Groups in Relation to
Predation
Solitary vs. Gregarious Life in Primates
Although when you think of mammals you probably think of group living
ones, actually most mammals are solitary. They live more like sloths.
However, mostly primates are group animals. Many prosimians are solitary,
but most other monkeys and apes live in groups. An exception is the
orangutan.
There are two basic questions regarding group living:
Why live in groups at all?
Why live in the size and composition that they do?
Costs and Benefits of Group Living
Why is group living beneficial? Many costs are pretty obvious.
For instance, feeding competition. Food is exhaustible and the more
people you get, the more competition there'll be. Also, the bigger your
group is, the father you're going to have to travel in a day to cover
enough food. For example, in studies of mangabeys, the larger their
groups were the larger their day ranges were. A third cost is increased
parasite and disease occurrence.
Summary of the costs:
1 feeding competition
2 increased costs of movement
3 increased transmission of diseases and parasites.
So what are the benefits which make it more worthwhile to live in
groups?
There are two main hypothesized benefits which will be subject of this
lecture and the next:
1 benefits associated with the acquisition of
food
2 benefits associated with reducing predation risk (read on)
Predation as an evolutionary force
Predation rates and problems of studying predation
Predation effects are difficult to document given problems of study.
First, because predation is a rare event. Not that it is unimportant, but
it is difficult to study. Most of the good data that we have come from
studies of the predator, not of the prey.
Also, when there's researchers around, predators are less likely to come
around, so just by observing you're usually changing the likelihood.There
was the 1993 study by Lynn Isbel in Kenya, studying vervets, during which
they noticed the 'Nairobi effect.' They noticed that a lot of vervets
would disappear when they went to Nairobi to get supplies. Not only would
more vervets disappear but there'd be more signs of leopards. The rate of
disappearances while the observers were present was .04/day, but the rate
of disappearances when they'd gone to town was .13/day. This is another
reason which makes it not easy to study predation.
Predation trends
Big snakes are common predators, as well as crocodiles waiting at
waterholes. The most important predators for primates are carnivores and
raptors. There are some eagle species which specialize in eating
primates. Big cats like lions, leopards, and tigers eat a lot of
primates.
Larger species are less vulnerable than smaller animals. Also, big
animals seem to have more different type of predators, but they get eaten
less often.
Mortality due to intra-specific killings is generally higher than rates
of inter-specific predation. So more primates are killed by members of
their own species than from members of other species. A particularly
common cause of mortality is infanticide.
Do primates respond behaviorally to predation risk? Three
behaviors:
- Alarm call
- A lot of animals use alarm calls, primates included. How important
they are depends on the size of the species. For instance, there are
studies on vervet monkeys' alarm calls by Cheney and Seyfarth. Vervets
have several different patterns of alarm calls. One call is for airborne
predators. When this call is heard, they'll run down from the trees.
There is a different alarm call for ground predators, which when heard
they run into the trees. The last is for snakes, and they don't
run anywhere but instead stand up on their hind legs and look around
carefully. Other primates, such as Gorillas for example, have alarm calls
too but they are used less often and aren't as specific.
- Mobbing
- A lot of animals do this too. You can see songbirds mobbing owls or
crows right around campus. They move toward the predator harassing and
screaming until they drive it off. Examples in primates: baboons will mob
any small carnivore, even cheetahs and jackals. When an animal lives in
large groups, it can mob large predators which it couldn't do if it was
alone. Chimps have been seen to mob leopards and lions. Rhesus monkeys
have been seen to mob tigers.
- Avoidance
- Longtailed macaque groups will go up into trees at night to sleep,
and they repeatedly use the same trees for sleeping. However, if they
encounter a python in a tree they're unlikely to use it ever again. The
interesting thing is, they don't show the same response in feeding trees;
they'll go back and eat at a tree even if they saw a python there before.
They do alter their behavior while eating, however; they stay closer to
other individuals after having seen a python.The difference probably is
that they feed in daytime so they can see better, so they're a little
more secure.
Primate examples- the two articles for today's lecture
Chimpanzee predation on red colobus- the facts:
1 There were 43 hunting episodes from 91 to 93 at Gombe.
2 28 of 43 hunts result in kills. This is a 65% success rate.
3 There were 42 kills, with multiple kills common.
4 The predation was on a population of about 50 animals.
5 Therefore about 8% of the colobus were killed by the chimps.
It also came out in this study that individual habits vary greatly. Some
chimps liked hunting better than others. One chimp in particular was
really good at it and he got about 20% of the colobus killed.
Crowned eagle predation on monkeys
Monkeys are about 84% of their prey. Struhsaker studied eagles and
checked out what they brought back and checked out the bones which fell
out of the nest. When the data was analyzed, they found that two species
of monkey were taken less often then their percentage of population in
the forest would predict. These two monkeys go around in poly-specific
groups, they run around together) and it has ben hypothesized that this
helps them avoid predation.
Possible predation-related benefits of living in groups
-
Dilution effects
- Also known as the "selfish herd" effect. You can protect
yourself by jumping into a group because your chance of being taken is
less when you're in a large group.
This theory is correct under the following conditions:
1 If predators only take a few individuals per attack.
2 If attack rate is independent of group size.
In other words, if you join a group with 10 individuals but the group
gets attacked 10 times more often, then it wouldn't pay off to be in the
group.
There's not a lot of data to back up these theories, but there is
suggestive evidence; When chimps are around, red colobus stay close to
their neighbors (less than 2 m to neighbor), but when chimps are not
around they're more like 2.5 m from their neighbors.
Increased group vigilance
- If you live in a larger group, there are more of you looking around
and it's more likely that when a predator arrives someone will happen to
be looking up.
Predator deterrence through mobbing
- Again, data from red colobus. Sometimes they mob chimps and sometimes
they don't. This data was collected at Gombe in 1974.
| defense | no defense
|
| successful | 2 | 29
|
| unsuccessful | 12 | 21 |
Of course there's also contradictory data:
| Site | Successful | Unsuccessful | Total
|
| Gombe | 16 | 3 | 19
|
| Tai | 2 | 8 | 10 |
Conclusion? You don't always see the predicted pattern.
Effects of predation on grouping patterns
A study was done on whether baboons live in larger groups when under
predation pressure. When predation risk was high, group sizes did seem to
be larger.
Another study was done on longtailed macaques: They live in Borneo where
there are a lot of predators, but there's also a group who lives on an
offshore island where there aren't so many predators. The researcher
compared the group sizes in these two places. The large groups that he
saw were only where there was high predation risk, while on the smaller
island there were only small groups.
In a final study on chimps in Tai national park, they didn't follow the
prediction. When predation pressures were high, most group sizes were
2-5. Under lower predation pressure, group sizes were 11-20, which was
the most. The thing is, most predators which prey on chimps and
sit-and-wait type predators, so it may be more helpful for chimps to be
in smaller groups.
So, predation, while not the only factor, it is an important factor for
determining why primates live in groups and what types of groups they
are.