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

1. 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
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. 2. 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. 3. 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. 4. 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. 5. 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.