Older mothers may do a better job raising their children than younger, less-experienced moms, at least among Killer Whales, researchers reported on Monday.They studied 30 years of data to show that calves born to the oldest Killer Whales were 10 percent more likely to survive the critical first year of life than calves born to younger mothers."Older mothers appear to be better mothers, producing calves with higher survival rates," Eric Ward of the Northwest Fisheries Science Center in Seattle and colleagues wrote in the journal Frontiers in Zoology.Killer Whale females become mature at around 15 and stop reproducing at around 40."Our work supports previous research showing that menopause and long post-reproductive lifespans are not a human phenomenon," the researchers said.For years researchers thought humans were the only creatures that had evolved menopause, and one theory was that having a healthy and unencumbered grandmother around to help take care of the babies benefited babies and mothers alike.But Killer Whales, Orcinus orca, also have menopause. The mammals live long lives, with males living up to 50 years and females living to be as old as 90.Ward's team used 30 years of data on the charismatic black-and-white carnivores to see which mothers did the best at raising calves."Older females may be more successful in raising young because of maternal experience, or they may allocate more effort to their offspring relative to younger females," the researchers wrote in their study.