Curtis R. Horne, Andrew. G. Hirst, and David Atkinson
Body size affects almost all aspects of life – from the speed at which organisms grow, breed and die, to how energy passes between different-sized organisms in food webs, such as from prey to predators. For this reason, variation in body size has fascinated biologists for over a century, leading to the formation of prominent ecological ‘rules’.
The ‘temperature-size’ rule applies to many ‘cold-blooded’ (ectothermic) species, from bacteria to insects to fish: according to this rule, species reared under controlled laboratory conditions frequently grow to a smaller adult size in the warm than in the cold. Similarly, individuals of the same species living nearer the equator tend to be smaller than those towards the poles.
Recent work demonstrated that animal groups, including insects, displaying some of the strongest reductions in size with warming in the laboratory are also those that show the strongest reductions in size towards the equator. These findings suggest that temperature plays a particularly important role in explaining size variation in nature.
Temperature also decreases with increasing elevation, but it remains unknown whether size variation is equally predictable across altitude, and whether this closely resembles body size responses to temperature measured in the laboratory.
We explored how body size changed with altitude in 121 insect species from 50 global locations – from butterflies in Costa Rica, to grasshoppers in the Swiss Alps, to bees at Mt. Kilimanjaro. We find widespread variation in the extent to which body size changed, and whether size increased or decreased, with altitude.
Unlike major gradients in adult body size with latitude and season, which are generally consistent with the temperature-size rule observed under controlled conditions, few clear trends in size emerge with altitude. Many environmental factors interact across elevations, such as season length and oxygen availability, likely obscuring size differences between localities. The movement of individuals between different altitudes may also result in population mixing and less local adaptation, further obscuring size differences.
With many species retreating to colder higher elevations with climate warming, our work also highlights the complexities associated with how altitude impacts the body size of species.
Horne et al. investigate changes in insect body size with altitude. Pictured: Hollyhock weevils, Rhopalapion longirostre. Photo credit: Curtis R. Horne