The neurobiology of Etruscan shrew active touch

Abstract

The Etruscan shrew, Suncus etruscus, is not only the smallest terrestrial mammal, but also one of the fastest and most tactile hunters described to date. The shrew's skeletal muscle consists entirely of fast-twitch types and lacks slow fibres. Etruscan shrews detect, overwhelm, and kill insect prey in large numbers in darkness. The cricket prey is exquisitely mechanosensitive and fast-moving, and is as big as the shrew itself. Experiments with prey replica show that shape cues are both necessary and sufficient for evoking attacks. Shrew attacks are whisker guided by motion- and size-invariant Gestalt-like prey representations. Shrews often attack their prey prior to any signs of evasive manoeuvres. Shrews whisk at frequencies of approximately 14 Hz and can react with latencies as short as 25–30 ms to prey movement. The speed of attacks suggests that shrews identify and classify prey with a single touch. Large parts of the shrew's brain respond to vibrissal touch, which is represented in at least four cortical areas comprising collectively about a third of the cortical volume. Etruscan shrews can enter a torpid state and reduce their body temperature; we observed that cortical response latencies become two to three times longer when body temperature drops from 36°C to 24°C, suggesting that endothermy contributes to the animal's high-speed sensorimotor performance. We argue that small size, high-speed behaviour and extreme dependence on touch are not coincidental, but reflect an evolutionary strategy, in which the metabolic costs of small body size are outweighed by the advantages of being a short-range high-speed touch and kill predator.

Active tactile exploration for adaptive locomotion in the stick insect

“…In contrast to these 'non-contact' situations, the present results reveal a marked change in antennal movement pattern in response to antennal touch: it becomes a repetitive tactile sampling pattern with a faster, smaller-amplitude rhythm in both antennal joints. The cycle frequency increases, much as has been observed in cockroaches [5] and similar to the tactually induced changes in whisking patterns of rodents [42][43][44][45]. The tactually induced change in inter-joint coupling, where HS levation leads SP levation, can be interpreted as an efficient strategy to sample a vertical edge, particularly when considering the slanted antennal joint axes in the stick insects [10,26].…”

Active touch sensing

“…If the star-nosed mole represents one extreme of the mammal class then our next animal, the Etruscan shrew, Suncus etruscus, considered in the article by Brecht et al [35], certainly represents another. The smallest terrestrial mammal and possessing the smallest mammalian brain, this animal again shows a remarkable facility for high-speed prey capture based on the sense of touch.…”

The Robot Vibrissal System: Understanding Mammalian Sensorimotor Co-ordination Through Biomimetics

“…Work in humanoid robotics might decompose this task as the following steps: (i) identify a potential target in peripheral vision based on a rapid analysis of superficial salient features (colour, shape, movement); (ii) orient to and fixate on the object using foveal vision to form an internal 3-dimensional model of the object and of its key properties (shape, size, texture, and so forth); (iii) in parallel, form a second set of representations of the position and orientation of the object in space relative to those of the body, arm, and hand; (iv) match the first, Òwhat?Ó, model with a variety of stored ÒtemplatesÓ in order to determine whether this particular item is, indeed, a suitable target for reaching; (iv) apply algorithms to the computed Òwhere?Ó representations of the object and body, and make use of knowledge of the kinematic and dynamic properties of the arm, hand, and digits, to determine appropriate movement trajectories; (v) execute the planned movements largely ballistically but using some sensory feedback in the final approach, to locate, enclose, and lift the object in an effective way. Now consider the capacity of an animal such as the Etruscan shrew, the smallest living terrestrial mammalÑand known to be a remarkably efficient predatorÑto localise, identify, and entrap an agile prey insect using only its whiskers (Brecht, Naumann et al, 2011). The problem is similar in many ways to that of human (or humanoid) sensory-guided reaching.…”