The whale species known for a “grandmother effect” among post-reproductive females is the killer whale. In resident killer whale societies, older females can live for decades after they stop reproducing and continue helping their family groups.
Research has found that post-reproductive grandmothers improve the survival of their grandoffspring. Their experience may help pods locate prey, especially during difficult conditions, while their social knowledge supports younger relatives. This provides one explanation for why female killer whales evolved such a long post-reproductive lifespan.
The pattern is closely related to menopause, which is unusual among mammals. Killer whales are one of the few nonhuman species in which females regularly live long after fertility ends; the other well-known examples include humans and some other toothed whales.
A common mix-up is treating the effect as a claim about every whale species. The strongest evidence concerns resident killer whales, and the benefit is social and evolutionary—not that grandmothers continue producing calves themselves.