Litter sizes of Ha(ICR) mice: a preference for even numbers?

The frequency distributions of litter sizes at birth for eight sublines derived from the same Ha(ICR) mouse stock with or without selection for large litters have their modes at even numbers of young. Each of the eight sublines produced a majority of even-numbered litters. An upward integral shift relationship among the medians of the frequency distributions was established. These findings are interpreted to mean that, in response to continued plus selection, ovarian and uterine performance of the Ha(ICR) bipartite reproductive system tends to be maximal bilaterally, which results in an excess of even-numbered litter sizes.