Competitive dominance of wild and domestic Norway rats (Rattus norvegicus)

Food competition and social experience effects on V1a receptor binding in the forebrain of male Long-Evans hooded rats

The present study investigated the effect of social status in Long-Evans hooded rats established during food competition on V(1a) vasopressin receptor (V(1a)R) binding in the lateral septum (LS), medial preoptic area (MPOA), bed nucleus of the stria terminalis (BNST), anterior hypothalamus (AH), and central/basolateral amygdala (CeB). Serum concentration of testosterone (T) and corticosterone (CORT) was also measured. In Experiment 1, thirty-two lever-trained weight-matched rat pairs were placed in operant chambers where a single bar press provided access to milk reinforcement. A dominant-subordinate relationship, determined by the duration of drinking, was evident in 88% of the pairs. Sixteen rats were lever-trained but did not interact and served as no-treatment (NT) controls. In the LS, V(1a)R binding in the subordinate (SUB) group was significantly higher than in the dominant (DOM) group. V(1a)R binding was significantly higher in the LS, BNST, CeB, and AH in the NT group than in the other groups. The levels of CORT and T were not affected significantly by group membership. Experiment 2 investigated whether the binding effect in the LS was related to differences in fluid consumption. The results did not indicate a significant effect of fluid consumption. In the rat, V(1a)R binding in several forebrain areas seems to be affected by brief periods of social interactions, and, in the LS, it also appears to be related to dominance status.

On the Evolutionary Significance of Encephalization in Some Eutherian Mammals: Effects of Adaptive Radiation, Domestication, and Feralization

Allometries of the brain to body size relationship in eutherian mammals are examined in this study as they can be used for comparative analyses concerning encephalization. In contrast with some modern presentations of this issue, an older concept is revived and expanded through this author's current study. Three allometries with clearly different slopes are valid and lead to reliable results: interspecific, intraspecific, and ontogenetic allometries. Interspecific allometries follow lines with slope values of 0.56 or 0.63 for larger and smaller species, respectively, and characterize different average encephalization plateaus with rodents and lagomorphs generally more strongly encephalized compared to basal insectivores. Artiodactyls, perissodactyls and carnivores as a whole are again on a higher but rather similar plateau. Several species of carnivores have reached different encephalization levels with respect to their average plateau indicating diverse radiations. A phylogenetic brain size increase from fossil to recent radiations is also evident. Intraspecific allometries have slope values of about 0.25. These are of help in comparing brain sizes of ancestral species with their domesticated relatives. Domestication has generally led to a brain size decrease, but species on higher encephalization plateaus show this trend more strongly than species on a lower level of encephalization. Several brain parts and the sense organs also decrease in size during the domestication process, but vary arbitrarily and to different degrees. Ontogenetic growth allometries are species-specific, but are especially different between altricial and precocial mammals. A very steep 1st phase slope of highly encephalized species is particularly useful for understanding evolutionary and adaptive phenomena. Domesticated mammals that have become feral do not show an increase in brain size despite living many generations in wild habitats.