Why are there apes? Evidence for the co‐evolution of ape and monkey ecomorphology

Abstract

Apes, members of the superfamily Hominoidea, possess a distinctive suite of anatomical and behavioral characters which appear to have evolved relatively late and relatively independently. The timing of paleontological events, extant cercopithecine and hominoid ecomorphology and other evidence suggests that many distinctive ape features evolved to facilitate harvesting ripe fruits among compliant terminal branches in tree edges. Precarious, unpredictably oriented, compliant supports in the canopy periphery require apes to maneuver using suspensory and non-sterotypical postures (i.e. postures with eccentric limb orientations or extreme joint excursions). Diet differences among extant species, extant species numbers and evidence of cercopithecoid diversification and expansion, in concert with a reciprocal decrease in hominoid species, suggest intense competition between monkeys and apes over the last 20 Ma. It may be that larger body masses allow great apes to succeed in contest competitions for highly desired food items, while the ability of monkeys to digest antifeedant-rich unripe fruits allows them to win scramble competitions. Evolutionary trends in morphology and inferred ecology suggest that as monkeys evolved to harvest fruit ever earlier in the fruiting cycle they broadened their niche to encompass first more fibrous, tannin-and toxin-rich unripe fruits and later, for some lineages, mature leaves. Early depletion of unripe fruit in the central core of the tree canopy by monkeys leaves a hollow sphere of ripening fruits, displacing antifeedant-intolerant, later-arriving apes to small-diameter, compliant terminal branches. Hylobatids, orangutans, Pan species, gorillas and the New World atelines may have each evolved suspensory behavior independently in response to local competition from an expanding population of monkeys. Genetic evidence of rapid evolution among chimpanzees suggests that adaptations to suspensory behavior, vertical climbing, knuckle-walking, consumption of terrestrial piths and intercommunity violence had not yet evolved or were still being refined when panins (chimpanzees and bonobos) and hominins diverged.

Segmental morphometrics of bonobos (Pan paniscus): are they really different from chimpanzees (Pan troglodytes)?

“…the most specialized suspensory ape), the need for mobility around the hip joint when moving in the trees. Because it is doubtful that bonobos are more arboreal and more suspensory primates than chimpanzees (Ramos, ; and Hunt, vs. Doran, ), our hypothesis about the size‐related muscular production capacity seems more appropriate to explain this difference between bonobos and chimpanzees.…”

“…This trunk elongation in bonobos may possibly increase the inertia for an improved control of the rotation about its vertical axis during orthograde locomotor modes, such as suspensory activities and bipedal walking (Preuschoft, ). However, bonobos are not more bipedal than chimpanzees (at least in captivity; Videan & McGrew, ), and the differences in the proportion of suspensory activities are called into question (Hunt, ). On the other hand, a longer and slender trunk in pronograde locomotion, although increasing the bending moments along the trunk, would also provide an advantage in fast locomotor modes such as bounding, galloping, jumping (Preuschoft, ), at least if there is mobility of the lumbar spine, and bonobos have been shown to exhibit impressive jumping performances (Scholz et al.…”

Mosaic evolution, preadaptation, and the evolution of evolvability in apes

“…Early‐Miocene ape and monkey species evolved in overlapping environmental conditions and shared similar ecomorphological and dietary habits. However, as their lineages diverged, apes evolved a diverse set of locomotor suites, while evolution in locomotor features has remained comparatively static across old world monkeys (Hunt ). The patterns in developmental and mosaic morphological evolution revealed here suggest that the substantial differences between the two taxa in phenotypic disparity were shaped more by differences in patterns in integration and constraint rather than from the ecological opportunities available to either.…”

Evidence of a chimpanzee-sized ancestor of humans but a gibbon-sized ancestor of apes

“…Taking a step back, suspensory behavior and increased body mass have been argued to be hominoid adaptations to a foraging strategy allowing them to compete with increasingly numerous old world monkeys since the Middle Miocene (reviewed in ref. 60 ). Our results suggest that these two adaptations occurred independently of each other and could have been part of an arms race with monkeys for fruit resources—suspensory behavior to access ripe fruit on compliant branches at the edges of foliage evolved first, followed by larger body sizes when direct physical competition was required.…”