The lava lizards of the genus Microlophus are reptiles endemic to the Galápagos Islands, where they are a common sight across the archipelago. These lizards exhibit one of the most pronounced examples of sexual dimorphism in the reptile world, meaning males and females have distinct differences in size, appearance, and behavior. These differences are the result of evolutionary pressures that favor specialized roles for each sex in their harsh volcanic environment. The disparities between the sexes allow for immediate identification and illustrate the varied reproductive strategies employed by these unique island inhabitants.
Differences in Size and Morphology
A primary distinction between the sexes is their substantial difference in body size, with males being significantly larger than females. Adult males can frequently reach lengths two to three times greater than their female counterparts, often attaining a total length of 22 to 25 centimeters, while females typically range between 17 and 20 centimeters. This male-biased size difference is linked to the intense competition for territory and mating access.
The male’s structure is noticeably more robust, featuring a heavier and broader head supported by a thicker neck. This powerful head structure is an adaptation for territorial fighting, where males engage in aggressive bouts involving biting and tail-slapping. Males also develop more pronounced features on their back, including heavily keeled dorsal scales and a short, spiny crest that runs along the ridge. These morphological characteristics are permanent features of the adult male, serving as signals of dominance and physical fitness.
Defining Coloration and Markings
Beyond size, male and female lava lizards are most readily distinguished by their coloration, a phenomenon known as sexual dichromatism. Male coloration is highly variable, often adapted to blend with the local substrate, ranging from uniform dark gray or brown to patterns featuring yellow-green markings or gold stripes. Males typically develop a dark or black throat patch, which they use to enhance visual displays during territorial defense and courtship.
In contrast, the female’s most defining visual characteristic is the development of bright patches of red, orange, or yellow on her throat, cheeks, or entire head. This striking coloration is often referred to as nuptial coloration, and it is most vivid when the female is reproductively receptive or gravid. This color signal is a direct indicator of the female’s hormonal and reproductive status, attracting males who are seeking a mate.
The intensity of these female color patches communicates her fertility, with males showing a preference for females exhibiting a normal range of this coloration. If a female is pregnant and no longer interested in mating, the color patches may remain visible, but her behavior changes to discourage further male attention. Females may also have an additional distinguishing mark, such as a black mark near the shoulder.
Divergent Social and Territorial Behaviors
Adult males are highly territorial, frequently patrolling and defending areas that can span up to 400 square meters. This territory is typically large enough to encompass the smaller home ranges of multiple females, allowing the dominant male to maintain a small group. Males advertise their ownership and dominance through conspicuous visual displays, such as rapid head-bobbing and repetitive “push-up” movements performed on rocks or other elevated surfaces.
These displays are used to intimidate rival males and communicate aggressive intent. If visual warnings fail to deter an intruder, the conflict can escalate to physical fighting, including biting and aggressive tail-slapping.
Female behavior is comparatively less aggressive, focusing more on basking and foraging for resources within their smaller territories. While females can also perform push-up displays, they do so less frequently and less vigorously than males, typically to defend their immediate foraging space from other females. During male-female interactions, a female uses her nuptial coloration to signal her readiness to mate, but she may also signal rejection by turning her back to the male and shaking her tail.