Behav Ecol Sociobiol(1991) 29 :277-282 Behavioral Ecology and Sociobiology © Springer-Verlag1991 Current indirect fitness benefits associated with philopatry in juvenile prairie voles Nancy G. Solomon* Department of Ecology,Ethology,and Evolution,ShelfordVivarium, 606 East HealeyStreet, Universityof Illinoisat Urbana-Champaign, Champaign, IL 61820, USA Received October 10, 1990 / AcceptedJune 10, 1991 Summary. Prairie vole (Microtus ochrogaster) family groups were examined to determine possible indirect fit- ness benefits from the presence of juvenile helpers at the natal nest. The reproductive performance of family groups retaining two juveniles was compared with that of families in which all juveniles were removed when the subsequent litter was born. Litter sizes at birth of the second litter and pup survival rates were the same in the two treatments. Offspring quality was affected by the presence of juveniles however. Pups reared with juveniles weighed 13% more at weaning than pups reared without juveniles. Pups also opened their eyes sooner when juveniles were present. Differences in growth and development may have been affected by the amount of time pups were alone in the nest since pups in families with juveniles were left alone less frequently than were pups without juveniles. Maternal behavior patterns were not affected by the presence of juveniles. In contrast, fathers in families with juveniles spent more time in non-parental behaviors such as feeding, drinking, and foraging. In families with large litters, mothers deliv- ered a subsequent litter sooner if juveniles were present. Subsequent litter sizes were the same in both treatments. Overall, both infants and parents benefited from the presence of juveniles, suggesting that helping may en- hance the helper's indirect fitness in multiple ways. Introduction The study of helping behavior has focused on two ques- tions: (a) under what conditions should an individual disperse or remain at the natal nest, and (b) if it remains at the nest, should it contribute to the care of young (Brown 1987)? Two hypotheses have been proposed to address the latter question. The individual fitness hy- pothesis states that helpers benefit directly by caring for young but breeders and young do not benefit (Ligon * Current address: Department of Zoology,Box 7617, North Car- olina State University,Raleigh,NC 27695-7617, USA 1981 ; Woolfenden 1981). According to the inclusive fit- ness hypothesis, helpers also benefit indirectly by in- creasing the fitness of nondescendent kin (Brown 1974; Emlen 1978). Although quantitative estimates of the effects of avian helpers are common, there are very few such stu- dies of mammals. In several avian and two mammalian species, the presence of nonbreeding juvenile or subadult family members (helpers) correlates positively with re- productive success of breeders (Rowley 1965; Parry 1973; Moehlman 1979; Brown and Brown 1981; Austad and Rabenold 1985; Gibbons 1987; Lennartz etal. 1987; Emlen and Wrege 1988; Rood 1990). Correlation- al studies do not demonstrate a causal relationship be- tween the presence of helpers and increased reproductive success. Other factors such as experience, age, or differ- ential territory quality may be responsible for the corre- lation (Koenig 1981; Brown et al. 1982). Accordingly, experimental manipulations are needed to assess directly the effects of helpers on the reproductive success of breeders. The present experiment was designed to investigate potential indirect fitness benefits that may accrue to ju- venile prairie voles, Microtus ochrogaster, as a result of natal philopatry and subsequent care of younger si- blings. Prairie voles are an excellent species for studying the effects of juvenile helpers. Individuals live in ex- tended family groups consisting of a breeding pair, one or two litters of young, and occasionally one or more unrelated adults (Getz and Hofmann 1986; Getz et al. 1990). Juveniles are highly philopatric: 68 % of the males and 75% of the females remain at their natal nest (Getz et al. 1987). Typically, one to three juveniles remain at the nest while parents produce a subsequent litter (Getz et al. 1987). All family members contribute to the care of pups by brooding, grooming, and retrieving pups (Thomas and Birney 1979; Gruder-Adams and Getz 1985). Natal philopatry appears costly to juvenile prairie voles. At least at low population density, female, and possibly also male, juveniles are reproductively sup- pressed and do not breed (Carter and Getz 1985).