Desert sand dunes represent an extreme environmental setting in which selective forces have apparently generated dune ‘ecomorphs’ in six lizard families. – Lamb et al. (2003) Biological Journal of the Linnean Society, vol. 73, p. 253

Several of the world’s deserts are characterised by vast fields of sand dunes, often called ‘sand seas’. Dune structures form by the continuous accumulation of windblown sand in hyper-arid regions, and for any animals they pose severe problems. These include low traction, high surface temperatures, scarce food and scarce shelter from predation.

Nevertheless, members of six lizards families have independently adapted to this extreme environmental setting, and although distantly related they have converged on many shared aspects of morphology and behaviour. These parallel desert lizard forms or ‘morphotypes’ are categorised together as unique, sand-dwelling (‘psammophilous’ in technical terms) ecomorphs, representing an excellent example of convergent evolution in the reptiles.

Sand lizard ecomorphs inhabit the deserts of North Africa, South Africa, North America, South America and Australasia, with relevant species including members of the distinct families Agamidae, Gekkonidae, Gerrhosauridae, Iguanidae, Lacertidae and Scincidae. In spite of the wide evolutionary and geographical distance between taxa, sand-dwelling ecomorphs all exhibit a suite of adaptive features that allow efficient locomotion and burrowing into loose sand. Most notable among these convergent features are:

1. Toe ‘fringes’ (laterally projecting spines or lamellae derived from scales) for stable locomotion and ‘swimming’ or ‘diving’ (i.e. burrowing) into sand

2. A pointed or spade-like snout and cylindrical body to facilitate burrowing.

3. Behavioural and anatomical adaptations for effective breathing and respiration when buried.

Toe fringes

Modification of scales on the toes (or other parts of feet that make contact with the ground) has occurred in all six of the lizard families mentioned above, and also on numerous independent occasions within each of the separate families. Toe fringes or spines come in various shapes (e.g. triangular, elongate or – in geckos only – conical), each apparently of equal adaptive value for running over dune surfaces and aiding rapid burrowing into sandy substrate.

Toe-fringed lizards of the Namib desert of South-West Africa exemplify the wide spectrum of taxonomic diversity within which a similar solution to desert living has evolved; in the Namib we find agamids (e.g. Agama etoshae), geckos (e.g. Pachydactylus rangei, P. vanzyli, Colopus wahlbergii and Chondrodactylus angulifer), lacertids (e.g. Meroles anchietae), skinks (e.g. Riopa sundevallii) and the gerrhosaurid Angolosaurus skoogi. Of note, recent molecular evidence has shown that within the Pachydactylus group of geckos, lateral toe spines evolved independently in each species, and are correlated with loss of the typical toe pad setae (microscopic hairs) that generate ‘dry’ adhesion in other geckos.

Head and body adaptations

Adaptations for ‘sand diving’ permit rapid burrowing into and locomotion within sandy substrates, which can be close to the surface or substantially deeper, indeed up to >1m depth; this capability is critical when escaping predators through concealment, sheltering from extreme heat, and in some cases pursuing prey. Burrowing among sand-dwelling lizards is made possible by their prominent shovel-shaped snouts and cylindrical bodies, as well as use of fringed feet and stereotyped body movements.

Lizards morphologically adapted to sand-diving occur throughout the world’s deserts and are exemplified by lacertids Meroles anchietae and Acanthodactylus in the Sahara, iguanids Uma and Phrynosoma in North America, agamids Phrynocephalus in Australia and Agama in Southern Africa, gerrhosaurid Angolosaurus in the Namib desert, geckos Ptenopus in Southern Africa and Teratoscincis in Northern Africa, and the ‘sand fish’ skink, Scincus phibyi, found in the African Sahara desert. In addition to a specialised head and body shape, many desert lizards (e.g. the Namibian gerrhosaurid Gerrhosaurus skoogi, North American iguanids Uma and Phrynosoma) have evolved enlarged scales on the cheeks (or ‘tympanic shield’) and/or upper forelimbs, providing an effective ratchet mechanism for support during movement through sand.

Breathing under sand

While actively moving or stationary within a sandy substrate, it is essential that buried animals receive adequate oxygen for respiration, to prevent suffocation. To facilitate respiration when entombed, sand-diving lizards have again converged on similar solutions. Breathing movements are ‘shielded’ or screened from surrounding sand, stopping sediment from blocking the space required for the ribcage to expand and deflate in concert with inhalation and exhalation of the lungs. Such shielding is often achieved by placement of forelegs along the ventral surface (‘venter’) of the body. This screening effect is enhanced by folds or ridges on the body trunk that prevent slippage of sand into the breathing space.

The distantly related iguanids Uma and Phrynosoma platyrhinos, and the skink Riopa sundevallii, are examples of species that employ forelegs and body ridges to exclude sand from exhalation depressions, and vertical venter movements are enhanced by possession of a concave ventral surface in the Saharan skinks Chaleides sepoides and Scincus officinalis, North American legless lizard Anniella pulchra, and even outside the lizards, in a number of African snakes (e.g. Chilomeniscus cinctus, Chionactis occipitalis). Thus, behavioural and anatomical adaptations to under-sand breathing provide yet another angle from which we can see the astonishing evolutionary convergence displayed by sand-dwelling lizards.