PRIMATES, 19(2): 363-369, April 1978 363

S H O R T C O M M U N I C A T I O N S

The Function of Ischial Callosities

JOEL A. VILENSKY The University of Wisconsin, Madison

ABSTRACT. Ischial callosities are generally related to the sitting positions of cercopithecoid monkeys, gibbons, and siamangs, but the selective advantage of ischial callosities to the sit- ting postures of these animals is uncertain. It is suggested that ischial callosities are adapta- tions that evolved for comfortable and stable sitting on thin branches during feeding in the peripheral branch zone. It is further suggested that prehensile tails in larger New World mon- keys also developed as mechanisms for exploitation of food resources found among terminal branches. Small platyrrhine monkeys do not possess any structural adaptations to peripheral branch feeding because their body size permits them to efficiently exploit small or large branches for food. Pongids are in the process of losing their ischial callosities because their large body size precludes sitting on thin branches.

INTRODUCTION

Ischial callosities are horny, epidermal thickenings located bilaterally below the anus of all Old World monkeys, gibbons, and siamangs (PococK, 1935). The callosi- ties are sharply circumscribed and firmly attached to the underlying ischial tuberosi- ties (MILLER, 1945). In chimpanzees, gorillas, and orang-utans, callosity-like thicken- ings over the ischial tuberosities are occasionally found (SCHULTZ, 1936; MILLER, 1945; ROSE, 1974a). Nothing resembling callosities is found in man or in platyrrhine monkeys (PoCOCK, 1920; SCI-IULTZ, 1936).

Ischial callosities are undoubtedly related to sitting positions and are commonly cal- led "sitting pads" (LE GROS CLARK, 1959). However, the selective advantage they confer to sitting is uncertain. WASHBURN (1957) suggested that ischial callosities are ".. . adaptations which make it possible for monkeys to sleep in a sitting position." He admitted that his hypothesis was speculative and that more data might invalidate it. Although much remains to be known about primate postural adaptations, enough data have become available in recent years to question WASHBURN'S hypothesis. ROSE (1974b) clearly demonstrates the essential role ischial callosities play in the daytime postural activities of Old World monkeys. Furthermore, the prehensile-tailed platyr- rhine monkeys can rest or sleep in sitting postures similar to those of catarrhine mon- keys despite their absence of callosities (EISENBERG & KUEHN, 1966; ROSE, 1974b; MITTERMEIER & FLEAGLE, 1976 ; MENDEL, 1976).

In spite of ROSE'S (1974b) demonstration, he fails to explain the selective advantage callosities confer to sitting Old World monkeys, gibbons, and siamangs. However, ROSE'S data is in agreement with my 51 hours of observations of the postural adapta- tions of six cercopithecoid species, eight ceboid species, and three hominoid species at the New York Zoological Park (VILENSKY, 1974). These observations suggest the

364 J.A. VILENSKY

hypothesis that ischial callosities are adaptations for feeding among peripheral branch- es. Ischial callosities allow the Cercopithecoidea and the Hylobatidae to sit comfor- tably and with stability on the small branches situated just central to the fine twigs which contain the peripheral foliage. From this position the animal can lean toward the peripheral foliage and use its hands to exploit the food resources of this region (see GRAND, 1972). The peripheral branch zone contains many concentrated food sources for non-insectivorous primates (RosE, 1974b; CARTMILL, 1974).

FEEDING POSTURES IN OLD WORLD MONKEYS AND HYLOBATIDS

All Old World monkey species that I observed assumed sitting postures on inclined branches. On these branches the frictional qualities of the callosities played a crucial role in allowing the monkeys to sit securely without the aid of the forelimbs. These Old World monkeys were also observed to sit on a branch with only a single callosity sup- porting most of their weight. Sometimes this single supporting callosity was planted on a pointed tree-knob which protruded from the surface of the branch. This ability to balance on a single callosity and on inclined branches suggests the advantage ischial callosities offer to flee-living monkeys feeding among small branches. Rose (1974a) states, "...the relative immobility of the callosities and their surface characteristics prevent the animal from slipping off the branch...but are particularly important when they feed among small branches." He also notes (1973) that the use of ischial callosi- ties flees the hands for food gathering. RIPLEY (1967) reports that langurs, when eating or sleeping at the periphery of a tree, will support themselves on the ischial callosities and grasp additional supports with their feet and one hand; or, they may balance entirely on the callosities. HU~L(1966)states that erect sitting permits the emancipation of the hands, thus enabling the animal to explore its environment.

Colobus guereza is reported to use sitting postures during 95 ~o of the time spent feeding in arboreal settings (ROSE, 19741o). Comparable figures are 94 ~ for Cercopi- thecus aethiops, 95 ~o for Cercopithecus ascanius, 94 ~ for Cercopithecus mitis, and 90 for Papio anubis (RosE, 19741o). Furthermore, ROSE states that feeding activity patterns are similar for all Old World monkeys that feed in arboreal settings. "The animal walks peripherally along a branch, stands briefly to inspect the vegetation for food objects, then sits and feeds on all objects within the range of its forelimbs" (ROSE, 1974b).

WELLS (1974) describes the arboreal sitting postures used by Cercopithecus aethiops. While sitting among small branches and reaching for food, these monkeys maintain balance by utilizing ischial callosity support plus footgrasps and contact between one forelimb and a surrounding branch. POIRIER (1972) also describes C. aethiops utilizing sitting postures when feeding from branch-ends.

RIPLEu (1970) notes that most of the foods that attract Presbytis entellus are located within two-three feet of the ends of branches. To eat these foods a langur sits on its eal- losities and grasps footholds, usually on branches at a lower level than the callosities. The monkey then systematically eats the surrounding food via a series of body position changes. The animal eats either directly from the attached branch by bending the branch toward itself and plucking off food with its fingers or mouth, or by break- ing off a twig and eating directly from it (RIPLEY, 1970).

The Function of Ischial Callosities 365

The detailed accounts of ROSE (1974b), WELLS (1974), and RIPLEY (1967, 1970) reveal the importance of sitting postures and hence ischial callosities to the peripheral branch feeding adaptations of Old World monkeys. Few studies other than these give even simple descriptions of the feeding positions of callositied primates. BOOTH (1957) notes how the olive colobus monkey sits on a leafy bough and consumes several oun- ces of foliage without shifting its position. GRAND (1972) describes how sitting is adap- tive for terminal branch feeding in macaques. MORBECK (1974) reports that for Colo- bus guereza sitting postures utilizing ischial callosities are usually associated with either feeding or resting activities. Similarly, MITTERMEIER and FLEAGLE 0976) describe sit- ting to be the most common feeding posture for Colobus guereza. Gibbons and sia- mangs are reported to feed both in sitting and in suspended postures (CARPENTER, 1940; GRAND, 1972; CHIVERS, 1974).

ISCHIAL CALLOSITIES, PREIIENS1LE TAILS, AND BODY SIZE

An evolutionary view of body size in anthropoids and knowledge of the effect of body size on feeding among peripheral branches substantiates my hypothesis. It is highly probable that the earliest anthropoids were relatively small (STANLEY, 1973), perhaps no larger than small cebid monkeys. One of the earliest known anthropoids (Apidium phiomense) from the Fayum deposits in Egypt is thought to be approximate- ly the size of the present day owl monkey (SIMONS, 1972). Due to a high ratio of branch size to body size and a low center of gravity as present in extant small arboreal mam- mals (NAPIER, 1967), Apidium and earlier anthropoids could have balanced atop thin branches with little danger of falling. In their feeding habits these early primates, like small New World monkeys, could have used their hands for gathering food from branches while perching solely on their hindlimbs. In fact, platyrrhine monkeys with- out prehensile tails rarely sit with their weight resting predominantly on the gluteal surface. Instead they rest and feed in perching or squatting positions with their weight on their fore- and hindlimbs, or on the hindlimbs alone (BEATTIE, 1927; FITZGERALD, 1935; ASI1TON & OXNARD, 1964; MOYNIHAN, 1964; HAMPTON, HAMPTON, • LAND- WEIIR, 1966; NAPIER & NAPIER, 1967; VILENSKY, 1974). Thus early anthropoids were probably quite adept at terminal branch feeding.

As body size increased in anthropoid primates, it became advantageous to develop structural adaptations for feeding among peripheral branches. One such adaptation was ischial callosities. DELSON (1975) also suggests that ischial callosities may have been an early adaptation of cercopithecoid monkeys. Thus the development of ischial cal- losities in evolving catarrhine lineages permitted an increase in body size without a loss in the ability to feed from peripheral branches.

For most New World primates the problem of balance during feeding among ter- minal branches did not arise because body size remained small. However, structural adaptations were eventually necessary in the Atelinae and the Alouttinae lineages if the animals were to continue feeding among peripheral branches. I suggest that in these two subfamilies prehensile tails, rather than ischial callosities, developed in response to the selective pressures imposed by increased body size and small branch feeding. In other words, the initial selective advantage offered by prehensile tails to the ancestors of the large platyrrhine monkeys was that the tail helped support and distribute the

366 J.A. VILENSKY

animal's weight when the animal reached for food among small branches. Both Alouatta and Ateles are reported to feed while being suspended by the hindlimbs and tail, or by the tail alone (CARPENTER, 1934; EISENBERG & KUEHN, 1966; CARPENTER & DURHAM, 1969 ; MENDIm, 1976 ; MIXTERMEIER & FLEAaLE, 1976). Furthermore, for spider monkeys, CARVENTER and DURHAM (1969) state that suspended postures are especially adaptive for feeding in terminal branches. Similarly, for the howler mon- key, MENDEL (1976) notes, "The prehensile tail is of utmost importance in the success- ful negotiation of this zone (the upper and peripheral part of the tree crowns) of the arboreal habitat, particularly the exploitation of the fruits and leaves on very small, flexible, terminal branches."

The above hypothesis suggests, therefore, that ischial callosities in Old World pri- mates and prehensile tails in large New World primates are different anatomical adap- tations that developed to solve a similar problem faced in both hemispheres of pe- ripheral branch feeding with increased body size.

]SCHIAL CALLOSITIES AND TIlE HYLOBATIDAE

Gibbons and siamangs utilize their ca]losities to feed in sitting postures but will also feed in suspended postures (CARPENTV, R, 1940; GRAND, 1972; CraVERS, 1974). CARVEN- TER (1940) notes that brachiation is a useful adaptation in the fruit eating habits of the gibbon. The probable explanation for the duality of feeding adaptation in the Hy- lobatidae is that by using above- and below-branch adaptations for feeding, gibbons and siamangs can increase their feed area at any one branch point by 100 ~ (GRAND, 1972). Further, body size and branch size impose a limit to the utility of ischial callosi- ties for feeding among small branches. CraVERS (1974) notes that siamangs will feed in suspended postures on branches too small for sitting. Therefore "brachiating" adap- tations give the hylobatids an advantage over monkeys for terminal branch feeding. However, the long forelimbs of gibbons and siamangs that permit brachiation and suspended feeding postures are probably a rather recent evolutionary development (SIMoNs, 1972). Thus ischial callosities represent a primary adaptation for terminal branch feeding in hylobatid lineages. Whether brachiation developed in gibbons as a feeding adaptation, an adaptation for locomotoryefficien cy (FLEAGLE, 1974), or more likely as a result of both, callosities have been retained as an important part of the feed- ing, resting, and sleeping adaptations of gibbons and siamangs.

]SCHIAL CALLOSITIES AND THE PONGIDAE

WASHBURN (1957)proposed that the common ancestor of chimpanzees, gorillas, and orang-utans had ischial callosities. He suggested that parallel evolution is occurring in these lineages because pongids have become too large to sleep in a monkey-like position. I concur with WASHBURN'S (1957) and SCltULTZ'S (1936) suggestion that the ischial callosities of pongids are in the process of disappearing. Callosity-like develop- ments occur in only 38 ~ of chimpanzees, and even less frequently in orang-utans and gorillas (SCHuLTZ, 1936). In these animals, the developments are not firmly attached to the ischial tuberosity, and the hamstring muscles originate more from the surface of

The Function of Ischial Callosities 367

the tuberosity than from its edge as in Old World monkeys and hylobatids (MILLER, 1945). Furthermore, ROSE (1974a) states that the callosity-like skin found in some pongids is histologically more similar to the skin surrounding the callosities in baboons than to the callosities themselves. Thus the callosity-like structures which occur in pongids represent anatomically an intermediate state toward the complete loss of ischial callosities.

Although I agree with WASHBURN'S (1957) suggestion that the ischial callosities of pongids are disappearing, this loss may be related to an inability of these primates to feed while sitting upon small branches, rather than to a change in sleeping positions. There is an obvious limit to the size that an animal can attain and still gain an advan- tage from callosities for sitting on small substrates. Further, it might be postulated that there was a slight locomotor advantage in having the hamstring tendons originate from the surface of the ischial tuberosity rather than from its edge as is characteri- istic of callositied primates. Thus the location of the tendons of pongids are returning to the earlier and now more advantageous condition with the concurrent loss of the ancestral callosities.

Despite the loss of callosities, pongids retain the ability to gather food from terminal branches through the use of suspended postures. The "brachiating" adaptations of the great apes permit them to reach out and pick fruit from peripheral branches (KORTLAND, 1975). As with the hylobatids, it thus appears that the arm-swinging adaptations of pongids are related to feeding and locomotor behaviors. Both NAPIER (1970) and KORTLAND (1975) have pointed out the intimate relationship that exists between diet and locomotion.

PROBLEMS

The most important postulate leading to my hypothesis is that non-human anthro- poid primates feed upon terminal branches. In the past, zoo experiments have suggest- ed that only pongids and hylobatids possess the ability to exploit peripheral branches for food (Avis, 1962). However, field observations by CARPENTER (1934), RIPLEY (1967, 1970), POIRIER (1972), GRAND (1972), WELLS (1974), ROSE (1974b), KORTLAND (1975), and MENDEL (1976) demonstrate that most and possibly all arboreal higher primates feed upon terminal branches.

An unanswered question concerns the retention of callosities by terrestrial monkeys that do not habitually sit on thin branches during feeding. Although callosities may have evolved for use in a small branch setting, their adaptive value on other substrates is not precluded. Thus, assuming terrestrial monkeys evolved from arboreal forms, the callosities in the latter were maintained because they were adaptive for sitting on rough surfaces. Further, terrestrial primates do, at least in part, utilize the food resources of peripheral branches (RosE, 1974b).

Another problem involves the talapoin monkey. This animal is only slightly larger than the squirrel monkey, yet it possesses ischial callosities (NAPIER t~ NAPIER, 1967). The talapoin evidently evolved from a larger form (STEPHAN, 1972) and maintained callosities because they still are adaptive for sitting postures adopted during resting, feeding, and sleeping (VILENSKY, 1974).

368 J.A. VILENSKY

CONCLUSIONS

It has been proposed that ischial callosities and prehensile tails both evolved in response to similar selective pressures of feeding in a small branch setting. However, it is not implied that the utilization of ischial callosities and prehensile tails is restrict- ed to small branches. These structures are important morphological characteristics with many uses among both small and large branches. Such uses include sleeping, grooming, resting, locomoting, and feeding. It is likely that these functions also played a role in the evolution of ischial callosities and prehensile tails.

Acknowledgements. I am sincerely grateful to W. LEUTENEGGER, K. A. BENNETT, R. A. MITTERMEIER, M. E. MORBECK, J. NODA, P. VAN TUINEN, M. BRANDON, and M. KATZ for criticizing versions of the manuscript. I would also like to thank the New York Zoological Society for allowing me to pursue my research at the Bronx Zoo.

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- -Received December 27, 1976; Accepted June 1, 1977

Author's Address: JOEL A. VILENSKY, Department of Anthropology, University of Wisconsin, Madison, Madison W1 53706, U.S.A.