amniotes, but they differ from reptiles in the structure of their skulls (especially the regions associated with chewing and hearing). Mammals also have hair and feed their young with milk produced by modified skin glands (mammary glands). [Source: Phil Myers Animal Diversity Web (ADW)]

Why are birds included within Reptilia, and how are they and other members of this group related to each other? Both the fossil record and comparative analyses of living species (especially those based on molecular evidence) convincingly establish that, among living reptiles, birds and crocodiles are more closely related to each other than they are to lepidosaurs (snakes and lizards). The position of turtles is more controversial; in the past they were thought to represent an early branch of Reptilia. Recent evidence suggests they may have a special relationship with crocodiles and birds. Because birds clearly arise from within the groups we traditionally consider to be reptiles, not separately from them, most systematists now formally consider birds (Aves) to be a subgroup within Reptilia.

Reptile Characteristics

lizard heart

Phil Myers wrote in Animal Diversity Web: In addition to being amniotes, all reptiles have (or did have, in their evolutionary history) horny epidermal scales made of a particular kind of protein, paired limbs with 5 toes, skulls with a single occipital condyle, lungs instead of gills for respiration, and a 3 or 4 chambered heart. Their eggs are covered with a leathery or calcium-based shell (partially or completely lost in some species that give birth to live young), and fertilization occurs inside the female, rather than outside, as it does in most amphibians. Members of Reptilia generally share many additional traits, for example in their nervous and excretory systems, locomotion, and reproduction. [Source: Phil Myers Animal Diversity Web (ADW)]

Reptiles have a distinctive hearts with three chambers — two rear chambers (auricles) and one front chamber (ventricle) — with a partition that divides the heart almost completely in two. By contrast mammals have a four-chambered heart and amphibians have a three-chambered heart without a partition.

Reptiles swallow rather than chew their food. Many have jaw bones that bend and pivot and even come unhinged to allow the reptile to manipulate large prey. These developments freed the tongue from manipulating prey and allowed it become a sense organ . The forked tongue that many snakes and reptiles possess dates back to around 65 million years ago. It picks up chemical clues in “stereo” which allows reptiles to locate things.

Reptiles shed their skin. Most have scales. Many are well camouflaged, which means they are given some protection from predators but are forced to stay to certain habitats. Those who blend in with leaves for example stand out on rocks. Their limited range results in many becoming fiercely territorial.

Reptiles such as iguanas show physiological changes associated with pleasure — a rise in heart rate and body temperature — like mammals but amphibians don't.

The saccule is a bed of sensory cells in the inner ear that detects certain kinds of vibrations. In many reptiles it connects directly to the brain. "A lot of snakes and lizards were thought to be 'mute' or 'deaf' in the sense that they do not vocalize sounds or hear sounds well," biologist Dawei Han told Science Alert. "But it turns out they could potentially be communicating via vibrational signals using this sensory pathway instead, which really changes the way scientists have thought about animal perception overall."

Cold Blooded Reptiles

Reptiles are ectothermic (cold blooded), which means they can not regulate their body temperature internally and can not create heat with their bodies like mammals can and are at the mercy of the sun and their surroundings for heat. This explains why reptiles are less common in cold and temperate areas. The few that live in these places often hibernate when the weather is cold.

Because they are cold blooded, reptiles can move vigorously when the weather is hot but are sluggish and move slowly when it is cool. At night a reptile’s body heat falls with dropping temperature of its surroundings. Each morning it must regain heat. Reptiles often bask in the sun to build up heat. Reptiles can also overheat. When it gets too hot for them they seek shelter in the shade.

lizard skin

Being cold blooded has its advantages. Because reptiles receive much of their energy from outside their bodies and do not have to create it, they are more energy efficient and need less food. This and the fact they have salt-excreting glands also means that they are more tolerant to salt, and thus are capable of surviving long periods at sea, which explains how they have traveled across seas and inhabited islands that no mammal, until the arrival of man, could ever make it to.

During the summer, the weather is often too hot for reptiles. At this time of the year reptiles become more active at night. In the middle of the day they seek shelter in the shade. As the weather cools they become more active. If they absorb enough heat in the afternoon they can remain active through the evening and night before seeking refuge in a burrow, in leaf litter or under a log or rock.

Reptile Reproduction

Reptiles were the first creatures to develop eggs. They passed on the ability to birds. Today, most reptiles lay eggs but not all of them. Boa constrictors, rattlesnakes and chameleons give birth to live young while pythons, cobras and iguanas lay eggs.

Reptiles that don't lay eggs produces eggs inside their bodies that either have very thing shells or no shell at all. In the case of the former the young hatch from eggs while still in their mother's body and emerge live.

Reptile eggs contain considerable amounts of yoke. Some reptiles species provide their young with additional nourishment by producing eggs that attach to the uterus. The uterus and the embryo generate interlocking blood vessels that allow nutrients to be passed from the mother to her young.

Most reptiles bury their eggs or conceal them somehow and then abandon them. Some snakes actively guard them and fight off predators.

Many reptiles have no sex chromosomes. Instead gender is determined by temperature. In crocodiles for example males are hot: eggs incubated in sand above a certain “pivotal temperature” almost always hatch males. This could spell trouble if global warming takes hold and female crocodiles — and reptiles — become scarce.

One-Fifth of Reptiles Worldwide Face Risk of Extinction

More than 1 in 5 species of reptiles worldwide are threatened with extinction, according to a comprehensive new assessment of thousands of species published in April 2022 in the journal Nature. Of 10,196 reptile species analyzed, 21 percent percent were classified as endangered, critically endangered or vulnerable to extinction. A total of 31 species are already extinct More than half of crocodiles and almost two thirds of turtles are threatened with extinction Reptiles are threatened throughout the world, but particularly in southeast Asia, West Africa, northern Madagascar, the northern Andes and the Caribbean. “This work is a very significant achievement — it adds to our knowledge of where threatened species are, and where we must work to protect them,” said Duke University ecologist Stuart Pimm, who was not involved in the study. [Sources: Christina Larson, Associated Press, April 28, 2022; Helen Briggs, BBC, April 28, 2022]

The study took more than 15 years to complete, because of problems getting funding for the work. Associated Press reported: Similar prior assessments had been conducted for mammals, birds and amphibians, informing government decisions about how to draw boundaries of national parks and allocate environmental funds. Work on the reptile study — which involved nearly 1,000 scientists and 52 co-authors — started in 2005. The project was slowed by challenges in fundraising, said co-author Bruce Young, a zoologist at the nonprofit science organization NatureServe. “There’s a lot more focus on furrier, feathery species of vertebrates for conservation,” Young said, lamenting the perceived charisma gap. But reptiles are also fascinating and essential to ecosystems, he said.

Six of the world's species of sea turtles are threatened. The seventh is likely also in trouble, but scientists lack data to make a classification. The Galapagos marine iguana, the world’s only lizard adapted to marine life, is classified as “vulnerable” to extinction, said co-author Blair Hedges, a biologist at Temple University. It took 5 million years for the lizard to adapt to foraging in the sea, he said, lamenting “how much evolutionary history can be lost if this single species” goes extinct.

Worldwide, the greatest threat to reptile life is habitat destruction. Hunting, invasive species and climate change also pose threats, said co-author Neil Cox, a manager at the International Union for the Conservation of Nature’s biodiversity assessment unit. Reptiles that live in forest areas, such as the king cobra, are more likely to be threatened with extinction than desert-dwellers, in part because forests face greater human disruptions, the study found.

Image Sources: Wikimedia Commons

Text Sources: Mostly National Geographic articles. Also David Attenborough books; Live Science, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated February 2025