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Abstract: Animals face trade-offs throughout life between competing functions, such as between self-maintenance, reproduction, and survival. Resource allocation between these competing functions leads to different patterns of life history traits, changes in investment in reproductive effort, and different patterns of reproductive success. Reproductive investment is also influenced by environmental factors, for example by resource availability or mating opportunities. In this thesis, I test for evidence of reproductive and life history trade-offs, as well as for whether individual- and population-level resource availability affect reproduction and life history traits. In my first chapter I tested whether there was evidence of age-related changes in reproductive success and a sexually-selected trait in male red squirrels, and in particular whether there was evidence of senescent decline in older ages. I also considered whether there was a trade-off between early life reproduction and late-life senescence in male red squirrels by testing whether age at first reproduction affected senescent decline. Theories of the evolution of senescence predict that this trade-off exists, but are typically tested in females. I found evidence of senescence, as well as evidence that males who delay reproduction achieve higher reproductive success throughout life. However, I did not find that age at first reproduction affected senescence rate. Thus, males do decline late in life but the rate of this decline was not dependent on the timing of the onset of reproductive success. In my second chapter I tested whether encountering a resource pulse affected life history traits. I also tested whether life history trade-offs and the fitness consequences of life history traits were affected by encountering a resource pulse called a mast year. I found that males who encountered a resource pulse as a yearling were more likely to breed. As mast years occur in the autumn but squirrels breed during the spring, this increase in reproductive effort is evidence of anticipatory reproduction in males. I also found that males achieved higher lifetime reproductive success when they encountered a mast year during their life. The fitness consequences of life history traits were also affected by encountering a mast year: males sired more pups when they bred early in life if they encountered a mast year in their lifetime, but they sired more pups when they bred late in life if they did not encounter a mast year. Encountering a mast year thus has significant consequences for fitness and for life history traits of male red squirrels. In my third chapter I tested whether there were different siring opportunities in mast years compared with non-mast years. I then tested whether this affected male mating behaviour. I found that there were more females who produced offspring during a mast year, and that in particular there were more second litters produced during a mast year. Male mating behaviour and siring patterns differed during a mast year, during a mast year males increased their reproductive effort. Reproductive success also differed between first and late litters during mast years: in late litters, males sired pups closer to their territories, with fewer females, and sired a larger proportion of the litter. I also showed evidence that males committed infanticide during a mast year. This suggests that resource pulses influence both mating opportunities for males, as well as the fitness consequences of reproductive investment. In my fourth chapter, I tested whether individual-level food availability was related with breeding season timing, mating behaviour, and reproductive success in male and female red squirrels. I demonstrated that there was an effect of cached cones on male reproductive success: males bred earlier, sired more pups, and sired more recruits when they had more cached cones in their midden. Males who cached more cones also had higher longevity and lifetime reproductive success. In contrast, the effect of resources on female reproductive success was weaker, as having higher cached resources was only associated with earlier breeding. There was also no effect of resources on female lifetime reproductive success.