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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Results

Habitat and ecological distribution

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Dendroaspis jamesoni is no doubt a common and widespread snake in southeastern Nigeria: it was found in 26 of 52 sites surveyed in the eastern axis of the Niger Delta (including sites along the courses of the rivers Imo, Bonny, New Calabar, Sambreiro, and Orashi), in several sites in the Delta, Edo, Abia and Imo States (surroundings of Aba, Okpala-Ngwa, and Owerri), in Eket, Uyo, Ikot-Ekpene, and in several sites along the Kwa-Ibo River (Akwa-Ibom State, cf. Luiselli et al., 1998c), and in Calabar, Itu, Akampka, Oban, Ekang, Ikom, and Basua (Cross River State, cf. Butler and Reid, 1986, 1990; Schmitt, 1996; Luiselli et al., unpubl. data).

FIG2

Slide 2. Rainforest in the surroundings of Buguma (Rivers State), a place where green mamba‘s are very common.

We obtained 83 records of D. jamesoni for which the precise habitat at the point of capture has been recorded. Snakes were found in secondary dry forest (n = 22, 26.5% of the total sample observed), primary dry forest (n = 10, 12%), primary swamp forest (n = 19, 22.9%), mangrove formations (n = 2, 2.4%), bushy spots in the forest-plantation mosaic (n = 3, 3.6%), farms and plantations (n = 23, 27.7%), and suburban areas (n = 4, 4.8%). As for the preserved specimens observed in high school and hospital collections (n = 11), it was impossible to establish the habitat of capture. However, based on interviews with school personnel, it is likely that these specimens came from highly disturbed areas around towns (suburbia and plantations). An adult roadkilled male was found in a business part of Port Harcourt city (“Mile two Diobu”), where only a few bushes and trees surrounding a wide grassy field were available to snakes.

The logistic regression model (-2 log likelihood = 72.087; for variables not in equation: residual χ2 = 3.5, df = 8, P = 0.89) showed that the presence of D. jamesoni was not correlated significantly with any macro-environmental parameters (P > 0.15 in all cases).

Seasonal distribution of records

We had 83 records of mambas for which the date of sighting has been recorded. There was no bias towards a particular season: 50.6% of the total number of specimens (n = 83) were observed in the dry season and 49.4% in the wet season (interseasonal difference at χ2 test with df = 1, P > 0.9).

Population size and density

During a 109-day-long research period in Eket, we captured and marked seven adult D. jamesoni (six males and one female), and later recaptured five of these specimens (four males and the female) for a total of eight recaptures. All the recaptured specimens exhibited sedentary habits. The female was contacted 23 days after the first capture in a tree adjacent to the one of capture (17 m of linear distance), whereas the four recaptured males were contacted again at a mean linear distance of 26.4 m from the sites of first capture, and in two cases inside the same tree where they were first captured.

According to a Lincoln-Petersen index calculation, population size was 11 adults (SE = 2), and density was approximately 0.11 adults per ha-1. It should be noted that, given (i) the apparently sedentary behaviour of these snakes and (ii) the relative isolation of the study forest (see “Methods”), the Lincoln-Petersen index seems to fit well with the population under study.

Sex ratio and sexual size dimorphism

Adult sex-ratio (males : females = 1.17 : 1, total n = 63) did not differ significantly from equality (binomial test with df = 1: P > 0.1).

Excluding young specimens (< 100 cm total length), we measured the total length of 22 males and 27 females. There was considerable sexual size dimorphism: the males (x_ = 166.4 cm, SD = 25.9) attained much larger sizes than the females (x = 146.9 cm, SD = 20.3) (Student t-test: t = 2.95, df = 47, P = 0.0049). The maximum length attained by males was 211.5 cm, and the maximum length attained by females was 174.7 cm.

Food habits

For dietary data 77 specimens (34 males, 29 females, 14 juveniles) were examined pooling together specimens captured alive in the field and specimens examined already dead. Identifiable food was found in 38.2% of the males, 13.8% of the females, and 28.6% of the juveniles, without any significant difference between wet and dry seasons (χ2 test, in all cases P > 0.1). Twenty-two specimens of D. jamesoni contained food in the guts; however, some individuals contained more than one prey item. From specimens shorter than 100 cm total length we removed 7 prey items, and from specimens longer than 100 cm we removed 27 prey items (Table I). Most snakes did not contain more than one prey category (i.e. birds and mammals, or birds and reptiles, etc), but a large male 208 cm long had one squirrel (Heliosciurus sp.) and one indetermined Passeriformes in the stomach. There was a positive correlation between log snake size (mass, in g) and log prey size (mass, in g) (Spearman’s r = 0.32, n = 20, P < 0.01), and the variance in prey size increased significantly with snake size (ANOVA: P < 0.05).

 

Table I. Summary of the dietary data collected from Jameson’s green mambas, shorter and longer than 100 cm TL, in southeastern Nigeria. For more details, see text.

Prey category

< 100 cm

> 100 cm

N

N

AMPHIBIA

Bufo sp.

1

0

REPTILIA

Agama agama

4

0

AVES

Dendropicos sp. (juv.)

0

4

Cisticola sp. (ad.)

0

3

Cisticola sp. (juv.)

0

3

Passeriformes indet.

0

6

Streptopelia semitorquata

0

1

Hatchlings (undet. species)

0

5

MAMMALIA

Crocidura poensis

1

0

Lemniscomys striatus

0

1

Rodentia (indet.)

1

Heliosciurus sp.

0

2

Scotonycteris zenkeri

0

2

TOTAL

727

Reproductive biology

Combats between males and matings were observed during the dry season (December, January, and February), both on the ground and on the trees. Combats involved two individuals in four cases, and three individuals in a single case, and always occurred during the central daylight hours (11.15 to 15.50). Dissection of female snakes found dead in the field, in bush-meat markets of local people (cf. Akani et al., 1998), and in school collections provided data on the seasonal reproductive timing and on the clutch sizes. Gravid females were collected in April (n = 4), May (n = 6), and June (n = 5). One additional gravid female was captured in an unknown date. Adult females captured in other periods of the year (1 in February, 1 in March, 3 in May, 1 in July, 1 in September, 4 in November, and 1 in December) were not pregnant. The smallest female carrying eggs was 119 cm total length, and there was a highly significant positive correlation between maternal length and litter size (Table II). Reproductive data concerning the examined females are summarized in Table II. We did not collect data on the period of egg-laying in free-ranging green mambas. However, oviposition of four gravid specimens housed in an outdoor enclosure in Calabar (Cross River State) during 1998 occurred respectively on 6, 16, 28 June, and on 22 July. Oviposition of free-ranging specimens should most probably occur also between June and July. Mamba eggs were found in the field just three times (24 June 1997, 14 July 1997, and 2 July 1998). In all cases they were excavated by chance from holes in abandoned termite nests at forest clearings.

 

Table II. Summary of the reproductive data collected from female Jameson’s green mambas in southeastern Nigeria. The statistics of the regression between maternal length and litter size are also presented.

Parameter

X (cm)

SD

n

Female total length

145.8 cm

22.6

16

Range (cm) 119 / 179

Litter size

10.9

3.11

16

Range 7 / 16

Regression statistics:
Equation: litter size = -7.89 + 0.128 x total length
r = 0.935; adjusted r2 = 0.865; n = 16
ANOVA: F = 91.13; df = 1,13; P = 0.0015