MV Blog


Maximum rate of mammal evolution

by Kate C
Publish date
30 January 2012
Comments (2)

"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.

Elephant and rabbit 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

Evolving the biggest mouth in history

by Kate C
Publish date
17 August 2011
Comments (1)

Imagine that your face was articulated so that your jaw could split down the middle and expand sideways until the tips were out as wide as your ears. Imagine that you could move all the bones of your face... not just the soft tissue, but the bones themselves.

Sound bizarre? Alien, even? Yet this is exactly what happens every time a Blue Whale takes a gulp of water. The filter-feeding whales, otherwise known as baleen whales or mysticetes, have feeding adaptations that are unique among mammals. Their intriguing evolutionary history is the subject of Dr Erich Fitzgerald's research, and today he's published a paper that overturns a long-held belief about how the baleen whales evolved.

Blue Whale Illustration of the biggest mouth in history at work. The Blue Whale can expand its mouth to gulp huge volumes of krill-filled water.
Image: Carl Buell
Source: Museum Victoria

For several years, he has worked on an extraordinary 25 million-year-old species known from fossils that were found in the 1990s near Jan Juc on Victoria's west coast. Called Janjucetus, this early baleen whale predated the evolution of baleen – the hairy structure used by modern baleen whales to filter tiny crustaceans from the sea. Instead, Janjucetus had the large eyes and ferocious teeth of a hunter.

Erich Fitzgerald with Janjucetus Dr Erich Fitzgerald holding the jaws of Janjucetus with Melbourne Museum's massive Blue Whale skeleton in the background.
Image: Jon Augier
Source: Museum Victoria

There are two key changes in the skull that permit the filter feeding of modern whales. The first is a lower jaw that can split down the middle. In humans, the seam (or symphysis) where the two halves of the jawbone meet at our chin is fused, thus our jaws are rigid. In contrast, baleen whales have greatly elongated jawbones that do not meet in the middle. The second change is in the width of the upper jaw; baleen whales have evolved a wide mouth, allowing them to engulf massive volumes of water.

"Previously it was thought that the origins of both features were intimately linked to filter-feeding and that's what differentiated baleen whales from toothed whales and dolphins," explains Erich. His research has just overturned this theory since Janjucetus had a wide upper jaw yet its lower jaw had a tightly connected, immobile symphysis. "So, the loose symphysis is not typical of all baleen whales, it's a later innovation. The earliest baleen whales could not expand and contract their lower jaws so were anatomically incapable of filter-feeding, yet they had these wide upper jaws."

Jaws of Janjucetus The fossilised jaws of Janjucetus, clearly showing the immobile symphysis at the tip.
Image: Jon Augier
Source: Museum Victoria

What Erich describes is an elegant example of an exaptation, where a feature evolved to serve a particular function but was later co-opted into a new role. Erich believes that its wide jaw helped Janjucetus to suck in large singe prey items, such as squid or fish, and didn't evolve for filter-feeding at all.

Says Erich, "Charles Darwin reflected upon this in The Origin of Species. He wondered how you could go from a whale that has big teeth like Janjucetus does and catching fish and squid one at a time, to something like a modern Blue Whale that feeds en masse. This is the kind of fossil palaeontologists dream of finding because it shows a transitional form."

"It's an exciting discovery, but actually not as surprising as you might think," concludes Erich. "Evolution by natural selection implies that we should expect to find these kinds of fossils in the rocks." The next question he looks forward to answering is how whales shifted from suction feeding to filter-feeding. "I think we're really close to finding a transitional series of fossils that illuminate this."

Erich's paper about this discovery, 'Archaeocete-like jaws in a baleen whale', is published today in Biology Letters.


Video: Erich discusses whale evolution

MV News: Ferocious fossil

Dr Erich Fitzgerald

Baleen and toothed whales

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