McNab’s “Resource Rule” and Adaptive Changes in the Skull of the Common Shrew (Sorex araneus L., Eulipotyphla, Soricidae) from Different Ecosystems

The adaptive features of the skull were compared with the resource of soil macrofauna in populations of three monomorphic chromosomal races of the common shrew from different ecosystems. In the severe climatic conditions of the shrubby tundra of the Vorkuta area (67°29′ N), with extremely low abundance of large oligochaetes (earthworms), the population of the Serov race is characterized by small sizes of the skull and brain capsule and a relatively low height of the lower jaw. Therefore, we should expect a small size of the temporal muscle and its contraction force (Fmt), and, with a short arm of temporal muscle strength (l1), also a weak jaw compression force (Fcomp) compared to shrews from other localities. In the tundra ecosystem, McNab’s “resource rule” is confirmed while Bergman’s rule (optimization of metabolism by a relative reduction of the body surface area) is violated. In the severe climatic conditions of the mountainous areas of the northern taiga in the northern Urals (62°05′ N, 520–650 m above sea level), with high numbers of earthworms, the Serov race is characterized by large sizes of both the skull and the brain capsule and an increased relative height of the lower jaw. Therefore, we should expect an increase in the mass and contraction force of the temporal muscle (Fmt) with an elongated arm of its strength (l1) and, consequently, an increase in the jaw compression force (Fcomp). In the mountain taiga, McNab’s “resource rule” and Bergmann’s rule are confirmed. In the optical conditions of spruce–broadleaved forests (55°44′ N) with high abundance of earthworms, small sizes of the skull and brain capsule are characteristic of the Moscow race. Therefore, we should expect a decrease in the mass and contraction force of the temporal muscle (Fmt). However, the lower strength of the temporal muscle is compensated for by an elongation of the arm of its strength (l1) (increased jaw height and decreased jaw length), so the jaw compression force (Fcomp) does not decrease. Metabolic optimization is also associated with the small size of shrews in more comfortable habitats; hence, Bergmann’s rule is confirmed, but McNab’s resource rule is violated. The results of this study show that climatic conditions are capable of directly affecting the body size of shrews, not only indirectly through the productivity of the environment, thus confirming Bergman’s rule. This conclusion should be considered preliminary and requires confirmation in populations of other chromosomal races.