Passive electroreception is a widespread sense in fishes and amphibians, but in mammals this sensory ability has previously only been shown in monotremes. While the electroreceptors in fish and amphibians evolved from mechanosensory lateral line organs, those of monotremes are based on cutaneous glands innervated by trigeminal nerves. Electroreceptors evolved from other structures or in other taxa were unknown to date. Here we show that the hairless vibrissal crypts on the rostrum of the Guiana dolphin (Sotalia guianensis), structures originally associated with the mammalian whiskers, serve as electroreceptors. Histological investigations revealed that the vibrissal crypts possess a well-innervated ampullary structure reminiscent of ampullary electroreceptors in other species. Psychophysical experiments with a male Guiana dolphin determined a sensory detection threshold for weak electric fields of 4.6 mV cm 21 , which is comparable to the sensitivity of electroreceptors in platypuses. Our results show that electroreceptors can evolve from a mechanosensory organ that nearly all mammals possess and suggest the discovery of this kind of electroreception in more species, especially those with an aquatic or semi-aquatic lifestyle.

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The skin of macroscopically distinct regions (hairy skin, vibrissal fields, buccal ridge, and rhinarium) of the head of the blind mole-rat, Spalax ehrenbergi, was studied by routine histological methods. Few guard and several soft vellus hairs are organized into tufts that grow from a group of hair follicles localized in an invaginated compound cavity. We suggest that this hair arrangement may be a burrowing adaptation to match frictional resistance. The follicles and the compound cavity possess either well developed complex striated musculature or errector pili muscles. There are no structural specializations (sweat glands, glomus bodies) to enhance thermoregulatory (heat dissipative) capacities in the hairy skin of the head. Vibrissae penetrate the epidermal surface as single hairs. They are microscopically normally developed and arranged in vibrissal fields according to a basal mammalian pattern. Most of them are, however, relatively short and inconspicuous. The mystacial vibrissal field is horizontally divided by a prominent buccal ridge which is probably involved in bulldozing. The hairs in the ridge leave the compound cavity singularly. The follicles of guard hairs and bristles are equipped with well developed pilo-Ruffini complexes indicating that the buccal ridge may serve also as a tactile organ. The glabrous skin of the rhinarium has a highly interdigitated dermal-epidermal interface. The dermal papillae possess simple lamellated and/or simple Meissner's corpuscles and few Merkel cell-axon-complexes indicating that the skin of the rhinarium may be particularly sensitive to perception of vibrations.

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