How body size and microclimate determine when solar absorptivity matters for insect body temperature

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Abstract
The net effect of solar absorption on body temperature is difficult to intuit because it depends on nonlinear interactions between the traits of organisms and their immediate environment. We used biophysical modelling to quantify solar absorption effects for Australian jewel beetles across their wide natural range of solar absorptivity (0.67-0.93), mass (more than 2 orders of magnitude) and shape under naturally co-occurring combinations of microclimatic conditions across Australia. Under hot, calm conditions, solar absorptivity could increase steady-state body temperature by 2.3°C on average. Higher wind reduced this effect by more than half, whereas increasing shortwave radiation amplified it. Transient simulations indicated substantial damping of these effects at a time scale of one minute. Across daylight hours during the spring–early summer period when adult jewel beetles are active, the absorptivity effect averaged 0.36°C across Australia. Under locally extreme high-radiation, low-wind conditions, the effect reached 5.69°C in the largest beetles and was more than twice as large at maximum as at minimum body mass. Whether solar absorptivity is of biological significance to heat budgets is therefore highly contextual. Experiments to infer the adaptive significance of insect colour for heat budgets should account for transient responses under realistic environmental combinations.

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