Adrian M. Shrader

Determining the relative importance of dilution and detection for zebra foraging in mixed-species herds

For many animals, a key benefit of group living is lowered predation risk. With increasing group size, individuals commonly reduce vigilance. This group size effect can arise from both dilution of risk and increased collective detection. To determine which was more important, we compared vigilance levels of plains zebra, Equus quagga, in areas inhabited by their main predator, lion, Panthera leo, as a function of herd size and composition (zebra-only versus mixed-species herds). For zebra-only herds, vigilance declined with increasing zebra numbers. In contrast, in mixed-species herds, zebra vigilance levels were significantly lower and did not vary with the total number of ungulates and/or zebra in the herd. This lower vigilance can be explained by detection benefits, since we controlled for the influence of herd size (i.e. dilution) statistically. Furthermore, we found that zebra in smaller herds were half as vigilant when they co-occurred with another preferred prey of lion, blue wildebeest, Connochaetes taurinus, compared with zebra-only herds. However, when zebra foraged with impala, Aepyceros melampus, a nonpreferred prey, there was only a slight reduction in vigilance compared with similar-sized zebra-only herds. Interestingly, the decrease in zebra vigilance when herding with wildebeest was influenced by the presence/absence, but not number, of wildebeest. In contrast, impala only reduced zebra vigilance when they comprised about 75% of the herd. Ultimately, our results indicate that for zebra in mixed-species herds, detection benefits are relatively more important than dilution over a larger range of herd sizes. However, detection only reduces vigilance when zebras herd with species that share a common predator (i.e. diluting partner). Potentially, detection by species that do not share a common predator (i.e. low-diluting partner) is not as reliable as detection by diluting species.

Dung odours signal sex, age, territorial and oestrous state in white rhinos.

Mammals commonly communicate olfactorily via urine. However, the extent to which they communicate via dung, another waste product, is unknown. Behavioural studies suggest that mammals can obtain information from dung odours but are unclear about the information transmitted. Moreover, an understanding of the volatile organic compounds (VOCs) released from dung is limited. To address this, we analysed the odours emitted from the dung of free-ranging white rhinos, and found that 2,3-dimethylundecane signalled an individuals sex, heptanal discriminated age class, nonane defined male territorial status and 2,6-dimethylundecane indicated female oestrous state. To validate these findings, we artificially reproduced key elements of the territorial and oestrous odour profiles (i.e. profiles likely to elicit behavioural responses from receivers). We then exposed free-ranging territorial males to these odours. In response, males elicited behaviours associated with the specific odours (e.g. territorial male (potential threat): reduced latency in assuming vigilance; oestrous female (potential mate): increased investigation). These results indicate that the VOCs identified from the dung of free-ranging individuals do transmit key information. Moreover, as white rhinos of all ages and sexes defecate communally, middens probably act as information centres. Furthermore, as many other mammals defecate communally, olfactory communication via dung odours is likely a widespread phenomenon.

Group-living herbivores weigh up food availability and dominance status when making patch-joining decisions.

Two key factors that influence the foraging behaviour of group-living herbivores are food availability and individual dominance status. Yet, how the combination of these factors influences the patch-joining decisions of individuals foraging within groups has scarcely been explored. To address this, we focused on the patch-joining decisions of group-living domestic goats (Capra hircus). When individuals were tested against the top four ranked goats of the herd, we found that at patches with low food availability they avoided these dominant patch-holders and only joined subordinates (i.e. costs outweighed benefits). However, as the amount of food increased, the avoidance of the top ranked individuals declined. Specifically, goats shifted and joined the patch of an individual one dominance rank higher than the previous dominant patch holder when the initial quantity of food in the new patch was twice that of the lower ranking individual’s patch (i.e. benefits outweighed costs). In contrast, when individuals chose between patches held by dominant goats, other than the top four ranked goats, and subordinate individuals, we found that they equally joined the dominant and subordinate patch-holders. This joining was irrespective of the dominance gap, absolute rank of the dominant patch-holder, sex or food availability (i.e. benefits outweighed costs). Ultimately, our results highlight that herbivores weigh up the costs and benefits of both food availability and patch-holder dominance status when making patch-joining decisions. Furthermore, as the initial quantity of food increases, food availability becomes more important than dominance with regard to influencing patch-joining decisions.

Legacy Effects of Different Land-Use Histories Interact with Current Grazing Patterns to Determine Grazing Lawn Soil Properties

Pastoralism and agriculture have affected rangeland ecosystems over the past millennia, including many ecosystems that are currently protected as reserves. However, the legacy of these land-use practices on current ecosystem functioning remains unclear. We studied legacy effects of former human land use on soil physical and chemical properties in a South African savanna. We did this by comparing soil properties in grazing lawns (patches of short grass maintained by the positive feedback between grazing intensity and forage quality) with the surrounding less grazed bunch grasslands within three different human land-use history contexts: (i) Abandoned bomas: permanent stone enclosures where livestock were kept overnight, and dung and urine accumulated for several years or decades. (ii) Old fields: areas where vegetation was cleared, soil tilled, and cultivated, but received little or no fertilization. (iii) Natural grasslands: not cultivated but grazed by livestock before the establishment of the reserve and wildlife thereafter. Former human land use rather than soil texture was the main determinant of grazing lawn location. Moreover, lawn soil properties also varied among land-use histories. In all grazing lawns, soil nutrient concentrations were higher than in adjacent grasslands but abandoned bomas contained three times more phosphorus, and twice as much nitrogen and carbon than old fields and natural grassland lawns. In addition to past land use, soil texture influenced lawn soil nutrients: Concentrations of phosphorus, potassium, calcium, magnesium, total nitrogen, and carbon in lawns were higher on clayey soils than sandy soils, whereas phosphorus, C:N ratio, and pH did not change with soil texture. Our study confirms previous findings on the effect of human land use on savanna heterogeneity, but also highlights how legacy effects may vary among different historic land-use practices.

Soil fertility on granite and sedimentary soils is associated with seasonal differences in foraging by elephants

Background and aimsIt is generally assumed that very large herbivores, such as elephants, make foraging decisions at large spatial scales, but the extent to which seasonal foraging decisions are driven by soil quality, and its link to plant nutrient levels, is uncertain.MethodsWe studied the diet selection of African elephants Loxodonta africana in Ithala Game Reserve in northern KwaZulu-Natal, South Africa, using data on elephant feeding preferences and spatial distributions from a published paper. Elephants were present in the eastern half with granite soils in the wet season, and in the western half with sedimentary soils in the dry season. The quality of these two soil types and of seven key tree species for elephants was assessed in both seasons.ResultsSoil quality was higher on the sedimentary soils in terms of total nitrogen, soil respiration, water-holding capacity, organic carbon and pH. Leaf quality was higher on the sedimentary soils in the dry season, while in the wet season there was no significant difference in leaf quality of the seven key tree species growing on the two substrates.ConclusionSoil quality may explain elephants’ foraging decisions in the dry season, but not in the wet season. Elephants preferred trees with higher protein and lower concentrations of fibre on both granite and sedimentary soils.

Is a reduction in the individual vigilance of mothers a key evolutionary driver of group formation in white rhinos

A key benefit and evolutionary driver of group living is reduced predation risk. In white rhinos, groups comprise adult females, their calves and one to six unrelated subadults. Subadults benefit from group living through exposure to novel areas, and protection from territorial males (i.e. ‘buddy system’). In contrast, it is unclear whether mothers benefit from group living. To determine if they benefit, or if there is simply no cost, we recorded the vigilance of white rhino mothers in different-sized groups. We predicted that as group size increased, calves would have lower predation risk and mothers would reduce their vigilance. In contrast, we found that vigilance did not decrease as group size increased. Our findings thus indicate that decreased vigilance is not a benefit that white rhino mothers gain from living in groups. Also, costs of group formation are minimal for mothers as their large body size and ability to feed on a wide range of grasses reduces competition with other group members. As a result, we suggest that the benefits obtained by subadults, coupled with the lack of costs to adult females, are the main drivers of group formation in white rhinos.

The role of middens in white rhino olfactory communication

White rhinos, Ceratotherium simum, use dung odours to transmit information about their sex, age, territorial status (males) and oestrous state. Moreover, as white rhinos defecate in communal middens (i.e. dung heaps, or latrines) it has been suggested that these middens may act as information centres. However, it is uncertain which individuals primarily transmit information via middens, or for whom this information is intended. Using video-recording camera traps, we investigated the behaviour of white rhinos at middens. We hypothesized that territorial adult males would visit, defecate and sniff other dung more than other adults. In line with this, we found that they visited and defecated more than other individuals. Moreover, territorial males and potential male challengers were the main individuals to investigate dung piles. These olfactory investigations focused primarily on territorial male and adult female dung (male–male and female–male communication). Although investigating less often, investigation by adult females and subordinate males was also focused on territorial male and female dung, suggesting male–female and female–female communication. In addition to olfactory signals, there was a spatial aspect to midden use, where territorial males defecated only in the centre of a midden, while other individuals defecated primarily around the periphery. Yet, subordinate males also tended to defecate in the centre, suggesting an indication of residency. Lastly, territorial males defecated more frequently than any other adult, and were able to do so by regulating their dung output (i.e. producing smaller volumes per deposit). Our results indicate that middens act as information centres, where the primary function seems to be for territorial males to transmit and obtain information. However, nonterritorial males may also assess female reproductive state, while females may be assessing the quality of all males, and the number of other females using a midden. Ultimately, our results highlight the importance of middens in white rhino communication.

Herbivores employ a suite of antipredator behaviours to minimize risk from ambush and cursorial predators

Prey species may adjust their use of antipredator behaviours to counter the hunting strategies (e.g. ambush versus cursorial) and the level of risk imposed by different predators. Studies of suites of behaviours across well-defined contrasts of predation risk and type are rare, however. Here we explored the degree to which six herbivore species adjusted their antipredator behaviours to two predator treatments (lion, Panthera leo, versus cheetah, Acinonyx jubatus, and wild dogs, Lycaon pictus). We focused on prey behaviour (vigilance, grouping, temporal use) at waterholes. We predicted that if the hunting strategy of the predator was the key driver of antipredator behaviour, ambushing lions would elicit a greater response than cursorial cheetah and wild dogs. Alternatively, if predator preference was the main driver, then we expected prey species to adjust their antipredator behaviours in response to the predators that specifically target them (i.e. preferred prey of the different predators). Overall, we found that the herbivores maintained greater vigilance, generally moved in larger groups and used waterholes less at dawn, at dusk or at night (when lions are active) when exposed to the potential threat of ambushing lions. However, some species within the accessible prey range of cheetah and/or wild dogs (i.e. red hartebeest, warthog, gemsbok) moved in larger groups when exposed to these predators. Yet, the magnitude of the differences in group size for these herbivores were small. Thus, we suggest that, overall, the potential threat of ambushing lions was the main driver of antipredator behaviour around waterholes, probably determined by prey weight preference and the possibility of being ambushed.

Similarities in perceived predation risk prevent temporal partitioning of food by rodents in an African grassland

One way in which animals coexist is through temporal separation of feeding activities. This separation directly reduces interference competition, but potentially not exploitive competition. To reduce exploitive competition, coexisting species tend to utilize different microhabitats and/or achieve different feeding efforts across microhabitats. However, 1 factor that has generally not been considered with regards to its impacts on competition, and thus coexistence, is predation risk. As different predators are active during the day and at night, the location of safe areas across the landscape can vary temporally. If so, then temporally separated prey species would likely forage in different areas thus reducing exploitive competition. However, if predation risk across the landscape is similar for nocturnal and diurnal species, then both could restrict their foraging to the same microhabitats, thus increasing exploitive competition. To explore these alternative possibilities, we manipulated grass height in an African grassland to create microhabitats that varied in predation risk. We then estimated perceived predation risk of both nocturnal and diurnal rodents in these microhabitats by recording giving-up densities (GUDs) in artificial resource patches. We found that despite differences in predators, both nocturnal and diurnal rodents preferred feeding in the same microhabitats, and they achieved similar feeding efforts within these microhabitats. This suggests that despite the prevention of interference competition through temporal partitioning, the spatial similarities in perceived predation risk in relation to cover likely increase exploitive competition between these rodents. However, as both nocturnal and diurnal rodents were present in the study area, it is likely that some other mechanism (e.g., varied diets) allows them to coexist.

Influence of density on the seasonal utilization of broad grassland types by white rhinoceroses

ABSTRACT We monitored seasonal use of grassland types by white rhinos at two sites within the Hluhluwe-iMfolozi Park (HiP). Thirty-two rhinos were removed from one site to reduce rhino density. Seasonal use of grassland types was similar at both sites, but differed to what a previous study reported. This was likely due to higher food availability during our study, coupled with lower white rhino density. These findings suggest that during high rainfall years, white rhinos at a density of 3/km2 will not overutilize food in the park. Dispersal, initiated by declining food resources, is used to manage white rhino population size in HiP. We suggest that during high rainfall years it is unlikely that dispersal rates will increase. The impact that a similar density would have on food availability and dispersal during low rainfall years is uncertain.