Heat shrink (Image: Image Quest 3d/NHPA)

Honey, we shrank the copepods. These tiny marine animals offer a clue to why a warming climate makes animals smaller.

It is well established that cold-blooded species get smaller as the climate heats up, says Andrew Hirst of Queen Mary, University of London. Experiments show that, on average, 1 °C of warming reduces their adult body mass by 2.5 per cent. The mystery is why.

To find out, Hirst pulled together data on 15 species of copepod that swim in the open sea, focusing on how they grew at different temperatures. As temperatures rose, the copepods got heavier faster. Hirst thinks that’s because physiological reactions accelerate at warmer temperatures, allowing the copepods to bulk up faster.


Growing up faster

But they also matured to adulthood faster, so their rapid growth ground to a halt at a young age. The overall effect was such that the warmer copepods wound up smaller.

It’s not clear why temperature has such a strong effect on the way these organisms mature, but Hirst suspects evolution favours organisms that are flexible in how fast they mature to adulthood. In a competitive environment, this increases the odds that individuals will reproduce before they are killed.

The idea that growth and maturation respond differently to temperature has been around for a while, but no one has tested it across a range of species before, says Tim Coulson of Imperial College London.

He says changes in the size of species could prove critical in that such changes will affect what food animals can eat and what can prey on them in turn. “If you start changing the copepods, you could cause all sorts of unpredictable knock-on effects in the ecosystem,” he says.

Journal reference: The American Naturalist, in press