Chapter 16
Monkeys seem to have an easy life. Up in the trees, they are surrounded by leaves – no need to wait for them to fall as is the case for ground-based ungulates. Succulent fruits ripen periodically, loaded with easily digested carbohydrates. At night, they can perch above the reach of most ground-based carnivores and climb a little higher than the leopards that may ascend after them. The smaller monkeys do need to watch out above for attacks that come from large eagles. In some places big apes, in the form of chimpanzees, hunt them. On shaded branches, it is rarely too hot or too cold. But this easy lifestyle is not viable in temperate regions where broad-leaved trees lose their foliage through winter and fruit production is narrowly pulsed at the end of summer.
Nevertheless, there are temptations drawing monkeys down from the trees. Many trees drop their ripe fruits, which accumulate on the ground, eaten by duikers if monkeys do not get there first. Insects may be extracted from the litter, adding protein spikes to a carbohydrate-rich diet. If trees are too far apart, terrestrial travel is necessary to move in between, or from one cluster bearing fruit to another. But as woodlands were transformed into savannas the tree spacing widened. Baboons successfully established a lifestyle as ground-based primates traversing savanna grasslands. They rely on being in large groups to detect or even cooperatively chase off threatening predators. There is usually a tree somewhere nearby to be ascended for safety from lions. Baboons ascend tall trees for nocturnal security. Where there are no trees, they employ their tree-climbing capabilities to ascend cliffs. Patas monkeys also occupy savannas, relying on their lengthened limbs for a speedy departure to the nearest tree, but are restricted to the tropics.
Actually, subsisting as a primate is not so idyllic. Primate ecologists raised the concept of fallback foods, eaten only when little else remains, because fruits are restricted seasonally in their availability, even in the tropics.1,2 Most tree leaves contain various secondary chemicals, which can be toxic and disrupt digestive enzymes. Evergreen foliage tends to be reinforced by fibre requiring bacterial fermentation to release its energy content. Rainforests retain seasonal variation dependent on rainfall. There is a period towards the end of the dry season when trees divert their resources to new leaf production and fruits become scarce. Miombo woodlands produce a wider range of succulent fruits than acacia savannas (Chapter 7),3 but still have a shortage of ripe fruits around the transition from the dry season into the wet season.
Africa’s apes currently remain forest dwellers restricted to the tropics. The chimpanzees rely on finding fruits through most of the year, falling back on new leaves and other plant parts during the times when fruits are gone. The gorilla is largely folivorous, eating fruits when available but able to survive on a diversity of herbaceous plants (but not tree leaves) in places lacking fruits. It is the colobine monkeys, not the big apes, that have taken the adaptive step of accommodating microbial fermentation to release energy even from mature leaves.
In this chapter, I will explore the consequences of dietary shifts from fruit-seeking or leaf-eating monkeys and apes to savanna-exploiting baboons. The aspects of ecology covered include areas traversed as home ranges and diurnal versus nocturnal tactics restricting the risks of predation. For whatever combination of reasons, primates as a group have evolved relatively larger brains than other herbivores and even carnivores. Large brains have costs, and the major one is to slow development and progression through life-history stages. Slower growth to reproductive maturity means that reproductive lifespans must be extended to compensate, making it especially crucial that mortality from predation is curtailed.
Primate Diets: Fruits Versus Foliage
All primates, even most humans, draw their diets largely from plant parts (Figure 16.1). Most monkeys are rather omnivorous, supplementing leaves, fruits and seeds with insects, eggs, and occasionally smaller mammals when opportunities arise. The lean time for fruit production in rainforests is around the beginning of the wettest months when trees flush new leaves and flowers. The flowers yield ripe fruits several weeks later. Tree species partition the times when their fruits ripen to span much of the year, including the drier months. Fig trees are especially valuable because they bear fruits through most of the year.4 Nevertheless, fruit crops can vary widely from one year to the next and trees bearing fruits are localised spatially. This generates lean periods when ripe fruits become scarce.5
Figure 16.1
Primates feeding. (A) Blue monkey eating berries up in a tree; (B) vervet monkey feeding on the ground (photo: Peter Henzi); (C) baboon feeding on the ground (photo: Peter Henzi); (D) gorilla eating fig fruits from the ground.
Colobus monkeys get through lean times through their capacity to digest even mature leaves. Nevertheless, they are selective among the tree species that they exploit, probably due to the varying secondary chemical contents presented. They even choose unripe fruits over ripe ones, because fermenting sugars would make their stomach contents too acid for microbes.6,7 Mangabeys rely on hard seeds, fruits and pods, frequently unripe, supplemented by insects, especially ants and caterpillars.8,9 Vervet monkeys inhabiting woodland strips within savannas consume acacia gum along with young leaves, flowers, unripe pods, and some insects when ripe fruit is unavailable.10 Patas monkeys inhabiting savannas also obtain much of their energy from acacia gum during the dry season, supported by insects, especially grasshoppers, for protein.11,12,13 They also eat the ants that live inside swollen thorns on ant-gall acacias. Geladas living in mountainous grasslands retain their dietary concentration on the leaves and seeds of grasses during lean periods, supplemented by underground corms.14 They depend on a high digestive turnover supported by efficient chewing to extract sufficient nutrition from grass blades. Fruits and insects make a small contribution opportunistically when available.
Savanna-dwelling baboons are particularly eclectic in their food habitats. Fruits are sought, but constitute only about a third of the diet of baboons, averaged across studies.15,16 Fruits are supplemented by leaves, seeds, flowers, corms, and invertebrates hidden under stones.17,18,19,20 In grassy savannas with few trees, baboons fall back on underground corms of graminoids (grasses plus sedges) and forbs along with grass rhizomes. Vertebrates such as hares and newly born antelope are caught and eaten opportunistically, but mostly by adult males and make little or no nutritional contribution to other classes. The C4 plant component in the diet of baboons, indicated by stable carbon isotopes in droppings, ranges from 20 percent to 40 percent in different study areas, but part of this comes from CAM succulents (see Chapter 7) rather than grasses.21
Chimpanzees seek out fleshy fruits as long as these remain available and gather at fruiting trees in noisy aggregations.22,23,24 When fruits become scarce, chimpanzees turn to young leaves, pithy stems, flowers, and other plant parts. Chimpanzees also probe for animal matter by poking termites from mounds and spear bushbabies hiding in tree cavities, using sharp sticks. Monkeys and baby antelope are hunted when opportunities present and are eaten mainly by males. Chimpanzees occupying wooded savanna in Senegal retain a mainly fruit diet, including some consumed even while still unripe. They supplement fruits with seeds and pods, underground plant storage organs and even some bark.25,26 Among the fruits sought are the large pods produced by baobab trees. Chimpanzees seek easily digestible fruits during the morning, switching to eating more leaves in the afternoon.27 Bonobos are somewhat more reliant on herbaceous plants than chimpanzees, converging on the diet of gorillas, which are absent from the bonobo distribution range.28
Gorillas subsist on an entirely vegetarian diet, consisting mostly of leaves and pithy stems of herbaceous plants common in the forest understorey. Fruits make a variable contribution, depending on their local availability.29,30,31,32 Lowland gorillas eat more fruit than mountain gorillas, but only while these are ripe, and pass seeds undigested in their droppings. Termites and other insects are eaten occasionally, but not any vertebrate flesh. Among lowland gorillas, males consume relatively more leaves and females more herbs and insects.30 The leafy diet of gorillas can provide more protein than these large animals actually need for maintenance.33
Hence most primates seek out fruits, some even still unripe, and fall back on vegetative plant parts, including underground storage organs or acacia gum, when fruit is unavailable. They include insects or small vertebrates in their diet opportunistically. Only gorillas and various colobus monkeys are strictly vegetarian.
Primates also need ready access to surface water for drinking, especially when ripe fruits are unavailable during the savanna dry season. Baboons drink regularly, travelling to pools to do so generally around midday.34 Colobus monkeys occupy home ranges next to rivers where they can readily obtain water.6,35
Home Range Extents
Home ranges encompass the places where animals find food, security, shelter and reproductive requirements. Bigger animals need larger areas to fulfil their greater individual food requirements. The home range extent depends also on the number of animals in the group sharing the food resources. Home ranges are smaller if leaves provide the main food source than if fruits are sought.
Colobus monkeys feeding on leaves can live within tiny home ranges covering only 2 ha, defended by pairs or family groups (Table 16.1).6,35 Forest-inhabiting guenons with more varied diets require much larger home ranges, typically around 50 ha.36,37 Vervet monkeys inhabiting savanna regions generally traverse home ranges exceeding 1 km2, but spend most of their time in smaller core areas.38,39 Home ranges of mangabeys extend up to 4 km2 to provide the fruits and seeds they need.40 Patas monkeys living in open grassy savannas cover especially large home ranges, encompassing around 4 km2 in Cameroon,41 but perhaps even as much as 50 km2 in Uganda.42
Table 16.1Comparative space use among primates
Additional reference: Clutton-Brock, TH, ed. (1977) Primate Ecology: Studies of Feeding and Ranging Behaviour in Lemurs, Monkeys and Apes. Academic Press, New York.
Baboon troops, which sometimes include over 100 individuals, move over home ranges varying in extent from 5 to 20 km2, smallest in the Okavango Delta and largest in Kenya (Table 16.1). Chimpanzees communities typically containing around 30 individuals inhabit home ranges covering around 15 km2 in forests,32 expanded to 45 km2 in savanna vegetation in Senegal.26 Home ranges occupied by gorilla groups including 5–15 animals cover 20–40 km2, with core areas contributing about half of this.31
Daily distances traversed are much less than a kilometre for colobus monkeys and around 1 km for forest-inhabiting guenons and mangabeys (Table 16.1). Savanna-inhabiting vervets travel 1–2 km per day, but patas monkeys up to 5 km. Baboon troops typically move 3–5 km per day, lengthened to 10 km in the dry savanna at Amboseli in southern Kenya. Chimpanzees at Gombe in western Tanzania travel about 4 km per day on average while foraging. Gorillas are more sedentary, travelling on average less than a kilometre per day, with a daily range varying between 0.25 and 2 km depending on the amount of fruit obtained.43 Bonobos with a more herbaceous diet than chimpanzees exhibit short daily ranges like gorillas.
Thus, home ranges traversed relative to group size increase with the fruit component in the diet and in savanna compared with forest habitats. Daily movement distances vary correspondingly. Home ranges covered by savanna-living monkeys and forest-dwelling apes are similar in size to those occupied by non-migratory ungulates, like kudu and sable antelope in Kruger NP (Chapter 10).
Evading Predation
By living in cohesive groups, primates reduce the risk of being killed by a predator. As is the case for ungulates, more eyes increase the chance of detecting a lurking predator before it gets too close. There is also a smaller chance of each individual being killed if it is a member of a group. Male monkeys and baboons may break away from groups temporarily, scouting opportunities to transfer between groups, but females and their offspring are never encountered alone.
During daylight, baboons usually detect lions at distances up to 100 m and either move away or ascend trees.44 Their main threat comes from leopards, nearly as adept at tree-climbing as they are. Baboons may respond to a threatening leopard with a coordinated attack led by adult males, which can even result in the leopard being killed.44,45 Hence leopards infrequently threaten baboons during daylight. Smaller monkeys must watch out for eagles swooping from the sky. Pythons can kill and eat monkeys, as can crocodiles when rivers are crossed.
Risks of predation increase greatly at night, particularly from leopards. Monkeys are somewhat safer than baboons because they can climb up onto smaller branches than those supporting the predator.46 In the absence of sufficiently tall trees, baboons seek nocturnal security on cliffs, as also do geladas. Chimpanzees and gorillas construct nests of branches high up in trees each evening, providing support for their large bodies. Only adult male gorillas, big enough to be a formidable opponent for a leopard, sleep frequently on the ground at night.47
More than half of the adult mortality incurred by vervet and patas monkeys in Kenya was inflicted by leopards.48 However, for red colobus monkeys about 20 percent of the mortality they experience can be due to chimpanzees.49 In Kruger NP, baboons killed by predators, including leopards, contributed only 0.1 percent of all found carcasses.50 Lions seldom eat baboons they have killed. Although about 40 percent of chimpanzee deaths in the Tai Forest in Ivory Coast were due to predation by leopards, some of this mortality was on ageing adults likely to have died soon anyway.51 All of the observed attacks by leopards on adult male gorillas were unsuccessful.52 Mortality rates incurred from predation amounted to only 5 percent per year for vervets, and 3 percent per year for baboons, during a long-term study in Kenya.46
Life Histories and Population Dynamics: Costs of Bigger Brains
Larger animals have their life-history stages prolonged (see Chapter 12). Gestation periods become longer, reproductive maturity is delayed, intervals between births get lengthened, and lifespans are extended. Among primates, the progression through life-history stages is slower than for other mammals of similar size, especially for apes and humans. Delayed maturity has been ascribed to the costs of building and maintaining relatively larger brains.53,54 Gestation generally lasts 5–6 months among African monkeys, including baboons, while chimpanzees show a 7.5-month gestation and gorillas an 8.5-month gestation. This closely matches the gestation periods of medium–large ungulates, which range between 6 and 9 months. However, baby ungulates are able to run shortly after birth, while primates are born at a feebler stage of development. Female monkeys generally delay the birth of their first offspring until they have attained 4–7 years of age and inter-birth intervals generally span 2–3 years, unless the previous offspring dies in infancy.48,55,56,57 In order for populations to be viable, the slower reproductive rates of primates must be coupled with lower rates of mortality and lengthened reproductive lifespans. Adult monkeys incur annual mortality rates ranging between 5 and 15 percent, while from 10 to 40 percent of infants die before they reach one year of age. Ground-dwelling patas monkeys exhibit higher mortality rates than other guenons, around 20–30 percent annually for adults coupled with 13–30 percent losses of infants during their first year. In compensation, female patas monkeys first reproduce at 3 years of age and typically give birth annually thereafter.48,56 Monkeys can live up to 35 years in the wild, although lifespans of around 18 years seem more typical. This is considerably longer than the lifespans of most ungulates. However, patas monkeys rarely lived longer than 10–15 years in places where they have been studied. My demographic modelling indicated that, in order to maintain a viable population, a female monkey reproducing only every other year and losing half of her offspring before they attain maturity at 5 years of age must expect to survive for 8 years or longer beyond her age at first reproduction (Table 16.2).
Table 16.2Modelled combinations of vital rates yielding either maximum or zero population growth for baboons, chimpanzees and gorillas. Zero growth has been achieved by density-dependent increases in mortality between birth and first reproduction affecting recruitment into the adult segment without much change in age at maturity, adult survival or birth intervals
The life-history patterns of baboons are similar to those of savanna-inhabiting vervets, despite the difference in body size. Female baboons first give birth at a median age of 6 years and show inter-birth intervals typically around 2 years.44,58,59,60 Juvenile baboons remain dependent on their mothers until they reach 2 years of age, and around 30–45 percent of them die during this juvenile stage.61 Much of the infant mortality is caused by male baboons.44,60 Generally, fewer than half of newborn baboons survive to reach adulthood. Annual mortality rates among adult female baboons during their early prime years may be as low as 3–4 percent annually,60 but rise as high as 9 percent annually in places where predation by leopards is more common. Baboons can live 26 years or longer in the wild. Under idealised conditions with zero mortality among both young and adult animals until the end of the lifespan, the maximum rate of population growth that could be achieved by baboons is about 12.5 percent per year, around half of the growth rate possible for ungulates (Table 16.2). Note that this takes into account terminal mortality at the end of the lifespan. Male mortality rates can be double those incurred by females.
Chimpanzees first reproduce as late as 11–14 years of age and exhibit inter-birth intervals averaging over 5 years in the wild but 4 years in captivity. Mortality rates estimated compositely from five studied populations averaged 21 percent during the first year, dropping to 5 percent annually among females during their prime stage between 15 and 30 years of age.62 However, in combination such demographic rates yield a declining population with a replacement rate (‘lambda’) of only 0.8, reflecting mortality imposed by viral epidemics as well as poaching. A specific assessment for the slowly increasing Kanyawara subpopulation in Kibale Forest, Uganda, indicated much lower mortality rates: 11 percent over the first year decreasing to 3 percent between 10 and 15 years and as low as 1 percent through the prime age range from 15 to 30 years.63 Male chimpanzees consistently incur greater mortality than females at all stages from birth through adulthood. A maximum longevity of 55 years has been recorded for a chimpanzee in the wild. The maximum potential growth rate for a chimpanzee population, assuming zero mortality throughout the adult lifespan, is around 5.5 percent per year (Table 16.2), similar to that of elephants and slower than that of white rhinos.64 Small increases in predation, infanticide or mortality among adults could pull the population growth rate below zero. In western Africa, targeted predation by leopards caused chimpanzee numbers to decline.51
Despite being larger, gorillas show a somewhat faster life history than chimpanzees. In the Virunga volcanoes, females first gave birth at 10 years and maintained birth intervals averaging 4 years.65 Annual mortality rates were 11 percent for infants, 2.5 percent for juveniles and 3.5 percent among prime-aged females. Most of the infant mortality was due to infanticide by adult males taking over groups, while some older juveniles were injured by snares. Gorillas in the Bwindi forest showed longer birth intervals averaging almost 5 years (with a minimum of 2.5 years recorded), but without any difference in offspring mortality.66 Maximum longevity attained by a gorilla is 45 years. A gorilla population could potentially grow at 6 percent per year in the absence of mortality prior to the end of the lifespan (Table 16.2).
Thus both of these great apes resemble elephants, rather than similar-sized antelope, in their life-history schedules. How can these and other primates keep predation low enough among both adults and offspring to make populations viable? This seems readily achieved in forests where there are few predators, but how could this be accomplished by savanna-inhabiting hominins?
How Is Primate Abundance Regulated?
Monkeys and baboons can attain high numerical densities locally within occupied home ranges, while densities of chimpanzees and gorillas are much lower and resemble those of low-density ungulates like kudu and sable antelope (Table 16.1). While food supplies constitute the basic limiting factor, predation may restrict the habitats that can be occupied, as noted for ungulates (Chapter 12).
The Amboseli baboon population in Kenya decreased from a density of 73 animals per km2 during the 1960s67 to under 2 animals per km2 eventually, following habitat loss. This crash was brought about by the disappearance of most of the fever tree woodlands they had depended on as their major food source, due to a rise in water table elevating soil salinity. The disappearance of these large trees also eliminated the treetop refuges they had occupied at night. Fifty-one separate troops present during the 1960s became reduced to eight over a period of 5 years. The median group size fell from 43 to 27 animals, with the largest and smallest groups being most reduced. The troops that survived did so by shifting their home ranges towards a region where umbrella thorn trees still remained. The subsequent recovery of this baboon population from a low of 123 animals in 1979 to somewhere between 1000 and 1200 animals around 201560 is consistent with a sustained annual growth rate of 6.5 percent, half of the potential maximum rate. However, an influx by baboons of a different subspecies contributed to the recovery in numbers.68 Fertility rates decrease in the largest troops and may lead to fissioning into smaller groups.69
Population declines among monkeys have been associated with drought conditions and habitat deterioration, causing elevated mortality particularly among juveniles.70 At Amboseli, vervet monkeys responded to the progressive disappearance of umbrella thorn trees as well as fever trees by expanding their territories initially, but later whole groups disappeared.71 In Cameroon, drought conditions led to substantially increased mortality among both infant and adult patas monkeys, producing a population decline.56
Although primate populations are limited fundamentally by food availability, regulation generally operates through social mechanisms effective within groups. More dominant females secure a greater share of resources than subordinate females. In both Botswana and Amboseli, high-ranking female baboons had shorter birth intervals than low-ranking ones.44 In Tanzania, infant mortality was greater in larger troops and among low-ranked females.61 However, larger groups of baboons are able to displace smaller groups into less-suitable habitat, imposing greater movement costs on the latter. Because competition gets intensified within larger groups of baboons, animals in intermediate-sized groups numbering 40–80 individuals seem to be most advantageously placed.72 When troops become much larger, low-ranked individuals may split off to form a new group, provided vacant habitat exists. Among chimpanzees, much of the mortality incurred is inflicted by adult males on juveniles, rising as density and hence social strife and inequality increases.
Overview
Primates depend to varying degrees on leaves, fruits and other plant parts as food resources. Their dietary protein content may be augmented by insects, although leaves alone can supply adequate protein. Both vervet and patas monkeys occupying savannas still obtain most of their food from trees. Chimpanzees are strongly dependent on forest fruits, while gorillas and bonobos rely largely on herbaceous plant parts obtained from the forest floor. Chimpanzees and baboons augment their carbohydrate-loaded diets with animal flesh obtained opportunistically, with males gaining most of the benefit.
Only baboons and geladas subsist substantially on the grass parts and forbs prevalent in savannas. To survive through the dry season, baboons fall back on carbohydrates stored in corms and bulbs underground. Their digging capabilities for deeper bulbs and roots are outdone by porcupines and undermined by mole-rats. Chimpanzees occupying savanna woodlands must cover substantially larger ranges than traversed by forest-inhabiting communities in order to support their frugivorous diet. Densities attained by the great apes in tropical forests are no greater than those shown by browsing ungulates in open savannas, despite the lush plant growth.
All non-human primates rely on their climbing capabilities to restrict predation, especially at night. They must keep mortality due to predation lower than incurred by ungulates because of their slow life-history progression, ascribed to building relatively bigger brains than other animal forms. Mortality losses incurred by gorillas and chimpanzees are indeed lower than those exhibited by most ungulates and resemble that of megaherbivores like rhinos and elephants, but these great apes inhabit forested regions where leopards are sparse and other large carnivores absent. Keeping mortality rates this low in savanna regions thronged with large carnivores is a more formidable undertaking.
SUGGESTED FURTHER READING
Boesch, C; Boesch-Achermann, H. (2000) The Chimpanzees of the Taï Forest: Behavioural Ecology and Evolution. Oxford University Press, Oxford.
Hohmann G, et al. (eds). (2006) Feeding Ecology in Apes and Other Primates. Cambridge University Press, Cambridge.
Schaller, GB. (1963) The Mountain Gorilla. University of Chicago Press, Chicago.
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