from their natal packs. Males are therefore more helpful towards their mothers, as they remain in the same pack as her and help provide food for her and her new offspring. The LRE the males provide is predicted to result in a male-biased sex ratio, which is the pattern observed in nature. Consistent with predictions of LRE influencing sex ratios, African wild dog mothers living in smaller packs were seen to produce more male-biased sex ratios than mothers in a larger pack, since they had fewer helpers and would benefit more from additional helpers than mothers living in larger packs. Evidence for LRE leading to sex ratios biased in favor of helpers has also been found in a number of other animals, including the Seychelles warbler (Acrocephalus sechellensis) and various primates. Trivers–Willard hypothesis The Trivers-Willard hypothesis provides a model for sex allocation that deviates from Fisherian sex ratios. Trivers and Willard (1973) originally proposed a model that predicted individuals would skew the sex ratio of males to females in response to certain parental conditions, which was supported by evidence from mammals. Though individuals may not consciously decide to have fewer or more offspring of the same sex, their model suggested that individuals could be selected to adjust the sex ratio of offspring produced based on their ability to invest in offspring, if fitness returns for male and female offspring differ based on these conditions. While the Trivers-Willard hypothesis applied specifically to instances where preferentially having female offspring as maternal condition deteriorates was more advantageous, it spurred a great deal of further research on how environmental conditions can differentially affect sex ratios, and there are now a number of empirical studies that have found individuals adjust their ratio of male and female offspring. Food availability In many species, the abundance of food in a given habitat dictates the level of parental care and investment in offspring. This, in turn, influences the development and viability of the offspring. If food availability has differential effects on the fitness of male and female offspring, then selection should shift offspring sex ratios based on specific conditions of food availability. Appleby (1997) proposed evidence for conditional sex allocation in a study done on tawny owls (Strix aluco). In tawny owls, a female-biased sex ratio was observed in breeding territories where there was an abundance of prey (field voles). In contrast, in breeding territories with a scarcity