Understanding the causes and consequences of biases in the sex ratio at birth and of adults in species of mammals that have unusual life histories may help us know whether sex ratios are adaptive responses to ecological and evolutionary forces and may be important in conserving endangered species. For example, have sex ratio biases at birth and in adults in species of mammals evolved as an adaptive response to environmental unpredictability? We investigated the sex ratio of populations of hairy-nosed wombats,Lasiorhinus latifrons, and the endangeredLasiorhinus krefftii. The social structure ofLasiorhinusincludes male philopatry and female dispersal, which are unusual traits among mammals. Reproduction in these wombats is often curtailed by unpredictable droughts, and so wombats may be a suitable target group for understanding the causes and consequences of biases in mammalian sex ratios. The sex ratio in populations of adultL. latifronsreported in the scientific literature varied widely, but the mean percentage of females in the 10 studies examined was 54.9 with a 95% confidence interval of 50.3 to 59.5. In the population ofL. latifronsthat we monitored for nearly twenty years beginning in 1994, the percentage of adult females varied between 66 and 37% in a manner that suggested homeostasis. The growth of the population of critically endangeredL. krefftiifrom 52 males and 29 females in 2000 to 79 males and 91 females in 2016 suggested that more females than males were born. The sex ratio of pouch young ofL. latifronswas biased towards females at times. The mean percentage of female pouch young in 19 breeding seasons was 56.4 (95% confidence interval of 50.4-62.4). Variation in the sex ratio of pouch young was examined regarding theories of sex allocation. When the percentage of adult females in the population was lower (37-50%) and had been declining,L. latifronsmothers had more female than male pouch young, but the sex ratio of pouch young was about equal when the percentage of adult females was higher (53-66%). This pattern is partly consistent with Fisher's principle, which is also known as the homeostatic hypothesis (HH). When the percentage of adult females was lower, the mothers of female pouch young were in better condition than the mothers of male pouch young, which is consistent with a reversed Trivers-Willard (TW) effect in part. The biases in the sex ratio of adult and pouch young wombats may be adaptive responses to their life history and to demographic and environmental cues. Significance statement Wombats are unusual among mammals in that males are philopatric and females disperse. Hairy-nosed wombats (two species within the genusLasiorhinus) exist tenuously in semi-arid environments because of infrequent reproduction, habitat degradation, and drought.L. krefftiiis critically endangered. This study suggests that following severe droughts, a female bias in the sex ratio of pouch young and of adults may result in populations increasing more rapidly than if the sex ratio was equal. Agencies charged with managing and conserving wombats could make use of this knowledge by undertaking more comprehensive demographic monitoring of populations to evaluate their risk of extinction; a persistent male bias in adults may be a signal of an increased risk of extinction. The unpredictability of rainfall and drought across their distribution has resulted in variation in abundance and in genetic, ecological, and social parameters associated with wombat colonies, making these species valuable for exploring the causes and consequences of biases in mammalian sex ratios.