It is first necessary to dwell on the sex ratio at birth, which we determined from the last stage of embryo development. Scan results 1633 embryos showed that among females predominate (53%). Predominance of females was also observed in the period of life strive to animals, but the sex ratio at older ages vary from year to year, depending on the habitat. In the end, young to hibernation, ie, obtained at the age of three months, the previous sex ratio – females predominate over males. At this time the animals lead an independent life. If a high percentage of deaths of male embryos can explain some features of fetal development, the high dropout rate of young males in the period of settlement – their high activity compared with females of the same age.
Among the semi-mature female squirrels always prevail (average over five years 56.5% of the 7855 individuals), and a much greater amount than in the young. This indicates a relatively high death of males in the period of settlement, and in the summer and autumn and winter months. Especially the few males in this age group on the island Barsakelmes observed in spring 1944 (28.5% of the total'' semi-mature, or 1.5% of all recovered animals), which accounts for the state breeding of rodents in the previous year.
As can be seen from the above data, the young before hibernation almost reach the size of hibernating animals, but not mature, because it is about the development of young squirrels nominal form, reaching sexual maturity at the age of 23 months.
Thus, the highest death among young males in a year marked by a depressed reproduction, apparently due to the increased activity of feathered foes. As a result, the sex ratio of semi-mature specimens in 1944, markedly different from that of the previous and following years.
In the adult population of gophers nominal subspecies in an isolated area (about-to Barsakelmes) dominate males (average over five years 59.5% of the 49,183 individuals). We attribute this (1952) method of field research. Trapping gophers produced from fishing to two months – since coming out of hibernation before molting. Adult males had awakened first, were caught 60 days, females – 50 and semi-mature specimens – about 10-15. Besides, during the rut males differed considerable mobility and activity, so for the period they were caught fishing in more than females, even if their numbers in the population is equal.
The greatest number of adult males produced in 1943 (67%), when was the lowest level of reproduction, than in all other years. It turned out that the weaker was breeding, the more common adult males due to their increased activity in search of females. Conversely, the more intense the reproduction occurs, the more females ready for fertilization, the smaller males produced as a result of moderate mobility caused enough females near their burrows.
On the mainland, several adult females predominate gophers. For example, the relative number of females for Volga squirrel in western Kazakhstan fluctuated over the years from 56 to 65% (Orlov, 1929). According to our observations, in Muyunkum it was equal to 65% in 1953, and in 1957, 59 in Dzhambul district, Almaty region – 53.5 (Kydyrbaev, 1959).
The age structure of populations of yellow squirrel from various points of its range is characterized by the following data. Prior to the release of the normal young, can be found on the surface of two age groups – adults and semi-mature specimens. The latter include animals born last year, which at the nominal subspecies is not multiply and make up a small part of the population (14%). Throughout the population of ground squirrels in the early spring is dominated adult mature individuals.