Amphibians

The Plethodontidae is by far the largest family of salamanders, comprising nearly 70% of all living species. In total there are 378 known plethodontids divided between four subfamilies and 24 genera. The plethodontids are united by the fact that they do not possess lungs and breathe entirely through their skin and mouth lining. They are often referred to as the lungless salamanders, although they are thought to have evolved from highly aquatic, lunged ancestors in the streams of the Appalachian Mountains in eastern North America. The earliest plethodontids were hypothesised to have lost their lungs because individuals with reduced, or absent lungs were less likely to float away in the swift mountain streams where they lived. The vast majority of other salamanders possess lungs, making the lungless salamanders an unusual and fascinating group of animals.

They are thought to have diverged from all other amphibian species 145 million years ago at the boundary between the Jurassic and Cretaceous periods. They are as different from all other amphibian lineages as wombats are from whales, evolving at a time when dinosaurs were still dominant. Overall, plethodontids are the most evolutionarily advanced salamanders, so it may at first appear odd that they should have lost their lungs, which are one of the most basic features of all vertebrates living on land. Lacking lungs and being dependent on their skin for respiration places a size restriction on these salamanders because large animals have a relatively small surface area of skin compared to their body’s volume, and have greater difficulty in supplying their body tissues with oxygen compared to smaller animals (which have a large surface area to volume ratio). The long, slender form of the lungless salamanders maximises the surface area available for gas exchange, and some species grow to lengths of over 300 mm.

Plethodontid salamanders occupy a great diversity of habitats, ranging from strictly aquatic to strictly terrestrial, exploring niches as diverse as caves, trees, mountain streams, and they are also found burrowing through the earth. Dependence on their skin for breathing places limitations on where and how lungless salamanders can live. Their skin must be kept moist at all times in order for oxygen to be taken up by the blood in capillaries beneath the skin. This means plethodontids are either confined to humid areas, or must find damp hiding places and only emerge in wet weather, typically at night. The life of a lungless salamander in less humid areas, like Europe and temperate North America, therefore comprises brief periods of activity interspersed with inactive phases that are often very long. They are able to survive the periods of inactivity because they have a very low metabolic rate and low energy requirements. Able to store much of what they eat as fat, they do not need to feed very often.

A further adaptation, present among many species of the lungless salamander subfamilies named “Plethodontinae” (from East and West North America) and “Bolitoglossinae” (from tropical Central and South America), is “direct development”. This is a method of amphibian development where the larval stage (e.g. the tadpole stage in a frog’s life history) has been eliminated. Early development takes place in eggs, which may be laid in moist places away from water, and the young hatch out as miniature adults. The well known amphibian metamorphosis, most commonly appreciated in the transition from tadpole to adult frog, does not occur outside of the egg. This means that certain lungless salamanders in these two subfamilies may live away from water bodies, allowing them to expand their ranges to new areas.

The history and characteristics of the lungless salamanders go some way to explaining their range. They are mostly found in the New World, where they are widely distributed in eastern and western North America, as well as Central and South America. However, continental drift over millions of years has also brought them to the Old World, where they are found in parts of Europe (e.g. Sardinia) and Korea. The existence of the Korean crevice salamander was unknown until 2005, when its discovery was a shock to science, indicating a long history of lungless salamanders in Asia. This is the only known species in Asia, suggesting that the rate of species generation in this part of the world is very low, especially compared to the huge radiation of lungless salamander species in the New World.

Comprising 50 known species, the genus Pseudoeurycea (commonly known as the “false brook salamanders”) is one of the largest genera in the Plethodontidae family, second only to the Bolitoglossa genus (the “mushroomtongue salamanders” – 93 known species). The false brook salamanders are present within the subfamily “Bolitoglossinae” (from Central and South America) and its members are very wide ranging in size, with some of the smallest and largest lungless salamanders included (total lengths from about 65 mm to 325 mm).

The false brook salamanders have recently been reorganised and expanded to absorb a number of other genera that are now considered to fit within the Pseudoeurycea genus. The taxonomy of these salamanders may not be finally resolved just yet, but it is known that the whole group diverged from all other salamanders in the Late Eocene period, at least 34 million years ago. This is around the same time that humans and monkeys shared a common ancestor.

The Smith’s false brook salamander is thought to be closely related to another top 100 EDGE plethodontid: the aquatic false brook salamander, the only known aquatic lungless salamander in between Northern Mexico and Panama, all the other species being terrestrial (or ground dwelling).

Smith’s false brook salamander, like all lungless salamanders in the Bolitoglossinae subfamily, possesses a slender body, long tail and prominent eyes. A distinctive feature of the plethodontid family is a narrow groove (the nasolabial groove) running from each nostril to the upper lip. Its function is to carry waterborne odours from the ground into the nasal cavity. Another curious trait of the lungless salamanders are mental (from the Latin “mentum”, meaning chin) glands. These are modified mucus glands and release pheromones (chemicals produced by an animal to influence the behaviour of other members of its species, often with regard to breeding receptivity). Mental glands are sometimes visible in males as raised bumps below their lower lip.

Lungless salamanders are very small to medium in size, usually measuring between 25 to 250 mm from the tip of the nose to the end of the tail, which salamanders retain throughout their life. They are unusual among the salamanders in that some species can detach from their tail as a predator-defence mechanism (also known as tail or caudal autotomy). It is therefore not unusual to see individuls missing part or all of their tail, which they may regenerate later. Lungless salamanders may have bold patterns on their skin as adults, or they may have a colouration more similar to their environment to aid camouflage. They have well-developed “costal” grooves (successive vertical grooves in the skin along the sides of the body), generally numbering between 12-15. Their limbs are slender and often have largely or completely webbed digits. Species, like the Smith’s false brook salamander, in the genus Pseudoeurycea (the “false brook salamanders”) are very similar in form to those in the genera Chiropterotriton (the “splayfoot salamanders”) and Bolitoglossa (the “mushroom-tongue salamanders”).

Smith’s false brook salamander is a fairly large species, reaching total lengths of about 145 mm, about 76 mm being accounted for by the proportionally long tail. The tail is constricted at the base and is slightly thicker and shorter in males than in females. The head is broad with strongly swollen parietal regions (the glandular areas either side of the head behind the eyes) and males possess a distinct mental gland. Thirteen “costal” grooves are present in the skin along either side of the body. The limbs are well-developed and the digits are partially webbed. The dorsal (or upper) surfaces of this species are dark brown, with the sides of the body being yellowish or greyish olive-brown tinged with red. A series of 13 vertical blackish bars are found on the sides of the body. The colouration of the chin is cream with a peppering of cinnamon-brown spots. The belly is greyish-cream with traces of darker lines following the costal grooves. There are numerous blackish spots along the sides of the tail.

Occurs only on the Sierra de Juarez, Cerro San Felipe and Sierra de Quatro Venados, north-western Oaxaca, Mexico, at an altitude of 2,500-3,000 metres above sea level.