Physiology &Behavior, Vol. 53, pp. 389-393, 1993 0031-9384/93 $6.00 + .00 Printedin the USA. Copyright© 1993Pergamon PressLtd. Sexual Maturation and Seasonal Changes in Plasma Levels of Sex Steroids and Fecundity of Wild Norway Rats Selected for Reduced Aggressiveness Toward Humans G. T. SHISHKINA, l P. M. BORODIN AND E. V. NAUMENKO Institute of Cytology and Genetics, Siberian Branch of the Academy of Science of Russia, 630090, Novosibirsk 90, Russia Received 24 February 1992 SHISHKINA, G. T., P. M. BORODIN AND E. V. NAUMENKO. Sexual maturation and seasonal changes in plasma levels of sex steroids andfecundity of wild Norway rats selected for reducedaggressivenesstoward humans. PHYSIOL BEHAV 53(2) 389- 393, 1993.--Parameters of sexual maturation and reproductive function were compared in two groups of Norway rats, one selectedfor reduced aggressiveness towards man and the other for aggressiveness. It was found that selectionfor reduced aggressiveness produced an accelerationof sexualmaturation asjudged by earliervaginalopeningin the femaleand presenceof mature spermatozoa in the testes of the male as well by earlier prepubertal increase in plasma testosterone in the male. In domesticated adults, seasonal variations in the parameters of the reproductive function narrowed; they showed neither the winter decrease in sexual activity nor the marked fall in plasma testosterone and estradiol exhibited by their aggressive counterparts. Aggressiveness Domestication Sexual maturation Fecundity Testosterone Estradiol Seasonal changes SEXUAL maturation occurs earlier and fecundity is higher in laboratory and captive-reared rats than in their wild counterparts (6,16). There is no obvious explanation for these observations. Behavior has much contributed to the hereditary reorganization of many functional systems in wild animals during domestication (1,2). With this mind, it appeared plausible that shifts in repro- ductive parameters may be correlated responses to behavioral changes associated with domestication. To test this assumption, we carried out experiments with two groups of Norway rats, one selected for reduced aggressiveness toward humans, the other showing the wild aggressive response. In the two groups we com- pared sexual maturation and seasonal variations in plasma level of testosterone and estradiol and fecundity in adulthood. METHOD Wild-trapped (20) Norway rats reared in the vivarium of this institute were divided into two groups. Each generation was se- lected through 17-24 successive generations. Group 1 was selected for reduced aggressiveness towards man with the use of the glove test (domesticated rats), and group 2 was selected for wild-type aggressiveness (aggressive rats) using the same test. The test was based on introduction of a hand into the home cage and its move- ment around the cage until the rat could be pinned against the back wall. The expression of savageness was rated on a 5-point rating scale where: 0--lack of defensive reaction to handling; 1- defensive reactions (motor excitement, vocalization) appeared only during handling; 2--the animal bit the glove and vocalized after the contact with the hand near the back wall of the cage; 3--the rat actively attacked the glove in the front part of the cage; and 4--where rage attacks occurred at the front door of cage in re- sponse to the appearance of the glove in front of the closed cage. On the basis of the glove test, two founder groups were selected from the initial population: domesticated rats with a weak expres- sion of defensive reactions toward humans (scores 0 and 1), and aggressive rats (score 4). These groups were selected in each suc- ceeding generation for low (scores 0-1) and high (score 4) reac- tivity, with the coefficient of selection being about 10-15% in both stocks. The stocks were maintained using a system which maximally avoided inbreeding. After 20 generations of selection, the coefficient of inbreeding is not higher than 0.13. Ratlings were weaned at the age of 4 weeks, males were set apart from females, and they were housed in groups of four. Requests for reprints should be addressed to G. T. Shishkina. 389