CHAPTER 1 General Introduction
4.4 RESULTS AND DISCUSSION .1 Aerial surveys
4.4 RESULTS AND DISCUSSION
November 1974, 1257 Nile Crocodile hatchlings were reared and released as part of a restocking program in NGR (Pooley, 1982). The first phase of the restocking program saw hatchlings (1967/1968 season) reared for two to three years, thereafter 700-800 individuals were released.
The second phase saw roughly 500 hand reared crocodiles (1969/1970 season) released into the reserve between 1972 and 1974. The crocodiles released in the restocking program would not have been noticed in the first series of aerial counts from 1971-1973 due to their small size as only individuals over 2 m in length are easily noticeable from the air (Games, 1994). According to literature it takes about 14 years for males to reach 2 m in length and about 17 years for females (Hutton, 1987a). However, individual growth rates are determined by temperature, resource availability and genetics and it becomes impossible to accurately age crocodiles according to body length after three years of age (Hutton, 1987a; Kofron, 1990). Hutton collected his data in Zimbabwe and it is likely that the Nile Crocodiles in NGR will show different growth rates because of these factors. Nonetheless, Hutton‟s study provides a good guideline in estimating an upper limit for when the released crocodiles would be visible in aerial surveys and we assume that after 20 years all released hatchlings that survived and are residing in NGR would be visible in aerial surveys.
The second series of aerial counts were initiated in 1985 (Ward, 1989). The first crocodiles released in the restocking program would have been 17 years old and would have been visible from the air explaining the sharp increase in crocodile numbers recorded between 1973 and 1985. The second batch of crocodiles released would have been between 12 and 14 years old during the 1985 survey and would only reach a size of 2 m between 1988 and 1990.
This may explain the second drastic increase in the population between 1989 and 1990. Ward (1990) who conducted the aerial surveys over this time suggests that an improvement in survey
method could have resulted in the increase in yield. However, it is unlikely that observer bias alone accounted for such a large change in survey yield.
The NGR Nile Crocodile population assumes a sequential, stepwise growth form in the 1990‟s. After the initial large increase of 1990, the population within the reserve increased evenly year by year, until 1994 when the population decreased. However, Mathews (1994) attributes the drop in crocodile numbers in 1994 to count technique. However, the pattern of population growth in the 1990‟s may be explained by the differential and/or sexually dimorphic growth rates of Nile Crocodiles (Hutton, 1987a). The faster growing individuals, in this case most likely the males would be the first to be noticed in aerial surveys, after this the females and slower growing individuals explaining the smaller, sequential steps of population growth.
4.4.3 Population stabilization
The earlier restocking program in NGR explains the pattern of population increase depicted by the aerial surveys but fails to explain the extended periods of zero population growth between the population increases (Table 1). While crocodilians are capable of displaying density dependent methods of population regulation such as cannibalism (Cott, 1961; Pooley, 1982) and exclusion (Hutton, 1989b; Richardson et al., 2002) this is unlikely the case in NGR because periods of population stability in the 1970‟s and 1980‟s are followed by dramatic increases in population size. Historical data show that nesting effort has declined as the NGR population has increased (Chapter 5). That is to say that the increase in population size has not translated into more nesting taking place within the reserve and recruitment is unable to sustain the artificially high population size brought about by the restocking program. Nesting effort is low at 21%
(Chapter 5) and Pooley (1982) estimated only a 1 – 2% survival rate of Nile Crocodiles past the first year at NGR.
All crocodiles captured (n = 103) were between 92.6 cm and 472 cm total length (TL) with a mean TL of 230.5 + 71.0 (SD). When compared with other demographic studies on Nile Crocodiles in Africa, NGR has a lower proportion of juveniles (13%) compared to sub-adults (39%) and adults (48%) (Table 2). We therefore suggest that the restocking program initiated in the late 1960‟s and early 1970‟s elevated the NGR Nile Crocodile population to an artificially high density and that cannot be sustained by recruitment within the reserve. The population has therefore stabilized and is not increasing but will in time decrease as the cohort released in the re-stocking programs age and die.
However, the future population decline in NGR may not only be attributed to natural causes. During this study we observed 15 Nile Crocodiles in Lake Nyamithi with snares around their necks (Fig. 1). These individuals were all large (> 2.8 m) and possibly represent a small proportion of snared individuals that manage to break free from traps by snapping the cable used to snare them. It is likely that many smaller individuals were caught and a percentage of larger animals were not able to escape the snares. Furthermore viable and historic crocodile nesting habitat within NGR is currently being destroyed and unavailable to crocodiles as illegal resource use and agriculture is taking place within the reserve (Calverley, 2009).
4.4.4 Sex ratios and population structure
Hutton (1987c) found that Nile Crocodiles in Zimbabwe had a significantly female biased sex ratio across all size classes. This was attributed to temperature-dependent sex determination in crocodilians and low nest temperatures (Hutton, 1987c). Similarly, Leslie (1997) found a
female biased (61.9%) sex ratio in sub adult and adult Nile Crocodiles at St. Lucia. Bourquin (2008) however, found a male biased sex ratio for yearling and juvenile Nile Crocodiles in the Okavango Delta in Botswana (61.8% and 61.2% respectively) followed by a slight bias towards females in the adult and sub adult size classes (54.8% and 55.3% females respectively).
Bourquin attributes the slight prevalence of females to a higher mortality rate in males due to aggressive male-male competition for reproductive rights. At NGR sex ratios were biased toward females in the juvenile (80.0%) and the sub-adult age group (63.0%) (Table 3). Adult sex ratios however were biased towards males (64.7%). Average sex ratio across all size classes was even (50.6% female and 49.4% male). The intercept of equal sex ratios fell in the 2.5 – 3.5 m size class at ± 3.1 m (Fig. 3). The female biased sex ratio in NGR may be a consequence of sex biased dispersal in juvenile crocodilians causing male juvenile males to disperse further away from nesting sites which are located at Lake Nyamithi (Tucker et al., 1998). This could explain why adult sex ratios are biased towards males as males would be more likely to disperse into NGR and Lake Nyamithi than females who stay in closer proximity to nesting sites (Tucker et al., 1998).
The interplay between sex biased dispersal and temperature determined sex (TDS) / environmental sex determination (ESD) could have interesting evolutionary implications when discussed in light of the selfish gene theory. In favorable nesting habitat more females should be produced so that philopatry would result in daughters being in close proximity to good nesting habitat. Comparatively fewer sons would then disperse to colonize new habitats and to spread genetic material. Un-favorable nesting conditions should produce more sons to disperse in search of better habitat. If (nesting) conditions are favorable in the new habitat benefits would be almost
substantial as the first generation of offspring would for the most part be female and would then remain closer to the new favorable nesting site.
The even sex ratio at the 3.1 m age class could also be a product of incubation temperatures selected for during the restocking program favoring neither sex over the other. If this is the case we would expect the development of a female biased sex ratio seen in the juvenile and sub-adult size classes to filter through to the adult size class in the future. Other causes of biased sex ratios throughout size classes may be due to differential mortality and habitat selection between the different sexes (Thorbjarnarson, 1997) and because only one habitat type was sampled in NGR (i.e. Lake Nyamithi). Nonetheless, the 1:1 sex ratio at the reproductive size class in NGR must be considered optimal for maximum recruitment and current sex ratios do not explain poor recruitment within the reserve. This is explained by the seasonal movement of the majority of the NGR population out of the reserve and during the nesting season.