Contributions to Zoology, 69 (3) (2000)Luca Luiselli; Francesco M. Angelici; Godfrey C. Akani: Large elapids and arboreality: the ecology of Jameson’s green mamba (Dendroaspis jamesoni) in an Afrotropical forested region

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Discussion

Based on (i) the wide variety of the habitats of sightings, (ii) the wide distribution of the species in the study region, and (iii) the results of the logistical regression model (no macro-environmental parameter influencing this species’ presence), we stress that D. jamesoni is no doubt a habitat generalist, which is very well adapted to persist even in very disturbed forest patches, including sites around towns, villages and human habitations. The fact that a single specimen was found in a strongly urbanized area of Port Harcourt city suggests that small sub-populations of Jameson’s mambas can survive in very small portions of suitable area (less than 2 ha surface). In this regard, the Jameson’s mamba fits well into the “general picture” of the green mambas (D. viridis, D. angusticeps), which are said to persist even in areas after forest has been felled, providing there are still thickets and trees to hide in (Spawls and Branch, 1997). The same is not true for the black mamba, D. polylepis, which occurs mainly in open savannas, but rarely around disturbed areas (cf. Spawls and Branch, 1997). The persistence of green mambas in strongly urbanized areas could locally place a serious problem to the human population, given that the fate of mamba envenomation is often lethal (Spawls and Branch, 1997).

The lack of interseasonal differences in the distribution of mamba sightings demonstrates that these snakes are active the year-round, without showing any diapause period during the dry months. This pattern is not general among large-sized arboreal snakes of the Nigerian rainforest: in fact, it is not consistent with that exhibited by Boiga blandingi (cf. Luiselli et al., 1998b), Boiga pulverulenta, Dispholidus typus, and Thrasops flavigularis (cf. Angelici et al., unpublished data), but it is with that exhibited by Rhamnophis aethiopissa (cf. Angelici et al., in prep.) and Pseudohaje goldii (cf. Angelici et al., unpublished data). It is likely that the interaction of the traits “large body size” and “arboreality” is not significantly correlated with any specific annual activity pattern (all-year-round-activity or dry-season-suspended-activity) in Afrotropical snake species.

Data on both population sizes and density of green mambas in Eket could be not entirely representative of the various populations of this species in southern Nigeria, given the wide range of habitats inhabited by D. jamesoni, and given also the many problems associated with such kinds of estimates applied to free-ranging snake populations (cf. Parker and Plummer, 1987). Assuming that the population density of mambas in our study site and that the densities of many other snake species studied elsewhere (cf. Parker and Plummer, 1987 for a review) are at least reasonable estimates of actual densities, it should be noted that D. jamesoni had a population density lower than that reported for most species. The low population density of mambas in Eket is likely to be due to multiple reasons. Firstly, it could be due to the very large size of these arboreal elapids, as higher densities of small snakes compared with large snakes should be expected (Peters and Wassenberg, 1983; Parker and Plummer, 1987). Secondly, it could be attributable to the scarcity of spatial and trophic resources availability, as Jameson’s mambas are syntopic with some other large arboreal snake species, which can be potential ecological competitors (Luiselli et al., 1998c). However, given that the Jameson’s mamba (i) is a ponderous climber whose great size enables it to go very high in big forest trees (20-30 m or more from the ground, Luiselli et al., unpublished observation) and (ii) has a cryptic green colouration, it is also possible that only a relatively small portion of the individuals of the area have been marked, despite the strong field effort (on average 10 man-hours per field-day) and the relatively long research period (109 field days). The fact that only one female was captured is intriguing but unexplicable at the present time. Based on pilot telemetric studies (Angelici et al., unpublished data), we suspect that the relative scarcity of females versus males depends on the fact that females prefer to rest in higher strata of the forest canopy, whereas males could be easily found at the lower branches or even on the ground. Intersexual differences in vertical utilization of the available space were observed also in another snake species from the Nigerian rainforest (Luiselli and Angelici, 1998). In any case, since no detailed data on population densities is available for any other Dendroaspis species, interspecific comparisons of these issues cannot be done. According to Spawls and Branch (1997), D. angusticeps populations from coastal Kenya and southern Tanzania average 2-3 snakes per ha, but it is not clear (i) whether juveniles are excluded from these estimates, and (ii) which type of statistical estimation was used to generate this mean density value.

FIG2

Slide 3. The border of a rainforest patch in the surroundings of Eket (Akwa-Ibom State), a typical habitat where green mamba‘s can be observed.

The lack of biased sex-ratio in adult green mambas is consistent with data available for many other snake populations studied to date, including green mambas from elsewhere (Phelps, 1989) and other large elapids as well (see Shine et al., 1996).

As previous authorities stated that male and female Jameson’s mambas attain similar body sizes (Schmidt, 1923; Isemonger, 1962; Pitman, 1974), the occurrence of a male-larger sexual size dimorphism (SSD) in our populations is surprising. The possible causes of the discrepancy between our and previous authors’ data sets are various. It is possible that this discrepancy depended on (i) geographical variation within the species, or (ii) flunk statistical results. If hypothesis (i) is correct, it is likely that such geographical variation may be related to the sexual behaviour of the various populations of this species (see Shine, 1994a), so that male-larger SSD should occur in the populations exhibiting male-male combats for access to females, whereas minor SSD should occur in the populations without such combat behaviours (see also Shine, 1994a). In fact, combats between male D. jamesoni have already been reported in the literature (Leloup, 1964), and male combat and male superiority in body sizes are common among large Australian elapids (Shine, 1977, 1978; Shine and Covacevich, 1983). If hypothesis (ii) is correct, the differences observed between our and previous authorities’ data sets should depend on the fact that we considered for our body size estimates only the adult specimens, whereas these previous authorities pooled together both adult and juvenile mambas or cited only the maximum length observed in the two sexes (cf. Schmidt, 1923; Isemonger, 1962; Pitman, 1974). The maximum size in our sample was also clearly attained by males. In this case, it is obvious that our method is the more reliable, as body sizes of immature snakes depend rather on their age than on their sex.

Although our records indicate that green mambas may prey on terrestrial organisms (e.g. shrews and toads), it is clear that most of their preys are arboreal (e.g. squirrels, fruit-bats, birds, etc.), especially when the snakes are adults. It is also clear that most of the prey eaten by green mamba species is warm-blooded (see also Schmidt, 1923; Wakeman, 1955; Lloyd, 1974; Pitman, 1974; Villiers, 1975; Phelps, 1989; Branch et al., 1995, and references therein). Moreover, our data partially confirm Chiszar et al.’s (1994) suggestion that juvenile and adult green mambas (D. angusticeps in the case of Chiszar et al.’s data) rely on partially different foods, with more ectothermic prey taken by the juveniles. However, based on our dietary data, we strongly agree with Branch et al. (1995) that it is unlikely that adult green mambas develop a taste for terrestrial rodents, and we disagree with Chiszar et al. (1994) that such a remarkable ontogenetic shift in foraging behaviour should be a widespread ecological trait in green mambas. In fact, the presence of a few terrestrial rodent prey in adult green mambas (one single case in our data: Lemniscomys striatus) need not infer terrestrial foraging as the prey items may have been ambushed from above, or have entered trees for feeding or breeding (Branch et al., 1995). The pattern of prey-size/snake-size relationships exhibited by Jameson’s mambas was the “ontogenetic shift in lower size limit” (sensu Arnold, 1993), which is not uncommon among large-sized snake species (for a review, see Arnold, 1993). Concerning the foraging strategy, it is likely that Jameson’s mambas exhibit a typical ambush predation (see also Angilletta, 1994, for D. angusticeps). In fact, we noticed sedentary habits both in the specimens recaptured during our capture-mark-recapture study in Eket and in the few radiotracked individuals monitored during 1998 and 1999 (Angelici et al., unpublished data).

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Slide 4. A highly altered plantation area near Rumuji (Rivers State), a habitat where green mamba‘s are still found although uncommon.

The strong seasonality of reproductive periods exhibited by Jameson’s mambas (mating in the dry season, and ovipositions in the wet season) mirrors data available for D. viridis, which is ecologically similar to D. jamesoni (Phelps, 1989), and is found in the rainforests of western Africa (Spawls and Branch, 1997). Moreover, it is likely that the majority of the adult females are able to reproduce every year, given that most of the females captured in April-June were carrying eggs. The number of eggs produced by Jameson’s mambas is similar to that reported for other Dendroaspis species (6-17, cf. Spawls and Branch, 1997), and is also consistent with that of the large-sized Oxyuranus species (Shine and Covacevich, 1983), which show strong morphological, behavioural, and ecological convergence with African mambas (Branch et al., 1995). The smallest female mamba to produce eggs was a relatively small specimen (119 cm total length) compared to the maximum size attained by the species (up to 2.5 m, probably even more, cf. Spawls and Branch, 1997). This may be explained by the general pattern of elapid snakes that large species mature when they are relatively small (i.e. at a low proportion of their maximum size) whereas smaller species tend to delay maturity until they have attained a higher proportion of their maximum body length (Shine, 1978; 1994b).

Offspring size in D. jamesoni is unknown, but is said to be possibly around 30 cm length (Spawls and Branch, 1997). Unfortunately, we did not collect any such small mamba from the wild (the smallest specimen found by us was 58.8 cm long, and was captured in late April). Thus, we cannot confirm whether the offspring size given by Spawls and Branch (1997) is correct, and we cannot establish the period of the year when the eggs actually hatch. This lack of information is a hard shortcoming to understand the rates of growth in these large-sized snakes. Despite direct evidence is lacking, we suspect that rapid growth is attained in free-ranging mambas. In fact: (i) green mambas are found in warm climates with activity and feeding occurring year-round (see data in this paper), (ii) of 102 specimens for which data are available (including specimens collected and measured, or just sighted) only two (1.9% of the total) were obviously less than 70 cm long. This scarcity of very small individuals suggests that snakes grow rapidly through this range of small body sizes, the same being true for other large-sized elapids from elsewhere (Shine and Covacevich, 1983). However, considering that Jameson’s mambas are arboreal and cryptically coloured, it is well possible that the presence of very young specimens in our sample of collected or just sighted records is strongly biased (underestimated) because of the difficulty to detect such animals in the forest canopy.