Mammalian monogamy is puzzling from and evolutionary perspective because it is unclear why males, which have the potential to father a great many offspring, should choose to associate with only one female. This project investigated the behaviour of a socially monogamous (pair-living) population of bat-eared foxes in Laikipia, Northern Kenya, and had two principal aims. The first aim was to identify the selective forces that operate to maintain social monogamy in the study population. The second aim was to determine whether bat-eared foxes mate exclusively with their social partners (i. e. if they are genetically as well as socially monogamous). Chapter I summarizes by background to the research: Broadly speaking, theories advanced to explain the evolution of monogamy fall into two categories; those that proposing that monogamy occurs when male assistance is required for successfW reproduction, and those that proposing that aspects of female spatial and/or temporal distribution make it impossible for even the most competitive males to gain more than one mate. Chapter 2 describes the study site and general methods employed. Chapter 3 examines whether a requirement for paternal care maintains social monogamy by investigating the parental roles of males and females: I found that females invest very heavily in reproduction, feeding at close to maximum rate throughout lactation and suffering increased mortality rates during this period. Consistent with previous studies of the species, I found that males are heavily involved in the rearing of young, spending significantly more time than females close to breeding dens, and contributing to all aspects of cub care. The importance of male care was revealed by the fact that, after statistically controlling for the confounding effects of territory quality, the male den attendance was significantly associated with cub survival. Chapter 4 investigates factors other than the requirement for male care that may prevent males from achieving polygynous status: Social monogamy was not enforced because males were incapable of defending sufficient resources to support more than one female, as some male territories contained sufficient food to support two or more females. I found, however, that because females occupied largely exclusive ranges and had synchronized fertile periods, it was probably impossible for even the most competitive males to successfully defend more than one fertile female. Chapter 5 investigates the mating tactics of bat-eared foxes by comparing their behaviour during and outside the mating season: Neither male nor female foxes increased their home range sizes during the mating season, demonstrating that they do not roam widely in search of extra-pair mates. Time-budget data suggest that this may be because bat-eared foxes have little time available to engage in activities other than foraging. The behaviour of mated partners wass highly coordinated, particularly during the mating season, and the close proximity of mated partners did not reduce their feeding rate. Chapter 6 uses DNA microsatellite analyses to establish the paternity of bat-eared fox cubs: We found that for the vast majority of cubs (42 of 44) social fathers were most likely to be their true fathers. These data demonstrate a high level of genetic monogamy in the study population. Chapter 7 summarizes data from the thesis: I conclude that, although male care enhances offspring survival, there are circumstances under which males may gain from polygyny. Males are probably unable to attain polygynous status, however, because the spatial and temporal distribution of females, combined with intense competition for mates makes it impossible for them to defend more than one mate. Consistent with observations of occasional polygynous breeding from other bat-eared fox populations, I conclude that polygyny could only a viable male strategy if compliant females were willing to co-ordinated their behaviour. I argue that the high levels of genetic monogamy observed are probably consequence of the species insectivorous diet, which leaves individuals with little time to engage in activities other than foraging, and makes it easy for males to guard their own partners.