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A study of breeding in grey kangaroos, Macropus giganteus Shaw and M. fuliginosus (Desmarest), in central New South Wales.

A total of 2750 grey kangaroos were examined at Mt. Hope, N.S.W., over a period of 4 years, commencing in 1962. Two colour phases of grey kangaroo were recognized but it was not until late in the study that concurrent investigations revealed that two sympatric species occurred in the area in the approximate ratio 1 : 8. As it was not possible to distinguish the species accurately in retrospect, all the kangaroos were treated as one group. A few males became sexually mature at 20 months of age, most by 36 months, and all before they were 72 months old. Females generally became sexually mature when between 20 and 36 months of age but onset of breeding was dependent on the breeding season (September-March). Most females were breeding by their second summer, some by the third summer, and a few not until the fourth summer. Most births occurred between September and March but some occurred in all months of the year. Of the total females carrying pouch young, 5% had dormant embryos in the uterus and all of these had pouch young more than 180 days old. The highest per- centage of females carrying dormant embryos was found in late autumn, possibly because of improved nutrition after heavy rains. No western grey kangaroos apparently carried a dormant embryo but 30% of the total eastern grey females did. A significant excess of pouch young were attached to an anterior rather than a posterior teat in multiparous females. When an anterior teat was already being suckled by a young-at-foot, the posterior teat on the same side was apparently shielded and a significant excess of young born to such females attached to the anterior teat on the non-suckled side. If a posterior teat was enlarged, almost two-thirds of the newborn young attached to anterior teats; the greater number on the non-suckled side. One female had two young in the pouch, separated in age by the length of one gestation period, indicating that suckling by the first young had failed to inhibit the next expected ovulation. Disparity between 713 male and female young was not significant at 105: 100 for pouch young and 84: 100 for young-at-foot. For sets of pouch young through the age range 113-280 days there was an estimated constant mortality rate of 1.82% per 28 days. No between-sex or between season variation in the mortality rate was revealed.