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Characteristics of the oestrous cycle of grey short-tailed opossums were studied by vaginal smears. The period of oestrus was identified by a sudden proliferation of epithelial cells which lasted about 6 days (range 3-12 days), followed by a leucocytic infiltration. Oestrous cycle length showed a bimodal distribution of 14.4 days (5 cycles, range 11-17 days) and 32.3 days (10 cycles, range 28-39 days). Females housed with males showed more days of epithelial cell proliferation than did females housed alone, and oestrous periods tended to occur in synchrony, suggesting that social factors may influence the oestrous cycle in this species.

Although social factors are known to influence reproduction in a number of eutherian mammals, the effect of these factors on marsupial reproduction has not heretofore been studied. In this study, the temporal pattern of births and of the appearance of vaginal estrus following pairing with a male were examined in the gray short-tailed opossum, a small Brazilian didelphid. Although the estrous cycle is approximately 32 days long in this species, 75% of conceptions occurred within 9 days and 91.6% within 13 days following pairing. When sensory contact was permitted although mating was prevented, females tended to show vaginal estrus 3 to 7 days following pairing with a male. These findings are discussed with respect to possible mechanism, adaptive significance, and evolutionary perspective.

Parental income was correlated significantly with performances on the MCAT and USMLE Step 1. These relationships may be particularly strong and persistent for minority women.

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