Between-gender differences in vigilance do not necessarily lead to differences in foraging-vigilance tradeoffs

When prey are time limited in their access to food, any trade-off involving time should ultimately affect their intake rate. In many herbivores, males and females experience different ecological pressures affecting their survival and reproduction because of differences in morphology, physiology and energy/nutrient requirements. If males and females have different vigilance strategies that affect their intake rates differently, they will suffer different foraging costs. This is particularly relevant in sexually monomorphic herbivores, where the two sexes have similar basal energy/nutrient requirements and risk of predation. We investigated how gender, reproductive status, age, group size, predation risk, and food biomass affected vigilance, intake rate, and their trade-off in a monomorphic species, the plains zebra (Equus quagga). Males were more vigilant than females, and lactating females were less vigilant than other females; the levels of vigilance were low (ca. 10 % of feeding time). The effects on time spent feeding, bite rates and intake rates were small and statistically not significant. Reproductive status did not affect the strength of the relationship between vigilance and intake rate, but intake rates increased with group size and, for adult females, were higher in tall grass. While gender and reproductive status were major drivers of vigilance, and group size and food biomass of the rate of food intake, males and females adjust their bite rates and food intake with vigilance in similar ways. Our results support the hypothesis that in monomorphic animals, males and females seem to make similar trade-offs (i.e. adjustments) between vigilance and intake rate

Facilitation between bovids and equids on an African savanna

Background: Equids, especially zebras and donkeys, and cattle (bovids) share habitats in many savanna ecosystems in Africa. The issue of competition for food between these ungulate guilds remains largely unresolved. Resolving it will provide insights into how wild zebra are likely to interact with cattle on shared landscapes and suggest best practices for cattle owners who must decide whether to tolerate wild ungulates, some of which are severely threatened (e.g. Grevy’s zebra, Equus grevyi). Aim: Determine whether an equid and a bovid compete in a semi-arid savanna in Kenya. Organisms: Boran cattle (Bos indicus) and donkeys (Equus africanus asinus) – the latter as surrogates for zebras. Methodology: Experiments to measure performance (weight gains), bite rates, diet quality (digestible organic matter and crude protein), and gastrointestinal worm burdens (parasite egg count per unit weight of faeces) of the two ungulate species when herded separately (single species) or together (mixed species). We used two stocking levels: low-density (one animal per 7 ha), a level typical of commercial ranchers; and high-density (one animal per 2 ha), a level typical of pastoral herders. Principal findings: When herded together, both species gained more weight, had higher bite rates (especially at low stocking density), and selected diets with a more favourable balance between digestible organic matter and crude protein, than when herded separately. In addition, parasite egg output in faeces of donkeys was reduced by 14–35% following shared foraging with cattle. Conclusion: Cattle (a ruminant) and donkeys (hindgut fermenters, closely related to zebras) showed no evidence of competion with each other. Rather, our results show a facilitative, rather than a competitive, interaction between them

Nutritional ecology beyond the individual: a conceptual framework for integrating nutrition and social interactions

International audienceOver recent years, modelling approaches from nutritional ecology (known as Nutritional Geometry) have been increasingly used to describe how animals and some other organisms select foods and eat them in appropriate amounts in order to maintain a balanced nutritional state max-imising fitness. These nutritional strategies profoundly affect the physiology, behaviour and performance of individuals, which in turn impact their social interactions within groups and societies. Here, we present a conceptual framework to study the role of nutrition as a major ecological factor influencing the development and maintenance of social life. We first illustrate some of the mechanisms by which nutritional differences among individuals mediate social interactions in a broad range of species and ecological contexts. We then explain how studying individual-and collective-level nutrition in a common conceptual framework derived from Nutritional Geometry can bring new fundamental insights into the mechanisms and evolution of social interactions, using a combination of simulation models and manipulative experiments