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Variation in the sex ratio of pouch young and adult hairy-nosed wombats (Lasiorhinus latifrons and Lasiorhinus krefftii)

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Abstract

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 endangered Lasiorhinus krefftii. The social structure of Lasiorhinus includes 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 adult L. latifrons reported 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 of L. latifrons that 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 endangered L. krefftii from 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 of L. latifrons was 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. latifrons mothers 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 genus Lasiorhinus) exist tenuously in semi-arid environments because of infrequent reproduction, habitat degradation, and drought. L. krefftii is 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.

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Availability of data

The datasets related to L. latifrons used in this study are available from the corresponding author on reasonable request. The L. krefftii datasets used in this study are available from the Director, Wildlife and Threatened Species Operations, Queensland Department of Environment and Science on reasonable request.

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Acknowledgements

We thank David Stemmer and the South Australian Museum for providing access to specimens and data from L. latifrons, and the Queensland Department of Environment and Science for allowing us to use their data on L. krefftii. We also thank Chris Johnson and Tim Clutton-Brock, Stephen Pruett-Jones, Philip Ryan and Maryam Montazerolghaem (statistical advice), and the reviewers and editors who made very helpful comments and suggestions about drafts of the manuscript.

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Authors and Affiliations

  1. School of Animal and Veterinary Science, University of Adelaide, Waite Campus, Urrbrae, SA, 5064, Australia

    Matt Gaughwin & David Taggart

  2. Department of Environment and Science, Wildlife and Threatened Species Operations, 61 Yeppoon Road, Parkhurst, QLD, 4072, Australia

    Alan Horsup

  3. School of Life and Environmental Sciences, University of Sydney, A08, Camperdown, NSW, 2006, Australia

    Christopher Dickman

  4. School of Biological Sciences, Flinders University, Bedford Park, SA, 5042, Australia

    Rod Wells

  5. School of Forestry, Northern Arizona University, Flagstaff, AZ, USA

    Faith Walker

  6. FAUNA Research Alliance Ltd., PO Box 98, Callaghan, NSW, 2308, Australia

    David Taggart

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  1. Matt Gaughwin
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Correspondence to Matt Gaughwin.

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Ethical approvals

All applicable national and institutional guidelines for the care and use of animals were followed in respect of the studies of L. latifrons on Kooloola Station from 1993 (University of Adelaide, Animal Ethics Committee) and L. krefftii on Epping Forest National Park (Animal Ethics Committee, Dept. of Agriculture and Fisheries Queensland). Other data from L. latifrons were obtained from published and unpublished reports and papers or from museum specimens collected in the 1960s before institutional ethics committees were established. Trapping of wombats on Brookfield Conservation Park was either undertaken before institutional ethics committees were established or was part of the management activities on the park.

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The authors declare that they have no conflict of interest.

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Communicated by K. Eva Ruckstuhl

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Gaughwin, M., Horsup, A., Dickman, C. et al. Variation in the sex ratio of pouch young and adult hairy-nosed wombats (Lasiorhinus latifrons and Lasiorhinus krefftii). Behav Ecol Sociobiol 74, 90 (2020). https://doi.org/10.1007/s00265-020-02864-7

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