Changes in selection of resources with reproductive state in a montane ungulate

Animals select habitats based on food, water, space, and cover Each of those components are essential to the ability of an individual to survive and reproduce in a particular habitat Selection of resources is linked to reproductive fitness and individuals likely vary in how they select resources relative to their reproductive state: during pregnancy, while provisioning young when nutritional needs of the mother are high, but offspring are vulnerable to predation, or if they lose young to mortality.

We investigated the effects of reproductive state on selection of resources by maternal female desert bighorn sheep (Ovis canadensis nelsoni) by comparing selection during the last trimester of gestation, following parturition when females were provisioning dependent young, and if the female lost an offspring. We captured, and recaptured each year, 32 female bighorn sheep at Lone Mountain, Nevada, during 2016–2018. Captured females were fit with GPS collars and those that were pregnant received vaginal implant transmitters.

We used a Bayesian approach to estimate differences in selection between females provisioning and not provisioning offspring, as well as the length of time it took for females with offspring to return levels of selection similar to that observed prior to parturition. Females that were not provisioning offspring selected areas with higher risk of predation, but greater nutritional resources than those that were provisioning dependent young. When females were provisioning young immediately following parturition, females selected areas that were safe from predators, but had lower nutritional resources.

Females displayed varying rates of return to selection strategies associated with access to nutritional resources as young grew and became more agile and less dependent on mothers We observed clear and substantial shifts in selection of resources associated with reproductive state, and females exhibited tradeoffs in favor of areas that were safer from predators when provisioning dependent young despite loss of nutritional resources to support lactation. As young grew and became less vulnerable to predators, females returned to levels of selection that provided access to nutritional resources to restore somatic reserves lost during lactation.

Authors

Marcus Blum

Kelley Steward

Kevin Shoemaker

Mike Cox

Cite

Brian Wakeling

Thomas Dilts

Joe Bennett

Vernon Bleich

We studied diet quality and forage selection by bighorn sheep before and following parturition to determine how nutritional demands associated with rearing offspring influenced forage acquisition. Selection of forage and habitats is driven by nutritional needs of individuals. Some species may sacrifice nutritional quality of forage for the mother in favor of safety of offspring (risk-averse strategy), immediately following parturition. We used desert bighorn sheep, Ovis canadensis nelsoni, to investigate that potential tradeoff.

We captured and radio-collared female bighorn sheep from 2016 to 2018. We used vaginal implant transmitters (VIT)s in pregnant females to identify parturition and to capture and radio-collar neonates to monitor survival of young. We collected fecal samples throughout the breeding season and throughout the year to understand diet quality and composition throughout those temporal periods. We determined diet quality and composition for pre-parturient females, females provisioning offspring, females that lost offspring, and nonpregnant individuals using fecal nitrogen and DNA metabarcoding analyses. Additionally, we compared the diet quality and composition of offspring and adult females during the spring, as well as summer and winter months.

Our results indicated differences in diet quality between individuals provisioning offspring and those whose offspring had died Females that were provisioning dependent young had lower quality diets than those that lost their offspring

Diet composition among those groups was also markedly different; females that had lost an offspring had a more diverse diet than did females with dependent young

Diet quality differed among seasons, wherein offspring and adult females had higher quality diets during the spring months, with decreasing quality as the year progressed. Diet diversity was similar across seasons, although spring months tended to be most diverse. Our results support tradeoffs associated with risk-averse strategies made by adult females associated with parturition. Nutritional quality of forage was linked to provisioning status, indicating that females were trading diet quality for safety of offspring, but those females whose offspring had died selected high quality forages. Those results help explain habitat selection observed in mountain ungulates around parturition and provide further insight into the evolutionary processes and adaptive significance exhibited by those specialized artiodactyls.

Authors

Marcus Blum

Kelley Stewart

Mike Cox

Kevin Shoemaker

Joe Bennett

Benjamin

Brian

Sullivan

Wakeling

Vernon

Bleich

Blum, M.E., K.M. Stewart, M. Cox, K.T. Shoemaker, J.R. Bennett, B.W. Sullivan, B.F. Wakeling, and V.C. Bleich. 2022. Variation in diet of desert bighorn sheep around parturition: tradeoffs associated with parturition. Frontiers in Ecology and Evolution, doi: 10.3389/fevo.2022.1071771.