Grandmothering hypotheses
Nakamichi et al report two cases of Japanese macaque grandmothers caring for their offspring:
In the first case, a 24-year-old grandmother provided essential care for the survival of her 2-month-old granddaughter for at least 6 days during which the mother had temporarily disappeared from the group for reasons unknown. In the second case, a 14-month-old granddaughter began sucking from her 23-year-old grandmother within 6 weeks after her mother gave birth to a younger sibling. For at least 6 months, the grandmother exhibited various patterns of maternal behavior toward her granddaughter. The behavioral data obtained in this study indicated that old yet healthy females without dependent offspring could contribute to the survival of their young grandchildren.
This study may roll into that bare valley of science: the anecdote. I don't have a problem with small sample sizes, personally, and short communications like these inform other researchers of what you saw and asks them (implicitly) to keep their eyes open.
Nakamichi, Onishi, and Yamada. 2009. Old grandmothers provide essential care to their young granddaughters in a free-ranging troop group of Japanese monkeys (Macaca fuscata). Primates (online first) doi:10.1007/s10329-009-0177-7
