The evolution and socio-ecology of two populations of the Vlei Rat, Otomys irroratus.
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Date
1993
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Abstract
This work investigated two important evolutionary
processes - speciation and adaptive variation - in two
chromosomally-distinct allopatric Natal Midlands populations
of the vlei rat Otomys irroratus. The two populations, at
Kamberg and Karkloof, differ in the presence of a tandem
fusion between chromosomes seven and 12 in the Kamberg
karyotype. Speciation studies considered possible
reproductive isolating mechanisms. In studies of adaptive
variation, socio-ecological characteristics of both
populations were investigated.
Data on breeding and postnatal development provided
evidence of post-zygotic barriers. Interpopulation pairs had
reduced breeding success compared to intrapopulation pairs,
and some hybrids died before weaning. Surviving hybrids had
reduced growth rates, and almost all were sterile.
In tests of pre-mating reproductive isolation,
ethological barriers were emphasized. Individuals preferred
same-population mates, suggesting the existence of mate
recognition, which was achieved by means of
population-specific courtship behaviour and communication,
particularly olfactory, tactile and visual cues. No evidence
of population-specific acoustic signals was found, although
acoustic cues were associated with agonistic interaction,
complementing other communicatory cues to promote increased
aggression during interpopulation pairings.
Laboratory studies of behaviour and morphology and field
work (trapping and habitat assessment) provided information
about socio-ecological parameters. The Kamberg habitat was
harsher than the Karkloof one, as revealed by differences in
seasonal and spatial availability of food and·cover. Cover
was the key determinant of the level of sociability of both
populations. Sparse, patchy cover selected for a partially
communal social system in Kamberg o. irroratus: females were
intrasexually tolerant and males were intrasexually highly
aggressive. This, in conjunction with male-biased sexual
dimorphism, implied that mating was polygynous. Abundant,
uniform cover selected for a dispersed social system in
Karkloof o. irroratus: females were intrasexually less
tolerant than males. Ritualized aggression between males and
a low degree of male-biased sexual dimorphism suggested that
male may have overlapping home ranges in nature and that
mating is promiscuous. Females possibly mated with dominant
males, however.
Contrasting social systems suggest that adaptation to
local environmental circumstances has occurred in allopatry,
and that Kamberg and Karkloof o. irroratus are undergoing
adaptive speciation. Post-zygotic and pre-mating
reproductive barriers appear to have evolved independently in
both populations, and could potentially impede gene flow
between the populations should they become sympatric. The
presence of the tandem fusion in the Kamberg karyotype which,
together with genetically-determined factors, may have caused
hybrid sterility, suggests that this population is a
chromosomally-determined incipient sibling species.
Description
Thesis (Ph.D.)-University of Natal, Durban, 1993.
Keywords
Vlei rat., Rats--Evolution., Rats--Ecology., Theses--Zoology.