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Calliope (Figure 1), an old, female chimpanzee at Mahale, is assumed to have died this past year (i.e., 2012). Her death is assumed because it has not been confirmed, but she has not been witnessed since the final observation on 5th April 2012. As it is rare to observe the deaths of wild chimpanzees directly, or to find the dead bodies, we usually assume individuals who have not been observed for more than 3 consecutive months in Mahale to be deceased. Calliope was estimated to be 52 years old in 2012, which is the oldest age at death so far recorded at Mahale. She was first identified in 1973 as a young adult. The starting age of adulthood in females is 13 years old. Therefore, she would have been at least 13 years old at that time. This gave the estimation that she was born in 1960, but her exact birth-year could be even earlier.

According to Nishida¹, Calliope had been very shy to human observers until the 1990s. Although she later became tolerant of observations from a certain distance, she continued to be more timid than the other females. She seemed reluctant to get too close to humans, especially when she had a small baby. Throughout her life, Calliope gave birth to a minimum of 5 offspring (i.e., 3 females and 2 males)². Except for 1 female offspring who died at 3 years old, the other 4 were weaned and reached the age of puberty. Thus, it can be said that she was a successful mother. After she gave birth to her last offspring in 1997, she did not give birth again throughout the last 15 years of her life. Excluding the 5 years during which she was nursing her last baby, she enjoyed her remaining post-reproductive life for 10 years.

Calliope was often observed traveling with her last daughter, Carmen, until Carmen emigrated from the M group in July 2011. Calliope also groomed often with her two mature sons, Carter and Cadmus, when she met them. After the emigration of Carmen, Calliope often ranged together with Wakusi, an older female who is estimated to be one year younger than Calliope. As the oldest duet, they usually kept a moderate distance from noisy males, even though they both had mature sons. However, in large, they ranged alongside the other chimpanzees. We sometimes observed that they would feed apart from the others, whose calls could be heard from a distance. Some moments later, they would travel in the direction of the calls in tandem. When they caught up with a group of males, the males would often groom the two older females. The females, however, seldom groomed the males in return, as if they were enjoying respects for the aged. Calliope might have suffered from presbyopia. For instance, when she groomed others, she straightened up her upper body so that her eyes were at a distance from the groomee’s body surface.

When MN observed Calliope on 15th December 2011, she was, again, travelling with Wakusi. When these two, and a mother-infant pair, were resting under a bush, there came her fully adult son, Carter. Then, Calliope and Carter started to play with each other. They tickled, slapped, and mouthed each other’s body and chased each other in a circle. Both of them showed a play face and play panted. A 51-year-old mother and a 26-year-old adult son were playing like two juveniles! The play lasted for 23 minutes.

HN was the last researcher to see Calliope on 4th March 2012, when he was following an adult male who was in consort with a young estrous female. On this day, again, Calliope and Wakusi were together. The two joined the party that HN was following. Although the observation time for Calliope was short, HN did not notice anything peculiar about her health or physical condition. After that day, she was not observed again for some time. This is not uncommon in March–April, however, as Mahale chimpanzees are usually dispersed in small parties throughout this season. On 5th April 2012, a research assistant observed Calliope with 10 other chimpanzees. This was the last day that she was observed.

Establishing the longevity, or the maximum life-span, of wild chimpanzees is difficult to ascertain. Chimpanzees typically have a long lifespan, and their ages are often estimated. Let us compare, here, the 52 years of life of Calliope (the longest documented lifespan at Mahale) with other wild chimpanzee populations. At Gombe, the oldest estimated age at death was 53 years for the famous chimpanzee, Flo³. At Bossou, the oldest female, Kai, died at an estimated 53 years of age⁴. At Taï, a female, Chanel, died when she was 46 years old (calculated from Appendix Table A.1. in ref. 5). These ages are all within the estimated range of the maximal lifespan potential of captive chimpanzees (48 ± 5 years; taken from a literature survey covering over 100 zoos worldwide; cited in ref. 6).

We have to be careful about the specific age at death for chimpanzee individuals in the wild because they are all estimates. For example, Flo’s age at death was first estimated to be 43 years old by Goodall⁷, but she was estimated to be 10 years older in the later literature³. This may be because Flo was already old, in her 30s or 40s, when she was first identified in 1962. Estimating the age of older individuals is particularly difficult because individuals differ in their appearance. Like humans, some chimpanzees look younger and others look older than they really are. Accordingly, we see, with a bit of suspect, that a 55-year-old mother gave birth to a child at Kibale⁸. This means that, if the estimation of age is correct, her longevity will be much longer than that or previously recorded individuals. Research at Kibale was initiated in 1983⁹, which means this female was in her 30s at that time. Thus, there may the possibility that her age was overestimated.

For the estimated age of Calliope, we expect a small proportion of error because she was estimated to be 13 years old when she was first identified. It is possible that one would estimate a 33-year-old female to be 43 years old, but it is not likely that one estimates a 3-year-old female to be 13 years old. From our experience, estimation error of a 13-year-old female can be within 3 years or so, thus we can more confidently say that Calliope was between 49 and 55 years old at the time of her death.

Finally, we should mention some aged individuals who are still alive. At the time of writing, there were several older chimpanzees at Mahale, such as Wakusi (female, 51), Gwekulo (female, 50), Fatuma (female, 49) and Kalunde (male, 49) (numbers in parentheses are their estimated ages in 2012, which are equally accurate as, or even more accurate than, that of Calliope because these individuals were in their young adulthood or adolescence when identified). Some of these individuals now look old in their appearance and show the typical behavioral characteristics of old age, such as slow traveling speed and a relative decrease in activity. However, at present, they all appear to be in good health. Similarly, at Bossou, 4 females and 1 male older than 50 years were still alive in 2012 (Ohashi G, personal communication). Looking at these figures, although the sample size is still small, it seems that more females experience old age than males at both sites. This assumption concurs with the fact that males, on average, show higher mortality than females¹⁰.

We hope that these older chimpanzees will live longer, well beyond the current records of longevity in wild chimpanzees. Then, our current knowledge about the longevity of wild chimpanzees would be revised.

We thank COSTECH, TAWIRI, and TANAPA for their permissions to conduct long-term research at Mahale; Prof. Y. Sugiyama for constructive comments on the manuscript; Mr. G. Ohashi for updated information on Bossou chimpanzees; and researchers of the Mahale Mountains Chimpanzee Research Project for their contribution to the accumulation of demography data of Mahale chimpanzees. MN’s field study was supported by Kakenhi (#19255008, #19107007, and #21770262) and HN’s field study was supported by the JSPS Institutional Program for Young Researcher Overseas Visits.