Fragmented habitat and climate warming could put large animals at a disadvantage because of their slower travelling speeds. (Photo by Soeren Stache/picture alliance via Getty Images)
Whether an animal is flying, running or swimming, its travelling speed is limited by how effectively it sheds the excess heat generated by its muscles, a new study has found.
An animal’s capacity to travel is a crucial part of its survival and dictates where — and how far — it can migrate, find food and mates and spread into new territories.
According to the researchers, this becomes even more problematic in a human-dominated world, which is characterised by increasingly fragmented habitats and limited food and water resources under climate change.
The study, published recently in PLOS Biology, was led by Alexander Dyer from the German Centre for Integrative Biodiversity Research and the Friedrich Schiller University Jena. Dyer is doing his PhD in Germany, but is originally from South Africa and studied at the University of Cape Town and Rhodes University.
He and his colleagues developed a model to look at the relationship between animal size and travelling speed, using data from 532 species, ranging in size from the fruit fly to the blue whale.
Some of the key findings of the study are that some large animals may struggle to meet dietary and food demands because of issues like habitat fragmentation in a warming world. Larger animals must also balance their muscles to not overheat when they travel.
Medium sized animals are the fastest
Although larger animals should be able to travel faster because of their longer wings, legs or tails, they found that it is particularly the medium-sized animals that typically have the fastest sustained speeds.
They attribute this to the fact that as larger animals require more time to dissipate the heat that their muscles produce while moving, they have to travel more slowly to avoid overheating. Any animal’s travelling speed can be jointly explained by how efficiently it uses energy and sheds heat.
“We found that the travel speeds that flying, running, and swimming animals can sustain over longer distances are related to their body size through a hump-shaped relationship, in which medium-sized animals typically sustain the highest travel speeds while the largest animals travel more slowly,” Dyer told the Mail & Guardian.
The researchers developed a biophysical model and discovered a mechanism to explain why the largest animals travel more slowly. He said that for animals to maintain stable body temperatures while travelling over long distances, the heat that their muscles produce needs to be balanced by the heat their bodies lose to the environment.
“Because of their size, large animals lose heat from their bodies more slowly than small animals. This means that they need to allocate more time — time where their muscles are not contracting — to avoid overheating while they travel.”
The only solution for the largest animals is to travel more slowly. Dyer said that while the travel speeds of running and swimming animals “saturate” between 1 000kg and 10 000kg — such as elephants and killer whales — the travel speeds of flying animals saturate much earlier, between 100 grams and 1kg, after which they continue to decrease with increasing body size.
“We can attribute these differences to the fact that flying animals such as birds and bats sustain much higher metabolic rates, and therefore, produce more metabolic heat while travelling than running and swimming animals of similar size.
“They therefore start to travel more slowly at a smaller body size. Ultimately, by jointly considering how efficiently animals use energy and shed heat, we can use their body size to predict the speeds that they can realistically travel at over longer distances.”
The study, according to Dyer, combines “our existing understanding of animal locomotion and thermoregulation” into a simple-to-understand framework. This can provide researchers and practitioners with more realistic estimates of how fast and how far animals can realistically travel, even when the details of their individual biologies are poorly understood.
“This information could be used, for instance, to predict which animals might be capable of moving between essential habitats that have been fragmented by deforestation and agricultural cultivation and to evaluate whether protected areas are sufficiently connected to each other.”
Dyer said Africa is home to many of the world’s largest flying, running and swimming animals. Large animals have higher total energy demands than small animals and therefore require larger areas of suitable habitat.
“Our model predicts that animals that weigh much more than 1 000kg (running and swimming animals) or 1kg (flying animals) are likely to experience an increasingly limited capacity to effectively meet their dietary demands by efficiently locating patchily distributed resources such as food and water.
“Because of their limited travel speeds, we also expect that large animals will be potentially more susceptible to the effects of habitat fragmentation in a warming world than previously thought and therefore at a higher risk of extinction; however, this still needs to be tested,” he said.