Grizzly (brown) bears (Ursus arctos) differ vastly in body mass, with some individuals reaching up to ten times the size of their counterparts. Understandably, much of this variation is to do with the age and sex of individuals; generally, older bears are larger than younger ones and males are bigger than females. However, a recent study in BMC Ecology shows that these factors only account for about half of the differences in bear body size. The authors of the report suggest that an additional influence comes from the quality of the environment into which cubs are born.
Every winter, grizzly bears are forced to spend five long months fasting in their dens as heavy snowfall mounts up outside. For the remaining seven months of the year, the grizzlies prepare for hibernation by devouring an immense amount of food. Unlike their kin in the west coast of Canada and in Alaska, ‘interior’ bears along the Rocky Mountains cannot rely on spawning salmon in order to bulk up. Rather, they depend on plant material, particularly fruits, nuts, roots and tubers, for the majority of their diet. However, the amount of food that is available to individuals can vary enormously from year to year and between different locations.
If a female is able to gain sufficient calories, then she may give birth to several cubs while in hibernation. It is this period, the year before and of a cub’s birth, that is crucial for determining an adult bear’s body size. In particular, this study points towards a strong link with environmental conditions, especially snowfall and temperature. High winter snowfall is thought to provide additional insulation in the den for vulnerable new-borns. Warm temperatures in the late summer can lead to a profusion of flowers the following spring. This has a knock-on effect of producing a glut of fruit and berries in the autumn which cubs can feast upon, giving them a positive start to life. These cubs then are more likely to grow up to be larger than their peers, with subsequent benefits for survival and reproduction.
The study confirms that conditions during a mother’s pregnancy can be as important as current conditions for determining a bear’s body size. This is known as the ‘silver spoon’ effect and it is not just restricted to grizzly bears. It can also occur in other mammals, including polar bears, red squirrels, caribou and even ancient human societies. With longer summers more likely due to global warming, it is thought that even more grizzly bears might come to benefit from these silver spoon effects.
Reference: Nielsen SE, Cattet MR, Boulanger J, Cranston J, McDermid GJ, Shafer AB, & Stenhouse GB (2013). Environmental, biological and anthropogenic effects on grizzly bear body size: temporal and spatial considerations. BMC Ecology, 13 (1) PMID: 24010501
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