Significance of diet type and diet quality for ecological diversity of African ungulates

1.We test two nutritional hypotheses for the ecological diversity of ungulates, the browser/grazer (diet type) and diet quality models, among free-ranging herbivores in a South African savanna, the Kruger National Park. Tests are based on assessment of relationships between diet type and diet quality with body mass and hypsodonty, two morphological features that have been associated with both elements. 2.We use stable carbon isotope ratios of faeces to reconstruct diet in terms of proportions of C3 plants (browse) and C4 plants (grass) consumed by different species in different seasons. These data are combined with proxies for diet quality (per cent nitrogen, neutral detergent fibre, acid detergent fibre, and acid detergent lignin) from faeces to track changes in diet quality. 3.Two statistical approaches are used in model selection, i.e. tests of significant correlations based on linear regression analyses, and an information-theory approach (Akaike’s Information Criterion) providing insight into strength of evidence for models. 4.Results of both methods show that, contrary to many predictions, body mass and diet type are not related, but these data confirm predictions that diet quality decreases with increasing body size, especially during the dry season. Hypsodonty, as expected, varies with diet type, increasing with increased grass intake. 5.These findings support both a diet type and diet quality model, implying some degree of exclusivity. We propose that congruence between models may be achieved through addition of diet quality proxies not included here, because hypsodonty is more likely a reflection of the abrasive properties of consumed foods, i.e. related to food quality, rather than food type. This implies that adaptation to diets of varying quality, through changes in body size and dental features, has been the primary mechanism for diversification in ungulates. 6. Our interpretation contrasts with several recent studies advocating diet type as the primary factor, exemplifying that further reconciliation between the two models is needed. We discuss the implications of this study for future approaches to achieve a more cohesive understanding of the evolutionary outcomes of herbivore nutrition

Evolution of body size in the genus Damaliscus: a comparison with hartebeest Alcelaphus spp.

In species with low levels of sexual size dimorphism, it may be relatively easy to detect the role of natural selection in the evolution of body size. Habitat primary production (HPP) appears to be a key factor in the divergence of size in the hartebeest clade (Alcelaphus spp.), such that subspecies in less productive savannahs are smaller than those in richer ones. Here I test whether a similar pattern exists within the genus Damaliscus (topi and their allies). Basal skull length was used as a surrogate of body size and measured in the seven allopatric subspecies of Damaliscus. Means for each subspecies and sex were regressed against climatic factors as surrogates of HPP. Variation in skull length across Damaliscus taxa was less than in hartebeest. Two clusters were present in both sexes and corresponded to the distinction between the species, Damaliscus dorcas and Damaliscus lunatus. This may reflect differences in productivity between edaphic grasslands, occupied by all D. lunatus, and dry grasslands, occupied by D. dorcas. Mean annual rainfall was the best predictor of body size in males and showed a non-significant positive tendency in females. After accounting for phylogenetic effects, these correlations were both non-significant. Edaphic grasslands might be less dependent on precipitation for primary production because the impeded drainage of their soil prolongs water availability after the end of the rains. Furthermore, they are probably more consistent in productivity across African regions than secondary grasslands and savannah woodlands, which rely on rainfall for grass growth. These properties of edaphic grasslands may explain why size in Damaliscus appears to be less sensitive to variation in rainfall and less variable across subspecies than in Alcelaphus

Forage fermentation patterns and their implications for herbivore ingesta retention times

1. Differences in digestive physiology between browsing and grazing ruminant feeding types have been discussed extensively. The potentially underlying differences in fermentative behaviour of forage plants have received much less attention. 2. In this study, different groups of temperate forage plants (grasses, browse leaves and twigs, herbs and legumes) were compared in their chemical composition and fementative behaviour. They were evaluated via an in vitro fermentation system (modified Hohenheim gas test), and relevant fermentation parameters such as maximal gas production and relative gas production rate were calculated. 3. Grasses generally had a higher NDF (neutral detergent fibre = total cell wall) content than browse leaves, herbs and legumes, while browse leaf cell wall was more lignified than that of herbs, legumes and grass. 4. With respect to fermentation parameters, grass had the highest maximal gas production, followed by herbs and legumes, and the lowest maximal gas production in browse leaves and twigs. Relative gas production rate was highest in herbs and legumes, while that of grass and browse was lower. As expected, browse twigs had the lowest nutritional value. 5. Dicot material reached given setpoints of absolute gas production rate like 1.0 or 0.5 mL gas/(200 mg dry matter x h) faster than grass material. Based on these results, a longer passage time of food particles seems to be adaptive for grazing ruminants, as over a wide range of fermentation times, absolute gas production rate is higher in grass compared with dicots. Especially for browse leaves, a higher intake level should be expected to balance energy requirements of animals relying on this forage type