ALTITUDINAL VARIATION FOR SENESCENCE IN MELANOPLUS GRASSHOPPERS

We describe differences in senescence among populations of grasshopper s that occur along an altitudinal gradient in the Sierra Nevada, Calif ornia. Experimental males from five populations of the Melanoplus sang uinipes/devastator sibling species complex were reared in each of two thermal culture conditions from eggs of field-caught females. In both culture conditions, we measured the survival of adult cohorts from eac h population. Differences in the physiological capacity to survive in a sheltered, common environment reveal genetic differences in underlyi ng rates of senescence, providing maternal effects do not affect the r ate of aging in offspring. At each temperature, mortality rates increa sed as a function of age, which indicates that senescence occurs under laboratory conditions. Within each culture condition, cohorts origina ting from low-elevation populations had greater survival than did coho rts from high elevations. Variation in body size along the altitudinal gradient did not account for the differences in survival. We suggest that high-elevation populations have evolved accelerated senescence as a direct or an indirect result of selection on reproductive schedules , which are potentially truncated by severe winter conditions at the e levated sites.