Variable responses to high density of female Mus musculus: A merger of two population regulation hypotheses.

Grouping of female Mus musculus resulted in an increase in mean estrous cycle length from 4 to 12 days, yet some short cycles were still observed. Short cycles were attributed to certain females who consistently maintained a short cycling pattern as well as females who exhibited both short and long cycles. Upon breeding, females that continued to show a short cycle when grouped produced and weaned more young than females with suppressed cycles. No differences in mean weaning weight or sex ratio of the litters was observed. The ecological and evolutionary implications of these findings are discussed.