Reproduction, Growth, and Development in Myotis Thysanodes and M. Lucifugus (Chiroptera: Vespertilionidae)

Studies of reproduction and pre— and postnatal growth and development in Myotis thysanodes and M. lucifugus showed that female M. lucifugus copulated in the fall prior to their exodus from the summer roost while M. thysanodes copulated sometime after leaving the summer roost but before their spring return. Ovulation, fertilization, and implantation occurred during the first 2 weeks of May for both species. Gestation has been calculated as 50—60 days for both species. Parturition in M. thysanodes is synchronous with nearly all births within a 2—week period at the end of June and beginning of July. Myotis lucifugus, however, gave birth over a period from June through August. Logarithmic regression lines were calculated for daily prenatal weight increase for both species. Myotis lucifugus grow more rapidly and weigh significantly less than M. thysonades. Regression equations for daily postnatal weight, forearm length, wing span, and fifth—finger length increase in M. thysanodes were calculated and found to be good criteria for estimating age to about 21 days, after which this species has attained adult size. Estimated neonate mortality was low, about 1% for M. thysanodes and 2% for M. lucifugus. Myotis thysanodes was capable of limited flight by 16.5 days of age and of adultlike flight by 20.5 days of age. Myotis lucifugus parallels this attainment of flight quite closely. Wing loading for both species increases slightly from birth to 6 days, followed by a decline to half the birth value at day 20. Aspect ratio for both species increases at steady rate from birth to about 16 days, when it decreases slightly to the adult value. There was no relation of fat index to age in either species, but significant decreases in water index were found with increasing age. Both species demonstrated significant decreases in percentage water and increases in percentage organic and mineral content with increasing age of juveniles. Myotis lucifugus exhibited a significant decrease in spleen weight with age, whereas M. thhysanodes showed no such trend. Neither species showed trends in adrenal weights through the growth period. Several guardian M. thysanodes females stayed with the young during the night. They nursed, retrieved, and in general protected the young throughout the night. Myotis lucifugus did not exhibit this behavior, but large groups of adults would return periodically throughout the night to nurse the young. While M. lucifugus juveniles exhibited practice flight behavior at night, M. thysanodes juveniles were never observed performing this learning process.