In rats, the long facial whiskers (mystacial macrovibrissae) are repetitively and rapidly swept back and forth during exploration in a behaviour known as 'whisking'. In this paper, we summarize previous evidence from rats, and present new data for rat, mouse and the marsupial grey short-tailed opossum (Monodelphis domestica) showing that whisking in all three species is actively controlled both with respect to movement of the animal's body and relative to environmental structure. Using automatic whisker tracking, and Fourier analysis, we first show that the whisking motion of the mystacial vibrissae, in the horizontal plane, can be approximated as a blend of two sinusoids at the fundamental frequency (mean 8.5, 11.3 and 7.3 Hz in rat, mouse and opossum, respectively) and its second harmonic. The oscillation at the second harmonic is particularly strong in mouse (around 22 Hz) consistent with previous reports of fast whisking in that species. In all three species, we found evidence of asymmetric whisking during head turning and following unilateral object contacts consistent with active control of whisker movement. We propose that the presence of active vibrissal touch in both rodents and marsupials suggests that this behavioural capacity emerged at an early stage in the evolution of therian mammals.

During exploration, rats and other small mammals make rhythmic back-and-forth sweeps of their long facial whiskers (macrovibrissae) [1-3]. These "whisking" movements are modulated by head movement [4] and by vibrissal sensory input [5, 6] and hence are often considered "active" in the Gibsonian sense of being purposive and information seeking [7, 8]. An important hallmark of active sensing is the modification of the control strategy according to context [9]. Using a task in which rats were trained to run circuits for food, we tested the hypothesis that whisker control, as measured by high-speed videography, changes with contextual variables such as environment familiarity, risk of collision, and availability of visual cues. In novel environments, functionally blind rats moved at slow speeds and performed broad whisker sweeps. With greater familiarity, however, they moved more rapidly, protracted their whiskers further, and showed decreased whisking amplitude. These findings indicate a strategy change from using the vibrissae to explore nearby surfaces to using them primarily for "look ahead." In environments with increased risk of collision, functionally blind animals moved more slowly but protracted their whiskers further. Sighted animals also showed changes in whisker control strategy with increased familiarity, but these changes were different to those of the functionally blind strain. Sighted animals also changed their vibrissal behavior when visual cues were subsequently removed (by being placed in darkness). These contextual influences provide strong evidence of active control and demonstrate that the vibrissal system provides an accessible model of purposive behavior in mammals.

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AcknowledgementsThe authors would like to thank Hazel Ryan for her continued help and support, especially in handling the dormice; also to Hazel Ryan and Angus Carpenter for their comments on the manuscript. We are extremely thankful to the Wildwood Trust for the use of their facilities and animals. The authors are also grateful to Ben Mitchinson for designing the climbing arenas and developing the portable set-up, and to Holly Langridge and Fraser Combe for data collection support. Thanks to Brendan O'Connor for finding and translating some of the literature. Video analysis was performed using the BIOTACT Whisker Tracking Tool which was created under the auspices of the FET Proactive project FP7 BIOTACT project (ICT 215910), which also partly funded the study, alongside a small project grant from the British Ecological Society (BES). A big thanks goes to the CEB Research Group at MMU for listening to results summaries and advising about the project direction. Animal careAll animals were part of a rescue and rehabilitation program, or from a breeding pool at the Wildwood Trust in Kent (UK) and approved by local ethics committees at each of the academic institutions and at the Wildwood Trust.2 Abstract Dormouse numbers are declining in the UK due to habitat loss and fragmentation. We know that dormice are nocturnal, arboreal, and avoid crossing open spaces between habitats, yet how they navigate around their canopy is unknown. As other rodents use whisker touch sensing to navigate and explore their environment, this study investigates whether Hazel dormice (Muscardinus avellanarius) employ their whiskers to cross between habitats. We analysed high-speed video footage of dormice exploring freely in flat and climbing arenas in near darkness and using infrared light illumination. We confirm that, like rats and mice, dormice move their whiskers back and forth continuously (~10 Hz) in a motion called whisking and recruit them to explore small gaps (<10 cm) by increasing the amplitude and frequency of whisking and also the asymmetry of movement between the left and right whisker fields. When gaps between platforms are larger than 10-15 cm dormice spend more time travelling on the floor. These findings suggest that dormice can actively and purposively move their whiskers to gather relevant information from their canopy at night. As this species is vulnerable to threats on the ground, we also provide evidence that joining habitat patches between dormouse populations is important for promoting natural behaviours and movement between patches. AbbreviationsRC -rostrocaudal DC -dorsoventral BWTT -BIOTACT Whisker Tracking Tool