The effects of food supply and nutrition on black bear reproductive success and milk composition
I investigated the relationship between food abundance and black bear reproductive success. Black bear reproductive potential had been hypothesized to be related to natural food abundance in other studies but the effect had not been tested.
Pregnant females should select dens of maximum thermal efficiency to conserve body fat used for thermoregulation. If this were so they could allocate more depot fat to lactation. I examined data on natal den type (open nest, brushpile, covered) and subsequent survival of cubs in Massachusetts and Minnesota over 11 and 13 years, respectively. I could not detect any differences in cub survival but the power of tests was low (power = 0.139 to 0.258).
I could not predict either litter size or MFYS from environmental and harvest variables. When natural food abundance was low black bears used cornfields intensely and did not differ in body weight from high food years. Litter size and MFYS were related to litter order; first litters were smaller and had lower survival than subsequent litters. Sixty percent of Massachusetts females had their first litter at 3 years old. Twelve of 20 first litters were of single cubs and 10 of 12 first litters were totally lost.
I hypothesized that female black bears operated according to the life history theory of tradeoffs between present and future reproduction. Having cubs is not very costly to bears, raising them is. Thus, females had cubs at an early age (3 years old) but often could not find enough food in spring to both lactate and continue structural growth. Thus, first litters were usually lost.
I provide the first direct test of the hypothesis of the effect of diet quality on milk composition and cub survival in free-ranging bears. Milk composition differed between years of varying spring diet quality. Milk fat was lower when diet fat was low but MFYS was not different between the high fat and low fat years. I conclude that in Massachusetts, and likely most of eastern North America, natural food abundance has little effect on bear reproduction because bears can access alternate food sources.