Why do giraffes occur in aggregated dispersion patterns

The Giraffidae are represented by only two extant species, the okapi (Okapia johnstoni) and the giraffe. Both have unusual and poorly understood social systems (reviewed by Dagg & Foster 1976; du Toit 2001; Pellew 2001). Although giraffes are typically observed in aggregations, they appear to join, and to leave, them independently of others, suggesting that they do not form long-term social bonds. It may be that adaptive benefits usually ascribed to social species have exerted selective pressure on what are essentially asocial animals to aggregate in this way. These benefits might include foraging efficiency (Krebs & Davies 1997, and see Bertram 1980) and/or collective vigilance (Pulliam 1973; Elgar 1989). Alternatively, giraffes may perceive their social environment in ways that are difficult for human observers to identify (Cameron & du Toit 2005). It may be that their behaviour is modified, not by the composition of whole aggregations, but only by the identity of and distance to their immediate neighbour/s (see e.g. Treves 1998). It may also be, however, that they are able to maintain contact with one another over long distances by means of visual, olfactory and/or infrasonic signals and that they spend much more of their time in stable groups (as they perceive them) than has been appreciated hitherto. The purpose of this study is to investigate the first of these two possibilities and to contribute to the elucidation of the second. It arises from and will extend the work of Cameron and du Toit (2005).

Hypotheses
Null hypothesis: (asocial) giraffes co-occur at sites of localised resources e.g. food patches.
Alternative hypotheses: benefits accrue to them (as in social species) from (i) sharing vigilance effort with others and/or (ii) from cueing on public information about food resources.

Predictions
1. The frequency and/or duration of individual vigilance is expected to decrease as a function of increasing aggregation size.
2. The time individuals spend foraging is expected to increase as a function of increasing aggregation size.

Research questions
1. Does aggregation size influence the time spent vigilant by individuals?
2. Does aggregation size influence the time spent foraging by individuals?
3. What is the frequency distribution of aggregation sizes?
4. What is the frequency distribution of aggregation compositions?
5. What is the frequency distribution of nearest/close neighbours distances?
6. What is the frequency distribution of nearest/close neighbours identities?