Oecologia (2003) 136:14–27 DOI 10.1007/s00442-003-1254-z ECOPHYSIOLOGY M. Clauss · R. Frey · B. Kiefer · M. Lechner-Doll · W. Loehlein · C. Polster · G. E. Rssner · W. J. Streich The maximum attainable body size of herbivorous mammals: morphophysiological constraints on foregut, and adaptations of hindgut fermenters Received: 22 July 2002 / Accepted: 1 March 2003 / Published online: 24 April 2003 Springer-Verlag 2003 Abstract An oft-cited nutritional advantage of large body size and increasing absolute energy requirements, their size is that larger animals have lower relative energy relative capacity has to increase in order to compensate for requirements and that, due to their increased gastrointes- this intake limitation. It seems that the foregut fermenting tinal tract (GIT) capacity, they achieve longer ingesta ungulates did not evolve species in which the intake- passage rates, which allows them to use forage of lower limiting effect of the foregut could be reduced, e.g. by quality. However, the fermentation of plant material special bypass structures, and hence this digestive model cannot be optimized endlessly; there is a time when plant imposed an intrinsic body size limit. This limit will be fibre is totally fermented, and another when energy losses lower the more the natural diet enhances the ingesta due to methanogenic bacteria become punitive. Therefore, retention and hence the intake-limiting effect. Therefore, very large herbivores would need to evolve adaptations for due to the mechanical characteristics of grass, grazing a comparative acceleration of ingesta passage. To our ruminants cannot become as big as the largest browsing knowledge, this phenomenon has not been emphasized in ruminant. Ruminants are not absent from the very large the literature to date. We propose that, among the extant body size classes because their digestive physiology offers herbivores, elephants, with their comparatively fast pas- no particular advantage, but because their digestive sage rate and low digestibility coefficients, are indicators physiology itself intrinsically imposes a body size limit. of a trend that allowed even larger hindgut fermenting We suggest that the decreasing ability for colonic water mammals to exist. The limited existing anatomical data on absorption in large grazing ruminants and the largest large hindgut fermenters suggests that both a relative extant foregut fermenter, the hippopotamus, are an shortening of the GIT, an increase in GIT diameter, and a indication of this limit, and are the outcome of the reduced caecum might contribute to relatively faster competition of organs for the available space within the ingesta passage; however, more anatomical data is needed abdominal cavity. Our hypotheses are supported by the to verify these hypotheses. The digestive physiology of fossil record on extinct ruminant/tylopod species which large foregut fermenters presents a unique problem: did not, with the possible exception of the Sivatheriinae, ruminant—and nonruminant—forestomachs were de- surpass extant species in maximum body size. In contrast signed to delay ingesta passage, and they limit food to foregut fermentation, the GIT design of hindgut intake as a side effect. Therefore, with increasing body fermenters allows adaptations for relative passage accel- eration, which explains why very large extinct mammalian M. Clauss ()) · B. Kiefer · W. Loehlein · C. Polster herbivores are thought to have been hindgut fermenters. Institute of Animal Physiology, Physiological Chemistry and Animal Nutrition, Keywords Ruminants · Browsers · Grazers · Passage Ludwig-Maximilians-University, Veterinaerstrasse 13, 80539 Munich, Germany rate · Fermentation e-mail: [email protected] Fax: +49-89-21803208 Introduction R. Frey · M. Lechner-Doll · W. J. Streich Institute of Zoo and Wildlife Research (IZW), Large body size provides a number of ecological advan- Berlin, Germany tages. The advantage cited most often is a reduced G. E. Rssner relative energy requirement for maintenance, and a Palaeontology Munich, relative increase in gastrointestinal capacity in relation Department of Geo- and Environmental Sciences, to energy requirements (Parra 1978). This allows, but Ludwig-Maximilians-University, does not oblige, larger animals to use forage of lower Munich, Germany 15 quality (Demment and Van Soest 1985) due to potentially (1974) labelled this concept the “Jarman-Bell Principle”. longer passage rates and consequently more thorough This principle has since been applied to other animals digestion. Larger animals can facilitate larger movement (e.g. Gaulin 1979; Demment 1983), and has been used to ranges (Altman 1987). Large body size enables large explain the phenomenon of sexual dimorphism in ungu- species to successfully compete with smaller species for lates (e.g. Barboza and Bowyer 2000). Parra (1978) the same resource (e.g. elephants; Fritz et al. 2002) by supplied a quantitative verification of the concept by means of interference (Persson 1985), or enables large demonstrating that gut capacity, measured as gut contents, species to use habitat resources unattainable for other increases linearly with body weight. Demment and Van species (e.g. the giraffe; Woolnough and du Toit 2001). Soest (1985) postulated that the main effect of increasing Finally, large body size provides an organism with the body weight, and hence gut capacity, should be an ability to avoid predation (Owen-Smith 1988). increase in ingesta retention (mean retention time, MRT) On the other hand, larger animals tend to have fewer and thus time available for digestion. Illius and Gordon offspring, mature sexually at a later age, and have longer (1992) finally provided the quantification of this influ- gestation times (Peters 1983), all of which lead to long ence, demonstrating that MRT (h) = 9.4 body weight0.255 generation intervals, which drastically prolongs the evo- in hindgut fermenters and MRT (h) = 15.3 body lutionary reaction time of larger species. In evolutionary weight0.251 in ruminants. terms, large mammalian species are “inert”, and probably Modifications to the general concept of the direct much more so than large dinosaurs in which the number correlation of body weight and digestive potential were of offspring was not as limited (Janis and Carrano 1992). added, e.g. for small (rodent) herbivores (Justice and Large animals obviously have high absolute energy Smith 1992; Smith 1995), to include effects of differences requirements and therefore need abundant forage. Due in relative intake and food sorting; Owen-Smith (1988) to their low surface-to-volume ratio, they can have and Van Soest (1996) also emphasized that larger difficulties in dissipating heat, a fact that accounts for ungulates can support their lower specific metabolic nocturnal habits and other adaptations of megaherbivores requirements either by accepting forage of lower quality, (Owen-Smith 1988); in fact, surface-to-volume ratio is or by ingesting lesser amounts of regular quality forage considered a constraint on the potential upper body size of per day, or some combination of both. Thus, larger terrestrial mammals (Alexander 1989). Finally, larger animals should eat diets of lower nutritional quality than animals are more conspicuous, a distinct disadvantage in those selected by smaller herbivores, but the difference the era of human predation. Humans have been held should be less marked than predicted simply on the basis responsible for the extinction of very large animals like of the body size-metabolic rate relationship. mammoths, giant ground sloths and temperate rhinoceros- es. Economos (1981) demonstrated that due to the metabolic costs of gravity, 20 tons should be the Open questions in large herbivore maximum attainable body size of terrestrial mammals. digestive physiology The relevance of body size for the morphophysiolog- ical design of different organ systems is well documented Why does the elephant deviate (e.g. Schmidt-Nielsen 1984; Damuth and MacFadden from the common body size scheme? 1990). In this review, we want to draw attention to interactions between body size and particular digestive In recent studies on the digestive physiology of elephants strategies. The omission of other factors important for the (Clauss et al. 2003; Loehlein et al. 2003), we raised the understanding of body size, such as resource availability, question of why elephants so obviously deviate from the or allometric constraints on other organ systems, reflects common digestive scheme postulated for herbivores of the limitations of both the space available for this review increasing body mass: they do not have long ingesta and our own expertise, rather than the assumption that passage rates, and achieve only comparatively low they have less significance. digestibility coefficients. This is not only true in free- ranging animals, where one could be tempted to explain low digestibilities on the basis of a very low quality diet, Ecophysiological theories on the digestive relevance but also in captive animals where food quality is higher. of body size The influence of the body sizes of large herbivores has Why is the largest ruminant smaller than been a major area of ecophysiological research. Jarman the largest hindgut fermenter, (1968, 1974) and Bell (1969, 1971) focused attention on and why is the largest ruminant a browser? patterns of food selection among African ungulates of varying body size. They noted that, while specific It has been repeatedly observed that