The Role of Hunting in Anthropogeny: Ian Gilby-Pan the Hunter: Explanations for Chimpanzee Predation

– [Announcer] We are the paradoxical ape. Bipedal, naked, large-brained. Long the master of fire,
tools and language. But still trying to understand ourselves. Aware that death is inevitable. Yet filled with optimism. We grow up slowly. We hand down knowledge. We empathize and deceive. We shape the future from our shared understanding of the past. CARTA brings together experts
from diverse disciplines to exchange insights on who
we are and how we got here, an exploration made possible by the generosity of humans like you. – Humans eat meat more often and in greater quantities
than any other primate. And as we’ve already heard today twice, meat consumption is seen by many as one of the defining
features of our species. And it’s been linked to the evolution of our large brains, bipedal locomotion, pair bonding and other
distinctly human traits. So therefore, it’s critically
important to understand what caused this dramatic dietary shift. So, to understand how and why meat eating became so prevalent in our lineage, we need to know something
about the ancestral state. In other words, we need to make inferences about the behavior of
the last common ancestor of humans and our
closest living relatives. So I’m talking about this creature here. It lived in Africa four to
eight million years ago. So, as our closest living relatives, and specifically as large-bodies, primarily fruit-eating apes, chimpanzees and bonobos provide clues about the behavior of
this last common ancestor. Of course, they are not
exact replicas or models of the last common ancestor. Instead, they help us to understand the ecological pressures
that this animal faced, and, in turn, can help us understand what changed in our lineage and why. Both chimpanzees and bonobos eat meat, but chimpanzees do so more often. So we know more about the factors that affect their hunting frequency. So that’s why I’m discussing
chimpanzees today. A little bit of background
on chimpanzee biology. Chimpanzee diet is
dominated by ripe fruit. Figs, like these here, are
particularly important. And the abundance and
distribution of ripe fruit in chimp habitat affects
grouping and travel patterns of the chimpanzees, which, as we will see, also affect hunting behavior. In addition to a largely fruit-based diet, chimpanzees also eat leaves,
specifically young leaves, which are easier to digest
and extract protein from. They eat flowers and stems as well as invertebrates
like termites and ants, and at some sites, they
use tools to do so. So here’s a chimpanzee at Gombe using a long blade of grass to extract termites from a termite mound. So it should come as no
surprise to this audience now that chimpanzees also eat meat. But compared to humans, they
do so relatively rarely. So this is a, and it’s
actually really surprising to find that there are relatively few published data on the proportion of diet spent eating meat. So this is just a quick
and dirty breakdown of diet from Gombe National
Park, which is where I work. And as you can see, the time spent feeding is dominated by ripe fruit and leaves, and meat makes up about 4.3
percent of feeding time. Insects are about 3.2 percent. Compared to Kanyawara, which
is Richard Wrangham’s site, there they spend less than one percent of their feeding time eating meat. And at Mahale, where they estimated it in terms of grams per day,
it’s only about 45 grams. So very very different to
what we see among humans. So, by far, the most
important prey species for chimpanzees is the red colobus monkey. These guys weigh from
eight to 10 kilograms, although at some sites the chimps target the infants and the juveniles, which are obviously much smaller. Red colobus are arboreal,
they live in trees. They rarely come down to the ground. And they live in large
groups, 50 or so individuals. And they fiercely defend themselves against the chimpanzee hunters. So when I say that they’re the most important prey species, you can see why. So here are five major chimpanzee long-term research sites, and at all sites where red
colobus and chimpanzees coexist, red colobus are the most
frequent prey species. So you can see here, at Mahale,
Tai, Gombe and Kanyawara, it’s about 80 percent of the prey taken are red colobus monkeys. And at Ngogo, at least until recently, 90 percent of the prey were red colobus. But this has changed in recent years, because the chimpanzees have decimated the red colobus population, which we may hear about
later from David Watts. So which species make
up the other category? The little gray bars at
the top on that last graph? Well, to some extent,
other arboreal monkeys. So from left to right, red-tailed guenons, black and white colobus
monkeys and blue monkeys. And then also they prey
upon small ungulates, infant bush pigs, at the
bottom left, bushbuck fawns, adult duikers, which are
small forest antelopes, and then small animals
that hide in tree holes, like this galago here,
and nestlings and so on. So hunts of the latter,
particularly the bottom row here, don’t really involve pursuit in the same way that hunts
of arboreal monkeys do. They’re much more of a quick grab as the chimps come across
them as they’re foraging. Now hunting frequency varies considerably within and between sites, ranging from as few as two kills per year in Budongo Forest in Uganda to as many as 136 per year
at Ngogo, also in Uganda. So this variation gives us the opportunity to study the ecological factors that affect hunting decisions. So clearly, much of it,
much of this variation is gonna be driven by what prey species are available in the habitat. So as we saw, red colobus monkeys dominate the prey profiles at the sites where they’re present. So if they’re not available, we’re gonna see lower rates
of hunting overall, obviously. So we can see here, at
Fongoli and Budongo, there are no red colobus monkeys, and their number of prey
killed per year are quite low. However, we see also, there’s considerable variation in the sites where
red colobus are present. So there is additional variation that’s not explained by the
presence of red colobus monkeys. So it’s not just gonna be the presence of a particular species that’s important, it’s also gonna be important how dense the population is, how many there are in the habitats. And this is a particular
challenge that we face, particularly when it
comes to the cryptic prey. So, for example, these galagos, they’re small, solitary,
nocturnal primates that are active at night, and they nest in tree
cavities during the day. Really hard to get a good sense of how many there are in the habitat. So we’re gonna hear in Jo
Fritz’s talk later today that these galagos are eaten relatively frequently at Fongoli, but this may simply be because, relative to other sites, perhaps they’re more common there than they are elsewhere. There’s also considerable
variation within sites as well. So I’ll use Kibale National
Park as an example here. Duiker, this small forest antelope, are frequently seen at Ngogo, but only rarely seen at Kanyawara, which is a chimpanzee community
in the same exact forest. So it’s really challenging to do so, but I think we really need
to have some systematic comparison of prey density, both within and between study sites, to fully understand prey choice. So with that in mind, what other factors are going to affect hunting frequency? So as a behavioral ecologist, I think in terms of the costs
and the benefits of hunting. The main costs are energy, especially when hunting arboreal monkeys, because it takes a lot of effort to climb trees quickly
and chase the monkeys. Time is important. Mean hunt duration at
Gombe, Craig Stanford found, is about 28 minutes, so
there’s opportunity cost. And also injury. From falling or from being attacked by prey who clearly
don’t want to be eaten. The benefits, as we heard
about from Alyssa, are clearer. Meat is always gonna be beneficial. Meat is concentrated source of easily-accessible
macro- and micro-nutrients However, for chimps, it does, it would be, it seems unlikely that meat
is essential to their diet, given that some communities rarely hunt, and even among communities
that do hunt frequently, there are some individuals
who rarely get meat. So they have other alternatives, presumably the insect diet that they have, to get some of these nutrients, although it’s just less efficient than getting it from meat. So, therefore, we expect chimpanzees to take advantage of low-cost
opportunities to get meat. They can afford to be picky because they don’t require it to survive, but when they can get
it, they’ll advantage of those low-cost opportunities. So I’m proposing that variation in factors that affect hunting costs should affect hunting frequency. So what evidence do we
have that chimpanzees are especially sensitive
to the costs of hunting? First, theory predicts that females should be more risk-averse than males. Female reproductive
success is more closely tied to their body
condition, compared to males, whose reproductive success is determined by the number of females
that they mate with. So, the costs of wasted effort, wasted effort on failed hunting, should be greater for
females than for males. So this predicts, then, that females should specialize in prey that
are less costly to capture. So indeed, at Gombe, females
capture over 60 percent, you can see the gray bar
on the right-hand side, 60 percent of the sedentary,
passive prey items, like fledglings and bushbuck fawns, things that don’t fight
back and don’t run away, compared to only about 10
percent of the arboreal monkeys, which require a lot of effort to capture, and they fight back. In between, we have
terrestrial aggressive prey like bush piglets who, it
doesn’t involve climbing through the trees, but they
have very very angry moms who you do not want to mess with. So at Gombe, females specialize in sedentary, non-threatening prey. Females do, as we saw,
sometimes hunt red colobus. But when they’re present at a hunt that’s already in progress, they are less likely than
males to participate. So this, again, suggest
that they are particularly sensitive to the costs of hunting. Also, there is less incentive
for females to hunt, because when they are successful, most often, a male just
comes and takes it away. So instead, females are more likely to sit and watch from below, and then beg for meat from
successful male hunters. At all sites, upon encountering
a red colobus troop, a hunt is more likely to occur if there are many adult male chimpanzees present in a party. Chimps have what we call a
fission-fusion social system, so if you think of that oval
as one social group’s range, the entire social group are
never all together at once. Instead, they form fluid subgroups that change in size and composition in relation to food and
female availability. So, when I talk about the
number of male chimpanzees, I’m talking about the
number of male chimpanzees in one of those subgroups. And, as I said, the
likelihood of a hunt occurring upon encountering red colobus monkeys is proportional to the number of males present in that party. So there is some debate about why this is. It could be that, when
there are more males there, there are more opportunities
to collaborate, and to coordinate one’s actions. However, as my colleagues
and I have argued, we think there’s a simpler alternative, that group hunting simply arises as the cumulative effect
of independent actions. The more hunters there are, the easier it is for one
individual to make a kill. Either way, hunting costs are lower as more individuals hunt. So large communities are certainly going to have, they’re more likely to form these large hunting parties, contributing to the
overall hunting frequency. So remember this chart
that I showed earlier with variation in hunting frequency within sites where red
colobus are present? So, some of this variation is certainly going to be due to community size. So over the years, when
these data were collected, there were 150 to 200 chimpanzees
in the Ngogo community, while Gombe, Mahale and
Kanyawara were all around 50. So it certainly seems as though the large population size at Ngogo had an important effect
on hunting frequency. Another factor is variation
in forest structure. So this is a high-res satellite
image of part of Gombe, and you can see, just by looking, that the density and
continuity of the forest canopy varies considerably within the range of a single chimpanzee community. So, we look in the valleys, we see evergreen forest, which looks like this. It’s a continuous canopy, tall trees. It’s really costly for
an individual to hunt. Lots of energy is going to be
spent climbing and chasing, and it’s easier for monkeys to escape, so hunts might take longer and be less likely to succeed. On the ridges, we see woodland habitat, which are characterized by short trees that are widely spaced, so it’s easy to trap monkeys in one single tree. Less energy spent catching them. And, as we expect from that, we see that forest structure does indeed have an effect on both the
probability of hunting, in the darker maroon bars,
both at Ngogo and at Gombe, and also affects the probability that a hunt succeeds as well. Finally, overall diet quality affects whether or not chimpanzees will take on the risk of hunting. So for example, Richard
and I showed at Kanyawara, that hunting is more frequent during periods when overall
high quality fruit is available. And this result here isn’t just because high quality fruit enables the
formation of large parties, as it does at Gombe and at Ngogo, but once you take that into account, we found that the probability
of a hunt occurring was simply higher after
controlling for group size during times when there was
high quality fruit available. So a party of six males,
you can see there, hunts twice as often as a party, a party of six males in
high quality fruit seasons hunts twice as often
as a party of six males in low quality fruit seasons. We’ve argued that this is because the costs of a failed hunt are lower when there is high quality
food to fall back on. So to conclude, there is considerable variation in hunting frequency
within and between sites. And this is because hunting frequency is affected by many ecological factors having to do with the prey, having to do with the predators, and the ecology of the site. So, if we agree that
it’s logical to conclude that the last common ancestor was faced with similar
ecological pressures, and would therefore have
exhibited similar patterns, how then do we explain the increase in hunting in the hominin lineage? So I suggest that the first step might have been an increase in low-cost opportunities to capture prey, which could likely come as a
combination of many factors, but I would argue that
certainly prey availability and habitat were key factors. Thank you very much.

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