Introduction

Nutrition involves a series of processes whereby an animal uses items in its external environment to support internal metabolism (Robbins, 1993). The nutrition and consequent nutritional status of an animal are basic to all aspects of health, including growth, reproduction and disease resistance. Thus, appropriate nutrition and feeding are essential to a comprehensive animal management and preventative medicine programme.

The giant panda's obligate dependence upon bamboo as a primary energy and nutrient source has been well described (Sheldon, 1937; Schaller et al., 1985). Many aspects of panda biology are directly related to its adaptations for utilisation of this highly fibrous, low energy density food, thus demonstrating the inseparable influence of nutrition on behaviour, reproduction and other physiological functions. There may be few other species that more effectively illustrate how an understanding of nutritional adaptations helps us interpret the species ecology.

This chapter describes insights into the nutritional adaptations of the giant panda while identifying priority research that will fill gaps in

Giant Pandas: Biology, Veterinary Medicine and Management, ed. David E. Wildt, Anju Zhang, Hemin Zhang, Donald L. Janssen and Susie Ellis. Published by Cambridge University Press. # Cambridge University Press 2006.

our understanding of these unique abilities. Historical and current strategies on feeding giant pandas in captivity are presented along with recommendations for improving nutrition and dietary husbandry to promote health and feeding behaviours.

ANATOMY, PHYSIOLOGY, GUIDELINES AND ASSESSMENT

Feeding ecology and anatomical adaptations to a herbivorous diet

More than 99% of the food consumed by the free-ranging giant panda consists of bamboo (Schaller et al., 1985). Yet the giant panda is unique in that it has the relatively simple gastrointestinal tract of a carnivore. More specifically, it lacks the modifications found in most herbivores that promote increased digesta retention to facilitate microbial fermentation of ingested plant materials (Schaller et al., 1985).

Anatomically, the giant panda exhibits several specialised adaptations for processing and utilising bamboo. Perhaps the most familiar is the panda's enlarged and elongated radial sesamoid bone, attached to the first metacarpal bone (Fig. 6.1) (Davis, 1964; Endo et al., 1996, 1999). Using three-dimensional computed tomography, this adaptation can be visualised as a double pincer-like apparatus between the radial sesam-oid and accessory carpal bones, allowing the panda to grasp and manipulate bamboo culm with remarkable dexterity, as though it had a thumb (Endo et al., 2001).

The giant panda's large skull, with wide flaring zygomatic arches and a prominent sagittal crest (Fig. 6.2), supports heavy craniomandib-ular musculature, giving the species the crushing power required to masticate fibrous, highly lignified bamboo. Working in combination with these muscles are large, flat cheek teeth with elaborate crown patterns (Fig. 6.3), characteristic of herbivores. These dental characteristics are modifications of typical carnivore dentition (I3/3 C1/1 P4/4 M2/3 = 42, with P1 degenerate in both jaws and sometimes absent from the upper jaw) (Chorn & Hoffman, 1978; Schaller et al., 1985).

The giant panda's gastrointestinal tract, evolutionarily one of the most plastic organ systems in the animal world, is remarkably unspecialised for herbivory and is largely characteristic of omnivores. Once masticated, ingesta pass into an oesophagus with a tough and horny lining, followed by an uncompartmentalised stomach with a long, thick-walled pyloric section folded back on the cardia. The pylorus

Figure 6.1. Paw anatomy of the giant panda with arrow depicting the enlarged and elongated radial sesamoid bone.
Internal Anatomy Giant Panda
Figure 6.2. Giant panda skull illustrating the flaring zygomatic arches (arrows) and prominent sagittal crest.
Body Organs Pbl
Figure 6.3. Giant panda skull illustrating the large, flat teeth.

remotely resembles a gizzard in birds (see Chapter 18) and may knead and mix food with digestive juices.

The small intestine is a much-reduced segment of the gastrointestinal tract, suggesting that limited digestion occurs in this region (Chorn & Hoffman, 1978). The caecum is absent. However, relative to other bear species, the surface area of the colon is enlarged (Chorn & Hoffman, 1978). Overall, intestinal length ranges from 4.1 to 5.5 times head and body length (Raven, 1937; Davis, 1964; Schaller et al., 1985). The characteristics of the large intestine are consistent with being populated by microbial symbionts, although their contribution to digesta fermentation is probably limited due to rapid transit rates (see below) (Hirayama et al, 1989).

Field biologists have noted that faecal boluses of the free-ranging giant panda are coated with a thin layer of mucus, which may lubricate the fibrous digesta, facilitating movement through the intestinal tract (Schaller et al., 1985). Histological examination of the panda's gastrointestinal tract has revealed that the large intestine has a significant number of mucous cells (Wang et al., 1982).

The consequence of consuming a highly fibrous diet and the rapid passage of ingesta through a relatively unspecialised gastrointestinal tract (i.e. without sacculations or compartments to retain digesta for microbial degradation) is reduced nutrient digestibility and absorption.

Food intake

Measurement of food intake is complicated by variability among pandas, food palatability and food items selected. Yet quantifying ad libitum intake as it relates to food quality is important in evaluating animal response to that food (Van Soest, 1994).

Ad libitum intake is typically measured in a controlled environment where food is offered at 15 to 20% in excess of the quantity usually consumed. Under these conditions, refused food may differ in composition from food offered, depending on the ability of the animal to select preferentially and ingest specific portions of that diet. This selective intake is of particular concern when evaluating nutrients supplied by bamboo because bamboo components (i.e. shoots, leaves, branches and culm) differ appreciably in composition.

Food intake quantified in giant pandas under field (free-ranging) and captive conditions is summarised in Table 6.1.

In conjunction with the CBSG Biomedical Survey of Giant Pandas (1998 to 2000; see Chapter 2), 24-hour food intake data were provided and/or measured for 34 pandas in captive facilities in China (Table 6.2).

Dry matter intake (as a percentage of body weight, BW) was comparable between the two groups of captive animals as summarised in Tables 6.1 and 6.2 (2.7 - 5.6% BW). However, dry matter intake (% BW) was lower across all captive specimens when compared to estimates of intake under field conditions. These differences were most likely due to divergence in digestible energy density of the dietary items consumed or to the dietary proportions of bamboo ingested (Van Soest, 1994). When increasing quantities of bamboo forage are consumed (as a percentage of total dry matter intake), digestible energy density of the total diet declines, requiring consumption of more food to maintain absolute caloric intake. Additionally, giant pandas in captivity would be expected to have lower dry matter intake and energy intake consistent with reduced activities (and, thus, lower energy requirements) compared to their wild counterparts.

A similar trend is seen among estimates of intake in animals under field conditions (see Table 6.1). Dry matter intake of bamboo

Table 6.1. Daily food intake of giant pandas expressed as kilograms of fresh weight (FW), kilograms of dry matter (DM) or DM intake as a percentage of body weight (BW)

Age class

Gender

Diet"

FW (kg)h

DM (kg)h

DM (X]ßW

Reference

Free-ranging

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