And Physiology

The giant panda is an enigmatic carnivore, adapted to a highly specialised ecological niche (see Chapter 1). Although the species looks, moves and is genetically similar to bears (see Chapter 10), many of its pheno-typic traits are unlike those of other ursids. For a thorough understanding of the anatomy of giant pandas, the reader is referred to the classic publication of Davis (1964). Many of the species' distinctive features are related to its highly specialised reliance on bamboo. For example, the bone structure and musculature of the head differ markedly from that of other bears. The cheek teeth are enlarged (as in herbivores) to provide large grinding surfaces (see Chapter 6), and an

Anatomy Skull Carnivore

Figure 16.1. Dorsal aspect of the adult giant panda skull displaying the large temporal muscles (arrows). The masticatory apparatus is massive relative to other carnivore species. The volume of the temporal fossa and the associated temporalis muscle are particularly large. Similar adaptations are seen in the red panda (a procyonid with a similar diet) and the hyaena (Davis, 1964). The relative mass of bone in the giant panda cranium and mandibles is also much greater than in other ursids. (See also Plate XIV.)

Figure 16.1. Dorsal aspect of the adult giant panda skull displaying the large temporal muscles (arrows). The masticatory apparatus is massive relative to other carnivore species. The volume of the temporal fossa and the associated temporalis muscle are particularly large. Similar adaptations are seen in the red panda (a procyonid with a similar diet) and the hyaena (Davis, 1964). The relative mass of bone in the giant panda cranium and mandibles is also much greater than in other ursids. (See also Plate XIV.)

articulated jaw with massive masticatory muscles and skull bones are adapted to the panda's herbivorous diet (Davis, 1964; Fig. 16.1; Plate

Given its bamboo diet, the giant panda's short and seemingly unspecialised intestinal tract is a physiological enigma (Fig. 16.2; Plate

XV). Most mammals maintain a correlation between gut length and body length such that the greater the herbivorous proportion of the diet, the longer the gut length. Intestinal length of four to five times body length is typical of a true carnivore (e.g. cat), whereas six to eight times is representative of an omnivore (e.g. raccoon), and the ratio in a fully herbivorous ungulate (e.g. deer) is about 10 to 22 (Davis, 1964). The giant panda diverges from this pattern entirely in that, despite its wholly herbivorous diet, its gut is only four times as long as its body

Figure 16.2. Intestinal tract of the adult giant panda illustrating uniformity in diameter along its entire length. There is no discernible demarcation between the small and large intestine and no caecum. The extent of the tract is only four to five times the body length, which is more characteristic of a true carnivore than a species that survives on a fibrous bamboo diet. (See also Plate XV.)

Figure 16.2. Intestinal tract of the adult giant panda illustrating uniformity in diameter along its entire length. There is no discernible demarcation between the small and large intestine and no caecum. The extent of the tract is only four to five times the body length, which is more characteristic of a true carnivore than a species that survives on a fibrous bamboo diet. (See also Plate XV.)

length, such as in true carnivores. The panda has a simple stomach (precluding the possibility of foregut fermentation) and no caecum (precluding the possibility of hindgut fementation). There is no recognisable demarcation between the small and large intestine and no anatomical or histological clue for its ability to utilise bamboo as a primary energy source. The high transit rate of food and an intestinal flora comprised largely of facultative anaerobes (Hirayama et al., 1989) preclude the microbial-enhanced utilisation of fibrous plants (as occurs in true herbivores). The giant panda presumably secures its major nutrient requirements by selecting leaves and shoots rather than stems, ingesting large quantities of food, digesting plant cell contents (rather than cell walls) and rapidly excreting undigested residues.

The unique diet of the giant panda and its lack of natural predators may have influenced skeletal structure evolution. For example, proficiency in locomotion is reduced as much as that in handling bamboo is increased, and the animal appears well adapted to sitting on its hindquarters for 10 to 12 hours per day, stripping and chewing bamboo. Modification of the radial sesamoid bone of the wrist to

a b MCWH

Figure 16.3. (a) Bladder and ductus deferens of an adult giant panda. The glandular ductus deferens (arrows) opens into the neck of the bladder (ยป). (b) Ovary of an adult giant panda with its characteristic convoluted surface. (See also Plate XVI.)

function as a sixth prehensile 'digit' (see Chapter 6 for depiction) allows the panda to strip leaves from bamboo stalks with remarkable dexterity (Davis, 1964). Other modifications include the concentration of the panda's mass toward the front of the body (with a concomitant gradient in bone density), relatively broad and heavy vertebrae, fewer thoraco-lumbar vertebrae (four compared to five in other carnivores) and unique pelvic size and shape (Davis, 1964).

The male giant panda lacks seminal vesicles and bulbourethral glands but has well-developed glands at the terminus of the ductus deferens which supply seminal fluid (Davis, 1964; Fig. 16.3a; Plate XVI). Davis (1964) indicated that the species lacks a prostate gland, an assertion disputed by Hildebrandt et al. (see Chapter 17) who have observed a bilobal prostate by ultrasound. The penis is relatively small, located ventral to the anus, is directed posteriorly (like in the cat) and is normally withdrawn into the prepuce. The baculum is small and differs from the rod shape in other carnivores in that it projects ventrolateral wings (Davis, 1964). The female reproductive tract is bicornuate and unusual only in the highly convoluted surface structure of the ovary (see Fig. 16.3b), an appearance that seems retained into the breeding season.

Figure 16.4. The renculate kidney of the giant panda. (a) Whole kidney and (b) sagittal section. Each renculus has a thick cortex (1) and small medulla (2) with the papillae of each joining into a common minor calyx (3). In the renal fossa, the minor calyces unite into two major ones which, in turn, join to form the proximal end of the ureter. There is no renal pelvis in this species. (See also Plate XVII.)

Figure 16.4. The renculate kidney of the giant panda. (a) Whole kidney and (b) sagittal section. Each renculus has a thick cortex (1) and small medulla (2) with the papillae of each joining into a common minor calyx (3). In the renal fossa, the minor calyces unite into two major ones which, in turn, join to form the proximal end of the ureter. There is no renal pelvis in this species. (See also Plate XVII.)

Giant panda kidneys share the lobular or renculate structure of other bears, although each kidney is comprised of fewer lobes and contains two or three papillae rather than one (Davis, 1964; Fig. 16.4; Plate XVII). In place of a renal pelvis, two major calyces drain the minor calyces of each renculus and join at the proximal end of the ureter.

Giant panda vocalisations more closely resemble bleats, barks and honks than the roars or growls of typical bears, which may be related to differences in the anatomy of the epipharyngeal pouches. These highly elastic, tubular diverticulations of the caudodorsal pharyngeal wall are lined with respiratory epithelium and are unique to Ursidae. Most bear species have two juxtaposed pouches, but the relative sizes and positions

Figure 16.5. Dorsal view of the pharyngeal pouches of the adult giant panda. These paired sacs (p, shown here filled with water to facilitate viewing) open into the dorsal nasopharynx and may play a role in vocalisation. Some authors (Davis, 1964) describe the left sac as being much smaller than the right (15 mm vs. 130 mm in length), while others (Weissengruber et al., 2001 and references therein) describe just a single medially located pouch. e, epiglottis; t, tongue. (See also Plate XVIII.)

Figure 16.5. Dorsal view of the pharyngeal pouches of the adult giant panda. These paired sacs (p, shown here filled with water to facilitate viewing) open into the dorsal nasopharynx and may play a role in vocalisation. Some authors (Davis, 1964) describe the left sac as being much smaller than the right (15 mm vs. 130 mm in length), while others (Weissengruber et al., 2001 and references therein) describe just a single medially located pouch. e, epiglottis; t, tongue. (See also Plate XVIII.)

of the pouches vary greatly (Weissengruber et al, 2001 and references therein). The anatomy of the panda's epipharyngeal pouches is somewhat controversial. Some authors describe two pouches of which one is much smaller than the other, while others found only one medially located pouch (Davis, 1964; Weissengruber et al., 2001 and references therein; Fig. 16.5; Plate XVIII).

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