Orientation to solar radiation in black wildebeest (Connochaetes gnou)

Abstract

We recorded the body axis orientation of free-living black wildebeest relative to incident solar radiation and wind. Observations were made on three consecutive days, on six occasions over the course of 1 year, in a treeless, predominantly cloudless habitat. Frequency of orientation parallel to incident solar radiation increased, and perpendicular to incident solar radiation decreased, as ambient dry-bulb temperature or solar radiation intensity increased, or wind speed decreased. We believe these changes were mediated via their effect on skin temperature. Parallel orientation behavior was more prominent when the wildebeest were standing without feeding than it was when they were feeding. We calculate that a black wildebeest adopting parallel orientation throughout the diurnal period would absorb 30% less radiant heat than the same animal adopting perpendicular orientation. Parallel orientation was reduced at times when water was freely available, possibly reflecting a shift from behavioral to autonomic thermoregulatory mechanisms. The use of orientation behavior by black wildebeest is well developed and forms part of the suite of adaptations that help them to maintain heat balance while living in a shadeless, often hot, environment.

Appendix

The heat absorbed from direct solar radiation was calculated for black wildebeest oriented parallel and perpendicular to incident solar radiation for each hour and then summed for the diurnal period for each survey. Heat absorbed was calculated as the product of surface area presented × measured incident solar radiation intensity × absorption coefficient.

• 1. Solar radiation intensity incident on a horizontal surface was measured at the field site (Fig. 1).

  Fig. 8

  

  The calculated surface area projected on to a horizontal surface, presented to incident solar radiation by a 130 kg black wildebeest at solar elevation angles between 10° and 90° when oriented perpendicular (squares) or parallel (circles) to incident solar radiation. See Appendix for details of calculations. The best fit for the relationship proved to be a power relationship, as indicated by the linear fit on a log–log plot

• 2. The absorption coefficient was assumed be 80%, the same as that measured for dark brown cattle (Riemerschmid 1943). The heat load from radiation in dark coats is influenced by wind speed, coat conductance, and the penetrance of radiation into the pelage (Maloney and Dawson 1995). Ignoring these factors would result in an overestimate of the heat load, and so the figures we arrived at would represent the maximal heat load of the orientations. However, the proportional difference between orientations would be independent of these errors.

• 3. The fraction of the total body surface area presented to incident solar radiation varies with body orientation and solar elevation. Solar elevation was calculated for each hour during each survey for the latitude and longitude of our study site using software available at http://www.susdesign.com/sunangle. The surface area presented at different solar elevations was calculated based on data from Riemerschmid (1943) obtained for an 800 kg bull (total surface area 6.5 m²) oriented either parallel or perpendicular to the solar beam. The mass of an average adult female black wildebeest is 130 kg (Skinner and Smithers 1990). The total surface area of a cow weighing 130 kg is 1.86 m² (Hogan and Skouby 1923). We assumed a similar geometry for wildebeest and cattle, and so the surface areas presented for cattle was scaled by 1.86/6.5. The best fit was a power equation (Fig. 8). For perpendicular orientation SA (m²)=26.9×solar elevation^−0.98 and for parallel orientation SA (m²)=2.54×solar elevation^−0.41, with solar elevation in degrees. For each hour (0700 to 1800) during each survey, the surface area presented to incident solar radiation was calculated from these equations for the appropriate solar elevation.

• 4. Solar radiation absorbed was then calculated as the surface area (m²) × the measured incident solar radiation intensity (W/m²)×0.8. For each day the total absorbed solar energy was calculated by summing the hourly values for over 12 h. These values represent the extremes of the range of orientations, because they assume a fixed orientation for the full day.