Reproductive Correlates of Aggressive Behavior in Female Peromyscus melanophrys

Social interactions may play a major role in determining population size for some species of Peromyscus by regulating dispersal (Fairbairn, 1977, 1978a, 1978b; Metzgar, 1979), recruitment (Healey, 1967; Metzgar, 1971; Sadleir, 1965), and spacing behavior (Halpin and Sullivan, 1978; Wolff, 1985, in press). Female aggression has been suggested as the proximate mechanism which controls juvenile survivorship, natal dispersal, breeding dispersal, and spacing behavior (Halpin, 1981; Savidge, 1974a, 1974b). Wolff (1985) predicted that the amount of female aggressive behavior, which may lead ultimately to territoriality, also functions to prevent infanticide. Both the estrous cycle and aggressive behavior of Peromyscus have been well documented. Clark (1936) and Pournelle (1952) described the estrous cycle for P. maniculatus and P. gossypinus, respectively, and showed that these species were spontaneous ovulators with a cycle of 4-5 days. The aggressive behavior of certain species of the genus has been described (Dewsbury, 1984; Eisenberg, 1968; Sadleir, 1965; Wolff, in press). However, the data available on the relationship between behavior and reproductive condition of female Peromyscus are limited (Cushing, 1985). The systematic studies of female aggressive behavior during the reproductive cycle by Gleason et al. (1979, 1980, 1981) and Broida et al. (1981) encompassed only the proestrus and estrus stages of the estrous cycle and did not measure the response of test females to stimulus males. Gleason et al. (1980) showed that lactating P. leucopus were highly aggressive toward female intruders. Although there is a wealth of information regarding the behavioral biology of certain species of Peromycus (see King, 1968), the information available about the plateau mouse, P. melanophrys, is limited (Dewsbury, 1979; Ferkin, 1987). This species is nocturnal and native to the semi-arid, rocky scrub deserts of the central Mexican Highlands, between 120 and 230N latitude (Baker, 1952; Hooper, 1955, 1958; Osgood, 1909, 1945). In light of the results of Gleason et al. (1980) and the predictions of Wolff (1985), the present study examined the relationship between aggressive behavior and the reproductive status of female P. melanophrys during paired encounters. Offspring (Fl) from wild caught P. melanophrys were obtained from Michigan State University and the resulting F2 generation mice bred in captivity at SUNY Brockport were used in this study. Non-lactating mice were individually caged, whereas lactating and pregnant females remained with their mates. Littermates, upon weaning, at 33 days of age (Ferkin, 1987), were housed individually. Closely-related mice (parents, offspring, siblings, first cousins) never faced each other in the dyadic encounters. Mice were housed in clear plastic cages (24 by 17 by 22 cm) with dried corn cob bedding and nesting material of wood shavings in a room maintained at 22 ? 20C. Purina Rat Chow and water were provided ad lib. The light cycle was white light from 2300 to 1100 h and dim red light from 1100 to 2300 h. All dyadic encounters were carried out under red light. The reproductive cycle of females was determined by inspection of vaginal smears taken twice daily (0900 and 2400 h) from 10 individually-caged females over four consecutive cycles. The location of the testes, either scrotal or abdominal, was used to indicate reproductive condition of males. Only adult males with scrotal testes were used in the experiment. Aggressive behavior of each focal female was observed during dyadic encounters and correlated with her reproductive condition (proestrus, estrus, metestrus, diestrus, pregnant, lactation). Vaginal smears were taken from the females prior to testing to determine their reproductive status. Three separate groups of focal females (n = 40) were tested: 16 non-lactating (cycling), 8 pregnant (days 5-21 prepartum), and 16 lactating (days 4-7 postpartum) mice. No female belonged to more than one group. All focal females were of a similar age, and at least 70 days old. All females were primiparous, and none of the lactating females was concurrently pregnant. Females underwent staged dyadic encounters such that each female interacted with a male and a diestrous female. The non-lactating mice underwent eight staged encounters, two in each of the four stages