Species whose body sizes are smaller versions of their ancestors and relatives are fairly widespread in the animal kingdom. But extreme examples are unusual.
The benefits of being very tiny are quite hard to fathom.
A diminished body size typically results in simplified morphology, such as fewer digits on hands and feet, fewer skull bones, and some species produce fewer offspring than larger relatives.
So when does it help to be mini?
The advantages of evolving a much smaller body size are still debated by scientists.
“To be truly small leads to a number of constraints – biomechanical, physiological and ecological. So, it must be evolutionary advantageous to develop a small size. In many cases, it may be a way to avoid competitive pressures in the environment or to take advantage of an ecological niche,” says Felisa Smith from the University of New Mexico, Albuquerque, US, who has studied animal body size extensively.
Theories for the benefits to being smaller, she explains, include: increased heat dissipation as body size decreases; earlier reproduction within a species’ lifespan, and possibly greater resistance to extinction, as smaller animals tend to be more abundant.
It’s generally thought when resources in an environment are limited, some species decrease in size to reduce their total energy needs.
Interestingly, evolution of a smaller body size seems to happen most often in insular habitats, most notably on islands – known as “island dwarfism”.
The most famous examples of this phenomenon have occurred in large prehistoric mammals.
The smallest known mammoth, Mammuthus creticus, reached just over 1m (3.2ft) at shoulder height – the size of a modern baby elephant – and is thought to have roamed the island of Crete 3.5 million years ago. And mammoths of a similar size that once lived on the California Channel Islands would have been towered over by their mainland relatives which grew up to a comparatively dizzying 4m (13.1ft).
Remains of red deer that lived on Jersey during the last interglacial period (about 132,000-116,000 years ago) have provided intriguing insight into island dwarfism: the deer reduced to one sixth of their body weight in less than 6,000 years, according to a study published in the 1980s comparing dwarf deer remains to those of full-size red deer found on the island.
And in Madagascar, semi-fossilised remains of an ancient dwarf hippo called Hippopotamus madagascariensis suggest the extinct animal was just a third of the size of the living animal Hippopotamus amphibious.
“A common pattern of dwarfing of large-bodied animals is seen in insular environments. This is known as ‘Foster’s rule’ – or the ‘island rule’,” explains Smith.
Curiously, on islands, while some large mammal species become smaller (dwarfism), small mammals may evolve to be larger (giantism), due to selection pressures.
“Over evolutionary time, [the island rule] has driven the evolution of miniature dinosaurs, rhinos, elephants, mammoths and the like,” adds Smith.
Research suggests island dwarfism might even have occurred in an early species of human.
Homo floresiensis – nicknamed “the hobbit” – was a small brained species that stood at just 1m (3.2ft) tall and once inhabited the Indonesian island of Flores. Although its origins are much debated, some scientists argue the creature could have descended from larger early human species Homo erectus, and that it shrunk over time as it adapted to its environment.
These cases of shrinking large mammals on islands represent some of the most dramatic decreases in animal size known.
But fossils show us that “selection for smaller body size” has happened “lots” in many taxa, says Smith. “Often, this is related to temperature shifts in the environment.”
Although selection for a tinier body size is sometimes referred to as “miniaturisation”, this phenomenon in its true sense – where animals evolve a truly diminutive body size – is unusual, says Smith.
“I’d consider it an extreme form of selection for smaller size because the term implies you are dealing with the lower boundaries of what a size can be for a taxon.”
A classic example of miniaturisation, Smith adds, is the bumblebee bat (Craseonycteris thonglongyai). Found in Thailand and Myanmar, the cute mammal is thought to be the world’s smallest, weighing in at just 2g, and as its name suggests, is about the size of a bumblebee.
A British one pence coin is nearly twice the weight of the tiny mammal.
“These very small mammals must eat all the time and typically subsist on high energy food [such as] nectar [and] insects,” explains Smith.
Dr Douglas Kelt from the University of California, US, an expert in small mammals says: “While small size does result in reduced total energetic needs, it also increases mass-specific energetic needs.”
“Because mammals and birds are endothermic, smaller size means proportionally more surface area over which we lose heat, so smaller species have higher metabolic rates to compensate.”
To keep the body warm, smaller mammals need proportionally more energy rich food.
“Hence, miniaturisation is great in some respects, but typically is a poor approach for smaller species,” says Kelt.
Other miniature animals within the realms of the truly tiny include four dwarf leaf chameleon species found in northern Madagascar in 2012 which showed “striking cases of miniaturisation and microendenism”, according to the scientists who described the little lizards.
The tiniest of them is Brookesia micra, whose total length is less than 30mm (1.2 inches), and was found on the small islet Nosy Hara. The researchers postulate it could represent a case of “double” island dwarfism: the island of Madagascar may have driven the evolution of the group of small leaf chameleons B. minima, and then the islet Nosy Hara might be linked to the extreme miniaturisation found in the species B. micra.
Madagascar, the authors of the chameleon study point out, is known for some cases of gigantism – such as extinct giant lemurs and elephant birds – but less so for its extraordinary dwarf species.
But smaller still, the frog Paedophryne amauensis – whose average adult length of 7.7, (0.3 inches), is thought to take the title of world’s smallest vertebrate.
The frog was recorded on the island of New Guinea in 2012 by sharp-eyed scientists who found the elusive tiny amphibian in leaf litter. The team followed its unusual calls, which sound more like those emitted from insects than amphibians.
The discovery of P. amauensis, along with slightly larger species P. swiftorum during the same trip, confirmed Paedophryne as the most diminutive genus of frogs and toads in the world.
According to the researchers, extreme miniaturisation appears to have evolved independently at least 11 times in terrestrial frogs, with the greatest examples found in tropical wet forest leaf litter habitats. This is possibly because they are especially vulnerable to water loss due to their larger ratio of surface area to volume, and frogs need to stay moist.
The scientists point out the most recently discovered species could play an important role in the New Guinea wet forest ecosystem, hunting invertebrates even tinier than themselves and as prey for larger animals.
Smith points out that interestingly, the lower limit to body size in mammals at least, seems to be relatively constant over time.
“The big changes are in the upper limit to body size.” she explains, citing the giant mammals of the Cenozoic (the era that started 65.5 million years ago and continues to the present day), which are thought to have grown “to fill ecological niches subsequent to the dinosaur extinction”.
But while the largest beasts to have walked or swum the Earth may grab headlines and spark the imaginations of many, it seems we’re able to learn just as much about the evolution of our amazing world by inspecting "life in miniature" more closely.
More about:
