Seasonal Patterns and Age-Specific Variation in the Surface Activity of a Population of Desert Scorpions in Relation to Environmental Factors

Abstract

Population density and surface activity of marked individuals of the desert scorpion, Paruroctonus mesaensis Staknke (Vaejovidae) were monitored on over 225 nights during 5 yr. Although scorpions appeared on the surface during all bimonthly periods of the year, surface densities were highest in spring and fall, and lowest in mid-winter and mid-summer. Emergence from burrows at dusk is correlated with decreasing levels of ambient light. Surface density rapidly increases in early evening and then significantly decreases in late evening (early morning). Adults are relatively more active and young relatively less active in early evening as compared with late evening. Low surface densities of scorpions occur during an immediately after periods of precipitation. Although moonlight does not affect surface density, both mating and feeding occur significantly less frequently during periods of full moon. The surface density of adults was highly correlated with insect abundance. Adult surface density was high during the mating season and low during the mid-summer period of birth and brooding of newborn. Age-specific differences in seasonal and nightly patterns of surface activity were discussed. Adult activity may be an evolutionary response to periods of high prey density, whereas the activity of the 2 younger age groups reflect long-term avoidance of adult and/or competition for food. Age-specific seasonal patterns are maintained by a differential response to surface temperature by each group. Low levels of overall surface activity are characteristic: only about 1/2 the individuals in any age group were present on the surface during an average night for any bimonthly period. This species may be a time-minimizer. Both intra- and inter-specific predation significantly influence foraging patterns.