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Home / Equine Research Bank / J Anim Ecol / Causes and consequences of an unusually male-biased adult se...
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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  • Non-U.S. Gov't

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

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Citations

This article has been cited 4 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.
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