Animals grow smaller in hot versus cold climates, and scientists now say they know why.

Most cold-blooded organisms are affected by this 'temperature-size rule', where individuals of the same species reach a smaller adult size when they grow up in a warmer climate.

But the how and why of these size changes were discovered for the first time through a new research conducted by Dr Andrew Hirst and colleagues from Queen Mary's School of Biological and Chemical Sciences.

The study used data collected over 40 years on marine planktonic copepods, the main animal plankton existing in the world's oceans as important grazers of smaller plankton and a food source for many animal species.

According to the findings, growth rates, measured by how fast mass is accumulated and development rates, measured by how fast an individual passes through its life stages, in a wide range of species are completely distinct. The researchers found how fast an animal underwent development was more sensitive to temperature than the growth rate was.

We've shown that growth and development increase at different rates as temperatures warm. The consequences are that at warmer temperatures a species grows faster but matures even faster still, resulting in them achieving a smaller adult size, explains Hirst.

He added that the distinction between how fast an animal develops versus how fast an animal grows could have important consequences for individual species and ecosystems. 

The findings suggest that fundamental rates such as mortality, reproduction and feeding may not change in synch with one another in a constantly warming world, according to the researchers.

The study could help in understanding potential impacts on animals from climate change, scientists said.

The study was published in the journal The American Naturalist.