IMPLICATIONS OF SENESCENCE PATTERNS FOR THE EVOLUTION OF AGE POLYETHISM IN EUSOCIAL INSECTS

Division of labor based on age, with workers delaying performance of risky tasks, evolved in all major lineages of eusocial insects, raising questions about possible selective advantages of this pattern. We present a model showing that patterns of senescence in workers can have profound implications for the evolution of age polyethism in insect societies. Controversy exists over the relative importance of programmed versus rate of living senescence in determining the longevity of animals. If programmed senescence occurs in eusocial insects, workers that delay performance of risky tasks until later in life have greater expected longevity and therefore perform more labor per unit production cost. If senescence does not occur or if the age of senescent death is determined solely by rate of living effects, then worker longevity is not increased by delayed performance of risky tasks. Several lines of indirect evidence suggest that high rates of work have negative effects on worker longevity, supporting the rate of living hypothesis. Critical empirical data on patterns of senescence in eusocial insect workers are lacking, however, particularly with respect to the occurrence of programmed senescence. Furthermore, programmed senescence may operate in conjunction with rate of living effects. In this case, a negative relationship between work rate and longevity will be evident in highly active workers, while longevity of inactive workers will reach an upper plateau and will not be related to differences in work rate. If programmed senescence is demonstrated in eusocial insect workers, then increased worker longevity may have favored the evolution of age polyethism with delayed performance of risky tasks.