Hill, Elizabeth, and Brian Greenhoe
The pace of behavioral development varies between and within species, correlating with growth and various life history characteristics, as well as reflecting the availability of food in the immediate environment. In order to measure differences in development rate among litters of mice, a standardized assessment was implemented. Behavioral development was measured using tests of sensory and motor functions, such as abilities to grasp and climb. Here we investigate the relationship of behavioral development to litter size and individual body weight.
The study used 59 juvenile mice (Mus musculus) from 3 commonly-used strains, C57BL/6, BALB/c, and CD-1. Mouse pups from 10 litters were tested when 10 days old. Pups of this age have fur, but their eyes and ears are still closed. Behavioral tests included righting reflex, grasp reflex, ability to grasp a 2.5 mm rod with fore and hind limbs, grasping a 9.5 mm stick, and ability to grip a horizontal mesh screen and climb a vertical screen. The tests scores were averaged for analysis.
The behavioral development score correlated significantly with both litter size (r=-.576, p<.001) and pup body weight (r=.321, p<.05). In turn, pup weight correlated inversely with litter size (r=-.770, p<.001). A regression analysis was run using litter size and pup weight to predict behavioral development score, which was statistically significant (R2=.301, p<.001). The independent effect of pup weight was not significant, however, once litter size was in the equation. Thus, there is a negative relationship between behavioral development and litter size. The larger the litter, the slower the pace of development, but the impact of litter size is not solely due to nutrition level, as measured by weight of individual pups.