"How fast can a mammal evolve from the size of a mouse to the size of an elephant?" This question introduces a new paper published today by a group of international researchers led by Alistair Evans of Monash University, including Dr Erich Fitzgerald, Senior Curator of Vertebrate Palaeontology at MV.
The world's largest mammal by weight, the Blue Whale, is about 61 million times heavier than the world's smallest, the Etruscan Shrew. Erich and his colleagues are interested in how such a range of body sizes evolved within the mammals, particularly the rate at which such evolution occurs.
Previous investigators have calculated rates of evolution using narrowly-defined parameters, whether within a shorter time scale or within a limited taxonomic group. This study is the first to tackle the larger picture, using data from a variety of species that lived over the last 70 million years.
The researchers found that it takes a minimum of 1.6 million generations for terrestrial mammals to increase their mass 100-fold. To increase by 5,000-fold, it takes at least 10 million generations.
In contrast, the researchers found that land mammals can decrease in size more than ten times faster than the time it takes to increase to the same degree. Hypothetically, it could take 5 million generations for a species to evolve from rabbit size to elephant size, whereas in just half a million generations it could shrink back down again if selective pressures directed it thus. Smaller body mass gives a competitive advantage under certain conditions; this phenomenon, known as insular dwarfism, is seen in the now-extinct dwarf elephants that were stranded on Mediterranean islands by rising sea levels.
Left: Children riding on Queenie, an Indian Elephant, at Melbourne Zoo in 1917 (MM 004061). Right: Rabbit, Oryctolagus cuniculus.
Image: Unknown | Alex J.
Source: Museum Victoria | Used under CC BY 2.0 from a_jo.
Interestingly, aquatic mammals such as whales evolved large body mass much faster than land mammals, taking about half as many generations to achieve the same scale of increase.
Says Erich, "Whales can get bigger because the water supports their bodies and so their maximum size is not limited by gravity." He explains that a huge body can also be an advantage for aquatic mammals because it loses less heat.
"There doesn't seem to be any slowing-down in evolution of maximum body size in whales. Land mammals may have reached a plateau enforced by gravity, but it's conceivable that the Blue Whale is not the largest possible whale. Nevertheless, energetic demands of feeding a body larger than that of a blue whale may mean that, in reality, the blue whale is as large as animals get."
Large land-dwelling mammals have a variety of solutions to the problem of gravity, explains Erich. "Some of the changes we see are extreme thickening of bones, changes in locomotion and major changes to organ systems." A gigantic rabbit wouldn't just be a large version of today's feral bunny; in fact, it would probably be unrecognisable as a rabbit. Fossils of an extinct giant rabbit described in 2011 show that it had a stiff spine to support its bulk, which meant it would not have been able to hop. Accordingly, we might need to rethink the way we portray the Easter Bunny.
Evans, A.R. et al. The maximum rate of mammal evolution. Proceedings of the National Academy of Sciences, published ahead of print on January 30, 2012.
Speed limits on the evolution of enormousness (Wired Science)
Science reveals the secrets of super-sized mammals (The Age)
Dr Erich Fitzgerald