By Richard Wrangham
In a new Current Biology paper based on almost 1000 photographs of social grooming, Kanyawara researchers have shown for the first time that variants of chimpanzee social behavior can be passed from mother to offspring, and last for a lifetime.
During the many decades that Jane Goodall spent observing chimpanzees in Gombe National Park, Tanzania, there is one kind of social grooming that she never saw. The missing kind is called high-arm grooming, a style whose significance was first realized by Gombe researchers Bill McGrew and Caroline Tutin when they visited another Tanzanian chimpanzee research site in Mahale Mountains National Park in 1972. There they saw high-arm grooming for the first time. They instantly suggested it to be a social tradition. Since then it has become renowned as the classic example of chimpanzee social culture.
The pattern was well known to the team of Mahale researchers led by Toshisada Nishida. Every now and again, a pair of grooming chimpanzees each raised their opposite arms (left to left, or right to right). They would hold them in contact for up to about two minutes at a time while continuing to groom with their other hands. Nishida had assumed that all chimpanzees practiced this striking salute.
However while high-arm grooming has now been recorded from eight wild populations across Africa, it is entirely absent in three. Strikingly, the three that lack high-arm grooming are found in different subspecies (Gombe and Budongo in the east, Bossou in the west), supporting the notion of the differences being due to culture rather than genes. In Kanyawara, and probably throughout Kibale, high-arm grooming can be seen almost every day.
High-arm grooming was originally called hand-clasp grooming, but that term does not capture the fact that the style of contact between the uplifted arms varies widely. Sometimes chimpanzees indeed clasp their raised hands together (called palm-to-palm clasping). However at other times their arms are in contact only at the wrist, or along the forearm. The frequency of palm-to-palm clasping varies extensively among populations from less than 5% to more than 80% of high-arm grooming events.
This variation in the frequency of palm-to-palm clasping has been an intriguing mystery. Could palm-to-palm clasping be a sign of mutual friendship? Does it signify membership in a particular social network? Or does it indicate shared ages, or dominance ranks? Our new paper rules out those ideas and finds a surprisingly simple pattern. Whether or not a chimpanzee clasps another’s palm during high-arm grooming depends on whether their mother did so. Palm-to-palm clasping is a social custom passing down matrilines.
There is much yet to find out. Kanyawara mothers vary in their frequency of palm-to-palm clasping with their family from less than 10% to more than 90% of occasions. Where does that variation come from? And most importantly, why do they high-arm groom at all? When members of matrilines with different rates of palm-to-palm clasping engage in high-arm grooming, one of them has to abandon their preference. So who wins? Our current data do not give any indication that the “winner” is higher-ranking, or older, or female, or anything else. But with 1000s more photographs, maybe a pattern will emerge. If we are lucky we might then find out whether this social custom has a social meaning larger than doing what Mum did.
Birth rates can be a good indicator of the viability and health of great ape populations. While apes reproduce slowly as a rule, richer and more stable food resources lead to faster production of offspring, putting members of these endangered species on a path towards population growth. In Kanyawara, chimpanzee birth rates have increased noticeably over the past 2 decades. Our alpha female, Outamba, has produced infants every 3.5 years, a startling 2 years faster than the average wild chimpanzee. But is there a downside to having all of these babies? Life history theory suggests that, because mothers have limited resources to devote to reproduction, they may face tradeoffs between producing many offspring and investing in the quality of each. While these tradeoffs are almost inevitable, evidence for them has eluded anthropologists studying humans and other primates.
In a recent issue of the Proceedings of the National Academy of Sciences, the Kibale Chimpanzee Project team tested this prediction by examining the influence of reproductive rates on offspring growth. We found that an immature chimpanzee’s size had a strong positive relationship with the interval to its next sibling’s birth. In other words, when mothers could afford to reproduce faster, they did so at the expense of their offspring’s growth. Remarkably, chimpanzee juveniles did not manage to make up for these early growth deficits later on, but remained small throughout their adolescent years. These tradeoffs could come about because mothers who reproduce quickly exhaust their supplies of energy to feed infants. Alternatively, when a mother produces two offspring in quick succession, she might withdraw nutritional support from the first in order to feed the second. Our data support the second interpretation. It was mothers like Outamba, who were in the best energetic condition during lactation, that produced the smallest offspring. Instead of using surplus energy to enhance their current offspring, these mothers appeared to bank it so that they could afford to produce a new infant sooner. Because wild chimpanzee mothers cannot afford to feed two infants at the same time, they have to wean one infant in order to feed the next. The weaned infants then have to feed themselves, even if they have not yet learned how to be efficient foragers or developed the digestive system to fully process raw plant foods. For humans, this is less of a problem. Human infants continue to receive nutritional support from their mothers and other family members long after they finish nursing, and cooked foods are both calorie rich and easy to digest.
KCP is dedicated to research that is non-invasive. One of the big challenges that we have faced is how to estimate the body size and energetic condition of our animals without weighing them. Weighing is both impractical and disruptive, as it would require baiting scales with food. Over the years, we have developed novel assays for urinary biomarkers of muscle mass and energy balance that enabled us to complete this study. With this toolkit, we can begin to answer many new questions about growth and body size. Stay tuned!
GRASP (Great Apes Survival Partnership) just released this short video on the illicit trade in great apes. Narrated by KCP co-director Dr. Richard Wrangham, it introduces the Apes Seizure Database, an important new tool to fight the global traffic in great apes.
Richard Wrangham discusses apparent doll play by young chimpanzees. See the striking footage from Kanyawara on BBC Earth.
By Drew Enigk and Kris Sabbi
Chimpanzees occasionally supplement their diet of fruit and herbaceous vegetation with meat, which they most frequently obtain by hunting red colobus monkeys. Rates of hunting and successful capture of prey vary across communities. For example, within Kibale National Park, chimpanzees in the Ngogo community have decimated local red colobus populations, while chimpanzees in Kanyawara have typically hunted less often. Since Eslom took over the alpha position in January, however, the Kanyawara chimpanzees have targeted monkeys more frequently, and caught many more per hunt.
Eslom instigates a large fraction of hunts in Kanyawara, taking after his father, Johnny, who was one of the community’s most active hunters before his death in 2014. A newly published study by Ian Gilby and colleagues uses over 70 years of collective behavioral data from three study sites, including Kanyawara, to examine the effects of “impact hunters” like Johnny on community hunting rates. The impact hunter hypothesis proposes that certain individuals hunt more readily than others, and that by seizing the initiative during encounters, they cause monkeys to scatter, creating opportunities for others to catch prey. The presence of impact hunters during monkey encounters is positively associated with the probability of a hunt occurring.
Several of the hunts in which Eslom was involved after his alpha takeover in early 2015 yielded more than five red colobus captures, with Eslom sometimes responsible for >50% of the kills. This trend continued into the summer. In early June, Eslom single-handedly captured a grey-cheeked mangabey and two black-and-white colobus monkeys. On June 24, the Kanyawara chimpanzees took down a record number of colobus in a single hunt!
Before the chaos started, the chimpanzees were lounging and playing on a hillside. Suddenly, chimpanzee hunting calls cut through the air, indicating that monkeys had been spotted and targeted. Suddenly, everyone was at full attention, sprinting toward the call.
Eslom was already up a tree beside Kanyawara’s second-best hunter, an adolescent male named Tuber. Other males displayed beneath them, dragging branches as they ran from one side of the group to the other. While female and juvenile monkeys retreated to the top of the tree, the male colobus formed a defensive line beneath them, squawking and screaming at their enemies below. Eslom and Tuber individually rushed at the monkeys, but each was forced back. More and more chimpanzees joined the offensive. With each attack, the colobus banded together, repelling one chimp at a time. Finally, a hole opened up in the monkeys’ defenses, and the chimpanzees rushed in toward the females.
The pace of the hunt escalated quickly, with panicked monkeys scattering from their perches. Soon they misjudged their leaps, falling from thin branches into the eager hands of the chimpanzees. They were seized so quickly that we researchers could barely keep up with the action.
Our voices rang out across the scene: “Eslom caught one!” “Tuke—Tuke has one!” “—And Tuber!” “Tacugama!!” Four males, four monkeys. By the fifth kill, even the young chimpanzees were catching monkeys! Adult males are the usual hunters among chimpanzees, so it was especially surprising that three of the kills were made by 6-11 year-olds, and that two of these young hunters were females. When the slaughter was over, nine red colobus lay dead, the most successful hunt ever recorded at Kanyawara, and particularly noteworthy considering the involvement of the young chimpanzees. (The record from Ngogo, with many more males hunting, is 13.)
Eslom killed two monkeys in this historic hunt, and a large party of chimpanzees spent the rest of the day sharing the spoils. Now that Eslom has attained alpha status and entered his early 20s – prime hunting years for male chimpanzees – he appears to have established himself as the new impact hunter at Kanyawara.
The Kibale Chimpanzee Project (https://kibalechimpanzees.wordpress.com) seeks a postdoctoral scholar, based at the Department of Anthropology at the University of New Mexico, to participate in an NIH-funded study of aging in wild chimpanzees. This project involves the development and application of non-invasive methods to study health and aging in wild chimpanzees in the Kibale National Park, Uganda. The successful applicant will coordinate activities in the endocrinology laboratory, organize incoming data, conduct visits to the field site, and work with PIs Melissa Emery Thompson and Martin Muller to analyze and publish findings. The scholar will be encouraged to develop their own research interests relevant to the broader aims of the project, and this may incorporate primary field work. This is a one-year position, renewable for at least one additional year pending renewal of funding and satisfactory performance. We seek a self-motivated candidate that can work independently but also function well as a member of a team.
Required qualifications: Ph.D. in anthropology, biology, ecology, zoology, or other related field by the time of position start date; experience with observational research methods; evidence of competence in statistics.
Preferred qualifications: expertise in executing laboratory protocols (hormonal or other); previous experience with primates, humans, or other social species; ability to travel internationally and function in field conditions; competency with Microsoft Access; excellent verbal and written communication skills.
The position is funded at $45,000 plus benefits; additional funding will be provided for research costs (travel, research fees, etc.).
Applications received by December 15, 2015, will receive best consideration. Anticipated start date for the position is March 1, 2016. Please send a CV, cover letter outlining your relevant experience and research interests, 1-2 professional writing samples, and the names of 2 recommenders to Melissa Emery Thompson (firstname.lastname@example.org).