Advantages of being a dark beetle – Insect colouration and thermoregulation

Thermal benefits of dark body colouration (melanism) are known for various groups of ectotherms like amphibians (alpine salamander Salamandra atra, A) and reptiles (melanistic morph of the adder Vipera berus, B).  By studying the body colouration and body size of ground beetles (like e.g. Carabus violaceus, C) on continental scale we highlight the potential thermoregulative importance of dark colouration for this hyperdiverse group of insects. Photos: Andreas Schweiger.

by Andreas Schweiger

Animal colouration and its reasons

The causes of animal colouration mesmerize scientists since centuries. Numerous mechanisms regarding the evolutionary reasons for animal colouration have been proposed including intra- and interspecific communication and signalling, predation avoidance as well as protection against harmful UV radiation and fungal/ bacterial pathogens.

Colouration and thermoregulation

For ectotherms, which comprise 99.9% of animal species on earth, colouration is furthermore proposed to have major implications for thermoregulation by affecting the energy balance of this group of animals which rely on external heat sources to sustain their metabolism. The heat balance of ectotherms strongly depends on the animal’s capability to conserve and/or gain heat and, thus, the animal’s flexibility in thermoregulation, an idea which is conceptualised in the heat balance hypothesis (Olalla-Tárraga and Rodríguez 2007).

Species’ thermal capacity which affects the energy budget of ectotherms is strongly affected by animal colouration. Especially melanism (development of dark coloured pigments in the cuticle), is stated to be an important trait for thermoregulation. The thermal benefit of melanism is based on the fact, that darker individuals heat up faster and reach higher body temperatures than light coloured ones due to the higher absorption of incoming solar radiation. This thermal advantage of dark body colouration, which was initially conceptualised in the thermal melanism hypothesis of Gates (1980), is reported for numerous ectotherms ranging from fossil sea reptiles to extant arthropods.

Species’ thermal capacity is furthermore strongly related to body size with bigger animals experience lower cooling rates (higher heat conservation) but also lower heating rates and (lower capability to gain heat). These size-dependent thermoregulatory mechanisms which are assumed to act non-linear with increasing body size are conceptualized in the size dependence hypothesis of Olalla-Tárraga et al. (2006).

A link between the thermoregulatory importance of ectotherms’ body size and dark colouration has been already proposed by Gates (1980) based on the theoretical assumption that the thermal benefit from dark body colouration might be a critical compensatory mechanism for the size-dependent thermoregulatory limitations. A positive relation between body size and dark colouration has been reported for individual species of various invertebrate taxa. However, evidence for the general relevance of a body size – colouration trade-off is still missing on large spatial and taxonomic scale.

Our study

To explore the generality of this body size – colouration trade-off we combined colour information with trait data and distributional as well as bioclimatic attributes for more than 1000 carabid species from the entire Western Palearctic. Carabids or ground beetles are the most species rich family of beetles worldwide and are famous for their very diverse and conspicuous colouration. We quantified species-specific body colouration from high-quality, standardised digital photographs from a picture database (www.eurocarabidae.de; Ortwin Bleich) using the Munsell colour system. Species-specific information for morphological traits including body size as well as ecological and distributional characteristics was taken from an extensive trait database for carabids (www.carabids.org; Katharina Homburg et al.).

In our study body size turned out to be the most importnant predictor of carabid  body colouration among all tested morphological, ecological and bioclimatic variables. This emphasises the trans-specific and large scale importance of the proposed body size – colouration trade-off. We detected a strong increase of colour darkness with body size from small to medium-sized carabids up to a body size threshold of 15 mm. This finding is consistent with the thermal melanism hypothesis. However, body size showed no effect on colour darkness above this threshold which is in accordance with previous ideas formulated in the size dependence hypothesis.

Effect of body size on body colouration in Western Palearctic carabids. Scatterplot depicts the relation (continuous piecewise regression and 0.95 confidence intervals) between mean body size and colour darkness. Spectra of colour (Hue, represented as different colours and their dominance) and colour lightness (value, red line, ranging from back: 0 to white: 1) are shown for selected species as smoothed density histograms. The B-index quantifies species ’ colourfulness. Photographic credit: Ortwin Bleich (www.eurocarabidae.de).

Why does it matter?

In this study we illustrate the interspecific relevance of body colouration for carabids as a cosmopolitan group of ectotherms by a showing a general trend towards darker body colouration with increasing body size. Our results show that a trade-off between body size and melanism seems to be of significant importance for carabids on a broad spatial scale and may be a general but still underestimated phenomenon for ectotherms in general.

Literature

Gates, D. M. 1980. Biophysical ecology. – Springer.

Olalla-Tárraga, M. A. and Rodríguez, M. A. 2007. Energy and interspecifi c body size patterns of amphibian faunas in Europe and North America: anurans follow Bergmann ’ s rule, urodeles its converse. – Global Ecol. Biogeogr. 16: 606 – 617.

Olalla-T á rraga, M. A. et al. 2006. Broad-scale patterns of body size in squamate reptiles of Europe and North America. – J. Biogeogr. 33: 781 – 793.

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