Cooperative breeding describes a system where group members provide regular, unsolicited allomaternal care for offspring that are not genetically their own (Emlen 1991 Hrdy 2009). Most cooperative breeders live in groups with high reproductive skew (i.e., singular breeders with helpers at the nest) however, skew falls along a continuum (Sherman et al. 1995 Vehrencamp, 2000), and, while rare, some cooperative breeders live in groups which experience low skew, subordinates commonly breed, and many if not all breeding individuals communally rear offspring (i.e., plural or communal breeders) (Keller and Reeve 1994). Though phylogenetically widespread, communal breeding is rare. Among birds, communal breeding is typical in only a few species (e.g., groove-billed ani, Crotophaga sulcirostris: Koford et al., 1990 pukeko, Porphyrio porphyria: Craig and Jamieson, 1990 Guira cuckoo, Guira guira: Macedo, 1992), while within mammals, this reproductive strategy is common only in the banded mongoose (Cant, 2000 Rood, 1975), some rodents (e.g., the house mouse, Mus musculus domesticus Norwegian rat, Rattus norvegicus various caviids: Hayes, 2000 Solomon and Getz, 1997), and social carnivores (lions, Panthera leo: Packer et al. 1990 spotted hyenas, Crocuta crocuta: Owens and Owens 1984). Within the Primate order, communal breeding is particularly rare in fact, humans are often regarded as the only communally breeding primates. Though understudied and rarely cited, this reproductive system has also been tentatively described in ruffed lemurs (Varecia sp.), a diurnal, 3-4 kg Malagasy strepsirrhine that lives in large, communally-defended territories characterized by fission-fusion dynamics (Morland 1991a Morland 1991b Rigamonti 1993 Vasey 1997 Vasey 2006). Although, like most primates, ruffed lemurs are characterized by relatively slow life histories, due in part to their strict patterns of seasonal breeding, they are distinctive in that they are the only known diurnal primate to bear litters of altricial offspring during seasonal reproductive events (Foerg 1982 Rasmussen 1985 Brockman et al. 1987). Mothers park litters in nests and tree tangles throughout early infant development and it is during this time that evidence of communal breeding has been reported, including use the of communal nests (i.e., crÌ¬ches) and cooperative infant care (Morland 1990 Vasey 2007). Among communal breeders, the evolution of non-offspring infant care, which can include grooming, guarding, predator protection, and energy transfer (e.g., food transfer, allonursing), has been explained by a number of adaptive hypotheses. It is argued, though rarely empirically demonstrated, that communal breeding might confer benefits to participating mothers and infants by enabling lactating females to increase food consumption, improve infant thermoregulation and/or growth, guard against predators, and improve competition later in life (see Koenig 1997 for references). Moreover, it is unclear whether mothers show preferences for communal nesting partners, why females select these particular partners, and whether these preferences vary. Moreover, while reciprocity, mutualism, and kinship have all been used to explain why communal breeders live and reproduce in groups, the benefits of communal breeding have yet to be established for many taxa. Thus, the goal of this dissertation was to investigate communal breeding in black-and-white ruffed lemurs (Varecia variegata) in an undisturbed primary rainforest habitat, Ranomafana National Park, to address whether and to what extent communal breeding conferred benefits to mothers and their offspring. This dissertation had three main research objectives: 1) to assess the spatial ecology of black-and-white ruffed lemurs using Global Information System (GIS) technology to determine community membership and establish what constitutes a ruffed lemur community 2) to then examine the genetic population structure of this same ruffed lemur population using molecular techniques (microsatellite analysis) to determine dispersal and patterns of within- and between-community relatedness and finally, 3) to address how patterns of space use, genetic relatedness, and affiliation influence patterns of ruffed lemur communal breeding. I also investigate the benefits of ruffed lemur communal care to both mothers and their infants. The use of Global Information System (GIS) analyses (Chapter 2) revealed broad patterns of communal and individual home range area and overlap which confirmed the presence of a fission-fusion social organization, as in previous studies of other wild ruffed lemur populations. I found that multiple males and females used independent, yet overlapping ranges which together comprised a large, communal territory that was more-or-less spatially distinct from other neighboring communities. Within this community, females used significantly larger home ranges than did males (MCP: U = 10, p = 0.04 kernel: U = 9, p = 0.03), though home range overlap did not differ between the sexes. Range use did not vary by reproductive season, as previously suggested, but rather by climatic season, though not in the predicted ways. However, reproductive seasonality, not climate, best predicted variation in daily distance traveled. Taken together, the patterns of ruffed lemur spatial ecology described herein do not adhere to those described for other primate fission-fusion systems (Wrangham 1979, Lehmann & Boesch 2005). Instead, ruffed lemurs appear to adhere to a new, previously unrecognized system of primate fission-fusion dynamics that combines aspects of both `female-bonded' and `bisexually bonded' systems of range use: both males and females are more-or-less evenly distributed throughout a female-defended communal range, with group members exhibiting equal and moderate home range overlap with other community members. Genetic analyses (Chapter 3) revealed that ruffed lemurs within this community were characterized by unbiased dispersal (i.e., both sexes likely disperse), though females likely disperse less frequently or at closer distances than do male conspecifics. On average, community members share relatedness close to zero (average R = -0.06) however, relatives lived in significantly closer proximity and shared greater home range overlap than did unrelated neighbors (proximity: females, Mantel R = 0.490, p = 0.007 males, Mantel R = 0.655, p < 0.001 overlap: males: Mantel R = 0.373, p = 0.03), resulting in close spatial networks of both male and female kin within the larger communal range. Finally, in the first systematic field study of communal breeding in ruffed lemurs to combine data on rearing behavior, genetic relatedness and infant survivorship (Chapter 4), I demonstrate that communal breeding in ruffed lemurs is biased towards kin and female affiliates, and that communal nesting significantly improves infant survival (Spearman's rho = 0.872, p = 0.03), particularly during early stages of infant development. As communal nesting allows an improved balance between maternal responsibility and foraging effort (communal nesters spent less time at nest than non-nesters: Mann-Whitney-U = 61, Z = 2.539, p = 0.01 more time feeding/foraging U = 56, Z = -2.049, p = 0.04, it is likely that communal breeding in ruffed lemurs results in improved maternal energy balance, and ultimately confers direct fitness payoffs to communally nesting females. Taken together, the suite of traits described herein (i.e., fission-fusion dynamics, unbiased dispersal, communal breeding) are uncharacteristic of most primates. While it is true that ruffed lemurs share these `rare' behaviors with a handful of other primate taxa, it is only in ruffed lemurs that we find this particular suite of social and reproductive traits (e.g., fission-fusion social organization and cooperative breeding are found in a few non-human primates, but never in tandem). Instead, it seems that ruffed lemurs exhibit a social system that has gone previously unrecognized in primates, and instead loosely resembles patterns found in other communally breeding mammals such as hyenas (e.g., fission-fusion dynamics, female dominance and territory defense, and crÌ¬ching behavior: Boydston et al. 2003 Henschel and Skinner 1991 Holekamp et al. 2000).