A digit, in the context of reptilian anatomy, refers to a finger or toe, the skeletal structure of which is composed of small bones called phalanges. Lizards belong to the highly diverse order Squamata, which includes over 12,000 species of lizards, snakes, and amphisbaenians, making them the largest group of reptiles. This enormous variety means that while a standard number exists, evolutionary pressures have led to a spectacular range of limb and digit configurations.
The Baseline: Pentadactyly in Lizards
The foundational answer to the question lies in a concept called pentadactyly, which is the condition of having five digits on each limb. This five-digit pattern is the ancestral and most common arrangement found across a vast array of terrestrial vertebrates, including most lizards. The limbs of terrestrial reptiles are generally composed of four segments, culminating in the manus (hand) and pes (foot), which feature five digits.
In lizards that maintain this standard configuration, these digits are used for essential functions such as walking, climbing, and grasping surfaces. The typical lizard limb features digits of varying lengths, often with the fourth digit being the longest and strongest, providing leverage and traction for locomotion.
Evolutionary Adaptation and Digit Reduction
While five digits per limb represents the baseline, the number is far from universal due to millions of years of evolutionary adaptation. The order Squamata is one of the most prominent lineages to exhibit significant modification of the standard pentadactyl limb, ranging from minor reductions in bone segments to complete limblessness. This reduction in digit number is directly tied to changes in the lizard’s environment and preferred method of movement.
For species that have adapted to a subterranean or burrowing lifestyle, having fewer or smaller digits is advantageous for streamlined movement through soil or sand. Similarly, lizards that navigate dense grass or use serpentine (snake-like) locomotion benefit from a reduction in limb size and complexity, as large limbs can hinder movement in these environments.
The process of digit reduction is a spectrum, meaning some species may only lose one or two phalanges, while others lose entire digits. The loss often follows specific patterns; for example, minor reductions frequently occur in the first and fifth digits (the innermost and outermost). These variations are linked to changes in genetic programming during embryonic development, allowing the lizard to better suit its ecological niche.
Extreme Examples of Limb Modification
The most dramatic examples of digit modification are found in various lizard families that have independently evolved highly reduced or absent limbs, a process that has occurred roughly 25 times in lizards. Many species of skinks, which are the largest lizard family, showcase this variation, with different species exhibiting a wide range of digit counts. For instance, some skinks in the genus Brachymeles display a full range of morphologies, from five-fingered limbs to complete limb loss.
Other groups, like the flap-footed lizards (Pygopodidae) of Australia, have completely lost their forelimbs. Their hind limbs, if present, are reduced to small, digitless flaps that may be used in courtship or defense. The genus Bachia within the microteiid lizards also contains species that have undergone drastic losses, where adults may have no externally visible limbs at all. These legless forms are often mistaken for snakes.