THE SIZE OF AXIS DEER FLUID GROUPS IN WILPATTU-NATIONAL-PARK, SRI-LANKA

In species which groups are ever-changing in size, a proper description of group size is not easy to produce. On the basis of 3290 observations of groups of Axis deer (Axis axis) in Wilpattu national park in Sri Lanka, I present and compare eight descriptions of group size. The best are Jarman's typical group size (+/- S.D.), (gBAR = 25.8 +/- 24.0), the frequency distribution of deer in each of 29 size classes (from 1 to 125), and the size of the group of the median deer (17.8). These three animal-centered measurements are better than the three corresponding observer-centered ones : the frequency distribution of groups, the median of groups (6.7) and the mean (10.8 +/- 12.5), to describe the animals' experience. Groups were much larger in the open prairie (gBAR = 28.8 +/- 25.3) than in the forest (gBAR = 6.1 +/- 4.0). Rather than jumping to a functionalist explanation to account for that difference, I rather suggest the structuralist alternative, namely that it primarily reflects that the forest hinders, whereas the prairie allows, the coalescence of smaller groups into larger ones. There was a clear relationship between the size of groups seen in the open prairie in a given isolated sub-population and the size of that sub-population. That relationship was better with typical group size than with the mean (Spearman's r(s) = 0.75, p = 0.02 versus 0.58, 0.10 > p > 0.05). Typical (or mean) group size was much larger in the rainy than in the dry season for a given population (gBAR was on average 1.37 +/- 0.18 larger, n = 10), obviously not an indication of an increase in population size but of the confounding effect of season on group size.