Reproduction in the Two Species of Grey Kangaroos, Macropus Giganteus Shaw and M. Fuliginosus (Desmarest). Ii. Gestation, Parturition and Pouch Life.

Abstract

Over a 10-y period, eastern and western grey kangaroos were bred in enclosures. All western subspecies interbred, and western males bred with eastern females to give hybrid young, but never the reverse cross. Hybrid males were sterile but females readily backcrossed with western males. Mean gestation lengths were: eastern greys, 36.41 ± 1.63 days; western greys, 30.56 ± 2.55 days; hybrids, 34.09 ± 0.81 days; backcrosses, 31.58 ± 0.93 days. Births occurred throughout the year in both species, but those of western greys were more concentrated in the summer months. Gestation length did not vary with month of year. Parturition in one backcross female is described. Young weighed just over 800 mg at birth and could reach 1000 mg within 24 h. With a free choice, they preferred an anterior to a posterior teat, but when one teat was already being suckled, they chose either the anterior or the posterior teat on the opposite side. The sex ratio was close to unity in all groups, the overall value being 102B : 100@. If dropped from the pouch, young of all groups over 200 days old (mean minimum age 243.86 ± 19.74 days) were usually capable of standing and regaining the pouch. Eastern and western young left the pouch for the first time at mean ages of 283.86 ± 24.70 and 298.40 ± 34.29 days respectively,and left it finally at about 320 days old in both species. Within the western species, pouch life was significantly longer in the subspecies ocydromus than in either fuliginosus or melanops. The intervals between parturitions in lactating females was close to 1 year (variation between species was probably significant). The mean interval in eastern females undergoing embryonic diapause did not differ significantly from that in western females not doing so; in western females it did not differ significantly between fuliginosus and ocydromus, but did so between melanops and both the other subspecies. Lactation usually exceeded 18 months. Mortality of pouch young accounted for 17% of known births, mostly within the first few days after birth. Accidental injury and coccidiosis were the major causes of death in the first months after leaving the pouch.