Linda F. Marchant

Intergroup aggression in chimpanzees and humans.

The occurrence of fatal attacks during intergroup encounters among chimpanzees suggests that certain aspects of chimpanzee and human intergroup aggression may be explicable in similar ways. This paper addresses three questions: What conditions favor the evolution of lethal raiding in intergroup aggression? Why is intergroup aggression in these two species predominantly the domain of males? Under what circumstances do groups compete over access to females as opposed to material resources? Examination of comparative data on nonhuman primates and crosscultural study of foraging societies suggests that attacks are lethal because where there is sufficient imbalance of power their cost is trivial, that these attacks are a male and not a female activity because males are the philopatric sex, and that it is resources of reproductive interest to males that determine the causes of intergroup aggression.

On the other hand: Current issues in and meta-analysis of the behavioral laterality of hand function in nonhuman primates

The last decade has seen a resurgence of interest in laterality of function in primates, especially in hand use as it links to handedness and language in Homo sapiens. Manual lateralization of behavior in humans reflects asymmetry in cerebral structure, which must have evolved from nonhuman progenitors. To what extent is hand function lateralized in our nearest living relations? First, we address current issues of theory and methodology: statistics, measurement, variables, setting, sensory modality, and sample size. Specific topics include preference vs. performance, posture, bimanuality, inheritance, and arm asymmetry. We categorize the published literature in a descriptive, classificatory framework of five levels that range from Level 1, ambilaterality, to Level 5, human-like handedness. In a meta-analysis we put 241 published data-sets to a methodological test of seven criteria and code the 48 survivors onto the levels framework, by taxonomic grouping (prosimian, New World monkey, Old World monkey, ape, chimpanzee). Primates at Level 1 are mostly wild or naturalistic populations performing spontaneous species-typical behavior patterns. Most primates are at Levels 2 and 3, that is, individually lateralized to either side, especially on complex, demanding or practiced tasks, usually as devised in captive settings. Only chimpanzees show signs of population-level bias (Levels 4 and 5) to the right, but only in captivity and only incompletely. We conclude that nonhu- man primate hand function has not been shown to be lateralized at the species level—it is not the norm for any species, task, or setting, and so offers no easy model for the evolution of human handedness. Yrbk Phys Anthropol 40:201- 232, 1997. r 1997 Wiley-Liss, Inc.

Laterality of Hand Use Pays Off in Foraging Success for Wild Chimpanzees

The aim of this study was to see if behavioral lateralization in hand use benefits a lateralized organism in nature. We recorded wild chimpanzees (Pan troglodytes schweinfurthii) at Gombe, Tanzania, fishing for termites (Macrotermes spp.), an extractive foraging task using elementary technology. We compared individual apes who were completely lateralized, using only one hand or the other for the task, versus those who were incompletely lateralized, using either hand. Exclusively lateralized individuals were more efficient, that is, gathered more prey per unit effort, but were no different in success or error rate from incompletely lateralized apes. This is the first demonstration of a payoff to laterality of behavioral function in primates in conditions of ecological validity.

Laterality of function in apes: a meta-analysis of methods

Abstract Asymmetry of cerebral structure in human primates is correlated with laterality of function, which by homology, predicts that the same will hold for other hominoids (Hylobatidae, Pongidae). To what extent is the cerebral asymmetry of living apes reflected in functional laterality? To test this hypothesis, we analysed all 58 published sources of relevance for Pan (bonobo, chimpanzee) Gorilla (gorilla), Pongo (orang-utan), and Hylobates (gibbons). From coding the methods in terms of eight variables (function, context, sample, age, task, number, trials, complexity), we found that most studies focused on: hand use rather than other organs; captive rather than wild subjects; small rather than large samples of subjects; immature rather than adult subjects; spontaneous rather than induced measures; few rather than many tasks; few rather than many trials; simple rather than complex tasks. Other independent or dependent variables have been treated inconsistently or rarely, e.g., sex, performance asymmetries. Thus, the hypothesis cannot yet be tested, and such issues as population-level handedness versus individual hand preferences remain unresolved.

Nest building behavior in the great apes: the great leap forward?

INTRODUCTION Over decades apes have served as either referential or conceptual models in attempts to reconstruct the path of human evolution (Ghiglieri, 1987; Wrangham, 1987). In the search for behavioral traits shared by all members of the great apes, few have turned out to be conservative, that is, common features seen in all extant hominoids, and by inference present in our common ancestor. Of these shared traits, skilled object manipulation has been of great interest in comparative analyses as a basic criterion for hominization. Tool use and tool production, however, vary tremendously not only among the four species but also within a single species. Thus the trait in common is not tool use itself, but the general ability for environmental problem solving (McGrew, 1992). Nest building is part of this ability. It is probably the most pervasive form of material skill in apes. Whether or not this trait should be considered as tool use is much disputed (Goodall, 1968; Alcock, 1972; Beck, 1980; Galdikas, 1982). Nest building is called ‘bed building’ by some investigators (Itani, 1979; Hiraiwa-Hasegawa, 1986). It is a daily habit of weaned great apes to build a place in which to rest. The technique employed depends on the site and on the available materials. Orangutans, chimpanzees and bonobos start their arboreal constructions by preparing a foundation of solid sidebranches or forks, bending, breaking and inter-weaving sidebranches crosswise.

Using the Tools at Hand: Manual Laterality and Elementary Technology in Cebus spp. and Pan spp.

Cebus and Pan appear to be a remarkable example of evolutionary convergence in behavioral ecology. We examine their apparently analogous solutions to problems posed by laterality of hand function and elementary technology. We scrutinize appropriate published data in a meta-analysis, focusing on Cebus apella and C. capucinus and on Pan paniscus and P. troglodytes. We compare behavioral data in terms of captive versus wild, and tool use versus non-tool use, but notable gaps exist in the data, especially for bonobos. Cebus and Pan spp. are equivalent tool users in captivity, but chimpanzees are notably more extensively so in nature. For hand preference, captive bonobos and wild and captive chimpanzees show ambipreference for non-tool-use patterns. For both Cebus spp. and Pan spp., there is a tendency for individuals to be committed exclusively to one hand or the other for tool use. The data for laterality of hand function fit consistently into the five-level model proposed by McGrew and Marchant (1996).

Ant fishing by wild chimpanzees is not lateralised

Right-dominant handedness is unique and universal in Homo sapiens, suggesting that it is a highly derived trait. Our nearest living relations, chimpanzees, show lateralised hand preference when using tools, but not when otherwise manipulating objects. We report the first contrary data, that is, non-lateralised tool-use, for ant fishing as done in the Mahale Mountains of Tanzania. Unlike nut cracking, termite fishing, and fruit pounding, as seen elsewhere, in which most individuals are either significantly or wholly left- or right-biassed, ant fishers are mostly ambilateral. The clue to this exception lies in arboreality; all other patterns of chimpanzee elementary technology are done on the ground. Arboreal tool use usually requires not only that one hand be used to hold the tool, but also that the other hand gives postural support. When the supporting hand is fatigued, then it must be relieved by the other. Terrestrial tool use entails no such trading off. To test the hypothesis, we compared frequency of hand changing with the incidence of major hand support, and found them to be significantly positively correlated. The evolutionary transition from arboreality to terrestriality may have been a key enabler for the origins of human laterality.

Manual Laterality in Anvil Use: Wild Chimpanzees Cracking Strychnos Fruits

Wild chimpanzees (Pan troglodytes schweinfurthii) at Gombe National Park, Tanzania, smash open the hard-shelled fruits of Strychnos spp. on anvils of stone or woody vegetation. In this food-processing task, most of the apes show exclusive use of one hand or the other, that is, strong individual hand preferences. Such extreme laterality of manual functioning corresponds to Level 3 on a five-level descriptive model of lateralisation that appears to reflect the increasingly skillful demands of object manipulation. There is precise congruence in laterality between anvil use and another subsistence task involving elementary technology-termite fishing-in almost all cases.

“An ape's view of the Oldowan” revisited

In 1989, Wynn and McGrew published an explicit comparison between Oldowan technology and what was then known of chimpanzee technology. 1 They compared the range and variety of tools, adaptive role of tools, carrying distances, spatial cognition, manufacturing procedures, and modes of learning. They concluded that everything archeologists had reconstructed about the behavior of Oldowan hominins could be accommodated within the ape adaptive grade; that is, a paraphyletic group united by overall similarities in anatomy and, in this case, behavior. The only Oldowan activities that were almost unknown for modern apes were the long‐distance transport of objects and direct competition with carnivores, which was implied by meat acquisition activities. “In its general features Oldowan culture was ape, not human. Nowhere in this picture need we posit elements such as language, extensive sharing, division of labor, or pair‐bonded families, all of which are part of the baggage carried by the term human.”1:394

Social scratch: Another custom in wild chimpanzees?

Chimpanzees (Pan troglodytes schweinfurthii) in the Mahale Mountains National Park, Tanzania, scratch other individuals bodies while they groom them. This behavioral pattern of “social scratch” is another example of locality-specific social behavior, or custom, as it is not found in the Gombe National Park, Tanzania, about 150 km north of Mahale, nor has it been reported from any other sites of chimpanzee study. Frequency of social scratch was correlated with frequency of social grooming, but not with frequency of self-scratch. Frequencies of social scratch per grooming bout among adult and adoles-cent males, and from lactating females to infants or juveniles, were high, and among males, higher-ranking males especially received more. These facts indicate some social function of the behavior. Social scratch was directed mostly to the dorsal side of the body. However, when lactating females social scratched to infants or juveniles, they scratched other body parts. Social scratch was not lateralized to left or right. We present four hypotheses on the functional origin and on the learning process of this cultural behavioral pattern.