The star-nosed mole's amazing appendages

THIS weird and wonderful creature is the star-nosed mole (Condylura cristata), a small, semi-aquatic mammal which inhabits the low wetlands of eastern North America. Like other moles, it ekes out an existence in a network of narrow underground tunnels, and digs shallow surface tunnels where it forages for insects, worms and molluscs. Living as it does in almost complete darkness, the star-nosed mole has poorly developed eyes, and is virtually blind. Instead, it relies heavily on its remarkable star-shaped nose. This organ enables the star-nosed mole to decide whether something is edible with astonishing speed - in fact, it recently entered the Guinness Book of Records as the world's fastest forager - and also to sniff out food underwater.

The star-shaped nose is a highly specialized sensory-motor organ, which consists of 11 pairs of fleshy finger-like appendages, or 'tendrils'. The star, which is less than half an inch in diameter, 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 0.11 cm² covered by the noses of other mole species. The star also contains a far higher density of receptors than the noses of other mole species; its surface is covered with 25,000 mechanoreceptors called Eimer's organs. (That makes it about 6 times more sensitive than the human hand, which contains about 17,000 receptors.) This makes the star ultrasensitive - it is, in fact, the most sensitive organ in the entire animal kingdom. 

The nose is innervated by 100,000 large diameter axons, so that tactile information from it is transmitted to the brain rapidly. Furthermore, the star-nosed mole's brain processes the information at a very high speed, which approaches the upper limit at which nervous systems are capable of functioning. The mole can therefore decide whether or not something is edible within about 25 milliseconds (ms, or thousandths of a second). Neurons in the mole's somatosensory cortex - that part of the brain which responds to tactile stimulation - respond to touch within 12 milliseconds, and it is estimated to take a further 5ms for motor commands to be conducted back to the star. By comparison, it takes us humans approximately 600ms to press the brake pedal in response to something that steps out in front of our car.

The star-nosed mole can touch 13 separate areas of the ground every second, and 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 2ms, respectively. Once prey has been identified, it is captured with tweezer-like incisors, whose movements are co-ordinated with those of the star.

The importance of the nose to this organism's lifestyle is reflected in the way its brain is brain is organized - approximately half of the brain is devoted to processing sensory information from the nose. The nose substitutes for the eyes, with the information from it being processed so as to produce a tactile 'map' of the environment under the mole's nose. Like the somatosensory cortex of other mammals, that of the star-nosed mole is said to be somatotopically organized, such that sensory information from adjacent parts of the nose is processed in adjacent regions of the somatosensory cortex. The tendrils of the nose are therefore "mapped" onto the brain, with the lower, most sensitive pair of tendrils having a larger part of somatosensory cortex devoted to them than the other less sensitive tendrils. And whereas the brains of other mole species contain two sensory maps, that of the star-nosed mole contains three; this may enable it to carry out highly efficient parallel processing of tactile information.

This amazing appendage also enables the mole to smell underwater, something which was previously thought impossible. The animals were filmed with high-speed cameras as they followed underwater scent trails which led to food. They were found to exhale between 8 and 12 small air bubbles per second, each of volume 0.06-0.1 milliltres, onto objects or scent trails they encounter while foraging underwater. The bubbles are then drawn back into the nose, so that odorant molecules in the air bubbles are wafted over the olfactory receptors. When a fine mesh was used to prevent the mole's exhaled bubbles from coming into contact with the scent trails, the accuracy with which the animals followed the scents dropped to about 50%, confirming that the mole can indeed smell by blowing bubbles, and suggesting that it has to come into contact with, or at least come into close proximity with, a scent trail in order to smell it while underwater.

The star-nosed mole evolved to inhabit a wetland habitat, and so was placed under selective pressure to exploit the dense populations of small insects it found in its new environment. It consumes its prey in large numbers, and the dazzling speed with which it does so counterbalances the low nutritional value of each individual piece of food,  and maximizes the time available for finding food. The proximity of the star-shaped nose to  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, K. (2006). Olfaction: Underwater 'sniffing' by semi-aquatic mammals. Nature 444: 1024-1025. DOI: 10.1038/4441024a.

Catania, K., & Remple, F. (2005). Asymptotic prey profitability drives star-nosed moles to the foraging speed limit. Nature 433: 519-522. DOI: 10.1038/nature03250.