The social substructure and the distribution of genetic variation among colonies of yellow-bellied marmots, when analyzed as an evolutionary system, suggests that this substructure enhances the intercolony variance and retards the fixation of genetic variation. This result supports a traditional theory of gradual evolution rather than recent theories suggesting accelerated evolution in social mammals.

The yellow-bellied marmot (Marmota flaviventris) is a social, ground-dwelling squirrel that lives either individually or in kin groups of from two to five adult females. Philopatry and daughter recruitment lead to the formation and persistence of matrilines at habitat sites. By using 37 years of demographic data for 12 habitat sites, we could determine long-term trends in the effects of group size on two measures of fitness, survivorship and net reproductive rate, which otherwise are obscured by annual fluctuations in these measures. Both size and number of matrilines varied among sites and survivorship and net reproductive rate varied among sites and among matriline sizes. The role of social organization was explored further by examining the effect of matriline size, averaged over all years and sites, on fitness. For both survivorship and net reproductive rate the relationship with matriline size was curvilinear. Fitness increased with the increase in matriline size and then decreased in the largest groups. Decreased fitness in matrilines of four or five was associated with agonistic behavior, a large number of 2-year-old females in the social group, and reproductive suppression. There is no evidence that females acted to increase their fitness by increasing indirect fitness; i.e., by assisting relatives, but attempted to increase direct fitness. Direct fitness increased when mortality and fission of large matrilines reduced group size and the surviving females increased reproduction. Because sociality in mammals is potentially costly due to intraspecific competition, parasite transmission, or suppressed reproduction (1), sociality must enhance inclusive fitness directly through the production of offspring and possibly indirectly through the production of nondescendant relatives when compared with nonsocial conspecifics. However, in many societies, sociality is associated with reproductive skew; a few individuals do all or most of the breeding (2). Reproductive skew may be associated with a trade-off between reproduction and survival. For example, highly social species of marmots, such as Marmota olympus and M. vancouverensis, which do not reproduce before age 3, have higher survival at ages 1-4 than the less social M. flaviventris, which reproduces at age 2 (3-5). Increased group size is widely associated with reduced per capita reproductive success; e.g., increased social complexity among species of ground-dwelling sciurids is associated with decreased litter size and a smaller proportion of females breeding (6). Among cooperatively breeding species reproduction may be limited to a single female in a group (3, 7), and per capita reproductive success is lower in large coteries than in small coteries of black-tailed prairie dogs (8). In contrast, by using 37 years of field observations, we report that net reproductive rate (R o ), a direct measure of fitness (9), initially increases with increased group size in the yellow-bellied marmot (Marmota flaviventris), then decreases in the largest groups. Furthermore, ...

Yellow-bellied marmots Marmota¯aviventris in the East River Valley of Colorado were live-trapped and individually marked annually from 1962 through 1993. These pooled data were used to produce a demography and life table for these years. Females had signi®cantly better survivorship than males beyond the ®rst-year age class, and the sex ratio became progressively female biased. The major mortality factors of predation and unsuccessful hibernation acted evenly on all age classes as shown by the constant rates of survivorship. The rate of senescence indicated that the probability of mortality did not increase with age. Females produced litters from ages 2 to 10 years. Mean litter size was 4.1 and did not differ among age classes. The female generation length of 4.49 years was 2.4 times the life expectancy and the median survivorship. The net reproductive rate (R o ) was 0.67, yet the population did not continually decline; adjustments to these data increased R o to 0.85. Reproductive values (V x ) were approximately equal across the reproductive age classes. The polygynous mating system is both cause and effect of the demography. Marmot population size is affected by weather factors that in¯uence reproduction and survival, by predation, and by movement into and out of the study area.