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Since the beginning of the 20th century, the number of saigas (Saiga tatarica tatarica L., 1766) sharply decreased twice as a result of human activity. After the first decrease, all four saiga populations (the Northwest Pre-Caspian, Ustyurt, Volga–Ural, and Betpakdala) recovered, and they numbered more than 1 million heads by the 1970–80s. However, after the second decrease (since the beginning of the 21st century), an increase in the number of these ungulates has been noted only for two populations, the Volga–Ural and Betpakdala. The goal of the study is to analyze the biological parameters of the four populations and to identify the differences that allowed the saigas inhabiting the Volga–Ural interfluve and Betpakdala to overcome population depression and to recover their numbers. At the end of the 20th century, the economic situation in the countries inhabited by saigas promoted poaching among the population, which led to a sharp decrease in the total number of these ungulates down to 50 000. The area of the main range decreased by 50 times for the Northwest Pre-Caspian saiga population and by 2.4–5 times for the other three populations. The distance of seasonal migrations, as well as animal involvement, also decreased. The saiga population of the Northwest Pre-Caspian has become practically sedentary. The low-number periods are characterized by a decrease in herds of different sizes found throughout the year; herds with less than 50 animals were registered in 56–100% of cases. In the early 2000s, the main biological parameters characterizing the four saiga populations were comparable. Moreover, the proportion of adult males and the yield of young animals per female in July–August were higher in the population of the Northwest Pre-Caspian in comparison to the other three (11.5 ± 1.9% and 0.81 ± 0.18 ind. vs. 8.7 ± 3.8% and 0.48 ± 0.40 ind., respectively). The increase in the Volga–Ural and Betpakdala populations is mainly due to their effective protection and the education of the local people with the support of the Kazakhstan government and international foundations, which significantly reduce the poaching pressure.

The abundance of the saiga antelope (Saiga tatarica tatarica L., 1766) in the Northwest Pre-Caspian region has remained at an extremely low level over the past 20 years. Comparison of some biological parameters of the population for periods with different numbers of animals made it possible to reveal a number of low-population features. Thus, along with a decrease in the number of saigas (from 800 000 heads in 1958 to 4500 in 2016), its habitat area also decreased (from 60 000–70 000 to 2000–3000 km2, respectively), and a significant part of the population transitioned to a sedentary lifestyle in the protected areas of the Chernye Zemli ecoregion. A long-time deficiency of sexually mature males in the population (less than 10%) led to a decrease in its growth and, consequently, to a decrease in its number. At the same time, the total fecundity of females decreased (from 1.27 ± 0.25 calves in years of high abundance to 0.92 ± 0.39 recently) due to an increase in barrenness in different age groups caused by the lack of males in the population. Males prevail over females in newborns (51.9 and 48.1%, respectively). The increased weight of newborn saiga calves (100 g on average for both males and females) and their decreased mortality in the first days after birth (under favorable weather conditions) indirectly indicate the normal course of embryogenesis and a sufficient quantity and quality of feed. An increase in the proportion of sexually mature stags will lead to an increase in the population number, as the reproductive functions of the population practically did not change during the period of its depression.

Comparative analysis of chymus transformation have been carried out in Saiga tatarica kept at the Center for Wild Animals of Kalmykia, as well as in four wild Bovidae species: the Siberian ibex Capra sibirica, the East Caucasian tur C. cylindricornis, the chamois Rupicapra rupicapra, and the bighorn sheep Ovis сanadensis. The size composition of cellulose fibers and the nutrient content were studied in different parts of the digestive tract and in feces of 12 animals. It was found that the relative cellulose value in the saiga rumen was higher than in other Bovidae species, whereas in feces it was lower. The most effective cellulose fiber reduction in the digestive tract was observed in the East Caucasian tur, while in the Siberian ibex it was the least effective. It was shown that in S. tatarica successful fiber reduction depended not only on the dental apparatus, but also on the enzyme system of the species.

The fodder resources, nutrition, and food supply of free-ranging Bactrian camels, Przewalski horses, and saigas on a fenced forb-grass steppe pasture are studied. The species composition of consumed plants, food selectivity, digestibility, and amount of consumed forage are estimated seasonally. All three species differ in all diet parameters.

The nucleotide sequences of the alleles of the DRB3 gene of the major histocompatibility complex (class II) of saiga antelope are described. A high degree of heterozygosity is found. The results of the analysis are consistent with the hypothesis of overdominance of heterozygotes. Phylogenetic relations of the DRB3 gene alleles of the saiga antelope and other Bovidae species do not correspond to the systematic position of Saiga tatarica. It is assumed that the polymorphism of the DRB3 gene alleles of saiga is primarily due to the variety of extracellular pathogens (bacteria, protozoa, helminths, and others) affecting its population during the evolutionary history of the species.

The features of Uralian saiga habitats are discussed. Periods of high and low abundance of the animals, which alternate depending on the productivity of vegetation communities and territorial humidity (the number of watering places) due to climate changes, are shown. After periodic cases of the mass death of saiga, their population is quickly restored to its previous level, which is likely to be facilitated by the existing feed and water resources.

An interfaced analysis of change of forage vegetation, nutrition of saigas (Saiga tatarica), and their state in the steppe regions of Russia is performed. It is noted that the percent of graminoids has increased considerably and the proportion of forbs has decreased in the vegetation of current pastures, which has had a negative impact on the nutrition and state of saigas. It is shown that the lower nutritional value of graminoids has caused a decrease in the digestibility of forage used by saigas and does not provide the physiological requirements of the animals. It is established that modern steppe pastures, where gramineous communities prevail, are of little use or absolutely unsuitable for the stable existence of saiga populations. Only separate dwelling places with an abundance of forbs continue to provide adequate nutrition for saigas and well-being for their local groupings.