Reproductive Adaptations of the Red Tree Mouse

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Abstract
Captive red tree mice (Phenacomys longicaudus) were maintained on a diet of Douglas Fir boughs, the sole natural food of the species. Survival was good when the boughs were moistened frequently even though no other water was provided. The average duration of vaginal cornification was 5.9 days with a range of from one to 21 days. For four females with long smear records the proportion of cornified days ranged from 14 to 59%. Ovulation is induced by copulation. Free blastocysts were recovered from the oviduct of one female 64 hours after mating. Receptivity, as judged by lack of initial aggression, a distinctive circling behavior by the female, and a lordosis-like posture, was most probable for females with cornified vaginal smears. Such female receptivity was noted in 7 of 8 observed matings which led to pregnancy. Great individual differences in male sexual responsiveness were noted. One male attempted to mount nearly every female he encountered regardless of her receptivity. The usual gestation period of non-lactating females is from 27.0 to 28.5 days, but one such female produced a litter 33 days after mating. Two lactating females extended gestation to 40 and 41 days. Postpartum receptivity occurs at least occasionally. Among captives parturition may occur throughout the daylight hours. In normally developing litters the eyes open between 18 and 23 days. Individuals from smaller litters are significantly larger at birth. The rate of weight gain drops during the period of total dependence on the mother for nourishment, while absolute weight gain is nearly constant during this period. Correlated evidence from comparisons of litter sizes of 2 and 3 suggests that during the initial phase of development the mother's milk supply limits the development of the young. In healthy captives weaning takes place at from 30 to 35 days. Coprophagy prior to eye opening may be an adaptation to proper microorganism innoculation before ingestion of solid food. It seems probable that small litters, unusually long gestation, delayed implantation during lactation, and exceptionally slow development of the nursing young are all features of the reproductive process adapted to the energy-conversion difficulties inherent in a fir needle diet.

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