Altitudinal variation in body size and population density of Nicrophorus investigator (Coleoptera: Silphidae)

Increasing altitude may influence insect communities, population sizes, life-histories, and morphology. Nicrophorus investigator Zetterstedt, a holarctic species, occurs over a range of elevations in the western Rocky Mountains of North America. This study examines changes in population density, seasonal activity periods, morphology, and reproduction at three sites over an elevational gradient in the southern Rocky Mountains of Colorado. N. investigator population sizes varied yearly at both low (≃2,800 m) and high (≃3,200 m) elevations, but were always greater at the lower elevations. Adult activity at all sites began in late June, remained relatively constant throughout July, and declined by late August. Elytron length was highly correlated both with body mass and pronotum width. There was no difference in elytron length between males and females (within years sites). However elytron length was significantly greater in 5 out of 6 yr at the higher elevation site, and in 1999, at two additional low and high elevation sites in separate drainages. Beetles at the low and high elevation sites differed slightly in their reproductive strategies under captive conditions at the high elevation the number of larvae did not increase with carcass size thus larvae from larger carcasses weighed more than larvae reared on smaller carcasses. At the low elevation, brood size tended to increase with carcass mass, resulting in similar-sized larvae across all carcass masses. On average, brood growth efficiency (total brood grams/carcass grams) was higher at the higher elevation, perhaps explained by developing at lower temperature. These results indicate that both life-history strategies and developmental processes may be involved in the elevational variation seen in this species.