Specific Features of the Size Structure of Marginal Populations of the Nominative Subspecies of the Secret Toadhead Agama (Phrynocephalus mystaceus mystaceus) (Agamidae, Reptilia)

The probability of Foster’s rule or manifestation of the island effect in a population of the nominative subspecies of the secret toadhead agama (Phrynocephalus mystaceus mystaceus Pallas, 1776) living on an isolated sandy massif of Sarykum, Dagestan, is considered. Materials on the 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 the village of Dosang in Astrakhan oblast, 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 Astrakhan oblast. Our analysis of the published data confirms the smallest possible size of this population of the nominative subspecies. Thus, the population of the secret toadhead 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 tend to become giants, respectively. In this case, we are dealing with dwarfism of the largest species of the genus Phrynocephalus on 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 the habitat, which may indicate the physiological dimensional optimum of this age group.