Pyrenean frog  (Rana pyrenaica)

The Pyrenean frog is classified as Endangered (EN) on the IUCN Red List (1).

The smallest of the European brown frogs (4), the Pyrenean frog (Rana pyrenaica) was only discovered in 1990 (5) (6). Key identifying features of the Pyrenean frog are its delicate build and weak markings, as well as its nostrils (4), which are wider apart than its golden-yellow eyes (2) (4).

The Pyrenean frog has long fore and hind limbs (2), and the feet are fully webbed (2) (4). Large wrinkles, which are believed to increase the area of skin available to absorb oxygen, are present on the sides and thighs. These wrinkles are also probably secondary sexual characteristics, as they are more pronounced in the male during the breeding season (5).

The upperparts of the Pyrenean frog are generally a uniform colour (3), typically ranging from creamy-white to olive-grey (2), or sometimes more reddish (2) (4). A v-shaped pattern is present in some individuals (2) (3).

The underside of the Pyrenean frog is paler than the upperparts, with the belly turning from yellowish in the spring to bluish during the summer (2). This colouration blends into a whitish to pinkish-grey colour on the throat (2) (3) (4), which is usually spotted (4).

The insides of the thighs of the Pyrenean frog are red during the breeding season (2). The males of this species also develop greyish or brownish nuptial pads during the breeding season, which are specialised swellings on the thumbs necessary for gripping the female during amplexus (3).

The tadpoles of the Pyrenean frog are strikingly black on the upperparts, while the sides of the body are dark brown to black (3). As a result of their similar colouration, Pyrenean frog tadpoles may easily be mistaken for common toad (Bufo bufo) tadpoles (6). The belly of the tadpole is usually greyish and transparent, while silvery, whitish-yellow or gold spots with irregular borders are also present on the underside. The dark tail, which is up to twice the length of the body, is marked with smaller spots (3).

The Pyrenean frog has a weak, low-intensity call which is formed of low-pitched grunts (2).

The Pyrenean frog is endemic to the Pyrenean mountains of France and Spain (2) (5) (6) (7), and is largely restricted to the southern slopes of the western-central areas of the Pyrenees (1) (8).

On the Spanish side, the Pyrenean frog occurs from the Roncal Valley, Navarra, eastwards to the Parque Nacional de Ordesa, Huesca (1) (2) (8). This species is also found in the Iraty Forest in the western French Pyrenees (1) (8).

The Pyrenean frog occurs at elevations between 800 and 2,100 metres above sea level (1) (2) (8), although it is more commonly found between 1,000 and 1,800 metres above sea level (2).

Rocky mountain streams which are cold, fast-flowing and well-oxygenated are the preferred habitat of the Pyrenean frog (1) (2) (4) (5) (8). It can also occasionally be found in roadside and forest edge ditches (1), as well as in small artificial pools or puddles (2) (5), including drinking troughs (1) (6).

Compared to other species within the Rana genus, tadpoles of the Pyrenean frog tend to prefer areas with a constant supply of fresh water, which is colder and more oxygenated (6).

The ecology and reproduction of the Pyrenean frog are different to other European species within the Rana genus (5) (6). It is the only known European frog or toad species which is restricted to high mountain habitats (3), and prior to 1997 only 17 populations of the Pyrenean frog were known (6).

Adults of the Pyrenean frog are more aquatic than the juveniles (2), and are most frequently seen underwater, either near the bottom or sheltered under rocks away from the water current (5). This species feeds on insects and other invertebrates (4).

The Pyrenean frog is active from February to July, and the breeding season begins once the snow cover melts, typically between February and April (2). Breeding takes place in water (1) (8), and the female always lays its eggs in permanent water bodies (5).

The female Pyrenean frog lays a relatively small clutch of approximately 150 large eggs (2) (4) (5) (6), which are sometimes laid in several separate clusters (2). The eggs of this species, which are the largest eggs of all European brown frogs (5), are laid under stones (2) (4), in fissures (2) (6) and sometimes on the river bottom where the current is weak (2). As an adaptation to living in flowing water, the eggs of the Pyrenean frog are heavy and dense (5) (6), which prevents them from floating away (2) (5) (6).

At approximately 10.5 millimetres in length (2), the tadpoles of the Pyrenean frog are also relatively large (5) (6). Their size, combined with the development of a muscular tail within a few days of hatching (6), may enable them to withstand water currents (5). The dark colouration of the tadpoles of this species is thought to be an adaptation to the cold climate in which they live, and also the high UV radiation exposure they endure at high elevations (3).

One of the key threats to the Pyrenean frog is the eutrophication of streams as a result of the intensification of agricultural practices (1) (8), which leads to increased levels of pesticides, fertilisers and other pollutants in local watercourses (2).  

Other threats to the Pyrenean frog include drought, habitat loss and alteration due to the development of tourism and transport infrastructure (1) (2) (8), as well as the potential introduction of predatory fish species (1) (2).

Long-term studies on the closely related Cascades frog (Rana cascadae) indicate that increasing levels of UV radiation as a result of the diminishing ozone layer are causing greater mortality in eggs, which is also a concern for the Pyrenean frog (3).

As a result of its specific ecological requirements, the Pyrenean frog is likely to be vulnerable to climate change (1) (5), as well as human disturbance (5).  

The Pyrenean frog is listed on Appendix III of the Bern Convention, which means that it should be protected (9). This species is found in several protected areas, including the Ordesa and Monte Perdido National Park (1) (3) (8), and Lizardoia Integral Reserve (1) (8).

Suggested conservation measures for the Pyrenean frog include long-term observation of this species (3).

Learn more about amphibians:

• IUCN/SSC Amphibian Specialist Group:
  http://www.amphibians.org/
• AmphibianArk:
  http://www.amphibianark.org/
• Gascon, C., Collins, J.P., Moore, R.D., Church, D.R., McKay, J.E. and Mendelson III, J.R. (2005) Amphibian Conservation Action Plan. The World Conservation Union (IUCN), Gland, Switzerland. Available at:
  http://www.amphibianark.org/pdf/ACAP.pdf

Find out more about the habitat of this species:

• ARKive - Mediterranean Basin:
  http://www.arkive.org/eco-regions/mediterranean-basin/