Key reproductive parameters

It is clear from the wide range in all of the key reproductive parameters found in the literature (table 1) that the hippopotamus is an extremely 'plastic' species. Age at first parturition varies from 3 years to as late as 20 years; calving intervals may be from less than 2 years to over 3 years; mortality for newborn animals may be as high as 45%; central mortality may be over 4%. If Laws' figures for mortality are used, the rate of growth of a hippo population is 1.55%. If Pienaar et al's value for juvenile mortality (16%) is substituted for Laws' (45%) and mortality for two-year olds is set at 8%, the population grows at 5.35%. All of this presents a quandary for modelling because at least three main variables need to be explored and, finally, values of these variables need to be selected to apply to the Namibian hippo population.

Martin suggests the following parameters for the Namibian hippo populations. The selected values are what might be reasonably expected for the Caprivi - which is the only area where hippo are likely to be exploited. These values result in a population growth rate of 7.13% and will be used to examine the response of the population to various management treatments

Comparison with elephant

Whilst it is tempting to think of hippo populations as having similar population dynamics to elephants, it would be erroneous to do so. The short gestation period (8 months versus 22 months for elephants), the high fecundity (one calf every 2 years versus 4 years for elephants) and the wide range of possible ages at first parturition (4-12 years versus 10-12 years for elephants) make hippo populations capable of much higher growth rates and productivity. Martins sensitivity analysis of the reproductive parameters of hippos indicate that with low central mortalities and early ages for sexual maturity hippo populations are theoretically capable of growth rates in excess of 10% per annum.

Implications for Management

Smuts & Whyte (1981) describe the reproductive strategy of the hippo as one well adapted to the semi-arid environments of Africa. When resources become limiting, populations are able to maintain stable populations by delayed sexual maturity and fecundity and so adjust to the carrying capacity of the environment: equally, populations are capable of rapid increase when resources become abundant. This finding has important implications for hippo management. It should be possible to maintain a hippo population in a highly productive state by harvesting: the corollary is that by not harvesting the population is unlikely to increase greatly - its own homeostatic mechanisms will come into play to limit population growth.

Key reproductive parameters for hippo in the literature

Variability of age at first conception and first parturition

Pienaar (et al 1966) and Smithers (1983) stated that the m ajority of hippo conceive in their third or fourth year and produce their first calves in the fourth year. Smithers may have been quoting Pienaar et al. However, Sayer & Rakha (1974) and Smuts & W hyte (1981) cast doubt on the ages assign ed to Pienaar et al's specimens. Sm uts & W hyte suggest that if Law s' (1968) age c riteria were applied to Pienaar et al's data it would transpire that cows m ature in their seventh year and calve in their eighth. Laws and Clough (1966) found the mean age at puberty for females in Queen Elizabeth National Park to be 9 years with a range from 7-15 years. Smuts and Whyte point out that the data of Laws & Clough suggest a range from 3-17 years. Sayer & Rakha found the mean age at maturity for a hippo population in the Luangw a Valley (1965-1970) to be 13 years with a range from 6-20 years. Smuts & W hyte found the mean age at maturity for hippo in Kruger (1974-75) to be 9-10 years. Hippo in captivity have produced calves at 3 years old.

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Population model parameters

The approach which has been adopted is as follows -

1. Fecundity: Since the Namibian hippo population is at far lower densities than any of the populations on which studies have been carried out and there is no reason to suppose that it is under any environmental stress, a mean calving interval of 2 years has been selected - (i.e. a fecundity of 0.5) which is slightly lower than that of 0.55 found by Smuts & Whyte (1981) in the Kruger National Park.
2. Age at first parturition: Although Both Sayer and Rakha (1974) and Smuts & Whyte (1981) found the mean age at maturity to be over 9 years, even in these dense populations animals as young as 3 years old were conceiving - suggesting that non-zero fecundities need to be assigned from 3 years old in increasing values until full reproductive capacity is achieved (in Kruger National Park this was at 11 years of age (Smuts & Whyte 1981)). This requires a curve-shaping algorithm which will permit asymmetry either side of the mean age at sexual maturity (Fig.4). Smuts and Whyte (1981) found a slight decline in fecundity after 25 years of age and the model fecundity profile has been shaped to provide this.

   Figure 4: Fecundity

3. Mortality: The mortalities postulated by Laws (1968) have been adopted initially, other than in the first year of life where the value of 16% (Pienaar et al 1966) has been substituted for Laws' value of 45% ( Table 1 ).

Sensitivity Test

Tests have been carried out to examine the sensitivity of the population to mortality, age at sexual maturity and age at full reproductive capacity.

Mortality Age-specific mortality in the model is set by means of a 'template'. It is only necessary to specify the central mortality for the population (mortality for years 3-30 shown as pale grey shading in Table 2) and the curves for juvenile mortality and senescence are adjusted automatically. In the example shown below, the mortality for each age class is derived by multiplying the number in the template by the central mortality of 4%.

Table 2: The mortality for each age class is derived by multiplying the number in the template by the central mortality.

These mortalities are then adjusted for animals above 10 years of age by increasing the male mortalities by 5% and reducing the female mortalities by 5%.

Martin (2006) examines the effect of variations in central mortality (MC) and juvenile mortality (the shaded columns in the table above) on the rate of growth of a hippo population (Table 3 and Figure 5). The fecundity of adult females is set at 0.5, age at sexual maturity at 9 years and age at full reproductive capacity at 11 years are (using Smuts & Whyte 1981). To obtain the maximum rate of growth for any given central mortality, the mortality for year 1 (M1) and year 2 (M2) is set equal to MC. For first year mortalities of 10% upwards, M2 has been set at half of M1 except where MC is higher than M1 where it has been set at the value of MC (e.g when MC is 7%, M1 is set at 10% and M2 is set at 7%). The additional data points in the first two columns follow the sequence MC = 1, M2 = 2, M1 = 4 and MC = 2, M2 = 4, M1 = 8.

Table 3: Response of a hippo population to changes in juvenile and central mortality

The area shaded in green in the table and highlighted in Figure 5 is the likely operating area which pertains for hippo populations in Namibia. All of the hippo populations on which the quoted studies have been carried out were considered overabundant and under stress: there is reasonable justification for reducing central mortality to 3% and setting M1 and M2 at 12% and 6% respectively.

As with elephant populations, a hippo population is far more sensitive to variations in central mortality than juvenile mortality. It can tolerate high levels of juvenile mortality (e.g. over 60% for a population with a central mortality of 3%). The same is not true for adult female survival. A mortality of more than 9% causes the population to decline regardless of juvenile mortality.

Age at sexual maturity and full reproductive capacity

For modelling purposes, age at sexual maturity is defined as the age at which 50% of the females will be either pregnant or lactating (i.e. they have already given birth) and age at full reproductive capacity is the age at which all females are pregnant or lactating. Because of the extreme variability in both of these parameters which is possible in hippo populations,Martin (2005) assumes that a small proportion of females will calve as young as 3 years of age whatever the mean age at sexual maturity and that the age at which full reproductive capacity is attained may be anything from 2 years to 8 years after achieving sexual maturity. Because central mortality has the greatest effect on the growth rate of hippo populations, tests have been carried out over a range of 1-9% for central mortality. In all cases, mortality in the first year of life has been assumed to be four times the value of central mortality and, in the second year, twice the value of central mortality.

Table 4: Effect of age at sexual maturity and full reproductive capacity on growth rates

Age at sexual maturity has a far greater effect on the rate of growth of a hippo population than does age at full reproduction (Table 4, Figure 6). In other words, once half of the females in a population are producing calves, it makes little difference to the population growth rate how long after that the remainder start calving noting of course that, whatever the time span, it is nevertheless an increasing proportion (Figure 4).