Resting metabolic rate, stress, testosterone, and induced immune response in spring- and fall-born males of Campbell’s dwarf hamsters: Maintenance in long-day conditions

We have studied morphological and physiological traits of equally aged young males of Campbell’s dwarf hamster ( Phodopus campbelli Thomas, 1905) born at the end of summer (fall males) and winter (spring males) in a vivarium with a constant 14-hour day length (LD 14: 10). Animals were removed from parental cages at the age of one month, kept in isolation in the same lighting conditions, and tested at the age of 2 to 3.5 months. Spring males had a higher resting metabolic rate, a higher body mass in the middle of the experiment, bigger testes and seminal vesicles, higher blood testosterone levels, and a stronger T-cell immune response to phytohemagglutinin injected intradermally than fall males. They did not differ significantly from fall males in basal blood cortisol levels or in antibody production in response to the sheep red blood cells (SRBC) antigen challenge, but they showed lower adrenocortical response to a social stressor and to an adrenocorticotropic hormone. A general linear model analysis showed that the blood cortisol level after a 10 min encounter of males in the open arena and the resting metabolic rate were the only factors significantly influencing humoral immune response to SRBC. In the general linear model including the intensity of T-cell immune response or basal testosterone level as dependent variables, birth season was the only factor causing a significant effect.