Specific features of the size structure of marginal populations of the nominative subspecies of the secret toad-headed agama (Phrynocephalus mystaceus mystaceus) (Agamidae, Reptilia)

Abstract

The article considers the probability of Foster's  rule, or the island effect manifestation in a population of the nominative subspecies of the secret toad-headed agama (Phrynocephalus mystaceus mystaceus Pallas, 1776) living on an isolated  sandy massif of Sarykum, Dagestan. Materials on morphometric data, body length and tail length were collected  at two points in the habitat of the nominative subspecies. Data on the Sarykum sand massif were collected in May 2021. At the second point of the range, in the vicinity of  v. Dosang, the Astrakhan region, data were obtained during the spring field seasons of 2010–2014. Our processing of the materials shows that mature individuals of the population on Sarykum are significantly smaller than lizards of the same age group from the Astrakhan region. Our analysis of the published data confirms the smallest possible size of this population of the nominative subspecies. Thus, the population of the secret toad-headed agama on the Sarykum sandy massif may be  a typical example of an island shift, in which large animals tend to become dwarfs, and small ones to become giants,  respectively. In this case, we are dealing with dwarfism of the largest species of  Phrynocephalus genus in the isolated territory of the Sarykum sandy massif. It should be  noted that immature individuals of the populations have no dimensional differences from other populations of th e habitat, which may indicate the physiological dimensional optimum of this age group.