Materials and methods
Study areanext section
The field study was carried out mainly from September 1996 to February 1999, with some data recorded even earlier, in some localities of southern Nigeria. These localities were situated in the territories of the Niger Delta (Delta, Edo, Bayelsa, and Rivers States), in Anambra State, Abia State, Akwa-Ibom State, Imo State, and Cross River State (Fig. 1). These territories are in general heavily populated, with patches of rainforest interspersed among wide plantations (yam, cassava, cocoa, pineapple, banana, plantain, oil palm, etc.). The forest patches may have dry soil, or may be permanently or seasonally inundated swamp-forests. Enormous extensions of mangrove formations (Avicennia marina, Rhizophora racemosa) are found in the brackish water tracts, mainly along the rivers Benin, Forcados, Ramos, Sangana, Nun, Brass, Saint Nicholas, Santa Barbara, San Bartholomew, Sambreiro, New Calabar, Bonny, Imo, Kwa-Ibo, and Cross. Much details of both study areas environmental characteristics and climate conditions are described elsewhere (Luiselli et al., 1998a, 1998b, 1998c).
Field trips were conducted in all types of weather. Each day, the field research lasted approximately from 8 a.m. to 6 p.m. (Lagos time). Night searches were done only rarely, because of security constraints related to the prevailing unstable political situation. Random routes to locate snakes were followed throughout every macrohabitat type available in each study area. The captured snakes were sexed and measured for snout-vent length (SVL, to the nearest 1 cm) and tail length (tL). Since a negligible proportion of specimens had the tail broken (< 0.5%), for all analyses we considered the total length, i.e. SVL + tL. Meteorological conditions, time (Lagos time), and habitat of each observation were recorded. Specimens found already dead (roadkilled or macheted by farmers), including those which were too damaged for any reliable body size measurement, were inspected by dissection for data on diet and reproductive condition. The same was true for some preserved specimens found in museum collections (Rivers State University of Port Harcourt, and Cross River National Park of Akampka) and in the laboratories of several high schools and hospitals (cf. “Acknowledgements”), where these specimens were transitorily stored to be used for demonstration to students. Free-ranging snakes were captured with the help of grabsticks, given the arboreal habits and the extremely dangerous nature of this species which is well-known to be highly venomous (Spawls and Branch, 1997), but, contrary to earlier reports, not very aggressive (Luiselli, Akani and Angelici, unpubl. obs.). All snake specimens captured alive were palpated in the abdomen until regurgitation of the ingested food or defecation had occurred. The prey items were identified to the lowest taxon possible. When the prey was in good condition, it was weighed by an electronic balance (to 0.1 g precision). In these cases, also the snake mass was measured. Voucher specimens are deposited in the herpetological collections of the authors (L.L. and F.M.A., collections in Rome and in Ikot-Ekpene, Nigeria; G.C.A., collection in Port Harcourt, Nigeria), in the collections of the “Monti Prenestini Natural History Museum” (Capranica Prenestina, Rome), and in the collection of the Department of Biological Sciences, Rivers State University of Science and Technology, Port Harcourt.
Slide1. Mangrove formations along the Bonny River (Rivers State). In mangroves green mamba‘s are present but quite rare.
Population size and density of green mambas were estimated by capture-mark-recapture procedures in a moist rainforest area of southeastern Nigeria (Eket, Akwa-Ibom State). The surface of the area used for the capture-mark-recapture procedure was 100 ha. The capture-mark-recapture study lasted 109 days, from early June to late September 1997. Snakes were individually marked by ventral scale-clipping, and painted with a white number on the back for further observations at distance without necessity of recaptures. We considered as “recaptures” (second sample) all of the later sightings of marked (i.e. dorsally painted) individuals, when at least 10 days from the date of the first capture had passed.
Population density was calculated by applying the Lincoln-Petersen index, with its relative 90% confidence limits (Caughley, 1977; Seber, 1982). For population size estimates, only adult specimens were considered. The Lincoln-Petersen index was used under the assumption that our mamba population was nearly without emi/immigrants. In fact, the study area is surrounded in part by a large river (Kwa-Ibo River) and in part by much deforested areas, that are hardly crossed by green mambas.
The effects of the macro-environmental parameters on the presence/absence of D. jamesoni in the study region were assessed by using a logistic regression model (forward stepwise conditional procedure) for discrete values (Hosmer and Lemeshow, 1989). For this analysis, we used data of presence/absence of the green mamba in 52 study sites which were accurately surveyed during our environmental works (Anonymous, 1998). The surface of each site was approximately 50 ha, and was separated from the closest surveyed site by at least 10 km of linear distance. Eight macro-environmental parameters were identified during these large-scale environmental studies (see Anonymous, 1998), and we checked for the eventual presence of any of them in each of the 52 sites surveyed. The designed macro-environmental parameters were as follows:
(1) primary dry forest (PDF); (2) secondary dry forest (SDF); (3) shrubland (SHL); (4) primary swamp-forest (PSF); (5) secondary swamp-forest (SSF); (6) mangrove (MGR); (7) farmland and plantation (FPL); (8) large water body (main river tract or wide lake, PWB).
The place-names, the geographic coordinates, and the macro-environmental parameters of each of the 52 study sites are omitted in this paper to save space, but are fully presented in Luiselli and Politano (1998).
In the logistic regression model, the study areas were the cases (total n = 52), the presence/absence of D. jamesoni was the dependent variable, and the environmental parameters were the covariates (total n = 8).
All data were statistically processed by means of a SPSS (version 4.5, for Windows) personal computer package, with all tests being two-tailed and with alpha set at 5%.