Seasonal patterns and age-specific variation in the surface activity of a population of desert scorpions in relation to environmental factors

TLDR: Activity patterns and certain life-history characteristics suggest that this species is a 'time-minimizer' and that both intra- and inter-specific predation significantly influence foraging patterns.

Abstract: SUMMARY Population density and surface activity of marked individuals of the desert scorpion, Paruroctonus mesaensis Stahnke (Vaejovidae) were monitored on over 225 nights during a 5 year period. (1) 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 midsummer. (2) 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. (3) Low surface densities of scorpions occur during and 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. (4) The surface density of adults was highly correlated with insect abundance. There are no correlations between the surface density of the two younger age groups and insect abundance. Further, adult surface density was high during the mating season and low during the mid-summer period of birth and brooding of newborn. (5) There exist age-specific differences in seasonal and nightly patterns of surface activity. These patterns tend to minimize co-occurrence on the surface with a subsequent decrease in intraspecific interactions (cannibalism and possible competition for food). It appears that adult activity is an evolutionary response to periods of high prey density, whereas the activity of the two younger age groups reflect long-term avoidance of adults and/or competition for food. Age-specific seasonal patterns are maintained by a differential response to surface temperature by each age group. (6) Low levels of overall surface activity are characteristic: only about one-half the individuals in any age group were present on the surface during an average night for any bimonthly period. These activity patterns and certain life-history characteristics suggest that this species is a 'time-minimizer' and that both intra- and inter-specific predation significantly influence foraging patterns.