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The relative sizes of different sensory representations in naked mole-rats S1. The chart on the right shows the percentage of cortex devoted to different body parts. On the left the different body parts are illustrated according to their cortical proportions. This ‘‘ mole-ratunculus ’’ provides a graphic illustration of the cortical magni fi cation of the incisors and head (illustration by Lana Finch). 

The relative sizes of different sensory representations in naked mole-rats S1. The chart on the right shows the percentage of cortex devoted to different body parts. On the left the different body parts are illustrated according to their cortical proportions. This ‘‘ mole-ratunculus ’’ provides a graphic illustration of the cortical magni fi cation of the incisors and head (illustration by Lana Finch). 

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We investigated naked mole-rat somatosensory cortex to determine how brain areas are modified in mammals with unusual and extreme sensory specializations. Naked mole-rats (Heterocephalus glaber) have numerous anatomical specializations for a subterranean existence, including rows of sensory hairs along the body and tail, reduced eyes, and ears sens...

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... the upper incisors because both upper incisors make close contact with one another. Thus, responses to stimulation of either of the upper incisors were common. However, stimulation with fine von Frey filaments often yielded responses solely to the contralateral tooth, suggesting that many responses to the ipsilateral tooth were a consequence of inadvertent stimulation of both incisors. Al- ternatively, a proportion of the neurons in the upper tooth representation might be bilaterally activated, as has been shown for slowly adapting dental receptors in cats (19) and for oral receptive fields in primates (20). As a whole, the amount of cortex devoted to the incisor representations occupied an average of 10% of the total neocortical area. A second obvious specialization of S1 in naked mole-rats was its much larger size compared with most other mammal species (21). In a previous study of the blind mole-rat ( Spalax ehren- bergi ), Necker et al. (22) were able to map much of the sensory cortex. Their preliminary estimates suggested that Spalax so- matosensory cortex was at least 20% larger than laboratory rats and was shifted caudally in cortex to occupy part of the area usually devoted to vision. However, the dentition was not mapped and the borders of sensory areas were estimated in limited detail and not related to cortical histology. Nevertheless, the results strongly suggested that S1 in blind mole-rats is enlarged and displaced caudally. By sampling cortex with a high density of microelectrode penetrations and relating microlesions to flattened cortical histology, we were able to revisit this issue in much greater detail for naked mole-rats. Our results reveal a remarkable expansion of the proportion of the neocortex taken up by the primary somatosensory area (S1), far greater than previously reported in any other rodent (Fig. 4). S1 in naked mole-rats took up approximately 31% of total neocortex and at least 47% of all possible sensory cortex (cortex caudal to the rostral border of S1). In contrast, S1 in laboratory rats takes up approximately 21% of neocortex and only 27% of all possible sensory cortex (Fig. 5). Assuming the sensory cortex of rats to be more typical of ancestral rodents, naked-mole rat somatosensory cortex ap- pears to have increased by as much as 50% (as a proportion of total neocortical area) as mole-rats became specialized for fossorial life (Fig. 5). A final specialization of mole-rat somatosensory cortex was its far caudal and medial extension. This extension is perhaps best appreciated by noting the locations of microlesions where neu- rons responded to mechanosensory stimulation of the tail in a section of flattened cortex (Fig. 4 C ). In most other mammals (Fig. 4 A ), much of caudal cortex is occupied by primary and secondary visual cortex (23, 24). In naked mole-rats, somato- sensory cortex extended to the caudo-medial pole of neocortex, clearly occupying most of the area that usually processes visual information in other species. This somatosensory expansion seems analogous to the com- pensatory plasticity and corresponding cortical reorganization that has been observed when normally sighted animals are experimentally deprived of vision (25 – 28). Of course the degree of somatosensory expansion is much greater in naked mole-rats. This is to be expected given the millions of years they have had to evolve innate developmental mechanisms that bias their sensory system to process information related to touch. These findings also raise the question of how subcortical structures might be similarly specialized. In blind mole-rats ( S. ehrenbergi ), which depend heavily on their auditory system to detect low frequencies transmitted through soil, the dorsal lateral genicu- late body was found to respond to auditory stimuli (29). This finding suggests that in addition to changes in overall size, nuclei and areas might be rewired to respond to new modalities in the course of evolution. The somatosensory cortex of naked-mole rats extends much further medially and caudally than in blind mole-rats (22) and appears to take up a much greater proportion of the brain (30). It is not clear why naked and blind mole-rats appear to differ in this respect. Part of the explanation for the larger expansion of naked mole-rat S1 may be related to the important postcranial sensory hairs that uniquely cover much of the naked mole-rat body and could require a large somatosensory territory much like the facial whiskers of other rodents (31). Presumably the dentition is also of great importance to blind mole-rats, but this has not yet been explored. Naked mole-rats have diverged extensively from their non- fossorial relatives and exhibit a number of unusual physical and behavioral adaptations for their underground habitat. Here we demonstrate that brain organization has evolved in parallel with these physical and behavioral changes, resulting in an equally specialized and unusual cortical organization (Fig. 3). These specializations include major changes in the proportion of cortex devoted to touch and the location of somatosensory cortex, as well as the extreme magnification of the behaviorally most important parts of the mole-rat periphery — the incisors. Al- though the dentition is important in virtually all mammals, the degree of cortical magnification of mole-rat incisors is unprec- edented. Such magnification is usually associated with a special- ized sheet of sensory receptors (e.g., retina or skin surface) rather than the receptors at the base of the tooth that are activated by transmitting distant contact along a rigid surface. These findings raise a number of important questions re- garding cortical evolution, plasticity, and development. For example, investigations of rodent somatosensory ‘‘ barrel ’’ cortex have revealed a close relationship between innervation density and cortical representational area (32, 33). This finding would suggest that mole-rats may have a particularly large set of densely innervated periodontal mechanoreceptors. A sec- ond possibility, suggested by more recent investigations in primates (34, 35), is that activity patterns may have inf luenced the size of cortical representations in mole-rat cortex. Perhaps, for example, greater use of the incisors by naked mole-rats relative to other mammals has shaped the size of cortical representations throughout their lifetime. Alternatively, the incisors and their innervation may have an important inf luence on the cortex primarily during early critical periods of devel- opment (36 – 38). Finally, whatever the mechanism of cortical magnification, it remains to be seen what is gained by devoting so much cortex to the teeth. Can mole-rats use the dentition for a range of subtle sensory discriminations? We believe these interesting small mammals will be useful in answering a number of basic questions about the sensorimotor function of ...

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The naked mole-rat (Heterocephalus glaber) is famous for its longevity and unusual physiology. This eusocial species that lives in highly ordered and hierarchical colonies with a single breeding queen, also discovered secrets enabling somewhat pain-free living around 20 million years ago. Unlike most mammals, naked mole-rats do not feel the burn of chili pepper’s active ingredient, capsaicin, nor the sting of acid. Indeed, by accumulating mutations in genes encoding proteins that are only now being exploited as targets for new pain therapies (the nerve growth factor receptor TrkA and voltage-gated sodium channel, NaV1.7), this species mastered the art of analgesia before humans evolved. Recently, we have identified pain-insensitivity as a trait shared by several closely related African mole-rat species. In this chapter we will show how African mole-rats have evolved pain insensitivity as well as discussing what the proximate factors may have been that led to the evolution of pain-free traits.
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Brain size fascinates society as well as researchers since it is a measure often associated with intelligence and was used to define species with high “intellectual capabilities”. In general, brain size is correlated with body size. However, there are disparities in terms of relative brain size between species that may be explained by several factors such as the complexity of social behaviour, the ‘social brain hypothesis’, or learning and memory capabilities. These disparities are used to classify species according to an ‘encephalization quotient’. However, environment also has an important role on the development and evolution of brain size. In this review, I summarise the recent studies looking at the effects of environment on brain size in insects, and introduce the idea that the role of environment might be mediated through the relationship between olfaction and vision. I also discussed this idea with studies that contradict this way of thinking.
... There is a rich body of interesting work on body perception and representation in nonhuman animals [see, for example (Catania & Remple, 2002;Fang et al., 2019;Wada, Takano, Ora, Ide, & Kansaku, 2016)]. Our studies in no way invalidate behaviourist approaches that seek to map body perception to neural function through observed behaviour. ...
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