Growth and Development of the Eastern Harvest Mouse, Reithrodontomys humulis

Abstract

Observations were made on the breeding history, growth, and development of the young, and parental care in captive eastern harvest mice, Reithrodontomys h. humulis. Breeding occurred throughout the year, gestation was presumably 24 days or less, and mean litter size was 2.2. The sex ratio at birth was even. The curves for body weight and total length became essentially asymptotic by the 7th week. Tail length increased only slightly after the 5th week, and the curve for hind foot length leveled off at 3 weeks. Mean ear length showed no significant increase after the 4th week. A marked, slowing at 3 weeks of the initially rapid growth in weight, total length, and tail length coincided with the period of weaning. Newborn young are pinkish and possess both vibrissae and a sparse scattering of hairs over the middorsal region. The pelage is sufficiently developed by the end of the first week to exhibit the typical juvenile coloration, and by the second week the juvenile coat is apparently fully grown. The postjuvenile molt takes place between the 3rd and 9th weeks. Full adult pelage is not attained until at least one further partial or complete molt. The eyes open from the 7th to the 9th or 10th days of age. The pinnae unfold on the 1st or 2nd day of age, and the external auditory meatus becomes patent from the 7th to 10th days. Claws are present at birth and are noticeably better developed by the 4th day. Incisors were above the gums on the 4th day in one mouse, but typically did not appear until the 6th or 7th day. They are faintly yellowish by the end of the 2nd week. All molars were apparently functional in a specimen 4 weeks of age. Weaning occurs between the 2nd and 4th weeks. The vulvas of females opened between the 4th and 7th weeks, and two females became pregnant for the first time at 11 and 20 weeks of age. The testes and cremaster of the males enlarged by the 7th or 8th week. The young have little coordination at birth. When 4 days old they can sit upright, and the prehensile nature of the tail is apparent. Week-old young scratch and attempt to wash. With the opening of the eyes they become more active and exploration, and by 2 weeks of age their behavior is similar to that of adults. Most individuals were nervous and wild. The young and adults were generally highly tolerant of one another, although some fighting occurred among males in one cage. Increased nest building and food storing activities may precede parturition in females. Mothers exhibited concern for their young through the 3rd week of age, although the intensity of parental care declined after the 2nd week. Instances of paternal care were observed, and behavior assumed to represent a weak retrieving response was noted in 4-week-old young. Comparison of the growth and development of R. humulis and R. megalotis indicates a close similarity of postnatal developmental patterns, although megalotis may have a somewhat slower growth rate.