Ant assemblages have darker and larger members in cold environments

Abstract

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 (n 5 274) 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 UV-B 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 UV-B 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.

Not simply red: Colouration of red wood ant Formica rufa (Hymenoptera: Formicidae) is polymorphic, modular and size-dependent

“…It is possible, there is a trade-off in the investment of resources in an individual in size or melanization, because melanins are costly to produce and require proteins (Blois, 1978). Recently, it was discovered that at the level of assemblages ants with dark cuticles are larger than those with pale cuticles (Bishop et al, 2016). Finally, opposite trends in the colouration of communities and species and size may refl ect different selection pressures and functional differences in colouration in each particular case.…”

Dark butterflies camouflaged from predation in dark tropical forest understories

“…Morphology is one of the key functional traits that determines the performance and fitness of an organism, and therefore may affect distribution and diversity patterns (Kingsolver & Huey, 2008;Bishop et al, 2016). Among insects, morphological functions such as mimicry, camouflage and thermoregulation are used to adapt to variable environments, which present multiple selection pressures via biotic (e.g.…”

“…Being able to heat up and reach optimum body temperatures quickly could increase mobility and minimise predation risk for insects in cool habitats (Lederhouse et al, 1987). Consequently thermoregulatory advantages are frequently proposed to explain organisms exhibiting different luminance patterns along thermal gradients (Zeuss et al, 2014;Bishop et al, 2016;Xing et al, 2016). These advantages could lead to evolutionary adaptations through which individuals found in cooler closed habitats are generally darker, and those found in hotter open habitats are lighter in colour.…”

Anthropogenic waste products as preferred nest sites for Myrmica rubra (L.) (Hymenoptera, Formicidae)

“…Apart from the size of colonies, the ants can react to the environmental factors such as temperature (Bishop et al 2016) or latitude (connected with temperature) (Heinze et al 2003) also by changing their morphology, including their body size. Taking this into account, it is good to investigate whether different density and colony structure may affect ant size.…”