The rationale for culling of rodent litters.

Based on a review of the pertinent literature and our own unpublished data, it is recommended that culling of rodent litters in the early postnatal period should be a standard practice in delivery-type reproduction studies. This, in turn, will reduce the litter size-induced variability in the growth and development of pups during the postnatal period and thus increase the sensitivity of statistical analyses to detect treatment-related effects. This will also ensure that any adverse effects on pup growth (body weight gain) and development (reflex and behavior development) are not masked by a treatment-induced reduction in litter size. The culling should be carried out randomly and no attempt should be made to selectively cull sick or underweight pups. Since male pups weigh significantly more than females and studies have shown differences in maternal behavior toward one sex over the other, whenever possible each culled litter should consist of an equal number of males and females.