Which factor is least likely to affect litter size in laboratory animals?

The factor least likely to affect litter size in laboratory animals is the time of year. While environmental factors such as temperature and photoperiod can influence breeding behavior and reproductive cycles, they have a lesser direct impact on the actual number of offspring produced compared to the other listed factors.

In contrast, the age of the parents can have a significant influence on litter size. Younger animals may not be fully mature or may have less experience, leading to smaller litters, while older animals might have declined reproductive capacity. Nutritional status is also crucial; a well-nourished breeding pair is more likely to produce larger and healthier litters, as malnutrition can lead to reduced fertility and smaller litter sizes. Genetic make-up is another strong determinant, as certain breeds or genetic lines may be predisposed to larger or smaller litters, directly influencing reproductive output. Thus, the time of year stands out as the factor least likely to affect litter size compared to the direct biological influences of age, nutrition, and genetics.