Body size and lean mass of brown bears across and within four diverse ecosystems

Abstract

Variation in body size across populations of brown bears (Ursus arctos) is largely a function of the availability and quality of nutritional resources while plasticity within populations reflects utilized niche width with implications for population resiliency. We assessed skull size, body length, and lean mass of adult female and male brown bears in four Alaskan study areas that differed in climate, primary food resources, population density, and harvest regime. Full body‐frame size, as evidenced by asymptotic skull size and body length, was achieved by 8–14 years of age across populations and sexes. Lean body mass of both sexes continued to increase throughout their life. Differences between populations existed for all morphological measures in both sexes, bears in ecosystems with abundant salmon were generally larger. Within all populations, broad variation was seen in body size measures of adults with females displaying roughly a 2‐fold difference in lean mass and males showing a 3‐ to 4‐fold difference. The high level of intraspecific variation seen across and within populations suggests the presence of multiple life‐history strategies and niche variation relative to resource partitioning, risk tolerance or aversion, and competition. Furthermore, this level of variation indicates broad potential to adapt to changes within a given ecosystem and across the species’ range.

Influence of maternal body size, condition, and age on recruitment of four brown bear populations

“…We located and anesthetized adult female brown bears (ࣙ8 yr of age) via helicopter darting using Telazol® (Fort Dodge Laboratories, Fort Dodge, Iowa, USA) during spring, according to Hilderbrand et al (2018). We recorded the presence and number, and visually estimated the age, of accompanying offspring at the time of capture.…”

“…At the time of capture, we weighed each female using an electronic load cell and measured the skull width (straight line distance between the widest portion of zygomatic arches), and skull length (straight-line distance from the upper incisors and occipital condoyle) using calipers, and measured body length (following dorsal body contour from the tip of the nose to the base of the tail with the bear in a sternal recumbent position) using a nylon tape measure (Hilderbrand et al 2018). We used the sum of skull width and skull length as an index of body size (hereafter referred to as skull size).…”

“…Understanding recruitment in brown bears (Ursus arctos) is challenging because of the variation across populations and individuals in age of first reproduction, litter size, weaning age, interval between litters (i.e., reproductive pause), partial loss of litters, and unknown fates of weaned individuals. Both maternal body size and condition have been shown to influence initial litter sizefemales with <19% body fat at den entry do not produce 8 email: ghilderbrand@usgs.gov offspring (López-Alfaro et al 2013), and body mass generally is positively related to the number of cubs produced (Hilderbrand et al 1999). Gonzalez et al (2012), however, found that maternal age was the primary determinant for cub litter size.…”

“…The Lake Clark study area included the Chigmit Mountains, subalpine tundra, spruce forest, and riparian areas (Mangipane et al 2017). The Katmai study area included a portion of the eastern Aleutian Range, coastal, intertidal, and island habitats (Hilderbrand et al 2018). The Kodiak study area was a portion of the Kodiak Archipelago and included forest, mountain, and riparian habitat (Deacy et al 2016).…”

“…These study areas differed in ecosystem drivers, especially the availability of salmon (Oncorhynchus spp. ); and thus, relative population density and productivity (litter size, age of first reproduction, interval between litters) is generally greatest in Kodiak and Katmai, intermediate in Lake Clark, and lowest in Gates (McLellan 1994;Hilderbrand et al 1999Hilderbrand et al , 2018Mowat and Heard 2006). In addition, hunting pressure varies with sport and subsistence hunting authorized on Kodiak, limited hunting in Gates and Lake Clark, and no legal hunting allowed in Katmai.…”