The importance of social condition in the hormonal and behavioral responses to an acute social stressor in the male Siberian dwarf hamster (Phodopus sungorus).

Social condition is an important factor in determining the behavioral and hormonal responses to a social stressor in the Siberian dwarf hamster (Phodopus sungorus). We predict that males housed with a female or a family (female and pups) will show an increase in the magnitude of the behavioral and hormonal responses to a male intruder compared to those of individually housed males. Three treatment groups were studied: individually housed males that had been previously group-housed in same-sex colonies (males, n = 10), males housed with a female (male + female, n = 9), and males housed with their female and pups (male + family, n = 12). Males were monitored for aggressive behavior toward an intruder male for 10 min. Blood samples were taken at baseline and after the encounter. Male + female and male + family groups spent more time in aggressive behavior (P < 0.05), such as attacking (P < 0.05) and fighting (P < 0.05), than did individually housed males. These same groups showed significant increases in plasma cortisol after the encounter (P < 0.01) whereas there were no significant increases in plasma cortisol in solitary males. All groups showed significantly lower levels of plasma testosterone (male, P < 0.001, male + female, P < 0.05; male + family, P < 0.01) whereas a significant increase in prolactin occurred only in the male + family group (P < 0.05). There was a significant positive correlation between postencounter cortisol levels and total number of minutes spent in aggressive behavior (P < 0.05). These results demonstrate that the introduction of a novel intruder male results in an activation of the hypothalamic-pituitary-adrenocortical axis and a suppression of the reproductive axis. Furthermore, pairing of a male with a female alters the behavioral and hormonal responses to an intruder male.