Kenneth Catania, a comparative neurobiologist at Vanderbilt University, is one of 25 recipients of this year’s MacArthur fellowship awards. Catania studies the nervous system of the star-nosed mole; he is also interested in the development and evolution of the creature.
The star-nosed mole (Condylura cristata) is native to eastern North America. It digs a network of tunnels in moist soil; these tunnels have an average diameter of about 5 cm and can extend for over 250 metres. It lives in a small nest within this tunnel system, and forages for food in tunnels dug near the surface. The mole is virtually blind, and uses its highly sensitive nose to explore its dark wetland habitat.
The mole’s extraordinary nose is a highly specialized sensory-motor organ which enables it to handle objects extremely quickly before deciding whether or not they are edible. Analyses of the mole’s gut shows that its diet consists primarily of small invertebrates, which are abundant in its habitat, and which it consumes in large numbers. The low nutritional value of each individual piece of food is counterbalanced by the astonishing speed with which prey is caught and eaten, which maximizes the time available for finding prey.
In order to study the foraging behaviour of the star-nosed mole, Catania and his colleagues built an artificial glass tunnel in which they could house the animals, and used a high-speed video camera, which shoots 500 frames per second, to record footage of them. Using this experimental set up, Catania’s team has made some astonishing discoveries.
It was found that the mole forages for food continuously, and can find and eat prey faster than any other mammal. Using its star-shaped nose, which consists of 22 fleshy finger-like projections, or ‘tendrils’, the mole can touch 13 separate areas of the ground every second. It can locate and consume 8 separate prey items in under 2 seconds. When the outer appendages of the star come into contact with a potential food source, the mole moves its nose so that the two lower tendrils, which are the most sensitive, come into contact with it. This pair of tendrils is supplied with more nerve fibres than the others and is therefore far more sensitive. When searching for prey, the mole performs repeated cycles of star movements and touches, typically lasting 50 and 25 milliseconds, respectively. Once prey has been identified, it is captured with tweezer-like incisors, whose movements are co-ordinated with those of the star.
This film clip shows a mole locating a segment of earthworm, identifying it as edible, and then eating it. The entire process occurs within about 150 milliseconds, and without a high-speed video camera would be seen as a blur of movements:
The star-shaped nose is what makes this mole capable of such extraordinary feeding behaviour. The star, which is less than half an inch in diameter, consists of 11 pairs of tendrils and is divided into a high resolution central fovea region and less sensitive peripheral areas. It is much larger than the nose of other mole species, covering 0.92 cm2 per touch, compared to the 0.11 cm2 covered by the noses of other mole species. The star (c in the figure below) contains a far higher density of receptors than the noses of other mole species (d); its surface is covered with 25,000 mechanoreceptors called Eimer’s organs and is innervated by 100,000 large axons. This makes the star ultrasensitive – it is, in fact, the most sensitive organ known. For example, it is about 6 times more sensitive than the human hand, which contains about 17,000 receptors.
The nervous system of the star-nosed mole processes tactile information at very high speeds, perhaps as quickly as nervous systems are capable of functioning. The mole can decide whether or not something is edible in about 25 milliseconds. Neurons in the mole’s somatosensory cortex respond to tactile stimuli within 12 milliseconds, and Catania estimates that it takes a further 5 milliseconds for motor commands to be conducted back to the star. Because tactile information from the star is processed so quickly, the mole makes a lot of mistakes, and often moves in the wrong direction. In Catania’s artificial tunnel, this occurred on more than one-third of occasions when the animals were presented with food.
Like the somatosensory cortex of humans, that of the star-nosed mole is somatotopically arranged – in other words, the mole’s brain uses information from the star to produce a tactile ‘map’ of the environment under the mole’s nose. Approximately half of the star-nosed mole’s somatosensory cortex is devoted to receiving inputs from the nose, and the organization of that part of the somatosensory cortex reflects the different degrees of sensitivity of the star, with a far larger part devoted to lower-most pair of tendrils than to the other appendages. However, whereas the brains of other mole species contain two such maps, that of the star-nosed mole contains three; Catania believes that this enables the star-nosed mole to carry out highly efficient parallel processing of tactile information.
Catania speculates that as it evolved to live in a wetland habitat, the star-nosed mole was placed under selective pressure to exploit the dense populations of small insects it found in its new environment; the star evolved so that this could be done at the highest possible speed. The presence of a touch organ near the mouth greatly reduces the handling time required before food can be ingested, and is a major factor in how the star-nosed mole can find and eat food so quickly.
Catania’s work not only gives biologists a better understanding of a fascinating organism, it also provides insights into how the mammalian brain develops, evolves and adapts.