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Read more: “Minimals: Meet the smallest critters of all“

WALKING back to camp as night fell in Papua New Guinea, Christopher Austin heard coming from the forest floor. He and his companions were there to hunt for new species, so they started searching through the leaf litter.

“But we didn’t find anything,” says Austin. “We repeatedly did that during the night and weren’t able to find out what was making the call. So we ended up just grabbing a whole handful of leaf litter and throwing it into a plastic bag.”

Back in camp, with the benefit of better lighting, they slowly went through their haul. It soon became clear why they hadn’t been able to find anything out in the rainforest. The creatures that were making the noise were just 7.7 millimetres long. Yet they were not insects but fully grown frogs. “It was obvious that they were adult male frogs, as they were calling to attract a mate,” says Austin.

This minute amphibian, found in 2009, is the latest in a string of miniature vertebrates discovered in the past few years. The discoveries have sparked something of a race to find the world’s smallest – and although they had no idea at the time, is a leading contender for that title. It could well be smaller than the previous record holder, a tiny freshwater carp from Sumatra in Indonesia described in 2006.

So are there even smaller vertebrates out there waiting to be found? What are the limits on how small a vertebrate can get? And what made these animals so small in the first place?

Many ordinary-sized species, from monkeys to deer, are still being discovered each year, so it is perhaps not surprising that the most diminutive vertebrates have eluded us for so long. The recent rash of discoveries might be partly due to growing awareness of their existence, and partly to new tools. “I used my digital camera as an impromptu microscope, allowing me to quickly recognise this frog as a distinct species,” says , who is based at Louisiana State University in Baton Rouge.

Back in the lab, DNA analysis often reveals that very similar looking animals are separate species. And we know the smallest fish were slipping through the net; when a couple of ichthyologists from Singapore happened to use a finer mesh, they started catching miniature fish. Other collectors soon followed suit.

Whatever the reasons, the discoveries are coming in thick and fast. In most groups, from fish and frogs to lizards and snakes, biologists have now found examples of extreme miniaturisation.

Different factors may have driven the evolution of different kinds of animals. Many of the tiny fish live in swamps, for instance, and it is thought their size enables them to survive in small pools during dry periods.

In the case of the frogs, they may have evolved simply because there was a niche available for a tiny predator. “We think one of the major driving factors in the evolution of small body size in these frogs is an abundance of really, really small prey, like mites in the leaf litter that aren’t being preyed on heavily by anything else,” says Austin. “That food resource, that guild, that ecological niche is something that has caused the independent miniaturisation of frogs throughout the world.”

This may help explain why most miniature vertebrates are found on islands rather than on continents. “Not everything makes it out to an island,” says Blair Hedges, an evolutionary biologist at Penn State University, whose work in the Caribbean has resulted in the discovery of some of the smallest vertebrates there are. “That means there are open niches and the species that do make it out can expand their ecological space a bit more than they normally would on the continent,” he says. “Sometimes that means being really small.”

Missing bones

The simplest way to become small is to stop growing earlier. The tiniest animals go a step further: they often stop developing early too, meaning some adult features never form. Mini frogs, for instance, have a somewhat simplified skeleton, probably because bones that appear late in development in larger species never form. “With really small frogs you often get a reduction in the number of digits on the hands and feet,” says Christopher Raxworthy, a herpetologist at the American Museum of Natural History in New York, who has helped discover several species of diminutive frogs in Madagascar.

Some of these frogs also bypass one or more steps during early development. Many are “direct developers”, that’s to say, they skip the tadpole stage entirely and hop straight out of the egg. This may be because they live in environments where there isn’t a lot of standing water and the aquatic stage is very dangerous, especially for animals that produce few offspring.

The story is similar for described in 2006. They look somewhat like fish larvae, but features such as the presence of eggs in females show they are indeed fully grown adults. When , an ichthyologist at the Natural History Museum in London, studied their skeletons, he found that compared with close relatives . Most of them are bones that normally appear late on in development, making the fish another example of so-called developmental truncation.

However, midget fish aren’t just larvae whose development has frozen. Many have also evolved unique features, such as the male mini-carp’s bizarrely modified pelvic fins, which have been transformed into two drumstick-shaped appendages with various hooks and flanges attached. “When I saw them I couldn’t really believe my eyes,” says Britz. They are unlike anything found in any of the other 3000 or so members of the carp family.

“Miniature fish aren’t just larvae whose development has frozen. Many have also evolved unique features”

In rare footage of the mating ritual of these fish captured by an amateur aquarist, the male actually seems to use these appendages like drumsticks, flipping upside down to rap out a rhythm on the underside of a floating leaf. This may play a role in attracting the female, says Britz, but nobody has yet looked at their precise function.

Britz thinks the fishes’ truncated development opened up new evolutionary possibilities. “Becoming larval has given them more freedom developmentally to do things with their skeleton that others cannot do,” he says. It’s like building a house. “The more floors you have the less freedom you have to put something on top, but if you go all the way back to the foundation stage then you can do whatever you want.”

There is a limit to how much a body can shrink, though. Many of the constraints derive from the fact that for any given shape, the ratio of surface area to volume increases with decreasing size. This is a major issue for warm-blooded birds and mammals: the smaller they get, the faster they lose heat, so they have to generate heat faster to compensate. Minute birds and mammals push their metabolism to the absolute limits. “The classic evidence of this size limitation is in the smallest shrews, where they are constantly eating to renew the energy that is being rapidly lost through their skin,” says Hedges.

This is why the smallest recorded bird, the 30-millimetre-long bee hummingbird of Cuba, and the smallest known mammal, the 40-mm Etruscan pygmy shrew found across Europe, north Africa and south-east Asia, are much larger than the smallest known reptile, a dwarf gecko from the Caribbean that measures just 14 mm from snout to anus. An impossibly cute chameleon from Madagascar was hailed as a contender for the title of tiniest reptile earlier this year, but Hedges says it is a fraction longer than the gecko, which he and a colleague discovered.

While heat loss isn’t an issue for cold-blooded creatures, water loss is. This is a special problem for amphibians, says Raxworthy. “If a really tiny frog gets out into dry air it could dry out in a matter of minutes,” he says.

This was something Austin was careful to avoid when testing the jumping abilities of his pint-size Papua New Guinean frogs. “With this incredibly porous skin and this really large surface area to volume ratio, it makes desiccation a real issue.” This is probably why the frogs stay in moist leaf litter.

Fish would appear to have things easier. Being cold-blooded and aquatic, heat loss and desiccation are not a problem. There are other constraints that kick in at such sizes, though. Losing a few bones here or there might not make much difference to a very small animal, but all its parts still have to work. It still has to be able to see and hear and so on. And there are fundamental limits on how far organs can be scaled down. One is that organs are made of cells, a certain number of which are needed to make complex organs like brains and eyes. The upshot is that an organ in a small animal is usually larger relative to its size than in a big animal.

As vertebrates get really petite, then, it becomes increasingly tricky to fit everything they need inside the body – especially offspring. “You can’t have half a baby. The only way you could go is to have smaller offspring but smaller offspring would struggle to survive,” Hedges says. So a reduction in size leads to reduction in the number of offspring.

“You can’t have half a baby. The only way to go is to have smaller offspring but smaller offspring would struggle to survive”

This is evident for the world’s smallest snake, the 100-mm-long Barbados threadsnake . The female produces only one egg at a time, and even then it’s a squash, taking up half of the body cavity. “The female has to fill up this tubular spaghetti-shaped body cavity with her offspring,” says Hedges. “The egg that’s laid is this long sausage that’s 10 times as long as it is wide.”

Smallest of them all?

In fact, most miniature amphibians, reptiles and mammals that go in for internal fertilisation have room only for one, relatively large offspring, a clear indication that they are about as little as they can be.

Body shape, then, may account for the fact that the smallest known amphibians are shorter than their reptile counterparts, despite the problem of water loss. “The frog is more like a sphere with limbs,” says Hedges, and this makes it easier to pack in the organs while still leaving room for an egg.

All these considerations suggest that while there might be even smaller vertebrates still out there waiting to be discovered, they are unlikely to be much smaller. In the meantime, biologists are still arguing about which of the known vertebrates holds the record.

Frogs have an obvious advantage in this competition. “Frogs don’t have tails. Fish do,” says Austin. With the smallest of his specimens from Papua New Guinea measuring just 7 mm from snout to anus, he says it surely beats Britz’s carp, where the shortest mature individual was 7.9 mm long from its snout to start of the tail fin, which is how ichthyologists measure body length.

Britz brushes aside this amphibian challenge. This is not comparing like with like, he says. “If I were to use the amphibian metric [of snout to anus] we would have the smallest by far,” says Britz. “It would be less than 5 millimetres.”

Austin is having none of it. “Smallest clearly refers to a length measurement and tails count in measurement of fish length as any angler knows,” he says. “This is no fault of the frogs, fish or scientists. Rather it is a biological and anatomical reality.”

Still, perhaps we should wait a few years before awarding any prizes. The frog might be least, but it’s probably not the last.