Overwinter mass loss of snowshoe hares in the Yukon: starvation, stress, adaptation or artefact?

Abstract

Summary 1.Overwinter mass loss can reduce energetic requirements in mammals (Dehnel's phenomenon). Alternatively, mass loss can result from food limitation or high predation risk. 2. We use data from fertilizer, food-supplementation and predator-exclusion experiments in the Yukon during a population cycle from 1986 to 1996 to test the causes of overwinter mass loss by snowshoe hares ( Lepus americanus ). In all years, some hares on control sites gained mass overwinter. During the increase phase the majority gained mass, but in all other phases the majority lost mass. 3. Snowshoe hares weighing < 1000 g in autumn always gained mass overwinter, as did the majority that weighed 1000 -1400 g. Hares weighing > 1800 g in autumn usually lost mass. 4. Snowshoe hares on the predator-exclosure + food site gained mass overwinter in all years. Hares on the food-supplementation sites lost mass during the decline but gained mass in all other phases. Fertilization had little effect on mass dynamics. 5. Snowshoe hares were more likely to lose mass during winters with low survival rates. Snowshoe hares on the predator-exclosure treatments were more likely to gain mass than were hares on control sites. 6. Overwinter mass loss was correlated with maximum snow depth. At equivalent snow depths, hares on food-supplemented areas lost 98 g ( ± 14·6 SE) less on average than hares on the controls and predator-exclosure treatment. 7. Bone-marrow fat was related to body mass and cause of death. Small hares had the lowest marrow fat. Hares killed by humans had higher marrow fat than those killed by predators; hares that simply died had the lowest marrow fat. Hares on food-supplemented sites had the highest kidney and marrow fat. 8. Overwinter-mass loss for snowshoe hares is explained interactively by winter conditions, food supply, predation risk and autumn mass. Some snowshoe hares lost mass overwinter in all years and on all treatments, suggesting that reducing body mass may facilitate survival, especially in cases where foraging costs are high energetically or increase predation risk.

Does handling reduce the winter body mass of the European hare?

“…For other hare species and for small mammal species, it has been shown that heavier individuals gained relatively less body mass and more often lost mass following handling or predator exposure, when compared to individuals with less initial body mass [4,9,19,20]. We expected similar responses following the handling of European hares.…”

“…As with other hare species [9], the studied population did not show clear trends of body mass loss over the winter. However, individual reaction on handling was related to the time of the first capture.…”

“…Heavier adults have a better chance of survival [9][10][11] and higher reproductive success in a given population [12]. On the other hand, a larger body size is more difficult to maintain [7,13,14], and the necessity for longer feeding increases predation exposure [2,[15][16][17].…”

Visitations by Snowshoe Hares (<em>Lepus americanus</em>) to and Possible Geophagy of Materials from an Iron-Rich Excavation in North-Central British Columbia

“…this time frame corresponds to periods when Snowshoe Hares have reduced access to high-quality forage and may be under increased nutritional stress (Hodges et al 2006). Murray et al (1998) speculated that in years of peak Snowshoe Hare population numbers, nutritional stress can be exacerbated by increased intestinal parasite loads.…”

The Effect of Environmental Storage Conditions on Bone Marrow Fat Determination in Three Species

“…NY State Conservationist 3:19-22). 1,[3][4][5] Despite numerous studies correlating bone marrow fat levels to clinical starvation in wildlife, there are currently no peerreviewed reports on the use of bone marrow fat for diagnosing starvation in domestic species.…”