Meet the bizarre toad that ‘sees’ with its fingertips and gives birth through holes in its back

There are many reasons some might consider the Surinam toad to be strange.

For one, there’s the birth. Videos of the fully aquatic toad delivering its babies show fully formed young erupting from tiny holes in the skin on their mother’s back.

Then there’s the toad itself: flat-bodied, nearly blind and tongueless with tiny eyes perched atop a triangular head. In the murky waters of the Amazon basin where it is primarily found, it can look almost like a dead leaf lying on the sea floor.

But now, scientists in California have discovered another unique feature of this freak of nature: the Surinam toad can, in a sense, see with its fingertips.

For nearly two centuries, scientists have puzzled over how this toad manages to be such a deft and fierce hunter. Despite its tiny eyes, poor vision and the low visibility of its turbid environment, it can detect nearby prey and suck fish into its mouth in a matter of hundredths of a second.

Measuring 15-20 centimeters wide, with outstretched forelimbs, the toad lies in wait. When some poor, unfortunate fish ventures close, it snaps open its jaws, creating a powerful suction that pulls the prey into its mouth.

But how does it know the fish is there in the first place?

In a new study, Duncan Leitch, a neurobiologist at the University of California, Los Angeles, and his colleagues found the answer: the frog — commonly called a star-fingered toad — uses its fingertips to sense the tiny movements of the water made by nearby prey.

“Frogs have four fingers on their hands,” Leitch told Quirks and Quarks host Bob McDonald. “Those [fingers] split into four different lobes on each of the fingertips, and then, subsequently, each of those lobes split four times again in sort of this fractural pattern.”

The new research, published in theJournal of Comparative Physiology A, has revealed those lobes are highly sensitive touch organs, working somewhat like the fovea, a small, high-resolution patch of sensors at the centre of the human eye. The lobes are so sensitive, they can process signals collected by neurons to identify the size and proximity of potential prey.

As a result, the toad can suck the prey into its mouth without even touching it, even in the dark.

“If a fish passes within about half a centimetre to a centimetre of these frogs, the frogs will launch into this ballistic suction feeding and then completely suck fish or worms or anything that is moving and excites the finger tips,” Leitch said.

“So it doesn’t actually require direct contact with the finger tips themselves.”

What’s so special about the fingertips?

Using a powerful microscope, Leitch says he and other researchers found that each frog has 128 tiny lobes across its finger tips.

The skin on these lobes is covered with four times as many dome-shaped bumps called papillae than elsewhere on the fingers. Like they do on the human tongue, papillae are known to increase touch sensitivity.

“They’re at least as sensitive as your own fingertips,” Leitch said.

According to the study, although the lobes only make up about eight per cent of the skin on the frog’s front legs, they contain about 60 per cent of the touch-sensitive nerves on those legs.

Researchers also found the toad’s brain gives the fingertips far more real estate than one would expect based on the frog’s tiny size. And even though the fingertips take up little of the body’s total surface area, a disproportionately large part of the brain is devoted to processing sensory information from them. 

Why this matters

Lea Randall, senior manager with the conservation program at the Wilder Institute in Calgary, says this finding is particularly important because it’s a kind of strategy previously thought to have only occurred in the cortex of more complex mammal brains, such as that of the star-nosed mole.

“Frogs don’t have complex cortexes like most mammals do, so they’ve kind of utilized other parts of their brain in the mid-brain area,” Randall, who was not involved in the research, told CBC.

According to the study, this seems to suggest that frogs and mammals — despite having evolved separately for more than 350 million years — still arrived at the same solution to an ancient problem: if an animal needs lots of information from a tiny patch of their body, give lots of sensory receptors to that area and lots of brain power.

“I think that [the findings] can help us understand how vertebrates have developed these systems,” Randall said.

“So even in lower vertebrates, such as amphibians, where their brain isn’t nearly as well-developed, having an understanding of how they’ve evolved to utilize different portions of their brain is really fascinating stuff.”