Effect of housing conditions on sex differences in spatial cognition in rats
Harris, Anjanette Patricia
Male mammals typically outperform females in tests of spatial ability. However, in laboratory rats (Rattus norvegicus), from which the majority of data in support of this difference come, sex differences are not consistently found. Since stress affects cognition in males and females differently, I investigated possible sources of stress (e.g. housing conditions, spatial tasks) and the impact they have on cognitive performance in male and female rats. Firstly, I investigated whether isolation housing, purported to be chronically stressful, affected the presence of sex differences in a dark-eyed and an albino strain of rat. Irrespective of sex and strain, I found that young or old rats isolated for long or short periods were not behaviourally or cognitively impaired relative to pairhoused conspecifics. I found, however, that behaviour caused by the acute stress of the task impeded performance. Furthermore, sex differences in performance were found only when the females were more stressed than the males during testing. Additionally, the degree to which the rats found the task stressful depended upon the age at which they travelled from the breeding establishment. In the dark-eyed strain, males were always less stressed than the females, but also out performed the females only if they travelled while young (4-5 weeks old). Both sexes seemed to be less stressed by the task if the rats travelled as adults. Conversely, in the albino strain, males outperformed females only if the rats travelled as adults, because in the young travellers both sexes were equally and highly stressed during testing. Therefore, the acute stress response, which seems to underlie sex differences in cognitive performance, was influenced by the age at which the rats travelled in a sex and strain dependent manner. Next, I considered the impact of the physical attributes of the home cage on a rat’s welfare and performance in a cognitive task. I found that, male and female rats housed with a barrier that reduced visual contact from their cage showed higher levels of behavioural stress in their home cage than did rats housed without a barrier between the cages. Rats housed with the barrier were also more stressed during spatial testing and had poorer cognitive performance relative to rats housed without the barrier. Pair housing did not ameliorate the effect of the barrier. Based on these data, although a rather unorthodox suggestion, I propose that single housing with a view may be preferable to pair housing without a view. One implication of this finding is that the number of animals used in an experiment could be significantly reduced if the home cages allow sufficient visual interactions. Lastly, I investigated the impact of environmental enrichment on spatial cognition and behavioural stress responses. I found, contrary to current opinion, that enriched rats outperformed non-enriched animals not because they had superior cognitive ability but because their behavioural stress response was reduced significantly during testing. Furthermore, withdrawing enrichment from rats for at least one week did not increase stress responses during testing or impair cognitive performance. Therefore, exposure to enrichment, even if later withdrawn, improves welfare by reducing stress during cognitive testing. In conclusion, a differential behavioural stress response during cognitive testing may explain why males outperform females and why enriched animals do better than non-enriched animals in tests of spatial cognition. Furthermore, variation in this behavioural stress response may in part explain why sex differences in performance are not consistently found in laboratory rats.