Reproduction by Peromyscus polionotus: Number, Size, and Survival of Offspring

Relationships involving female size, litter size, offspring size, and offspring survival were examined using records available from a laboratory colony of Peromyscus polionotus. Female size was positively correlated to (1) birth size of offspring, (2) number of offspring per litter, (3) litter mass at birth, (4) size of offspring on day 21, and (5) postnatal gain from day of birth to day 21. In contrast to female size, increased litter size led to (1) reduced birth size, (2) reduced size on day 21, (3) reduced postnatal gain, and (4) reduced survival of offspring. On day 21, mice in litters experimentally reduced to three from four or more individuals were larger, gained more, and were more likely to have survived than offspring in unmanipulated litters, but were similar in the three characteristics to mice in natural litters of three. Results of litter reductions indicate that female P. polionotus do not simply prorate "potential" parental investment into a smaller number of offspring, but rather the interaction between a female and the number of nursing young leads to changes in both investment per progeny and total investment per litter.