INTRODUCTION

Primate habitats in south-east Asia span the two main forest formations of this part of the Oriental region – the deciduous monsoon rainforests of mainland Asia north of the isthmus of Kra and the evergreen rain forests of the “islands” of the Sunda Shelf (Whitmore, 1975). The evergreen rainforests of the Sunda Shelf comprise the main primate habitat, but significant numbers of taxa and individuals occur in the more seasonal forests of mainland Asia, mostly concentrated in pockets of evergreen forest surviving in the moister areas under maritime influence, e.g., Indo-China, Thailand, south India and Sri Lanka, Burma, and Bangladesh (Table 2.1). Intermittent volcanic activity over the last 100 million years has thrown up several arcs of mountains, creating the Sunda Shelf only about 12 million years ago; they run mainly from the north-west to the south-east, with the largest down the west of Sumatra.

The Sunda Shelf owes its uniquely rich fauna (and flora) to an admixture of immigrants. First came the Siva-Malayan fauna from the Indian subcontinent, nearly two million years ago during the Villafranchian. Then, nearly one million years ago, the Sino-Malayan fauna came from southern China, while the spread of tropical rainforest and/or the spread of the cold influence of the Himalayas blocked the Indian route.

It is important to note that during periods of lowest sea level, the center of the Shelf dried out. The key rainforest relicts, into which primates and other forest animals retreated and out of which they spread when sea level rose, were in eastern Indo-China and southern China, north-eastern Borneo, west Java, north Sumatra and southern Burma, as well as the Mentawai Islands.

The aim in this volume is to present a synthesis of a wide range of data pertinent to the digestive system in mammals, focussing on three areas, food, form and function. There are introductions to each of these three sections and a final discussion encompassing all aspects but working from the viewpoints of the three groups of contributors. The essence of this volume is that it brings together very different lines of research, and that an integrating process is initiated to develop a broader understanding of the digestive system.

In this preliminary section we set the scene and discuss general, often problematic, issues and then Langer (Ch. 2) presents an evolutionary perspective. This is followed by introductions to modelling gut function by Martinez del Rio, Cork and Karasov (Ch. 3) and optimum gut structure for specified diets by McNeill Alexander (Ch. 4).

As we focus on the details of diets and digestive systems (structure and function), we need to think about the nature of the niches to which they are adapted: in particular, as to how broad or narrow these may be or how variable in the short (days) or long term (seasons, years). To what extent is form and function accounted for by preferred food and body size? To what extent are they moulded by critical events (‘bottlenecks’ in food availability). We need to think about them against an evolutionary background of increasing environmental diversity over the last 50 million years in relation to the dramatic radiation of angiosperm (flowering) plants.

When classifying food, the following questions are often asked:

1. What are the fields of study that need food classification?

2. What criteria have to be considered in this classification in different fields of study?

3. What is the discrimination (or resolution) of classifications in different fields: is it always similar, or is it necessary to recognise coarser- or finergrain classifications in different fields of study?

Food classifications are needed and applied in animal and human nutrition science, comparative physiology and ecology; they discriminate food characters with as much detail as necessary and/or possible. When classifying food materials, one should keep in mind that categorizing is always dangerous because it can restrict thoughts and ignore the origins of data used and required for the categories (Winkler, 1984).

The resolution (or discrimination) used in the classification of food can be based on physical properties, as well as chemical composition and food availability and shows the following tendencies in different areas of study.

Ecology Modern ecological studies emphasize, among other criteria, the availability or abundance of food in relation to area and time (e.g. Rattray, 1960; Osbourn, 1980; van Dyne et al., 1980; Owen-Smith, 1982, 1988; Duncan, 1991). Chemical and physical properties, and food availability, have to be given in as much detail as possible. Detailed data are often not available, however, and the ecologist has to work with low resolution (coarse grain) data. Perrin (1988) of the University of Natal, Pietermaritzburg supplied a highly differentiated trophic classification for the mammals living in the Eastern Cape Coast area of South Africa.

We have considered the spectrum of factors involved in food, form and function, and the interrelationships between them. Thus, diets, gut morphology and digestion are each classified in detail, in terms of the nature of foods, the mechanics of their processing in the mouth and the chemistry of their digestion in the alimentary canal. Attempts have been made to relate them to each other and to indicate future directions for research. The aim in this concluding chapter is to synthesise the results of these deliberations as they have developed during the production of this book.

This must be done in an evolutionary context: the integrating framework for the diversity seen among mammals with respect to any body system and its interaction with the environment. This should be especially true for the digestive system and the wide range of foods exploited by mammals in the whole range of available environments. The biomes of the world are characterised by floral and faunal associations that provide trophic resources with changing profiles of potential diets, so that many configurations observed today may not be typical of the past. Nutritional niches tend to be narrow because of constraints on the digestive systems, but some mammals, especially primates, exhibit a wide range of diets because of a relatively unspecialised digestive system. Much can be gained by comparing the adaptations of primates with the specialisations of other animals for eating either animal matter or foliage. For such reasons, we do not restrict our attention to mammals but try to put them into a broader vertebrate perspective, especially by comparisons with the great radiation of birds.