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The Journal of animal ecology2020; 89(12); 2909-2921; doi: 10.1111/1365-2656.13349

Causes and consequences of an unusually male-biased adult sex ratio in an unmanaged feral horse population.

Abstract: The adult sex ratio (ASR) is important within ecology due to its predicted effects on behaviour, demography and evolution, but research examining the causes and consequences of ASR bias have lagged behind the studies of sex ratios at earlier life stages. Although ungulate ASR is relatively well-studied, exceptions to the usual female-biased ASR challenge our understanding of the underlying drivers of biased ASR and provide an opportunity to better understand its consequences. Some feral ungulate populations, including multiple horse populations, exhibit unusually male-biased ASR. For example, research suggests that the feral horse Equus ferus caballus population on Sable Island, Nova Scotia, Canada may exhibit a male-biased ASR. Such exceptions to the rule provide a valuable opportunity to reveal the contributions of environmental context and trait differences to ASR bias. We aimed to test for bias in Sable Island horse ASR, identify the demographic drivers of bias, and explore its demographic and social consequences. To do this, we used life history, movement and group membership information for hundreds of horses followed through a long-term individual-based study between 2007 and 2018. Sable Island horse ASR is male biased and this skew has increased over time, reaching 62% male in 2018. Our life table response experiment suggested that ASR skew was driven predominantly by male-biased adult survival. Further analyses pointed to sex-biased survival being driven by reduced female survival post-reproduction. Male-biased ASR was associated with reduced harem sizes, an increase in the number of social groups on the island, and reduced reproduction in young females. Our results support the idea that male-biased ASR in feral ungulate populations may be caused by a combination of high population density and high reproductive output. We suggest that female-biased mortality may be caused by females continuing to reproduce at high density, and thus being more susceptible to resource shortages. Thus, our results highlight the strong context dependence of ASR. Furthermore, our work indicates the potential for ASR to substantially alter a population's social organisation. Such changes in social structure could have knock-on consequences for demography by altering the formation/stability of social relationships, or competition for matings.
© 2020 British Ecological Society.
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Publication Date: 2020-10-16 PubMed ID: 32996590DOI: 10.1111/1365-2656.13349Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The study examines the unusually high male to female ratio (ASR) among feral horses on Sable Island, Nova Scotia. It aims to identify why this bias is occurring and what effects it has on behaviour, demography, and evolution. The research indicates that the main cause may be that females reproduce at such high density they may be more susceptible to resource shortages, thus possibly reducing their survival rates.

Objective of the Study

  • The primary objective of this study was to investigate the sex ratio bias among the population of feral horses on Sable Island, to identify what led to this bias and to understand its impact on demographic and social implications.

Methodology

  • The researchers used a life history, movement, and group membership information approach for a large number of the horse population over a decade-long individual-based study, from 2007 to 2018.
  • They then used a life table response experiment to identify the drivers of this sex ratio bias, which pointed towards higher rates of male survival.

Findings of the Study

  • The study found that the feral horse population on Sable Island was indeed male-biased, with the bias increasing over time, reaching a percentage of 62% males in 2018.
  • More specifically, the research pointed towards higher male survival rates as a predominant factor behind the sex ratio skewness, with the sex-biased survival driven mostly by reduced female survival post-reproduction.
  • The male-biased ASR was associated with smaller harem sizes, an increase in the number of social groups on the island, as well as decreased reproductive rates among young females.

Implications of the Study

  • The study asserts that male-biased ASR in feral ungulate populations might be caused by a combination of high population density and high reproductive output.
  • The researchers suggest that increased mortality among females may be due to continued reproduction at high density, making females more susceptible to resource shortages.
  • The male-biased sex ratio could substantially alter the population’s social dynamics, possibly impacting the formation and stability of social relationships and competition for mating.

Cite This Article

APA
Regan CE, Medill SA, Poissant J, McLoughlin PD. (2020). Causes and consequences of an unusually male-biased adult sex ratio in an unmanaged feral horse population. J Anim Ecol, 89(12), 2909-2921. https://doi.org/10.1111/1365-2656.13349

Publication

The Journal of animal ecology
ISSN: 1365-2656
NlmUniqueID: 0376574
Country: England
Language: English
Volume: 89
Issue: 12
Pages: 2909-2921

Researcher Affiliations

Regan, Charlotte E
  • Department of Biology, University of Saskatchewan, Saskatoon, SK, Canada.
Medill, Sarah A
  • Department of Biology, University of Saskatchewan, Saskatoon, SK, Canada.
Poissant, Jocelyn
  • Department of Ecosystem and Public Health, University of Calgary, Calgary, AB, Canada.
McLoughlin, Philip D
  • Department of Biology, University of Saskatchewan, Saskatoon, SK, Canada.

MeSH Terms

  • Animals
  • Female
  • Horses
  • Islands
  • Male
  • Nova Scotia
  • Population Density
  • Reproduction
  • Sex Ratio

References

This article includes 71 references
  1. Archie EA, Tung J, Clark M, Altmann J, Alberts SC. Social affiliation matters: Both same-sex and opposite-sex relationships predict survival in wild female baboons. Proceedings of the Royal Society B: Biological Sciences 281, 20141261.
  2. Berger J. Predation, sex ratios, and male competition in equids (Mammalia: Perissodactyla). Journal of Zoology 201, 205-216.
  3. Berger J, Gompper ME. Sex ratios in extant ungulates: Products of contemporary predation or past life histories?. Journal of Mammalogy 80, 1084-1113.
  4. Berteaux D. Female-biased mortality in a sexually dimorphic ungulate: Feral cattle of Amsterdam Island. Journal of Mammalogy 74, 732-737.
  5. Bonenfant C, Gaillard JM, Coulson T, Festa-Bianchet M, Loison A, Garel M, Loe LE, Blanchard P, Pettorelli N, Owen-Smith N, Du Toit J. Chapter 5 empirical evidence of density-dependence in populations of large herbivores. Advances in Ecological Research 41, 313-357.
  6. Bonenfant C, Gaillard JM, Klein F, Maillard D. Variation in harem size of red deer (Cervus elaphus L.): The effects of adult sex ratio and age-structure. Journal of Zoology 264, 77-85.
  7. Boness DJ, Bowen WD, Iverson SJ. Does male harassment of females contribute to reproductive synchrony in the grey seal by affecting maternal performance?. Behavioral Ecology and Sociobiology 36, 1-10.
  8. Bowling AT, Touchberry RW. Parentage of great basin feral horses. Journal of Wildlife Management 54, 424.
  9. Burnham KP, Anderson DR. Information and likelihood theory: A basis for model selection. In K. Burnham & D. Anderson (Eds.), Model selection and multimodel inference: A practical information-theoretic approach (2nd ed., pp. 49-97). Springer-Verlag.
  10. Cameron EZ, Linklater WL, Stafford KJ, Minot EO. Aging and improving reproductive success in horses: Declining residual reproductive value or just older and wiser?. Behavioral Ecology and Sociobiology 47, 243-249.
  11. Cameron EZ, Setsaas TH, Linklater WL. Social bonds between unrelated females increase reproductive success in feral horses. Proceedings of the National Academy of Sciences of the United States of America 106, 13850-13853.
  12. Caswell H. Matrix population models: Construction, analysis, and interpretation (2nd ed.). Sinauer.
  13. Caswell H. Life table response experiment analysis of the stochastic growth rate. Journal of Ecology 98, 324-333.
  14. Caswell H, Weeks DE. Two-sex models: Chaos, extinction, and other dynamic consequences of sex. The American Naturalist 128, 707-735.
  15. Christie B. The horses of Sable Island (2nd ed.). Pottersfield Press.
  16. Clutton-Brock TH, Illus AW, Wilson K, Grenfell BT, MacColl AD, Albon SD. Stability and instability in ungulate populations: An empirical analysis. The American Naturalist 149, 195-219.
  17. Clutton-Brock TH, Lonergan ME. Culling regimes and sex ratio biases in highland red deer. Journal of Applied Ecology 31, 521-527.
  18. Contasti AL, Tissier EJ, Johnstone JF, McLoughlin PD. Explaining spatial heterogeneity in population dynamics and genetics from spatial variation in resources for a large herbivore. PLoS One 7, e47858.
  19. Duncan P, Harvey PH, Wells SM. On lactation and associated behaviour in a natural herd of horses. Animal Behavior 32, 255-263.
  20. Dyson EA, Hurst GD. Persistence of an extreme sex-ratio bias in a natural population. Proceedings of the National Academy of Sciences of the United States of America 101, 6520-6523.
  21. Eberhardt LL. A paradigm for population analysis of long-lived vertebrates. Ecology 83, 2841-2854.
  22. Eberhart-Phillips LJ, Küpper C, Carmona-Isunza MC, Vincze O, Zefania S, Cruz-López M, Kosztolányi A, Miller TE, Barta Z, Cuthill IC, Burke T. Demographic causes of adult sex ratio variation and their consequences for parental cooperation. Nature Communications 9, 1-8.
  23. Eberhart-Phillips LJ, Küpper C, Miller TE, Cruz-López M, Maher KH, Dos Remedios N, Stoffel MA, Hoffman JI, Krüger O, Székely T. Sex-specific early survival drives adult sex ratio bias in snowy plovers and impacts mating system and population growth. Proceedings of the National Academy of Sciences of the United States of America 114, E5474-E5481.
  24. Eggert LS, Powell DM, Ballou JD, Malo AF, Turner A, Kumer J, Zimmerman C, Fleischer RC, Maldonado JE. Pedigrees and the study of the wild horse population of Assateague Island national seashore. Journal of Wildlife Management 74, 963-973.
  25. Feh C. Long-term paternity data in relation to different aspects of rank for camargue stallions, Equus caballus. Animal Behavior 40, 995-996.
  26. Frère CH, Krützen M, Mann J, Connor RC, Bejder L, Sherwin WB. Social and genetic interactions drive fitness variation in a free-living dolphin population. Proceedings of the National Academy of Sciences of the United States of America 107, 19949-19954.
  27. Gaillard JM, Festa-Bianchet M, Yoccoz NG. Population dynamics of large herbivores: Variable recruitment with constant adult survival. Trends in Ecology & Evolution 13, 58-63.
  28. Garrott R. Sex ratios and differential survival of feral horses. Journal of Animal Ecology 60, 929-936.
  29. Goodloe RB, Warren RJ, Osborn DA, Hall C. Population characteristics of feral horses on Cumberland Island, Georgia and their management implications. Journal of Wildlife Management 64, 114-121.
  30. Grange S, Duncan P, Gaillard JM. Poor horse traders: Large mammals trade survival for reproduction during the process of feralization. Proceedings of the Royal Society B: Biological Sciences 276, 1911-1919.
  31. Hamel S, Festa-Bianchet M, Côté SD. Offspring sex in mountain goat varies with adult sex ratio but only for mothers in good condition. Behavioral Ecology and Sociobiology 70, 123-132.
  32. Hamel S, Gaillard JM, Yoccoz NG, Loison A, Bonenfant C, Descamps S. Fitness costs of reproduction depend on life speed: Empirical evidence from mammalian populations. Ecology Letters 13, 915-935.
  33. Isaac JL. Potential causes and life-history consequences of sexual size dimorphism in mammals. Mammal Review 35, 101-115.
  34. Jenouvrier S, Caswell H, Barbraud C, Weimerskirch H. Mating behavior, population growth, and the operational sex ratio: A periodic two-sex model approach. The American Naturalist 175, 739-752.
  35. Johnsson M, Gering E, Willis P, Lopez S, Van Dorp L, Hellenthal G, Henriksen R, Friberg U, Wright D. Feralisation targets different genomic loci to domestication in the chicken. Nature Communications 7, 1-11.
  36. Kaseda Y, Khalil AM. Harem size reproductive success of stallions in Misaki feral horses. Applied Animal Behaviour Science 47, 163-173.
  37. Lawson Handley LJ, Perrin N. Advances in our understanding of mammalian sex-biased dispersal. Molecular Ecology 16, 1559-1578.
  38. Le Galliard JF, Fitze PS, Ferrière R, Clobert J. Sex ratio bias, male aggression, and population collapse in lizards. Proceedings of the National Academy of Sciences of the United States of America 102, 18231-18236.
  39. Legendre S. Age structure, mating system and population viability. In R. Ferrière, U. Dieckmann, & D. Couvet (Eds.), Evolutionary conservation biology (pp. 41-58). Cambridge University Press.
  40. Lemaitre JF, Gaillard JM. Male survival patterns do not depend on male allocation to sexual competition in large herbivores. Behavioral Ecology 24, 421-428.
  41. Liker A, Freckleton RP, Székely T. The evolution of sex roles in birds is related to adult sex ratio. Nature Communications 4, 1587.
  42. Liker A, Freckleton RP, Székely T. Divorce and infidelity are associated with skewed adult sex ratios in birds. Current Biology 24, 880-884.
  43. Linklater WL, Cameron EZ, Minot EO, Stafford KJ. Stallion harassment and the mating system of horses. Animal Behavior 58, 295-306.
  44. Linklater WL, Cameron EZ, Minot EO, Stafford KJ. Feral horse demography and population growth in the Kaimanawa Ranges, New Zealand. Wildlife Research 31, 119-128.
  45. Loison A, Gaillard JM, Pélabon C, Yoccoz NG. What factors shape sexual size dimorphism in ungulates?. Evolutionary Ecology Research 1, 611-629.
  46. Martin JG, Festa-Bianchet M. Bighorn ewes transfer the costs of reproduction to their lambs. The American Naturalist 176, 414-423.
  47. Medill SA. Sociality of Sable Island horses: Population, group and individual interactions (PhD thesis). University of Saskatchewan.
  48. Morton Boyd J. The Boreray sheep of St. Kilda, outer hebrides, Scotland: The natural history of a feral population. Biological Conservation 20, 215-227.
  49. Nuñez CM, Adelman JS, Rubenstein DI. Immunocontraception in wild horses (Equus caballus) extends reproductive cycling beyond the normal breeding season. PLoS ONE 5, e13635.
  50. Ojanguren AF, Magurran AE. Male harassment reduces short-term female fitness in guppies. Behaviour 144, 503-514.
  51. Owen-Smith N. Comparative mortality rates of male and female kudus: The costs of sexual size dimorphism. Journal of Animal Ecology 62, 428-440.
  52. Owen-Smith N, Mason DR. Comparative changes in adult vs. juvenile survival affecting population trends of African ungulates. Journal of Animal Ecology 74, 762-773.
  53. Plante Y, Vega-Pla JL, Lucas Z, Colling D, De March B, Buchanan F. Genetic diversity in a feral horse population from Sable Island, Canada. Journal of Heredity 98, 594-602.
  54. Promislow DE, Montgomerie R, Martin TE. Mortality costs of sexual dimorphism in birds. Proceedings of the Royal Society B: Biological Sciences 250, 143-150.
  55. Ransom JI, Powers JG, Garbe HM, Oehler MW, Nett TM, Baker DL. Behavior of feral horses in response to culling and GnRH immunocontraception. Applied Animal Behaviour Science 157, 81-92.
  56. Réale D, Boussès P, Chapuis JL. Female-biased mortality induced by male sexual harassment in a feral sheep population. Canadian Journal of Zoology 74, 1812-1818.
  57. Regan CE, Medill SA, Poissant J, McLoughlin PD. Data from: Causes and consequences of an unusually male-biased adult sex ratio in an unmanaged feral horse population. Dryad Digital Repository .
    doi: 10.5061/dryad.tht76hdxcgoogle scholar: lookup
  58. Richard E, Simpson SE, Medill SA, Mcloughlin PD. Interacting effects of age, density, and weather on survival and current reproduction for a large mammal. Ecology and Evolution 4, 3851-3860.
  59. Ruckstuhl KE, Manica A, MacColl AD, Pilkington JG, Clutton-Brock TH. The effects of castration, sex ratio and population density on social segregation and habitat use in Soay sheep. Behavioral Ecology and Sociobiology 59, 694-703.
  60. Ruckstuhl K, Neuhaus P. Sexual segregation in ungulates: A new approach. Behaviour 137, 361-377.
  61. Scorolli AL, Cazorla ACL. Demography of feral horses (Equus caballus): A long-term study in Tornquist Park, Argentina. Wildlife Research 37, 207-214.
  62. Silk JB, Alberts SC, Altmann J. Social bonds of female baboons enhance infant survival. Science 302, 1231-1234.
  63. Solberg EJ, Loison A, Ringsby TH, Saether BE, Heim M. Biased adult sex ratio can affect fecundity in primiparous moose Alces alces. Wildlife Biology 8, 117-128.
  64. Stearns SC. The evolution of life histories (Vol. 1). Oxford University Press.
  65. Székely T, Weissing FJ, Komdeur J. Adult sex ratio variation: Implications for breeding system evolution. Journal of Evolutionary Biology 27, 1500-1512.
  66. Toïgo C, Gaillard JM. Causes of sex-biased adult survival in ungulates: Sexual size dimorphism, mating tactic or environment harshness?. Oikos 101, 376-384.
  67. Torres R, Drummond H. Female-biased mortality in nestlings of a bird with size dimorphism. Journal of Animal Ecology 66, 859-865.
  68. van Beest FM, Uzal A, Vander Wal E, Laforge MP, Contasti AL, Colville D, Mcloughlin PD. Increasing density leads to generalization in both coarse-grained habitat selection and fine-grained resource selection in a large mammal. Journal of Animal Ecology 83, 147-156.
  69. Veran S, Beissinger SR. Demographic origins of skewed operational and adult sex ratios: Perturbation analyses of two-sex models. Ecology Letters 12, 129-143.
  70. Welsh D. Population, behavioural and grazing ecology of the horses of Sable Island (PhD thesis). Dalhousie University.
  71. Zhang SJ, Wang GD, Ma P, Zhang LL, Yin TT, Liu YH, Otecko NO, Wang M, Ma YP, Wang LU, Mao B, Savolainen P, Zhang YP. Genomic regions under selection in the feralization of the dingoes. Nature Communications 11, 1-10.
    doi: 10.1038/s41467-020-14515-6google scholar: lookup

Citations

This article has been cited 12 times.
  1. Medill SA, Janz DM, McLoughlin PD. Hair Cortisol Concentrations in Feral Horses and the Influence of Physiological and Social Factors. Animals (Basel) 2023 Jun 27;13(13).
    doi: 10.3390/ani13132133pubmed: 37443930google scholar: lookup
  2. Watson VT, Kehler D, Medeiros AS. A paleolimnological context of ecological vulnerability for the freshwater ecosystems of Sable Island National Park Reserve, Canada. Sci Prog 2022 Oct-Dec;105(4):368504221126865.
    doi: 10.1177/00368504221126865pubmed: 36254564google scholar: lookup
  3. Gavriliuc S, Reza S, Jeong C, Getachew F, McLoughlin PD, Poissant J. Targeted genome-wide SNP genotyping in feral horses using non-invasive fecal swabs. Conserv Genet Resour 2022;14(2):203-213.
    doi: 10.1007/s12686-022-01259-2pubmed: 35673611google scholar: lookup
  4. Gavriliuc S, Stothart MR, Henry A, Poissant J. Long-term storage of feces at -80 °C versus -20 °C is negligible for 16S rRNA amplicon profiling of the equine bacterial microbiome. PeerJ 2021;9:e10837.
    doi: 10.7717/peerj.10837pubmed: 33854827google scholar: lookup
  5. Pethig L, Ozgul A, Heistermann M, Fichtel C, Kappeler PM. Prenatal sex determination illuminates the unusual adult sex ratio of a group-living lemur. Biol Lett 2025 Feb;21(2):20240418.
    doi: 10.1098/rsbl.2024.0418pubmed: 39999891google scholar: lookup
  6. Ahn S, Redman EM, Gavriliuc S, Bellaw J, Gilleard JS, McLoughlin PD, Poissant J. Mixed strongyle parasite infections vary across host age and space in a population of feral horses. Parasitology 2024 Oct;151(12):1299-1316.
    doi: 10.1017/S0031182024001185pubmed: 39663810google scholar: lookup
  7. Crowther C, Adams CIM, Fondren A, Janzen FJ. Adult Sex-Ratio Bias Does Not Lead to Detectable Adaptive Offspring Sex Allocation Via Nest-Site Choice in a Turtle With Temperature-Dependent Sex Determination. Ecol Evol 2024 Nov;14(11):e70543.
    doi: 10.1002/ece3.70543pubmed: 39539677google scholar: lookup
  8. Dentinger JE, Dolan Todt LA, Schultz EA, Helferich JN, Demarais S, DeYoung RW, McKinley WT, Strickland BK, Boudreau MR. Testing secondary sex ratio bias hypotheses in white-tailed deer in Mississippi, USA. Ecol Evol 2024 Oct;14(10):e70296.
    doi: 10.1002/ece3.70296pubmed: 39398629google scholar: lookup
  9. Stothart MR, McLoughlin PD, Medill SA, Greuel RJ, Wilson AJ, Poissant J. Methanogenic patterns in the gut microbiome are associated with survival in a population of feral horses. Nat Commun 2024 Jul 22;15(1):6012.
    doi: 10.1038/s41467-024-49963-xpubmed: 39039075google scholar: lookup
  10. Driscoll RMH, Beaudry FEG, Cosgrove EJ, Bowman R, Fitzpatrick JW, Schoech SJ, Chen N. Allele frequency dynamics under sex-biased demography and sex-specific inheritance in a pedigreed jay population. Genetics 2024 Jul 8;227(3).
    doi: 10.1093/genetics/iyae075pubmed: 38722645google scholar: lookup
  11. Hagen RV, Scelza BA. Sex ratios and gender norms: why both are needed to understand sexual conflict in humans. Evol Hum Sci 2024;6:e10.
    doi: 10.1017/ehs.2024.3pubmed: 38414809google scholar: lookup
  12. Ram M, Sahu A, Srivastava N, Chaudhary R, Jhala L, Zala Y. The semi-arid ecosystem of Asiatic Lion Landscape in Saurashtra, Gujarat: Population density, biomass and conservation of nine wild prey species. PLoS One 2023;18(9):e0292048.
    doi: 10.1371/journal.pone.0292048pubmed: 37768920google scholar: lookup
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