The black rhinoceros (Diceros bicornis (L.)) in Ngorongoro Crater, Tanzania, are relatively sedentary (Klingel & Klingel, 1966; Goddard, 1967a; Kiwia, 1983), but a few individuals move between the floor and the Northern Highland Forest Reserve, where they
A total of twenty-five rhinos were counted in the area during the study (Table 1). Of the twenty-five rhinos, fifteen were permanent residents of the Crater floor and the remaining ten were temporary visitors. The sex ratio (male:female) for rhinos of a
The birth rate of the present population was 13.6%, compared to one of 24.1 % reported for the period 1964-1966 by Goddard (1967a). However, during my second visit to the area, a year later (December 1983), five more calves had been born, thus raising the birth rate to 24.2%, a value very close to that recorded by Goddard. The intercalving interval of two known females in the area was observed to be 27-28 months. Goddard (1967a) reported an intercalving interval of 27 months. By September 1982, the mortality rate was 8% and the natality rate (number of births per year as a percentage of all rhinos) was 6%, showing a decline in the population of 2%. However, by December 1983, the natality rate changed to 10.7% resulting in a population increase of 2.7%.
A total of twenty-five rhinos were counted in the area during the study (Table 1). Of the twenty-five rhinos, fifteen were permanent residents of the Crater floor and the remaining ten were temporary visitors. The sex ratio (male:female) for rhinos of all age classes combined was 1: 1.2 and for adults 1: 1.8 (Table 2). Three calves (two males and one female) were born in the area, whereas four adults (two males and two females) were killed by poachers (three were shot and one speared) leaving twenty-one individuals by the end of the field work. Table 2. Sex and age ratios of Black Rhinoceros inTanzania and Kenya (all individuals known by. observer) Sample Sex ratio Age ratio Source of Location size Male:Female Cow:Calf information Ngorongoro Crater 24 1:1.2 100:45 This study 105 1: 1.1 100:72 Goddard. (1967a) Serengeti Plains 63 1:1.0 100:79 Frame, ( 1980) Olduvai Gorge 74 1:0.8 100:79 Goddard. (1 967a) Masai Mara 97 1:0.8 - Mukinya, (1973) Tsavo medium density 52 1:1.2 - Goddard. (1970) Tsavo high density 124 1:1.0 100:79* Goddard, (1970) Tsavo high density 89 1 0.8 - Goddard. (1970) Tsavo high density 119 1:0.9 - Goddard, (1970) Tsavo high density 77 1:1.0 - Goddard. (1970) Note: All unsexed calves not included. *all Goddard's data combined. The sex ratio also does not differ significantly from those observed in ten other studies (X2 =3.13, P>0-05).
The study was confined to Ngorongoro Crater (35?25'E, 3?5'S), which has an area of about 310 km? with the floor occupying 250 km? (Fig. 1). The geology of the Crater is summarized by Fosbrooke (1972) and Anderson & Herlocker (1973). Drainage is complete
The birth rate of the present population was 13.6%, compared to one of 24.1 % reported for the period 1964-1966 by Goddard (1967a). However, during my second visit to the area, a year later (December 1983), five more calves had been born, thus raising the birth rate to 24.2%, a value very close to that recorded by Goddard.
Table 2. Sex and age ratios of Black Rhinoceros inTanzania and Kenya (all individuals known by. observer) SampleSex ratio Age ratio Source of Location size Male:Female Cow:Calf information Ngorengoro Crater 24 1:1.2 100:45 This study 105 1: 1.1 100:72 Goddard. (1967a) Serengeti Plains 63 1:1.0 100:79 Frame, ( 1980) Olduvai Gorge 74 1:0.8 100:79 Goddard. (1 967a) Masai Mara 97 1:0.8 - Mukinya, (1973) Tsavo medium density 52 1:1.2 - Goddard. (1970) Tsavo high density 124 1:1.0 100:79* Goddard, (1970) Tsavo high density 89 1 0.8 - Goddard. (1970) Tsavo high density 119 1:0.9 - Goddard, (1970) Tsavo high density 77 1:1.0 - Goddard. (1970) Note: All unsexed calves not included. *all Goddard's data combined. Likewise the cow:calf ratio did not differ significantly from those recorded in four other studies (X2 = 0.91, P> 0.05) (Table 2).
To date, information on the Minimum Viable Population (MVP) for ver- tebrates is mostly theoretical and controversial (Soul?, 1987). No attempt has been made to calculate the MVP for the rhino, for since the animal is long-lived, it will take time before a viability analysis for the species is possible. However, from a genetic perspective, a population of twenty-five individuals in the Crater is small enough for its viability to be doubted. However, the Crater is not a closed ecological unit for the rhinos. Observations show that the temporary visitors interbreed with the floor residents and possibly with the Northern Highland Forest Reserve population, thus minimizing the effects of inbreeding. Given a proper protection from poaching, and if there is no significant change in the habitat, the present rhino population has the potential to increase in size.
The birth rate of the present population was 13.6%, compared to one of 24.1 % reported for the period 1964-1966 by Goddard (1967a). However, during my second visit to the area, a year later (December 1983), five more calves had been born, thus raising the birth rate to 24.2%, a value very close to that recorded by Goddard. The intercalving interval of two known females in the area was observed to be 27-28 months. Goddard (1967a) reported an intercalving interval of 27 months. By September 1982, the mortality rate was 8% and the natality rate (number of births per year as a percentage of all rhinos) was 6%, showing a decline in the population of 2%. However, by December 1983, the natality rate changed to 10.7% resulting in a population increase of 2.7%.
All individual black rhinoceros in the Crater were photographed at close range, on both sides, and identified using various morphological characteristics. The photographs of each individual were mounted on an index card, below which a detailed description of the individual was written. Some of the features used for identification were: sex (external genitalia), age class, horn structure, ear marks and permanent scars. The same method was used with success by Klingel & Klingel (1966), Goddard (1966), Mukinya (1973, 1976) and Frame (1980). The characteristics for horns proved very useful for identifying individuals in the field especially when they were at a distance or lying down. Makacha, Mollel & Rwezaura (1979) compiled data to show the trend of the rhino population in the Crater since 1958. However, since three different census methods were included in the compilation (aerial, vehicle and individual recognition), the data should be interpreted with caution because possible biases were not allowed for. Goddard (1967b) and (Western 1982), for example, found that aerial counts consistently under-estimated the actual number of rhinos in an area. Single-day vehicle or foot count methods also underestimate the population size of rhinos in the Crater. Throughout the study period, rarely could more than ten individuals be located in a single day despite the fact that each rhino's locality in the field was roughly known to the observer. In fact, on some days no rhinos were seen. The Ngorongoro staff use vehicles once or twice a year to conduct total animal censuses in the Crater. In September 1981, I participated in one such census and not a single rhino was spotted by the five groups involved in the exercise. Thus, it seems likely that the best method for counting rhinos in any area is that of individual recognition. This method was feasible here because the Crater floor is relatively small with good visibility, the rhinos were relatively sedentary and few in number, and the observer had enough time to conduct the study.
Ngorongoro, Tanzania. The intercalving interval of two known females in the area was observed to be 27-28 months. Goddard (1967a) reported an intercalving interval of 27 months.
