Social ecology of Otomys irroratus, Rhabdomys pumilio and Praomys natalensis.
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Date
1982
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Abstract
This work sets out to describe the socio-ecological niches of otonya
irroratus, Rhabdomys pumilio and Praomys natalensis in the Natal midlands.
This objective necessitated a broad-based approach in which aspects of the
ecological niches, and the social behaviour and social organization of the
three species were investigated in field (habitat and trapping) and
laboratory (experimental and observational) studies, during the period
January 1976-May 1978.
To test the assumption that the commonly sympatric O. irroratus,
R. pumilio and P. natalensis are primarily adapted to habitats which differ
mainly in respect of water availability, an attempt was made to define
their positions on a mesic/xeric continuum. The results suggest that in
terms of the parameters measured (responses to water and cover availability,
and to food and water deprivation), O. irroratus is more nearly mesically
adapted than is R. pumilio, with P. natalensis positioned between the two
extremes. However, in the case of P. natalensis it is apparently of
overriding importance that this species is adapted to disturbed habitats.
The mesic habitats preferred by O. irroratus are often of limited size,
but resource availability within these areas is apparently high and stable.
Extensive areas of suitable habitat are available to R. pumilio, but
availability of resources is seasonally variable. The disturbed habitats
preferred by P. natalensis arise unpredictably in nature and, depending
on the rate of ecological succession, may be short-lived; hence availability
of resources to this species is highly unpredictable.
Social behaviour was studied by means of dyadic encounters in the
laboratory. Communication in O. irroratus, R. pumilio and P. natalensis
appears to be dominated by auditory, visual and olfactory signals respectively, although O. irroratus also has well developed visual signals
in its communicatory repertoire. These differences are explained mainly in
terms of patterns of diel activity and habitat preferences: R. pumilio is
adapted to bright light, and its visual signals are subtle; O. irroratus
is adapted to dim light, and its visual signals involve gross changes in
posture, while the use of loud, low frequency vocalizations would allow
conspecifics to know their precise location in relation to one another in
dense vegetation; P. natalensis is nocturnal, and hence appears to emphasize
olfactory (and possibly ultrasonic) communication, which would permit
transfer of information in the dark. O. irroratus is overtly highly
aggressive, but agonism is ritualized; ritualization of aggression has
probably evolved to allow high densities of this species to exploit limited
areas of prime habitat. R. pumilio is less overtly aggressive than
O. irroratus, but aggression tends to be unritualized; direct aggression
probably acts as a spacing mechanism, and is possible in view of the large
areas of habitat available to this species. Agonism is poorly developed
and ritualized in P. natalensis, permitting high sociability and group
formation in this species, and maximum exploitation of locally abundant
resources.
Social organization was studied in the field (trapping studies) and
by means of dyadic encounters in the laboratory. The social system of
O. irroratus appears to include temporal territoriality, which would
permit animals to live in close spatial association (in small areas of
habitat) while seldom actually meeting. Hierarchical ranking occurs in
male O. irroratus and R. pumilio, with competition in both most likely
being for mating opportunities. Breeding females of these two species are
intrasexually territorial, in the case of Q. irroratus thereby providing
dispersing young with adequate space for establishment of a home-range in prime habitat, and in R. pumilio protecting the young against conspecific
female aggression until dispersal. R. pumilio tends to aggregate in mesic
refuge habitats in winter, which apparently reflects the seasonality of
resource availability to this species in drier environments. P. natalensis
is colonial, an adaptation facilitating maximum exploitation of temporarily
abundant resources in transitory disturbed habitats. In response to intense
courtship by the male, female R. pumilio appear to undergo reflex ovulation;
this strategy would maximize the chances of fertilization occurring during
occasional meetings between males and females. Courtship intensity is low
in P. natalensis, and presumably because the colonial social system of this
species ensures frequent association between potential mates, females
appear to ovulate spontaneously. Sexual activity was not observed in
O. irroratus.
The life-history tactics of the three species are such that o. irroratus
and P. natalensis respectively appear to be K- and r-selected, with
R. pumilio falling between these two extremes on an r-K continuum.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1982.
Keywords
Muridae., Rats., Mice., Rodents., Theses--Zoology.