Importance of apparatus scaling in novel object recognition for juvenile and adult rats.

The growing interest in ontogenetic studies of learning and memory, along with early-life perturbations, has led to the use of younger rodents as a key biological variable in many investigations. This development raises an important question about whether procedures and apparatus designed for studying learning and memory in adults should be simply adapted for use with juvenile and smaller rodents. The current study examined how apparatus size, including the arena and object stimuli, affects novel object recognition (NOR) performance in postnatal day 24-25 juvenile and 90-91-day-old adult rats. Here we found that commonly used larger apparatus reliably produced novel object preference in adults but not in juveniles. Adjusting apparatus size to account for age-group differences in size improved the consistency of NOR performance in juvenile rats. Sex differences were observed: adult males performed reliably across both apparatus sizes, whereas adult females required a larger apparatus to demonstrate effective NOR performance. These findings highlight the importance of tailoring arena and object dimensions to developmental stage and sex for accurate cognitive assessment. Specifically, they support the use of size-based apparatus scaling as a methodological approach in developmental neurobehavioral research and emphasize the importance of careful design when studying male and female rodents. Future studies should explore similar environmental adjustments for other behavioral tests in juvenile and female populations.