Behaviour And Population Dynamics Of Maasai Giraffe (Giraffa Camelopardalis) On A Kenyan Game Ranch
A study was carried out to obtain information on behaviour, social organisation, movement pattern, feeding habits and population dynamics of the Maasai giraffe (Giraffa camelopardalisi on Game Ranching Limited (GRL), Athi River, Kenya; and to use modelling and simulation techniques to assess different options for harvesting the GRL's giraffe population. An existing giraffe photo-file was regularly updated during the course of the study. Individually known focal giraffes were followed for periods of 6 or 12 hours during daytime and their activity budgets continuously recorded. Activities recorded included: walking, feeding, standing, lying, drinking, running, urine testing, fighting and suckling. The names of other giraffes accompanying the focal individual, its location, vegetation type and catena were recorded at half-hour intervals. Browse production by Acacia drepanolobiwn, Acacia seyal and Acacia xanthophloea, and Balanites glabra and its consumption by giraffe were studied. The study found that feeding, walking and resting (standing and lying) constituted more than 95% of the giraffe's daytime activity budget. Males spent more time walking and resting while females spent more time feeding. Females spent 67% and males 39-54% of their daytime feeding. Both males and females spent more than 90% of their feeding time browsing on A. drepanolobium, A. seyal, A. xanthophloeaand H. glabra. Females and young allocated 67-84% and males allocated only 23-34% of their browsing time to feeding below their shoulder height. This height differentiation was also accompanied by a marked female preference for A. drepanolobium, leading to resource partitioning, a factor that reduced competition between the two sexes. Axanthophloea, A. seyal and B. glabra showed clear signs of over-browsing by giraffe, but A. drepanolobium didn't. Browse availability of on A. drepanolobium, A. seyal, A. xanthophloea and B. glabra exhibited seasonal fluctuations, being 1I10st abundant two months after the rains commenced and most scarce towards the beginning of the following rainy season. GRL's giraffe responded to this fluctuating food availability by forming two sub-populations, adjusting time spent on feeding, walking and resting and, adjusting their daily movement pattern. The seasonal variation in birth and death rates of the GRL's giraffe population and the frequent occurrence of population crashes after prolonged droughts documented in this study were also attributed to the seasonal fluctuations in browse availability. Annual calf mortality during the study period was about 17%, while sub-adult and adult mortality was less than 2%. Average calving interval, on the other hand, was 21.5 months. The annual growth rate of GRL's giraffe population was estimated at 10.2%, suggesting a giraffe population growing at almost its maximum potential growth rate. The analysis of browse availability, on the other hand, suggested a population at its habitat's carrying capacity. These two findings casted doubt about the suitability of the logistics population growth model (Seidl and Tisdell 1999) in describing the growth pattern of the GRL's giraffe population. This study therefore concluded that the J-curve and not the logistics model best describes the growth pattern of GRL's giraffe population. The modelling confirmed 'the notion that age-selective harvesting has little effect , in improving sustained yield whether the latter is measured in terms of animals killed, kilograms of meat sold or financial returns. The only exception is when the products from the different age classes , , vary in their market value. Male-biased harvesting, on the other hand, increased sustained yield by 26-48%. Benefits accruing from male-biased harvesting were a Tunction of how far the sex ratio is skewed in favour of females. Maximum sustained yield was achieved at a sex ratio of one male to nine females.
CitationDoctor of Biology of Conservation
University of Nairobi