Exploratory behaviour, memory and neurogenesis in the social Damaraland mole-rat (Fukomys damarensis)

Laterality in the Damaraland Mole-Rat: Insights from a Eusocial Mammal

Lateralization is the functional control of certain behaviors in the brain being processed by either the left or right hemisphere. Behavioral asymmetries can occur at an individual and population level, although population-level lateralization is less common amongst solitary species, whereas social species can benefit more from aligning and coordinating their activities. We assessed laterality (individual and population) through turning biases in the eusocial Damaraland mole rat, Fukomys damarensis. We considered factors such as breeding status (queen or subordinate), environment (wild-caught or captive), sex (male or female), colony and body mass. All individuals together demonstrated significant left-turning biases, which was also significant at the population level. Wild-caught animals were more strongly lateralized, had a wider spread over a laterality index and lacked the population-level left-turning bias as compared to captive mole rats. Subordinate animals were more lateralized than queens, demonstrating social status differences in turning biases for social mole rats. This emphasizes the importance of animal handling and context when measuring and interpreting behavioral asymmetries.

Social Isolation Does Not Alter Exploratory Behaviour, Spatial Learning and Memory in Captive Damaraland Mole-Rats (Fukomys damarensis)

Exploratory behaviour, spatial learning and memory affect the survival of animals and appear to be dependent on the specific habitat that a species occupies. Good spatial navigation and memory are particularly important for subterranean animals, as it is energetically expensive to inhabit this niche. Damaraland mole-rats are subterranean mammals that live in colonies with organised social structures. Damaraland mole-rats have been maintained in the laboratory for many years and can be housed in groups or individually. We evaluated the effect of social isolation on the exploratory behaviour and spatial memory of single-housed and colony-housed animals and also considered potential differences in animals with different social statuses. We predicted that solitary housing would increase anxiety-like behaviour and result in higher activity and more errors when solving a maze. Exploration by colony- and single-housed mole-rats was tested in an open-field test, where all individuals explored readily. Single-housed queens and non-breeding females showed increased activity and spent more time in tunnels, which can be explained by increased anxiety. In the Y-maze, improvements in solving the maze were observed in all experimental groups, except in single-housed non-breeding females. In addition, all males showed a decrease in the number of errors in the maze. Spatial learning is thus apparent but could not be conclusively proven. It was possibly underestimated, as magnetic cues that may be used by mole-rats as stimuli for navigation were removed in the experimental setup. Overall, it appears that social isolation has a limited effect on the exploratory behaviour and spatial learning of Damaraland mole-rats.

Postnatal and Adult Neurogenesis in Mammals, Including Marsupials

In mammals, neurogenesis occurs during both embryonic and postnatal development. In eutherians, most brain structures develop embryonically; conversely, in marsupials, a number of brain structures develop after birth. The exception is the generation of granule cells in the dentate gyrus, olfactory bulb, and cerebellum of eutherian species. The formation of these structures starts during embryogenesis and continues postnatally. In both eutherians and marsupials, neurogenesis continues in the subventricular zone of the lateral ventricle (SVZ) and the dentate gyrus of the hippocampal formation throughout life. The majority of proliferated cells from the SVZ migrate to the olfactory bulb, whereas, in the dentate gyrus, cells reside within this structure after division and differentiation into neurons. A key aim of this review is to evaluate advances in understanding developmental neurogenesis that occurs postnatally in both marsupials and eutherians, with a particular emphasis on the generation of granule cells during the formation of the olfactory bulb, dentate gyrus, and cerebellum. We debate the significance of immature neurons in the piriform cortex of young mammals. We also synthesize the knowledge of adult neurogenesis in the olfactory bulb and the dentate gyrus of marsupials by considering whether adult-born neurons are essential for the functioning of a given area.

Spatial Learning and Memory in the Naked Mole-Rat: Evolutionary Adaptations to a Subterranean Niche

Evolutionary adaptation to a subterranean habitat consisting of extensive underground tunnel systems would presumably require adept spatial learning and memory, however, such capabilities have not been characterized to date in naked mole-rats (Heterocephalus glaber) which, like other members of Bathyergidae, are subterranean rodents. The goal of this study was to develop a method for effectively assessing spatial learning and memory by modifying a Hebb-Williams maze for use with these subterranean rodents. Established behavioral tests to assess spatial learning and memory have primarily focused on, and have been optimized for, more typical laboratory rodent species such as mice and rats. In the current study, we utilized species-appropriate motivators, analyzed learning curves associated with maze performance, and tested memory retention in naked mole-rats. Using a modified Hebb-Williams maze, naked mole-rats underwent 3 days of training, consisting of five trials per day wherein they could freely explore the maze in search of the reward chamber. Memory retention was then tested 1 day, 1 week, and 1 month following the last day of training. Performance was analyzed based on latency to the reward chamber, errors made, and distance traveled to reach the reward chamber. Overall, this study established a behavioral paradigm for assessing maze navigation, spatial learning, and spatial memory in subterranean rodents, including optimization of rewards and environmental motivators.