This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Surinam toads (Pipa pipa) are strange, flat frogs with minuscule eyes that live in the murky waters of the Amazon basin and carry their eggs on their backs. The way they catch prey is even weirder.
They lie in wait with outstretched forelimbs until a small fish approaches, then snap open their jaws to create enough suction to draw the fish into their tongueless mouths and swallow it whole, all in a few hundredths of a second. But how do these frogs hunt in low visibility with poor vision? A new study by UCLA biologists points to an answer that has been hiding in plain sight for two centuries: the frog's peculiar fingertips.
In fact, they are commonly called star-fingered toads. The research, published in the Journal of Comparative Physiology A, revealed a sensory system that processes signals gathered by neurons in unusual lobes on the frog's front fingers to identify the proximity and size of potential prey. Because vision is not useful in their turbid environment, the frogs position their arms in front of and just to the sides of their mouths, fingers spread wide, to detect prey through the movement of water as a fish swims.
Each finger ends not in a claw or pad but in a small cluster of softer lobes, an anatomical oddity noted by 19th-century naturalists but never satisfactorily explained. The new work shows that these lobes are dedicated touch organs, the amphibian equivalent of the fovea—a high-resolution patch of sensors at the center of the eye. "I was reading an anatomy report that described how the tips of the fingers of Surinam toads split into four, and each of those four little lobes split again into four," said Duncan Leitch, a corresponding author and UCLA assistant professor of integrative biology and physiology.
"It immediately seemed to me like these lobes might be somewhat analogous to antennae that the frogs extend so they can feel the space around them." Leitch and his co-authors examined the anatomy of the finger surfaces using a scanning electron microscope and identified a total of 128 mini-lobes, called lobules, per frog. The skin on each lobe was covered with nearly four times as many dome-shaped bumps, called papillae, as skin elsewhere on the fingers. Papillae, also found on the human tongue, are known to increase touch sensitivity.
They then touched the frogs' hands with calibrated filaments to see how easily neurons from each skin area became excited by precisely known forces. They found that the neurons became increasingly sensitive to touch, peaking at the ends of the fingers, where thresholds fell into the same range as human fingertips. Next, the researchers studied the electrophysiology of nerves in the arms and found dense clusters of touch-receptive nerves on the fingertip lobules.
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