Breakfast of champions: Spatiotemporal variation in the quality of ant nests for bear consumers

Abstract

Black bears in the Colorado Rocky Mountains forage for western thatching ants as a food source. However, there is little knowledge about the nutritional benefit of ants for black bears, particularly with regard to changes in nutritional value during the ant life cycle. From late spring to mid-summer, thatching ants go through two separate stages of reproduction that greatly influence the composition of individual ant nests. In early summer, the ant colonies produce sexual ants, resulting in large-bodied immature (larvae and pupae) and adult ants (drones and queens) being present in the nest. During mid to late summer, ant colonies produce nonsexual ants, resulting in smaller immature and adult ants (workers). We conducted a study to measure how these nutritional values change from early to late summer, and how the varying phenology of different locations affects these values. We did this by simulating bear attacks on ant nests in two sites with differing phenology in early summer (June) and late summer (July) and collecting samples from the central brood chamber in each nest. We also wanted to see how a prior bear attack on a nest may affect its nutritional value later in the season, which we addressed by comparing nests that were attacked once (in July) vs. nests that were attacked twice (once in June, once in July). The samples were sorted into castes and life stages of ants (adult worker, pupae, queen, or drone) and weighed. These masses were used to approximate food mass per L of nest material for bears and converted into fat content to measure nutritional value. We found that there was significantly greater total mass and fat content in the ant nests in June compared to July. This was driven by large masses of sexual pupae found within the June nests. We found that there was no significant difference in the nutritional value of nests that were previously attacked vs. those that were not previously attacked. This lack of variation suggests that ant colonies can recover quickly from attacks made by bears, and that bears can revisit ant nests as a replenishable source of food. Altogether, these results indicate that seasonality in ant life cycles should be taken into account when evaluating the nutritional value of ants for bears.

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