AIM : In ectotherms, the colour of an individual’s cuticle may have important
thermoregulatory and protective consequences. In cool environments,
ectotherms should be darker, to maximize heat gain, and larger, to minimize
heat loss. Dark colours should also predominate under high UV-B conditions
because melanin offers protection. We test these predictions in ants
(Hymenoptera: Formicidae) across space and through time based on a new,
spatially and temporally explicit, global-scale combination of assemblage-level
and environmental data.
LOCATION : Africa, Australia and South America. METHODS : We sampled ant assemblages (n5274) along 14 elevational transects
on three continents. Individual assemblages ranged from 250 to 3000 m a.s.l.
(minimum to maximum range in summer temperature of 0.5–35 8C). We used
mixed-effects models to explain variation in assemblage cuticle lightness.
Explanatory variables were average assemblage body size, temperature and UVB
irradiation. Annual temporal changes in lightness were examined for a subset
of the data.
RESULTS : Assemblages with large average body sizes were darker in colour than
those with small body sizes. Assemblages became lighter in colour with
increasing temperature, but darkened again at the highest temperatures when there were high levels of UV-B. Through time, temperature and body size
explained variation in lightness. Both the spatial and temporal models
explained c. 50% of the variation in lightness.
MAIN CONCLUSIONS : Our results are consistent with the thermal melanism
hypothesis, and demonstrate the importance of considering body size and UVB
radiation exposure in explaining the colour of insect cuticle. Crucially, this
finding is at the assemblage level. Consequently, the relative abundances and
identities of ant species that are present in an assemblage can change in
accordance with environmental conditions over elevation, latitude and
relatively short time spans. These findings suggest that there are important
constraints on how ectotherm assemblages may be able to respond to rapidly
changing environmental conditions.