Male group size, female distribution and changes in sexual segregation by Roosevelt elk

Abstract

Sexual segregation, or the differential use of space by males and females, is hypothesized to be a function of body size dimorphism. Sexual segregation can also manifest at small (social segregation) and large (habitat segregation) spatial scales for a variety of reasons. Furthermore, the connection between small- and large-scale sexual segregation has rarely been addressed. We studied a population of Roosevelt elk (Cervus elaphus roosevelti) across 21 years in north coastal California, USA, to assess small- and large-scale sexual segregation in winter. We hypothesized that male group size would associate with small-scale segregation and that a change in female distribution would associate with large-scale segregation. Variation in forage biomass might also be coupled to small and large-scale sexual segregation. Our findings were consistent with male group size associating with small-scale segregation and a change in female distribution associating with large-scale segregation. Females appeared to avoid large groups comprised of socially dominant males. Males appeared to occupy a habitat vacated by females because of a wider forage niche, greater tolerance to lethal risks, and, perhaps, to reduce encounters with other elk. Sexual segregation at both spatial scales was a poor predictor of forage biomass. Size dimorphism was coupled to change in sexual segregation at small and large spatial scales. Small scale segregation can seemingly manifest when all forage habitat is occupied by females and large scale segregation might happen when some forage habitat is not occupied by females.

Evaluating the summer landscapes of predation risk and forage quality for elk (Cervus canadensis)

“…This problem is further complicated by additional unmodeled sources of heterogeneity in resource selection patterns. Prior work has demonstrated that heterogeneity in movement patterns arises from differences among populations, sex, and ecological dynamics (e.g., variation induced by density‐dependent process or predation; Anderson et al, 2005 ; McCorquodale, 2003 ; McLoughlin et al, 2010 ; Peterson & Weckerly, 2017 ).…”

Influences on food supply from elk abundance and precipitation early in the growing season

“…The herds were habituated to people, and so all observations were carried out within 200 meters of elk using binoculars or the naked eye. We did not include adult males because they do not graze the Davison meadow complex to the same degree as female herds [35,36]. We used the highest count across the 10 surveys as our estimate of population abundance for each year.…”

When can cropping rate compensate for increased vigilance?

“…Coastal redwood (Sequoia sempervirens), Douglas-fir (Pseudotsuga menziesii), Sitka spruce (Picea sitchensis), and Western hemlock (Tsuga heterophylla) were among the most abundant tree species in the study area. Boyes meadow complex is 70 ha in size and Davison meadow complex lies approximately two km away and is 51 ha in size (Peterson & Weckerly, 2017). The meadow complexes had flat terrain and were dominated by annual and perennial grass species such as California oatgrass (Danthonia californica), redtop (Agrostis gigantean), soft chess (Bromus hordeaceus), and reed canary grass (Phalaris arundinacea) (Weckerly et al, 2001;Starns et al, 2015).…”

“…Bites taken were counted during the focal observation and cropping rate was (the number of bites taken)/(focal observation length). Sex, age class of the focal elk, and group type (male in a male-only group, females in female group, male in female group, and juveniles in female group) were recorded to capture any potential influences these factors might have on cropping rate (Townsend & Bailey, 1981;Weckerly, 2001;Peterson & Weckerly, 2017). Males were included in two group types to assess if their behaviours varied when grouped with females because males are socially dominant to females.…”

“…As the proportion of time spent vigilant while foraging increases, elk might strategically increase their cropping rate (bites per unit time), effectively increasing instantaneous intake rate during the foraging period and offsetting some of the cost of lost foraging time due to increased vigilance (Figure 1). Elk are a sexually dimorphic grazing species that adjust vigilance and foraging levels in response to social factors including group size and group sex composition (Peterson & Weckerly, 2017), predator presence (Laundre et al, 2001, and individual body condition (Laundre et al, 2001;Winnie & Creel, 2007). Elk dominance hierarchies dictate much of their social interactions; less dominant individuals such as females and juveniles increase vigilance in the presence of their dominant male counterparts, presumably to avoid potentially aggressive interactions (Weckerly et al, 2001).…”