The ecology and life history of the vlei rat, Otomys irroratus
(Brants, 1827), were studied in a population on the Van Riebeeck
Nature Reserve near Pretoria. Two main procedures were used to collect
the basic information and material for the project. First, a permanent
live-trapping grid was established where 333 animals were captured,
marked, released, and recaptured over a period of 26 months, Second,
snap-trapping and live-trapping were conducted elsewhere in the study
area for supplementary material. Because Otomys angoniensis Wroughton,
1906, a sibling species of 0. irroratus, also occurred in the study area and grid, it was necessary to define the differences in morphology and ecological distribution between the two species.
The ecology and life history of 0. irroratus are described and discussed in regard to the following major topics: taxonomy and morphology, distribution, population dynamics, reproduction, postnatal growth and development, activity, behavior, and economic importance.
In addition to the presently used means of distinguishing 0. irroratus and 0. angoniensis, it was found that the hind foot length of the two species differed, being 29-34 mm in 0. irroratus and 25-28 mm in O. angoniensis. The baculum of the two species was also found to be distinct in both shape and size.
In Southern Africa 0. irroratus occurs primarily in the grassland
subregion of the Southern Savanna biotic zone, while O. angoniensis
occurs primarily in the woodland subregion of the same biotic zone.
The preferred habitats of the two species on the study grid were
determined. There was little overlap of niches and each species had
a high degree of association with a different veld type, O. irroratus
being associated with the more mesic habitat.
The population size of O. irroratus reached a peak in May and a
low in September. The population also fluctuated considerably from
one year to the next, with rainfall possibly being the main contributing cause. The ratio of juveniles, subadults, and adults fluctuated primarily in respect to the season when the young were born. The sex ratio did not depart significantly from 1:1. Males comprised 49,5 per cent of the total and females 50,5 per cent.
The rate of disappearance after one month from first capture was
41 per cent, indicating that a large number of transients and individuals
dispersing from their birth site were involved. The overall rate
of disappearance, excluding the high first month rate of disappearance,
was 3,1 per cent per month.
The mortality rate appeared to be at its highest between August
and September when the food and cover were of very poor quality and
quantity. This was also the period when males demonstrated a significant
loss in body weight. The major factors causing mortality appeared to be flooding of
the habitat and owl predation. Floods not only reduced the population
but significantly interrupted breeding. Otomys irroratus was one of
the most prominent prey species of both barn and grass owls.
The major factors causing mortality appeared to be flooding of
the habitat and owl predation. Floods not only reduced the population
but significantly interrupted breeding. Otomys irroratus was one of
the most prominent prey species of both barn and grass owls.
Recapture data revealed that the mean home range size was 1 443m2, while that of males (1 730 m2) was highly significantly different
(P<O,Ol) from that of females (1 252m2). Home range size decreased
significantly in winter but only slightly in relation to a doubling of the population size. Of 18 individuals recaptured after a flood, 12 (66,7 per cent) had returned to their previously determined home range. Despite a small mean litter size (2,33) and relatively long gestation period (about 40 days), the long breeding season (nine months) and
precocial birth, rapid development, and nipple-clinging behavior of the
young together give O. irroratus a good reproductive potential.
Several litters of young were reared. They are precocial at
birth and development proceeds rapidly. At birth the incisors are
erupted, enabling the young to cling firmly to the nipples of the
mother, thereby reducing mortality losses. Most adult behavior
patterns are developed before weaning at 13 days of age. Sexual maturity
is reached at approximately three months of age. Otomys irroratus is primarily crepuscular, but activity tests conducted in the field and laboratory indicated some activitv throughout the day and night. Individual and social behavior are described and discussed. The
vlei rat is shy and retiring in captivity and in nature is not easily
captured by the use of snap-traps.
Interactions of adults were tested and they were found to be antisocial,
with intrasexual aggression occurring when caged. Complex
threat and communication patterns exist, a feature of asocial behavior.
Mating failed to occur in captivity, probably as a result of their
antisocial nature.
Marking behavior is very distinctive and, combined with urination
and possibly defecation, would appear to be useful in delineating
territories. It would also appear from their social habits, marking
behavior, and considerable overlap of home ranges that their interactions
with conspecifics in nature are represented by a dominance
hierarchy.
The influence of O. irroratus upon the environment was found to
be slight. The species serves as a common prey item for a number of
predators and was found to harbor several endo- and ectoparasites of
public health importance. / Thesis (DSc)--University of Pretoria, 1973. / gm2013 / Zoology and Entomology / Unrestricted
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/32453 |
| Date | January 1973 |
| Creators | Davis, Richard Marcy |
| Contributors | Nel, J.A.J, Meester, J.A.J |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | © 1973 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria |
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