Foraging Effort and Life Span of Workers in a Social Insect

Abstract

SUMMARY (1) An inverse relationship between the effort an animal invests into current activities and its expected future life duration is important for life-history traits and, in particular, would provide a connection between foraging strategy and life-time fitness. Energetically low-cost foraging strategies such as those practised by nectar-collecting workers of the honeybee (Apis mellifera) are considered to be adaptive if foraging effort reduces life span (Schmid-Hempel et al. 1985). We here tested whether there is an inverse relationship between foraging activity and life span. (2) Workers of a colony were individually marked, placed at random into one of five treatment groups and their activities and life span observed. Treatments differed in the length of time individual bees were allowed to leave the hive to forage within an 8-h period each day (with schedules systematically altered from day to day). During the treatment period, individuals of the 0-h group could never leave the hive, those in other groups were allowed to exit the hive for 2, 4, 6 or 8 h (i.e. always). Sliding doors at the hive entrance were operated by an observer to ensure that only the appropriate individuals were kept back while others could forage freely. (3) There were no differences among groups in average adult life span (eclosion to last observation) (for individuals in the 0-h group: 41 6 + 2-0 S.E. days, N= 49; 2 h: 41-3 + 1.9, N=59; 4 h: 41.9+1 8, N=57; 6 h: 45.1 +22, N=46; 8-h: 39.0+2.3, N=49), despite differences in the number of foraging trips per individual/treatment period, and despite a threefold increase across treatment groups in the total number and duration of trips over the entire life span. In pairwise comparisons, average life span of workers in the 8 h group was shorter than those of workers in the 6 h group. (4) Within groups, however, there was a significant negative correlation between life span and number of trips per period, or time out of the hive per period, respectively. This relationship was largely due to hard-working individuals. We also found a significant positive correlation between the average amount of time a bee spent inactive in the hive, rather than being active, and its life span. We conclude that a reduction in the average work load below what bees accept under natural conditions (such as tested here) would not significantly lengthen life span of workers, but that life span might be reduced if bees increased their work load.