As climate warms, some animals shrink, others grow

While evolution tends to make creatures larger over time — because bigger animals have the edge when it comes to survival and reproduction — climate change is messing with that trend in some cases. Around the globe, scientists are finding that some species are growing smaller thanks to a warming climate, while other animals are in fact becoming larger.

So which creatures are shrinking? They include:

Sheep

The wild Soay sheep on the Scottish island of Hirta have become measurably smaller — by an average of 5 per cent — over the past 24 years. Researchers believe it’s because the winters there have grown more mild, making it more likely that smaller and slower-growing sheep will survive and go on to reproduce. “In the past, only the big, healthy sheep and large lambs that had piled on weight in their first summer could survive the harsh winters on Hirta,” said lead researcher Tim Coulson. “But now, due to climate change, grass for food is available for more months of the year, and survival conditions are not so challenging — even the slower growing sheep have a chance of making it, and this means smaller individuals are becoming increasingly prevalent in the population.”

Fish, algae, plankton and bacteria

Scientists at the Leibniz Institute of Marine Sciences have confirmed that marine species tend to become smaller as the climate warms. They’ve found evidence of that trend among bacteria, plankton-algae, zooplankton and fish in the North Sea and the Baltic Sea, as well as rivers across France. “This development affects the functioning of the whole eco-system: The body size is decisive for what animals can eat and by whom they are eaten,” said researcher Ulrich Sommer. “A shift to smaller species and individuals within the fish population could lead to a reduction of zooplankton because small fish will eat less fish and more zooplankton. This could pave the way fvor massive and unpleasant algal blooms. Furthermore, with smaller fish the economic value of fishery declines.”

Birds

Comparing modern birds with museum specimens from the past, Australian researchers have found that at least eight species are now smaller — by up to 4 per cent — than they once were. Smaller birds have an advantage in warmer climes, as they can dissipate excess body heat more quickly than larger birds.

While a warmer climate is driving some species to shrink, it’s showing the opposite effect for other animals such as:

Rodents

Human population density and climate change appear to be working together to cause some types of wild mice to increase in size, according to research from the University of Illinois at Chicago. Again, by comparing modern specimens with those from museums, ecologist Oliver Pergams found that some physical traits among mice have changed by as much as 50 per cent over the past 80 years.

Jellyfish

In oceans around the world, researchers are finding that jellyfish are taking advantage of a changing environment that benefits them over fish. Overfishing, for example, reduces the number of fish around to eat jellyfish or compete with them for food, so jellyfish numbers can grow. Jellyfish can also thrive in ocean dead zones where fish and other marine species can’t. Off the coast of Japan, scientists are finding increased populations of such jellyfish as the Echizen, which can grow to five feet across.

Snakes

While research isn’t yet showing that today’s snakes are growing larger because of climate change, scientists are warning that a warmer climate could cause Burmese pythons — an invasive species in south Florida — to expand their range into areas that were previously too cool for them. And, earlier this year, researchers from the University of Toronto caused a stir when they announced they’d found the fossils of a monstrous snake that lived some 60 million years ago, when the climate was warmer than today’s. The boa constrictor-like snake, which roamed the region that is now Colombia, weighed more than 1.25 tonnes and measured more than 13 metres in length.