Intraspecific Strategic Responses of African Elephants to Temporal Variation in Forage Quality

Abstract

Mammalian herbivores adopt foraging strategies to optimize nutritional trade-offs against restrictions imposed by body size, nutritional requirements, digestive anatomy, physiology, and the forage resource they exploit. Selective or generalist feeding strategies scale with body size across species. However, within species, where constraints should be most similar, responses to limitation have rarely been examined. We used African elephants (Loxodonta africana) to test for changes in seasonal diet quality of individuals of differing body size and sex through measurement of fecal nitrogen and phosphorus. We measured physiological stress response of these age and sex classes to seasonal change by fecal glucocorticoid metabolite levels (i.e., stress hormones). Large body size increased tolerance to lower-quality forage. Adult males and females exhibited divergent trends; females had higher diet quality than males, irrespective of body size. When limited by forage availability or quality during the dry season, diet quality declined across all body sizes, but weaned calves ingested a higher-quality diet than larger-bodied adults. On release from restriction during the wet season, weaned calf nitrogen concentrations were consistently high and stress hormone levels decreased, whereas adult female phosphorus levels were highest and less variable and stress hormone levels were unchanged. The ability to adjust forage quality is an important strategy used to ensure adequate nutritional intake according to body size limitations. Although body size is a key determining factor of dietary differences between adult elephants, foraging strategies are also driven by specific nutritional requirements, which may override the body size effects driving foraging decisions in some cases. The diversity of intraspecific response highlights ecologically segregated entities within a species and should be a concern for population management planning, particularly for threatened species. Fecal diet quality and stress hormone analysis could provide an early and sensitive indicator for monitoring age and sex class responses to resource restriction in high-density elephant populations.

Soil ingestion, nutrition and the seasonality of anthrax in herbivores of Etosha National Park

“…Although Wrench et al (1997) had zebra as a study species, most zebra samples were below their recommended thresholds, a result which was not discussed. Dietary thresholds of fecal nutrition indices determined for ruminant species are unlikely to apply to non-ruminant herbivores (e.g., elephants, Woolley et al 2009), since they only take into account the proportion of nitrogen or phosphorus per unit feces. Ruminants have lower intake rates, greater digestive efficiency, and select higher quality forages than nonruminant herbivores of similar body mass (Foose 1982, Duncan et al 1990).…”

Examining Human Perception of Elephants and Large Trees for Insights Into Conservation of an African Savanna Ecosystem

“…Elephants such as large bulls potentially have a higher impact on vegetation than other age and sex classes (Hiscocks, 1999). The large males have a tendency to break and bite stems with larger diameters than females or subadult males (Greyling, 2004;Stokke & du Toit, 2000) as well as requiring larger amounts of food (Woolley, Millspaugh, van Rensburg, Page, & Slotow, 2010).…”

“…Females in breeding herds have added nutritional demand for high quality diets during pregnancy or lactation suggesting a higher impact on trees as food sources (Woolley et al, 2010), and an increasing number of elephants such as that in a breeding herd is presumed to have a greater impact compared to a single individual of that herd. The effect of elephants on the vegetation plays an important role in shaping the environment.…”

African elephant in a cleft stick : choosing between starving or dying from thirst in arid savanna

“…The difference in forage decisions based on gender has been reported in other studies. For instance, Woolley et al (2009) found that diet quality differed between elephant of different sexes, ages and body sizes.…”

“…This difference also exists within conspecifics based on behavioural differences between the sexes and the age classes (Woolley et al, 2009). For instance, male elephant have prolonged growth , and have to incur additional energetic costs in the annual reproductive musth phase where they spend less time foraging and more time searching for the females, hence incurring energetic costs of contests and fights .…”