Effects of spring acorn availability on black bear diet, milk composition, and cub survival

We investigated the role of changing abundance of spring foods on female American black bear (Ursus americanus) diet, milk composition, and cub survival in western Massachusetts. We hypothesized that diets would change, percentage milk fat would be higher, and cub survival would be higher in a year when overwintered hard mast was more abundant. We obtained paired samples of milk from 7 adult female bears across consecutive reproductive cycles during which spring diets differed; I year followed a bumper acorn (Quercus rubra) crop and estimated spring diets were > 25% acorns; the other year followed an extremely poor acorn crop and spring diets were estimated to be 99% skunk cabbage (Symplocarpus foetidus). Postdenning milk was higher in fat (26.7% versus 18.2%; P = 0.0557) during the spring when acorns were abundant, but we did not identify any carryover affect to Cub Survival. We suggest that adult female black bears compensate for changes in food availability by eating greater Volumes of alternative foods, and perhaps allocate resources primarily to milk production; the bears appear to be able to produce milk of adequate quality to sustain cubs, regardless of spring diet.