The size of Axis deer fluid groups in Wilpattu national park, Sri Lanka

Abstract

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.), (g = 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 (g = 28.8 ± 25.3) than in the forest (g = 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 (g 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.Resume. -Chez les especes dont la taille des groupes est tres labile, il est difficile de decrire adequatement la taille de ces groupes. Sur la base de 3 290 observations de groupes de cerfs Axis, Axis axis, dans le pare national de Wilpattu, au Sri Lanka, je presente et compare huit mesures de la taille de ces groupes. Les meilleures sont d'abord ce que Jarman appelle la taille typique des groupes (± E.T.), (g = 25.8 ± 24.0), puis la distribution de frequence des cerfs dans 29 classes de taille (de 1 a 125), et la taille du groupe du cerf median (17.8). Ces trois mesures centrees sur 1'animal decrivent son vecu de fa?on plus signifiante que les trois mesures correspondantes qui sont centrees sur 1'observateur : la distribution de frequence des groupes, la mediane des groupes (6.7) et la moyenne (10.8 ± 12.5). Les groupes etaient beaucoup plus grands en prairie (g = 28.8 ± 25.3) qu'en foret (g = 6.1 ± 4.0). Au lieu de simplement conclure que cette difference est fonctionnelle, je considere plutot une explication structuraliste voulant que la difference resulte dans une large mesure du fait que la prairie permet, alors que la foret empeche la formation de grands groupes. II y a une relation tres nette entre la taille des groupes d'une sous-population isolee et la taille de cette meme sous-population. Mammalia, t. 55, n° 2, 1991. Brought to you by | University of Alberta Library Authenticated Download Date ...

Group size, sex and age composition of chital (Axis axis) and sambar (Rusa unicolor) in a deciduous habitat of Western Ghats

“…In Mudumalai, average mean group size of chital differed significantly between seasons and this may be attributed to the availability of food varying between seasons. The observed mean group size of chital in Mudumalai was second highest after Pench (Acharya, 2007) in the sub-continent followed by KarnaliBardia, Chitawan, Wilpattu, Sariska, Gir, Guindy Ramthambhore, and Bhadra (Dinerstein, 1980;Mishra, 1982;Barrette, 1991;Sankar, 1994;Khan et al, 1995;Raman, 1997;Bagchi et al, 2003;Jathanna et al, 2003). Increased chital mean group size and crowding occurred during the wet season when there was increase in food availability due to monsoon in the study area.…”

“…Differences between species in the use of habitat, temporal and spatial resources are known to promote species to have different ecological adaptation. Ungulate groups tend to be larger in open habitats than dense vegetation thereby hindering the formation of larger groups and this is assumed to be a biological adaptive response (Lagory, 1986;Barrette, 1991). Most of the Cervids are gregarious animals and groups are centered around adult females.…”

“…Indeed, in such species most group members are not cemented to each other by social bonds but are free to leave or join a given group. The size of their groups is more likely to be a sensitive reflection of the immediate effect of such important ecological parameters as habitat structure, spatio-temporal distribution of food and predation pressure on group formation (Barrette, 1991;Raman, 1997). Current hypotheses about grouping led to the reasonable expectation that individual fitness varies with group size (Rodman, 1981).…”

An example of segregation between age and sex classes only weakly related to habitat use in mouflon sheep (Ovis gmelini)

“…As in many other ruminants (Woolf, O'Shea & Gilbert, 1970;Feer, 1979;Schaal, 1982;Fichter, 1987;Barrette, 1991;Estes, 1991;ToõÈ go, Gaillard & Michallet, 1996;Mechkour et al, 1997), mou¯on groups are unstable, frequently splitting and merging (Le Pendu et al, 1995). Accordingly, if mou¯on associated independently of their sex, the number of adult females in the groups including exactly n adults could be expected to follow a binomial law with parameters n and p, where p is the proportion of adult females in the groups including n adults.…”

Seasonal changes in group size and composition of Chinkara (Gazella bennettii shikarii) (Mammalia: Bovidae) in central Iran

“…For herding ungulates, the size and composition of groups are the most basic elements of social organization (Barrette 1991). Environmental factors have been considered crucial in explaining ungulate social organization, although other factors, such as predation risk and reproductive strategies, have been identified as being equally important in shaping group sizes (Roberts 1996;Bon et al 2001).…”