Naked Mole-Rats: Blind, Naked, and Feeling No Pain.

Around the world and across taxa, subterranean mammals show remarkable convergent evolution in morphology (e.g., reduced external ears, small eyes, shortened limbs and tails). This is true of sensory systems as well (e.g., loss of object vision and high frequency hearing). The naked mole-rat (Heterocephalus glaber) displays these typical subterranean features, but also has unusual characteristics even among subterranean mammals. Naked mole-rats are cold-blooded, completely furless, very long-lived (> 30 years), and eusocial (like termites). They also live in large colonies, which is very unusual for subterraneans. Their cortical organization has reduced area for visual processing, utilizing 30% more cortex for tactile processing. They are extremely tolerant to oxygen deprivation, and can recover from 18 min of anoxia. Their pain pathway is reduced and they feel no pain from acidosis. They are the only rodent tested to date whose pheromone-detecting vomeronasal organ shows no postnatal growth. These features may be a result of this species' "extreme subterranean lifestyle" that combines living underground and living in large colonies. Many respiring animals cramped together in unventilated burrows elevates CO2 levels, high enough to cause acidosis pain, and depletes O2 concentrations low enough to kill other mammals. The naked mole-rat may be an extreme model of adaptation to subterranean life and provides insights into the complex interplay of evolutionary adaptations to the constraints of subterranean living. Anat Rec, 2018.