Selective Value of Characteristic Size Parameters in Hylobatids. A Biomechanical Approach to Small Ape Size and Morphology

Abstract

Small apes of the family Hylobatidae are with 5–8 kg and 10–12 kg (siamangs) within the size range of many arboreal simian primates and at the upper size limit of extant prosimians. On the other hand, hylobatids are much smaller than their close relatives the large-bodied African apes and orangutans where even females weigh three times as much as a siamang or more, not to speak of the much larger body sizes of great ape males. Among living apes, hylobatids also seem to be morphologically more specialized compared to other hominoids, which raises the question if biomechanical reasons can explain hylobatids’ special morphological traits. Body size is best described as body mass, but several length measurements are also highly informative of the hylobatid morphotype. Three sources were used in this chapter: (i) published observations of postural behavior, (ii) other authors’ and the authors’ own dissections and measurements, and (iii) knowledge of mechanical conditions and physical laws that govern posture and ideomotoric movement as well as locomotion in animals. By applying physical laws, attempts are made to pin down selective pressures that act on the body and give it its characteristic shape and size. Biomechanical formulae were developed to show the selective advantages of size-related traits. We suggest that characteristics of the arboreal habitat and conditions of substrates keep body size at limits. Bending rigidity of twigs is much less than their tensile strength, and therefore suspension below branches allows fairly large, flightless animals access to resources in the periphery of trees. Suspension by the forelimbs permits maintaining an upright body posture, and arm-swinging is a useful, energy-saving mode of locomotion. The famous deep cleft between digit rays I and II of hylobatid hands allows seizing bigger stems or branches or thicker bundles of thin twigs than would be possible without the deep cleft trait. Moreover, the narrow shape of the hand allows increasing compression between seized substrate and the hand so that friction does not decrease with hand length. Length of the forelimb in pendulous progression yields high speed at low energy expenditure and forelimb length makes slapping with the hand a powerful, dangerous threat in agonistic encounters. During feeding, the length of the forelimb determines the ‘feeding envelope’, which grows by the third power of arm length. Longer-armed animals need to shift their feeding position less frequently than shorter-armed animals and overall they need fewer changes of feeding stations to exploit the same volume as an animal with a shorter forelimb length. These features associated with long arms contribute to safety and allow saving energy. There are, however, limits to forelimb elongation. While the limiting factor ‘support strength’ can only be determined empirically, necessary muscle forces associated with longer forelimbs can be calculated. The greatest forces between substrate and animal occur during acceleration and especially deceleration of the body or its parts, and any increase of the required muscle force also entails an increase in weight, not only of the muscle but also of supporting bones, which leads to higher forces operating between substrate and body. The insights gained from a biomechanical perspective to understand the hylobatid morphotype and its functioning in a complex, three-dimensional arboreal environment are interpreted as selective advantages/disadvantages during hylobatid evolution.