Struktura wiekowa a organizacja socjalna populacji gryzoni

Gliwicz, Joanna

doi:ISSN 0013-2969

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Title: Struktura wiekowa a organizacja socjalna populacji gryzoni

Abstract:

Occurrence of social organization in higher vertebrate populations and its importance for unequal resource partitioning has been widely recognized. It is also agreed that in free-living rodent populations it affects size of individual home ranges, their localization in richer or poorer patches of habitat, frequency of visiting baited (therefore attractive) traps (trappability), time of daily activity peak, etc., with individuals of higher ranks having larger home ranges, situated in better habitats, being more trappable than their lower rank neighbours.Recently the model indicating the basic importance of such individual differences between animals for regulation of population density was presented by Łomnicki (1978a). But till now it is not elear which are the most important characteristics allowing the individual to gain a high position in population social hierarchy.Here I am presenting the hypothesis that social hierarchy is strongly based on the population age structure: the old (but probably not the oldest ones) individuals holding the highest positions and the youngest (but already independent) ones having the lowest ranks. If it ds so, one must accept that: (1) the fate of individual in population is determined by non-genetic trait; (2) rank of progeny cannot be related to age-based social position of its parents; (3) all other profitable characteristics -including genetic traits favoured at present by natural selection may assure only that their "owner" will hold a higher position among other individuals of the same age i.e. among the individuals of similar position in the social hierarchy of entire population.In favour of this hypothesis the following arguments may be brought:(1) It is supported by results of earlier observations indicating that the higher ranks are usually ga1ned by m.ature individuals against immature, those longer belonging to the population against newcomers, settled on their home ranges against intruders, etc. (Alee et al. 1958, Petrusewicz 1959). All these properties. allowing for high er ranks are at the same time the properties of older individuals when compared with younger ones.(2) Hierarchy based upon age is well known in other mammal groups. Behaviourists studying the herd organization of primates large carnivores or ungulates usually refer to older individuals as to the herd leaders. ; (3) Several examples 3 were found indicating that individual differences in respect to home range size (Fig. 1), to its localization (Fig. 3), and to trappability (Fig. 2) -usually assigned to the rank in social hierarchy -are simultaneously the differences between individuals of different age (belonging to different cohorts Ko-K4). Additionally the example is given (Fig. 4) showing that home range size differences based upon age structure are much more pronounced under norma! food conditions (natural food supply) than under the surplus of food (supplementary food added). At the same time indirect evidence was found of more tolerant social relationships between individuals in the period of experimentally increased food supply.(4) The hypothesis of age-based social organization is helpful for explaining the ability of the population to maintain its stability in fluctuating environment.When the population is in the state of balance with its resources for some time, the age structure of the population is stable and so is its social hierarchy governing the unequal partitioning of the resources among individuals. But under critical (suddenly worsened) conditions the best chance for survival is given to the old individuals -holding the highest social position, having largest home ranges and occupying the richest patches of habitat -not to the ones best genetically suited to the new situation. This results in retardation of genetic and demographic response of population known from field and laboratory observations and named ''population inertia". But it also makes possible for a population to rebuild itself according to the mode from before the crash. It is possible to achieve as surviving the crash old individuals maitain their highest ranks preserving both the former organization and the "conservative'' gene pool. (If we assume that the strongest selection occurs during the highest early mortality period, then those old individuals were selected long time ago, probably under different to present environmental conditions.) So, those old, long-living thanks to such social organization individuals preserve specific population "memory'' which helps to keep the stability of population organization and processes despite environment fluctuations.

References:

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