According to the paper published on Primates, Nakamichi et al (2009) observed that these grandmothers, without dependent offspring, were observed taking care of their granddaughters and even suckling them. The first case was a 24 year-old grandmother who provided essential care to her 2 month-old granddaughter after her mother temporarily disappeared from the group (the author cited unknown reason for her disappearance). The second case was a 23 year-old grandmother who suckled her 14 month-old granddaughter after her mother gave birth to a younger sibling. In summary, these behavioral data indicate that healthy grandmothers without dependent offspring could contribute to the survival of their grandchildren thus supporting the Grandmother Hypothesis.
The grandmother (GM1) is retrieving her granddaughter (GD1) (a), and GD1 is holding GM1’s nipple in her mouth (b) during the period of the mother’s (M1) temporary disappearance (21 July 2008). GM1 is grooming M1 who is nursing GD1 on the first day when M1 returned to the group (28 July 2008) (c). Photo from Nakamichi et al. (2009)
Read more about the article, Old grandmothers provide essential care to their young granddaughters in a free-ranging group of Japanese monkeys (Macaca fuscata) on Primates. Also, BBC ran a story about this article, Grandmother monkeys care for baby.
"To our knowledge, there have been no reported cases in which, instead of a mother, a grandmother without dependant offspring has continuously provided essential care for the survival of her dependant grandchild, which is in accordance with the grandmother hypothesis," Dr Nakamichi and colleagues write in the journal Primates. BBC Earth News, 2009.
Nakamichi, M. Onishi, K. Yamada, K. 2009. Old grandmothers provide essential care to their young granddaughters in a free-ranging group of Japanese monkeys (Macaca fuscata). Primates Retrieved November 24, 2009, from http://www.springerlink.com/content/a30977860p50wt76/ doi: 10.1007/s10329-009-0177-7.
Walker, M. 2009. Grandmother monkeys care for baby. BBC Earth News Retrieved November 24, 2009, from http://news.bbc.co.uk/earth/hi/earth_news/newsid_8370000/8370743.stm.