The Star-Nosed Mole Decides What Is Food in Eight Milliseconds

A blind mammal the size of a hamster decides whether a passing object is food in roughly eight thousandths of a second, faster than a human can register a flash of light.

The eight-millisecond verdict

The animal in question is Condylura cristata, the star-nosed mole, a small semi-aquatic burrower of the moist, low-elevation marshes of the northeastern United States and eastern Canada. According to the Wikipedia entry on the species, when the mole's snout contacts a possible prey item, it identifies whether the object is edible in approximately 8 milliseconds. The journal Nature has described the animal as the "fastest-eating mammal" known. Guinness World Records lists it as the world's "fastest forager." These two attributions are unusual on the same creature, and more unusual still on one most people would not recognise from a photograph.

The surrounding numbers are worth holding in view. The mole touches between ten and fifteen separate patches of ground every second as it forages, sampling its environment by repeatedly pressing the star against the substrate. Once it commits, the full prey-handling cycle, from detection through bite, can be completed in as little as 120 milliseconds, with an average of 227 milliseconds. Behavioural studies have measured a minimum processing threshold of about 25 milliseconds, beyond which the mole cannot distinguish two separate touches. In one filmed sequence the animal was documented consuming eight separate prey items in under two seconds, which is among the fastest feeding rates ever recorded in a mammal.

A blind hunter in the wet edges of North America

The eyes of the star-nosed mole are tiny and its optic nerve is reduced. The animal is considered functionally blind, and uses the structure on its snout, not its vision, to navigate and hunt. The habitat is consistent with that bias. Peat, standing water, and root-tangled mud are environments in which sight does not offer much. Most populations stay at low elevation, but the species has also been recorded in the Great Smoky Mountains at elevations up to 1,676 metres, navigating soils where the air itself is thinner.

The diet is broad. Caddisfly larvae, midges, dragonfly and damselfly larvae, crane flies, horseflies, predaceous diving beetles, stoneflies, earthworms, leeches, annelids, molluscs, aquatic crustaceans, and occasionally small amphibians and small fish all appear in the prey list. None of these items announces itself with a flash of colour or a clear silhouette. The identifications have to be made by touch, quickly, against a confusing background of inedible debris.

Twenty-two appendages and more than 25,000 receptors

The mechanism that makes this possible sits at the tip of the snout. Twenty-two fleshy, finger-like appendages are arranged radially into a "star" roughly 1.5 centimetres across. The Wikipedia entry describes the surface as carrying "more than 25,000 minute sensory receptors," called Eimer's organs. A typical human hand carries about 17,000 mechanoreceptors. The mole therefore packs roughly six times as many touch receptors as the entire human hand onto a surface barely larger than a fingertip.

Even compared with its closest relatives, the star-nosed mole is a statistical outlier. The much more common European mole carries more than 5,000 Eimer's organs on its snout, served by about 105,000 nerve fibres, which is already a remarkable density and still far below the star-nosed mole's count.

Each Eimer's organ is a bulbous papilla, 0.1 to 0.2 millimetres in diameter, formed from modified epidermis. Inside, three receptor populations encode different signals. Central fibres respond to skin indentations, peripheral fibres respond to pain, and lamellated corpuscles are tuned to vibration. The Pacinian-type corpuscles found in those lamellated layers are known across mammals to register rapid vibrations in the 200 to 300 hertz range. The star is not a single sensor. It is a layered instrument that reads pressure, pain, and vibration on top of one another, more than twenty-five thousand times over, every time it touches the mud.

Half a brain spent on a nose

The brain devotes itself in proportion. Roughly 52 percent of the star-nosed mole's somatosensory cortex is occupied by input from the nose alone. Each cerebral hemisphere contains eleven distinct cortical stripes, with each stripe mapping onto a single appendage on the opposite side of the star. The result is a precise, one-to-one neural map of the touch organ.

That cortex runs fast. Neurons in the somatosensory area respond to a touch on the star with a mean latency of 11.6 milliseconds, reported elsewhere as roughly 12. Motor commands are issued within another 5 milliseconds. The arithmetic is unforgiving. Detection, identification, and the order to strike are all completed in about the time it takes a household lightbulb to settle into full brightness. The eight-millisecond figure for prey identification sits inside that arithmetic, not outside it.

Theodor Eimer, Kenneth Catania, and a bubble in the dark

The receptors that make the whole system possible were not discovered on this mole. The Eimer's organ was first isolated and described by the German zoologist Theodor Eimer in 1871, working on the European mole, and was later named in his honour. The version found on the star-nosed mole is the same structure scaled up by an order of magnitude and rebuilt around a different anatomical centrepiece.

Most of what is known about the modern animal traces back to the neuroscientist Kenneth Catania of Vanderbilt University, who has spent more than twenty years studying the species and has produced the bulk of the published research on its sensory system. Catania's work is also the source of one of the strangest findings in mammalian biology. The star-nosed mole can forage underwater. While submerged it exhales between eight and twelve tiny air bubbles per second, each 0.06 to 0.1 millimetres across, and then re-inhales them, sampling the scent molecules carried back by the bubble. This bubble-sniffing behaviour was the first demonstration of underwater olfaction in any mammal.

The picture that comes together is of an animal that does not lean on a single sense at all. It has rebuilt the front of its face into a touch organ, dedicated more than half of its sensory cortex to reading that organ, and then borrowed the same apparatus to taste the water it swims through. The fastest forager on Earth, operating the most densely packed tactile system among mammals, has been doing this quietly in the muddy edges of North America for far longer than anyone has been watching.

Sources

• Wikipedia: Star-nosed mole
• Wikipedia: Eimer's organ
• Wikipedia: Mechanoreceptor