Behav Ecol Sociobiol(1990) 27:183-190 Behavioral Ecology and Sociobiology © Springer-Verlag1990 Inter-litter competition and communal nursing among Norway rats: advantages of birth synchrony Julie A. Mennella*, Mark S. Blumberg**, Martha K. McCiintock, and Howard Moltz Department of Psychology, The Universityof Chicago, 5730WoodlawnAvenue, Chicago,IL 60637, USA Received November9, 1989 / AcceptedMarch 23, 1990 Summary. The effect of inter-litter competition on pup survival was investigated in pairs of female rats (Rattus norvegicus) living and breeding in the same environment. If a female gave birth when a 0- to 14-day-old litter was already present in the environment, her pups had a very high chance of surviving, similar to the situation in which no other litter was present. Moreover, the mother was likely to nurse communally with the mother of the 0- to 14-day-old litter. This communal nursing benefitted the newborn pups as evidenced by their being heavier at weaning than litters that were not nursed com- munally. In contrast, if a female gave birth when a 15- to 28-day-old litter was already present in the environ- ment, her newborn pups were likely to die within 3 days postpartum, owing to the fact that they were often pre- vented from suckling at their mother's teats, resulting in milk deprivation, and were often beneath the older pups, resulting in physical trauma. These findings sug- gest that inter-litter competition is an important source of pup mortality when litters are born 15-28 days apart. The data are discussed in terms of the advantages of birth synchrony. Introduction In many species of mammals, lactating females build a nest jointly and nurse communally (Crowcroft and Rowe 1963; Dieterlen 1962; Emlen 1984; Gurski and Scott 1980; MacDonald et al. 1987; McShea and Mad- ison 1984; Southwick 1969; Spencer-Booth 1970; Zim- merman 1974). The likelihood, however, of two females nursing communally is often dependent upon their being pregnant at the same time and subsequently giving birth at approximately the same time, customarily referred to as birth synchrony (Gurski and Scott 1980; Packer * Present address: Monell Chemical Senses Center, 3500 Market Street, Philadelphia,PA 19104, USA ** Present address: Department of Psychology, IndianaUniversity, Bloomington,IN 47405, USA Offprint requests to: M.K. McClintock and Pusey 1983a; Rood 1980; Sayler and Salmon 1971). Females and their young should benefit from birth syn- chrony and the subsequent communal nesting because the mothers can share the energetic burdens of protect- ing and nursing young (Baldwin 1970; Boinski 1987; Gurski and Scott 1980; McClintock 1981; Sayler and Salmon 1969, 1971). Furthermore, it has been demon- strated, although as yet only in mice, that growth rate is enhanced when young are nursed in a communal nest, even when the ratio of young to lactating females re- mains the same (Sayler and Salmon 1969, 1971). In the lion (Panthera leo), another advantage of birth synchrony is evident: cub litters of similar age (inter- birth interval < 2 months) have a lower mortality rate than cub litters of dissimilar age, owing in part to the fact that older cubs can prevent younger cubs from gain- ing access to teats (Bertram 1975; Packer and Pusey 1983b; Rudnai 1973; Schaller 1972). Thus birth syn- chrony minimizes inter-titter competition. The mechansims that affect the timing of breeding among female rats have been well documented (Aron 1973; Gudermuth et al. t984; McClintock 1978, 1983a- c, 1984). The consequences of synchronous or asynchro- nous breeding, however, are less well understood (McClintock 1981, 1983 a-c). In this study, we followed the reproductive histories of pairs of adult female rats while each pair was living with a male in the same envi- ronment. Specifically, we monitored the timing of mat- ing and births, the incidence of communal nursing, and the incidence of pup mortality over a period of some 4 months. From literature on other species, we expected communal nursing to occur when females gave birth synchronously and expected pup mortality to be high when they gave birth asynchronously. Methods Animals. Wistar rats were born in the authors' laboratory and reared in mixed-sex litters. Followingweaning at 30 days of age, they were housed with animals of the same sex and age in groups of 6-8. A lighting schedule of 12L:12D was in effect throughout the study. The ambient temperaturewas 22 ° +_ 2 ° C, with a relative humidityof 60%-90%.