A reward for those of you who stayed awake to the end of my last post…some photos of Negra enjoying a forage on the hill this afternoon:
Archives for March 8, 2013
It’s one of the most commonly asked questions about chimpanzees and for most of the last 15 years I’ve been answering it incorrectly. In my defense, I was not alone. Search the internet for “chimpanzee lifespan” and you will often read that chimpanzees live 40-50 years in the wild and 50-60 years in captivity, or something to this effect. This was the standard line when I first started to learn about chimps, but it now appears to be wildly inaccurate. How could this be?
For such a simple question, the answer for many years was difficult to come by. If, for example, you asked, “How long do fruit flies live?” the answer would be cut and dry. Just follow a bunch of fruit flies from birth to death and record their lifespan, which is measured in days. But for a species like the chimpanzee that has only been studied on a large scale for a handful of generations and whose lifespan is measured in decades, good data was hard to come by. Over the years, however, researchers have collected more and more data on captive and wild chimpanzees and a new picture has emerged, one which has drastically changed my own understanding of chimpanzees.
So…how long do chimpanzees really live?
For chimpanzees in captivity, the best information we have is this: For those who survive to their first birthday, median life expectancy is 31.7 years for males and 38.7 years for females. These figures were provided to us by Lincoln Park Zoo’s ChimpDATA as part of a unique program we participated in to help project future demographic trends in sanctuaries, and are based on 35 years of records from Association of Zoos & Aquariums (AZA) institutions.
A couple of technical points, if you’re interested: First, you’ll notice that this excludes infant mortality. If infant deaths were included, life expectancies would be even lower (closer to 32 years for males and females combined, I believe). The reason why infant deaths are excluded is because infancy remains a particularly high-risk period, even in captivity. If we want to know how long a yet-to-be-born chimpanzee might live, we might include it. But most of the time we are trying to determine the expected lifespan of chimpanzees who are already young adults or adults (typical of chimpanzees coming to sanctuaries), who by definition have already beaten the odds of that early high-risk period. Second, these figures are median ages. The way to interpret this is that for a given figure, half of all chimpanzees in that category will die before that age, and half will die after that age. So you would expect to see many chimpanzees live past the median age – in fact, one chimpanzee in the AZA group lived to 72. However, you would also expect an equal number to die before the median age. Finally, we don’t know how different captive environments and life histories affect life expectancy, so it’s possible that future data from sanctuaries will change our understanding of this unique population.
Life expectancy in wild chimpanzees tends to be the same or lower than in captive chimpanzees. One study found that average life expectancy for chimpanzees across five field study sites was only 15 years. But for those chimpanzees who survived to adulthood, which in this study was defined as 12 years old, their life expectancy was an additional 15 years. Of course, many wild chimpanzees live well beyond 27 years – the oldest wild chimpanzee was estimated to be about 63 years old when she died. Direct comparisons between these types of studies can be difficult because they use different methodologies and analyses, but it gives you a rough idea.
The importance of this issue is not just academic – it has the power to shape the way we think about many different aspects of captive chimpanzee care. As we learned through ChimpDATA, information about life expectancy can help us predict future capacity in sanctuaries, as well as the costs associated with caring for a chimpanzee across his or her lifetime. But just as importantly, it can help give us more realistic expectations about the lives of the chimpanzees that we all care for and support.