Andrew W. Illius

The allometry of food intake in grazing ruminants

(1) A simulation model of grazing mechanics in ruminants shows that, due to the allometric relations of bite size and metabolic requirements to body size, small animals are able to subsist on shorter swards than large animals. (2) The density of nutrients in the grazed horizon of the modelled swards markedly affected the ability of animals of a given body size to satisfy their energy requirements. (3) By extension, the allometric relationships would be expected to apply in selective grazing and browsing species in their choice of food items of different size and nutrient content. (4) The results support the argument that sexual segregation and habitat choice of dimorphic species is an effect of scramble competition for limited resources, the males thus being excluded from mutually preferred swards. (5) The model provides an explanation for two interspecific phenomena amongst grazers: grazing succession and grazing facilitation.

The ecology and management of grazing systems

Part 1 Plants and plant populations: tissue flows in grazed plant communities, D. Chapman and G. Lemaire survival strategies under grazing, D. Briske plant competition and population dynamics, J. Bullock community and ecosystem processes under grazing, S. Archer. Part 2 Animal and animal populations: foraging strategies of grazing animals, M. Demment biochemical aspects of grazing behaviour, K. Launchbaugh ingestive behaviour and diet selection, E.D. Ungar nutritive value of herbage and nutrient requirements of large herbivores, H. Dove animal populations in grazing systems - intra- and inter-specific interactions, M. Murray. Part 3 Grazing systems and their management: complexity and stability in grazing systems, N. Tainton management of temperate pastures, G.W. Sheath and D. Clarke management of rangelands, M. Stafford Smith management of Mediterranean grasslands, N. Seligman management of tropical grasslands, M.J. Fisher.

Incisor arcade structure and diet selection in ruminants

The structure of the incisor arcade in ruminant ungulates is likely to affect the degree to which animals can be selective in choosing food items. Species of ruminant which feed predominantly on grasses have broader more flattened incisor arcades when compared at the same size with species which browse on woody dicotyledonous plants. This is likely to reflect the differential dispersion of food items on the food plants of these two different groups of ruminants.

Influences of feeding level and physical restriction on development of stereotypies in sows

Abstract In attempting to understand the mechanisms underlying the development of stereotypic behaviour patterns it is important to determine those environmental factors that contribute to the development of such behaviour. The effects of restraint and food restriction on the development of stereotypies in pregnant nulliparous sows, Sus scrofa , were investigated. In the two housing conditions used in the present study, food restriction, but not restraint, was a major factor in increased levels of activity and in particular high levels of drinking and chain manipulation. Food-restricted sows also increased their feeding rate significantly over the experimental period. Restrained sows spent more time in behaviour such as standing inactive and sham chewing that did not involve manipulation or exploration of substrates. Hughes & Duncan (1988, Anim. Behav. , 36 , 1696–1707) proposed that stereotypies result from long-term performance of elements of appetitive behaviour. Their model suggests that, in the present study, stereotypies resulted from both environments preventing the animal proceeding to the consummatory phase of foraging behaviour and ‘switching off’ the underlying motivation. This explanation, however, is not consistent with the observation that time spent in appetitive behaviour did not decrease when the level of stereotypy increased, and that stereotypies were concentrated in the post-feeding period. It is also not consistent with the high levels of drinking and chewing of substrates that the food-restricted sows developed, as these activities appear to be more closely related to consummatory behaviour than to appetitive behaviour. It is concluded that there remain a number of aspects of stereotypies in pigs and other species that cannot be wholly explained by current models.

STABILITY AND INSTABILITY IN UNGULATE POPULATIONS: AN EMPIRICAL ANALYSIS

While many populations of large mammals are stable from year to year, some show persistent oscillations associated with high mortality. This article investigates the causes of variation in population stability in ungulates by comparing the contrasting dynamics of two naturally regulated island populations: the Soay sheep population of Hirta (St. Kilda), where numbers fluctuate by 60%-70% every 3 or 4 yr as a result of overcompensatory winter mortality, and the red deer population of the North Block of Rum, where numbers have been stable for over a decade. We suggest that the contrasting dynamics of these two populations are caused by differences in fecundity and maturation rate. Red deer show relatively low fecundity and slow maturation, and increases in population density depress both fecundity and juvenile survival. In contrast, the relatively rapid development of Soay sheep allows them to evade densitydependent effects on reproduction and survival until the population exceeds winter carrying capacity by a substantial margin, subsequently triggering overcompensatory mortality in late winter in all age classes.

Prediction of intake and digestion in ruminants by a model of rumen kinetics integrating animal size and plant characteristics

Simulation modelling was used to investigate interactions between forage degradation characteristics, rumen processes and body weight, and to predict the voluntary food intake and digestion of a range of forages. Predicted voluntary intake and digestion agreed well with empirical data, explaining 61 and 70%, respectively, of variance in observed values. Since the data covered a wide range of animal weights and forage qualities, these results suggest that the model is a useful means of integrating the effects of animal and forage variables. Interactions were examined between animal weight and diet quality, as defined by the proportion of potentially digestible cell contents and cell walls and their rates of digestion. Retention time of food in the digestive tract was shown by regression to scale with W⁰·²⁷. The time taken to comminute large fibre particles also scaled with W⁰·²⁷. Longer retention of digesta by large ruminants increases digestive efficiency compared with small animals and would permit them to survive on lower-quality foods. The model showed that maximum intake of metabolizable energy scales with c. W⁰·⁸⁷, greater than the scaling of maintenance with W⁰·⁷³

Megaherbivores influence trophic guilds structure in African ungulate communities

We used a data set of ungulate censuses from 31 natural ecosystems from East and Southern Africa to test two hypotheses: (1) megaherbivores should dominate ungulate communities in ecosystems with high rainfall and low soil nutrient status because of their ability to survive on poor quality food resources, and (2) the abundance of megaherbivores affects the abundance of the mesoherbivores, distinguishing the different feeding guilds: mesograzers, mesobrowsers and mesomixed feeders. Two axes of a multivariate analysis (77% of the variance) discriminated the sites well, the first separating sites dominated by megaherbivores from those dominated by mesoherbivores, and the second representing a gradient between mesograzers and mesobrowsers. Our analysis shows (1) that megaherbivores can be considered to be a separate trophic guild and (2) that mesograzers and mesobrowsers respond differently to variation in their trophic environments. The metabolic biomass density of megaherbivores increased with annual rainfall, but was not related to soil nutrient status, and as predicted, megaherbivores comprised a larger proportion of the biomass of ungulate communities in ecosystems with high rainfall and low nutrient soils. The metabolic biomass density of mesoherbivores increased with rainfall and soil nutrient status. Within the mesoherbivores, the metabolic biomass density of mesograzers showed the same trend, and seemed unaffected by megaherbivores. Conversely, mesobrowsers and mesomixed feeders appeared to be unaffected by rainfall or soil nutrient status, but mesomixed feeders declined when megaherbivores were abundant. This suggests that megaherbivores may compete with the mesomixed-feeder species for food or they may alter the vegetation communities unfavourably. A similar analysis using elephants alone instead of megaherbivores as a group showed that both mesobrowsers and mesomixed feeders were affected significantly by elephant, which is consistent with the fact that most of the effect of megaherbivores on browse resources or woodland habitat is due to elephants. This study shows that the different trophic guilds within African ungulate communities react differently to environmental factors (rain and soil), and that megaherbivores, and particularly elephants, appear to compete with mesomixed feeders and mesobrowsers. These results are relevant for the understanding of the functioning of African ungulate communities and call for further testing with longitudinal data.

The functional significance of the browser-grazer dichotomy in African ruminants

The allometric relationships for the fermentation rate of dry matter, the total energy concentration of volatile fatty acids (VFAs), the energy supplied from VFA production and the mass of the digesta contents within the rumen or caecum and proximal colon (hindgut) were used to test whether the digestive strategies of grazing and browsing African ruminants differ. The wet and dry mass of the contents of the rumen and hindgut were allometrically related to body mass (BM). These relationships did not differ between browsing and grazing ruminants. The fermentation rates in the rumen were strongly allometric and the intercepts of the relationships did not differ between browsers and grazers. The fermentation rates in the hindgut were not allometrically related to BM and did not differ between ruminants with different feeding habits. Likewise, the total energy concentration of the VFAs in the rumen and hindgut showed no allometric scaling and did not differ between browsing and grazing ruminants. The energy supplied by VFA production in both the rumen and hindgut of African ruminants scaled at around 0.8 with BM. Only in the case of the energy supplied by VFAs in the rumen were there significantly different intercepts for browsing and grazing ruminants. The energy supplied by VFA production in the rumen was inadequate to meet the energy requirements for maintenance of browsers and small grazers. The retention time of digesta in the alimentary tract was positively related to BM although there was no difference in the allometric relationships for grazers and browsers. The results of these analyses suggest that, after controlling for the effects of body mass, there is little difference in digestive strategy between African ruminants with different morphological adaptations of the gut.

DIET SELECTION IN GOATS: A TEST OF INTAKE‐RATE MAXIMIZATION

The mechanisms of diet choice by herbivores are poorly understood. We tested whether the preference of goats among five grass species was accounted for by differences in intake rate or differences in species-specific attributes of the grasses. When offered a choice between grass species, the goats selected diets that tended to maximize intake rate. Only a small amount of the residual variation was explained by the individual preference of goats for each grass species. The animals exhibited partial preferences, only leaving the lower intake-rate alternative ungrazed in a quarter of the trials. It is argued that the most likely explanation of this is the difficulty of discriminating between alternatives of similar intake rate. Discrimination error is likely to be a general explanation of mixed diets in herbivores, regardless of whether other explanations, such as rate maximization subject to nutrient constraints, also apply.

Spatial and temporal variability modify density dependence in populations of large herbivores

A central challenge in ecology is to understand the interplay of internal and external controls on the growth of populations. We examined the effects of temporal variation in weather and spatial variation in vegetation on the strength of density dependence in populations of large herbivores. We fit three subsets of the model ln(Nt) = a + (1 + b) x ln(N(t-1)) + c x ln(N(t-2)) to five time series of estimates (Nt) of abundance of ungulates in the Rocky Mountains, USA. The strength of density dependence was estimated by the magnitude of the coefficient b. We regressed the estimates of b on indices of temporal heterogeneity in weather and spatial heterogeneity in resources. The 95% posterior intervals of the slopes of these regressions showed that temporal heterogeneity strengthened density-dependent feedbacks to population growth, whereas spatial heterogeneity weakened them. This finding offers the first empirical evidence that density dependence responds in different ways to spatial heterogeneity and temporal heterogeneity.