Thomas S. Kraft

Tree climbing and human evolution

Paleoanthropologists have long argued—often contentiously—about the climbing abilities of early hominins and whether a foot adapted to terrestrial bipedalism constrained regular access to trees. However, some modern humans climb tall trees routinely in pursuit of honey, fruit, and game, often without the aid of tools or support systems. Mortality and morbidity associated with facultative arboreality is expected to favor behaviors and anatomies that facilitate safe and efficient climbing. Here we show that Twa hunter–gatherers use extraordinary ankle dorsiflexion (>45°) during climbing, similar to the degree observed in wild chimpanzees. Although we did not detect a skeletal signature of dorsiflexion in museum specimens of climbing hunter–gatherers from the Ituri forest, we did find that climbing by the Twa is associated with longer fibers in the gastrocnemius muscle relative to those of neighboring, nonclimbing agriculturalists. This result suggests that a more excursive calf muscle facilitates climbing with a bipedally adapted ankle and foot by positioning the climber closer to the tree, and it might be among the mechanisms that allow hunter–gatherers to access the canopy safely. Given that we did not find a skeletal correlate for this observed behavior, our results imply that derived aspects of the hominin ankle associated with bipedalism remain compatible with vertical climbing and arboreal resource acquisition. Our findings challenge the persistent arboreal–terrestrial dichotomy that has informed behavioral reconstructions of fossil hominins and highlight the value of using modern humans as models for inferring the limits of hominin arboreality.

A natural history of human tree climbing.

Walking and running have dominated the literature on human locomotor evolution at the expense of other behaviors with positive and negative fitness consequences. For example, although modern hunter-gatherers frequently climb trees to obtain important food resources in the canopy, these behaviors are seldom considered within the existing framework of primate positional behavior. As a result, inferences about the arboreal performance capabilities of fossil hominins based on a resemblance to humans may be more complicated than previously assumed. Here we use ethnographic reports of human tree climbing to critically evaluate hypotheses about the performance capabilities of humans in trees compared with other primates. We do so by reviewing the ecological basis of tree climbing behavior among hunter-gatherers and the diversity of human climbing techniques and styles. Results suggest that the biological and adaptive significance of human climbing has been underestimated, and that some humans are surprisingly competent in trees, particularly during vertical climbing and activities in the central core of trees. We conclude that while hominins evolved enhanced terrestrial locomotor performance through time, such shifts may have imposed only minor costs on vertical climbing abilities. The diversity of the locomotor repertoire of modern humans must therefore be taken into account when making form-function inferences during the behavioral reconstruction of fossil hominins.

Phenotypic Plasticity of Climbing-Related Traits in the Ankle Joint of Great Apes and Rainforest Hunter-Gatherers

Abstract The “negrito” and African “pygmy” phenotypes are predominately exhibited by hunter-gatherers living in rainforest habitats. Foraging within such habitats is associated with a unique set of locomotor behaviors, most notably habitual vertical climbing during the pursuit of honey, fruit, and game. When performed frequently, this behavior is expected to correlate with developmentally plastic skeletal morphologies that respond to mechanical loading. Using six measurements in the distal tibia and talus that discriminate nonhuman primates by vertical climbing frequency, we tested the prediction that intraspecific variation in this behavior is reflected in the morphology of the ankle joint of habitually climbing human populations. First, to explore the plasticity of climbing-linked morphologies, we made comparisons between chimpanzees, gorillas, and orangutans from wild and captive settings. The analysis revealed significant differences in two climbing-linked traits (anterior expansion of the articular surface of the distal tibia and increased degree of talar wedging), indicating that these traits are sensitive to climbing behavior. However, our analyses did not reveal any signatures of climbing behavior in the ankles of habitually climbing hunter-gatherers. These results suggest that the detection of fine-grained differences in human locomotor behaviors at the ankle joint, particularly those associated with arboreality, may be obscured by the functional demands of terrestrial bipedalism. Accordingly, it may be difficult to use population-level characteristics of ankle morphology to make inferences about the climbing behavior of hominins in the fossil record, even when facultative arborealism is associated with key fitness benefits.

Hunter-gatherer residential mobility and the marginal value of rainforest patches

Significance Hunter-gatherers are notable for their high levels of mobility, but the ecological and social cues that determine the timing of camp movements (residential mobility) are poorly understood. Using models from foraging theory, we found that, for one population of hunter-gatherers, camp movements coincided with the point at which resource acquisition declined to a critical threshold level, but before local resources were completely depleted. These results suggest that hunter-gatherer residential mobility is constrained in a predictable fashion by rates of local resource depletion. The residential mobility patterns of modern hunter-gatherers broadly reflect local resource availability, but the proximate ecological and social forces that determine the timing of camp movements are poorly known. We tested the hypothesis that the timing of such moves maximizes foraging efficiency as hunter-gatherers move across the landscape. The marginal value theorem predicts when a group should depart a camp and its associated foraging area and move to another based on declining marginal return rates. This influential model has yet to be directly applied in a population of hunter-gatherers, primarily because the shape of gain curves (cumulative resource acquisition through time) and travel times between patches have been difficult to estimate in ethnographic settings. We tested the predictions of the marginal value theorem in the context of hunter-gatherer residential mobility using historical foraging data from nomadic, socially egalitarian Batek hunter-gatherers (n = 93 d across 11 residential camps) living in the tropical rainforests of Peninsular Malaysia. We characterized the gain functions for all resources acquired by the Batek at daily timescales and examined how patterns of individual foraging related to the emergent property of residential movements. Patterns of camp residence times conformed well with the predictions of the marginal value theorem, indicating that communal perceptions of resource depletion are closely linked to collective movement decisions. Despite (and perhaps because of) a protracted process of deliberation and argument about when to depart camps, Batek residential mobility seems to maximize group-level foraging efficiency.

Lagged effects of early-season herbivores on valley oak fecundity

The seasonal match between folivore and leaf phenology affects the annual success of arboreal folivore populations because many folivores exploit developing leaves, which are an ephemeral resource. One strategy for folivores to exploit early-season leaves is to anticipate their emergence. The consequence of this behavior for trees is that individuals that set leaves earlier may experience greater rates of folivore damage, with potential negative fitness consequences. To test this hypothesis, we surveyed the early-season phenology, insect folivore damage, and acorn crop of a population of valley oaks (Quercus lobata) over a 3-year period. We found that trees that set leaves earlier experienced greater rates of folivore damage than trees that set leaves later in the season. In addition, we observed a lagged effect of folivore damage on acorn production, whereby trees with greater leaf damage produced fewer acorns in the subsequent year. These results indicate potential negative fitness consequences of earlier leaf phenology. Our study suggests that folivore pressure may be one factor that affects the optimal timing of leaf set in oaks.

Seed size and the evolution of leaf defences

Summary Leaf defences vary widely among tree species, affecting rates of herbivory, survival and reproduction. Two contrasting hypotheses account for variation in leaf defences among species. The first predicts that a slow life history, which is characteristic of larger seeded species adapted to resource-limited environments, is associated with well-defended leaves. The second, apparency theory, predicts that elevated leaf defences are necessitated for species that are more detectable to herbivores. Here we use comparative methods and a global data set to test (i) the relationship between seed size and leaf defences and (ii) the relationship between clumping (spatial apparency) and leaf defences. We found that seed size was positively related to leaf fracture toughness, but not phenolics or tannin concentration and that spatial aggregation was unrelated to leaf defences. Synthesis. Our results suggest that larger seed size and increased leaf toughness are correlated as part of a trait syndrome associated with a slow, resource-limited life history, not clumped dispersion and increased spatial apparency.

Could plant extracts have enabled hominins to acquire honey before the control of fire

Honey is increasingly recognized as an important food item in human evolution, but it remains unclear whether extinct hominins could have overcome the formidable collective stinging defenses of honey bees during honey acquisition. The utility of smoke for this purpose is widely recognized, but little research has explored alternative methods of sting deterrence such as the use of plant secondary compounds. To consider whether hominins could have used plant extracts as a precursor or alternative to smoke, we review the ethnographic, ethnobotanical, and plant chemical ecology literature to examine how humans use plants in combination with, and independently of, smoke during honey collection. Plant secondary compounds are diverse in their physiological and behavioral effects on bees and differ fundamentally from those of smoke. Plants containing these chemicals are widespread and prove to be remarkably effective in facilitating honey collection by honey hunters and beekeepers worldwide. While smoke may be superior as a deterrent to bees, plant extracts represent a plausible precursor or alternative to the use of smoke during honey collection by hominins. Smoke is a sufficient but not necessary condition for acquiring honey in amounts exceeding those typically obtained by chimpanzees, suggesting that significant honey consumption could have predated the control of fire.

William Hoppitt and Kevin N. Laland: Social Learning: An Introduction to Mechanisms, Methods, and Models

In a seminal moment for the field of primatology, in 1953 an 18-month-old female Japanese macaque (Macaca fuscata) named Imo was observed performing a simple but highly innovative behavior: she washed a dirt-covered sweet potato in the water before eating it. Imo’s behavior subsequently diffused through her troop, being adopted both by her close relatives as well as other, unrelated individuals (Kawai 1965). Primatologists have since reported many parallel cases of apparent social diffusion of innovative behavior. Recently, for instance, researchers studying gelada monkeys (Theropithecus gelada) in the Ethiopian highlands observed an unusual behavior in their primarily gramnivorous study subjects: an adult female gelada named Rerun discovered bird eggs as a viable food and this egg-feeding behavior then spread through the troop (Fashing et al. 2014; V. Venkataraman, pers. comm.). While examples like these may involve some form of information transfer, how are researchers to be sure that such naturally observed behavioral diffusions actually involve social learning, wherein individuals copy the behavior of others? Are there methods of distinguishing different mechanisms of social learning, either observationally or experimentally? And what insights can mathematical models of social learning offer to empiricists working in the field or laboratory? William Hoppitt and Kevin Laland tackle these fascinating questions in their accessible and comprehensive book Social Learning. This book synthesizes current Int J Primatol (2015) 36:1060–1064 DOI 10.1007/s10764-015-9868-7