From a purely reductionist ecological perspective, it is impossible to fully understand giraffe without understanding the plants that they eat. At the most fundamental level - on the molecular scale - giraffe are comprised entirely of the same proteins, molecules and atoms that comprise the browse they consume. In this way, from an evolutionary view, diet selection is an mechanism to ingest some type of molecular stoichiometry that is most conducive to survival and reproduction. Perhaps
ore simply stated, nutrition is key for making more giraffe and healthier giraffe.
In other giraffe populations , researchers have suggested that populations may be food limited, and once these "bottom -up" regulations are relaxed, the population can continue to grow. As part of his seminal work, Robin Pellew documented an increase in the Serengeti giraffe populations and demonstrated an association with increasing biomass in the area. Clearly, however, it was not purely an increase in biomass that precipitated this population growth - (which Pellew acknowledges). Instead, the type of biomass, the relative abundance of browse species and their nutritional value were all important aspects in contributing to individual health and subsequent population growth
In determining the role that forage availability plays on individual behaviour, two questions become immediately important:
What specifically do giraffe eat and why do they choose to eat these specific items?
Determining what giraffe eat is a (relatively) simple task. Numerous studies have examined giraffe diet composition in different populations across Africa. Although diets vary in accordance with the plant communities in which giraffe live, giraffe have been shown in numerous instances to forage on a wide range of available browse species. Although traditionally, diet studies have relied on direct observation of foraging individuals, recent technological advances have allowed for innovative new approaches to understanding diet composition for cryptic species and species that might forage inconspicuously: DNA metabarcoding allows for the identification of consumed plant species by matching DNA sequences of plant bits in the feces with the sequences of DNA of known plants in a reference library, and animal-borne video cameras have allowed for the behavioural observations of animals without having to be anywhere near the study organisms.
Determining why giraffe eat specific plants is a bit more complicated. In many natural systems, giraffe navigate a remarkably complicated resource landscape, where the quality and quantity of forage resources vary considerably over space and time. Like many large-bodied herbivores, giraffe are thought to respond to subtle variations in the nutritional quality of their forage resources. Additionally, many species of plants have evolved defense mechanisms to limit the loss of leaves and branches to herbivores; thorns and chemical defenses limit the amount of leaves a browser can consume and the rate at which it can consume them. Although undoubtedly a complicated task, beginning to tease apart the basis of diet selection may perhaps provide deeper insights into the mechanisms for habitat use and the role of resource availability on population dynamics.
To begin to address these issues, we need to know what giraffe are eating - at both the species and chemical level - how much giraffe are eating and in what sequence are giraffe consuming them . Since we need to know precisely how much of each plant species giraffe are consuming, the diet composition insights revealed by DNA metabarcoding techniques would provide an insufficiently coarse view of overall diet. Additionally, the behavioural insights offered by animal-bourne video cameras are not yet logistically feasible for the scale at which we hope to operate. Instead, in the tradition of the great field biologists of the last century, we employ continuous focal behavioural observation. Over the span of a day from sunrise to sunset, we follow a single individual giraffe and note how much it consumes of every plant, the location of every plant that it consumes and the order in which it consumes these plants. Unlike many other herbivores, giraffe are rather conspicuous foragers, which allows us to observe and catalogue foraging from a distance, thereby minimizing any disturbance to natural behaviors. In addition to providing key data for foraging ecology studies, this process also quite literally allows us to experience a day in the life of a giraffe. These hours of observation have provided us with innumerable unintended insights into individual variations in behavioural repertoires and fresher perspective on giraffe social dynamics.
To better understand the role that plant nutritional and chemical properties might play in influencing giraffe foraging decisions, we collect samples of the plant species that giraffes regularly consume. In attempt to mimic the potential role of mechanical manipulation on the , we process the plants in a manner similar to the way that giraffe consume them. Carefully picking the leaves from the thorny Acacia drepanolobium or Acacia senegal certainly engenders a sense of admiration for the dexterity and efficiency of the giraffe tongue in overcoming these formidable plant defences. Once processed, we bring these plant samples to the laboratory to analyze them for common nutrient
and biochemical properties that are thought to be important for ruminant physiology. By understanding the relative nutritional value of plant species and how they vary over space and time, we hope to develop a deeper understanding of the giraffe 'foodscape' in Murchison Falls National Park.
To date, we have collected nearly 160 hours of preliminary observational data and have collected samples for nearly 100 individual plants to be analyzed. In examining piece together a better understanding of how plant variation at the species level and nutrient level can influence giraffe foraging behaviour. Combined with the spatially explicit population data from our demographic surveys, we then hope to draw connections between this nutritional landscape and spatial variation in demographic parameters. Through these methods, we hope to better inform ecological theory relating to foraging behaviour and indentify key resources for giraffe that might be important for individual health and sustained populations.