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Ecological Studies on a Population of the Water Snake Grayia Smythii in A Contributions to Zoology, 70 (3) 139-146 (2001) SPB Academic Publishing bv, The Hague studies Ecological on a population of the water snake Grayia smythii in a of rainforest swamp the Niger Delta, Nigeria Godfrey+C. Akani¹ & Luca Luiselli² 'Department ofBiological Sciences, Rivers State University ofScience and Technology, P.M.B. 5080, Port 2 Harcourt, Rivers State, Nigeria, e-mail:[email protected]; Centre of Environmental Studies “Demetra”, via dei Cochi 48/B, 1-00133 Rome, Italy: F.I.Z.V., via Olona 7, 1-00198 Rome, Italy; and Mu- nicipal Museum of Natural History, piazza A. Frezza 6, 1-00030 Capranica Prenestina, Rome, Italy, email: [email protected] (corresponding author) Keywords: Grayia smythii, Colubridae, snakes, ecology, Nigeria Abstract Introduction The few of snakes of the The ecology of the water snake, Grayia smythii (Reptilia: species aquatic genus occurring in a seasonal rainforest of the Colubridae) swamp Grayia constitute a systematically problematic Niger Delta (southern Nigeria) was investigated between De- group of Colubridae that is endemic to tropical cember 1998 and March 2000. Females and males were similar Africa (Boulenger, 1909). They have been con- in body sizes (SVL) and head sizes, but males had tails the sidered as belonging to subfamily Grayini significantly longer than females. The diet was constituted only and fish. The but also with some by frogs majorprey type was Xenopus tropicalis, (Meirte, 1992), “lycodontines” followed and Clarias Adult sex-ratio by Tilapia sp. sp. was morphological affinities to South American 1:1. Sloughing ofskin and ovipositions occurred in dry season, xenodontines (e.g. McDowell, 1987). Although in the humid enclosure of buttress roots amongst leaf litters. these snakes notori- locally abundant, aquatic are from 8 14 with of Fecundity ranged to eggs per female, a mean ously difficult to catch, and their ecology is known 10 = smallest female eggs (SD 1.8). The gravid was 78.2 cm SVL. laid in batches ofthree in for a few and anecdotal comments Eggs were to four eggs at a site, only general at three different sites. The size ofthe least two to eggs averaged (e.g. see Cansdale, 1961; Isemonger, 1962; Pit- 3.1 cm in length, 2.1 cm in width, and had a fresh weight ranging man, 1974; Villiers, 1975). from 18,2 22.1 Maternal size influenced the to g. significantly Grayia smythii (= smithii, cf. Meirte, 1992, for of number eggs produced by female,but not their average size. a discussion on its correct name) is one of the com- There was a statistically significant negative correlation bet- monest water snake of seasonal number of and size. Predators ofthis snake species rainforest ween eggs mean egg at area herons fishermen. and bodies of the the study were and swamps permanent water Niger Delta basin and south-eastern flank of Nigeria (Butler and Reid, 1986, 1990; Luiselli et ah, 1998; Contents Akani et ah, 1999). Like other rainforest snakes, its and there persecution goes on unabated, are Abstract 139 conservation considerations hardly any (Akani et Introduction 139 ah, 1999). Moreover, the meat was described as a Study area , 140 several rural households and delicacy by may Methods 141 be traded on. Results 141 possibly Limited data Morphometry 141 ecological on Nigerian specimens Food habits 142 are available only in Dunger (1971), Butler and Sex-ratio and reproductive biology 142 Reid (1986, 1990), and in more recent studies by Sloughing cycles 144 Luiselli and Akani (1999), Akani et ah (1999), and Discussion 144„ Luiselli et ah (1998). Acknowledgements 146 In view of References 146 the significance of Grayia smythii in the ecosystem dynamic of the seasonal rainforest Downloaded from Brill.com10/08/2021 09:46:15AM via free access 140 G.C. Akani & L. Luiselli - Ecology of water snake (Colubridae) in Niger delta this Table I. Means and Standard Deviation of physico-chemical swamps (Luiselli et al., 1998), study was con- parameters ofthe seasonal freshwater at Rumuosi (Rivers ducted in order to provide basic information on its swamp State, Nigeria), where the ecology of Grayia smythii has been biometrics, habitat characteristics, food habits, studied. Ranges are in parentheses. reproduction and behaviour. Parameter Wet Season Dry Season = (N = 65) (N 57) area Study Air Temperature (°C) 25.6 ± 1.5 30.8 ± 1.8 (24 - 29) (27.6 - 33) The Water (°C) 25.1 + 0.8 28.5 ± 1.1 data presented here are based on a field study Temperature (24 - 26.1) (26.2 - 30,4) from carried out during the wet and dry seasons, PH 5.6 ± 0.9 6.2 + 1.3 December 1998 to March 2000. The study area (4.8 - 6.6) (5.0 - 6.8) 30 ha of is a seasonal rainforest (about surface) Electricity pS/cm 15.7 ± 5.4 24.7 ± 6.8 located 5 km - - swamp at Rumuosi, approximately 1 Conductivity (10.0 23.0) (12.8 30.5) Salinity 0(0) 0(0) on the northern flank of Port Harcourt (04° 45' N, Transmittance at 98 ± 1.3 97+1.8 004° OT E), the capital city of Rivers State, Nige- 420 p (%) (97.2 - 99.5) (96.0 - 98.9) ria. The vegetation is essentially secondary forest, Dissolved Oxygen (rag/1) 0.5 + 0.2 0.5 ± 0.2 characterized such flora as by Pterocarpus sp., (0- 1.0) (0- 1,0) 14.1 Raphia sp., Triumphetta eriophlebia, Mitragyna TDS (mg/1) 12.2 ± 2.5 ± 2.0 which broad and (10.0 - 13.1) (12.0 - 16.8) stipulosa, etc. have leaves, grow Total Alkalinity as adventitious roots as the water level rises. On the CaC03 (mg/1) 14.8 ± 3.2 16.2 ± 2.6 slightly elevated spots, the trees Triplichiton sclero- - (12.0 - 18.6) (13.1 19.3) Terminalia xylon, Khaya sp., superba, Mitragyna ciliata are found. The undergrowth is lush and dominated by pterophytes (ferns), shrubs, herbs, and sedges. Due to the shading effect of the crown, Shrimp, leeches, amphibians (Xenopus tropicalis, similar to light conditions were twilight even at Ptychadena spp., Hoplobatrachus occipitalis, Bufo midday. Another significant feature of the swamp maculatus, Bufo regularis, and undetermined is that the forest floor is interspersed with several Hyperoliidae tree-frogs), fish (families Clariidae, natural and man-made ponds of varying depths and Cichlidae, Malapteruridae, Phractolaemidae, Cypri- surfaces, often bordered by hydrophytes. nodontidae), reptiles (Agama agama, Mabuya affi- The Rumuosi on cf. swamp depends exclusively nis, Psammophis phillipsi, Natriciteres variegata, rainfall during wet season (May-October) for its Natriciteres fuliginoides, Gastropyxis smaragdina, link all and small water supply and has no at to other water Naja nigricollis, Causus maculatus), bodies. Inundation of the swamp usually occurs mammals(Crocidura nigeriae, Crocidurapoensis, / which area of over 25 by July August, during an Mus musculoides, Lemniscomys sp., Rattus rattus) be encroaches into in the ha may underwater. Floodwater are found in the swamp or immediate sur- farmlands serious dam- neighbouring causing crop roundings (Akani and Luiselli, unpublished trap- age. The water is lentic, generally shallow but deeper ping data). Apart for fishermen (who catch these in in the ponds, and is brown colour. By dry sea- snakes with no return valves), the major natural 85% the son (Novcmbcr-April) over of swamp dries predators of Grayia smythii at the study area were the Thick that out, leaving water only in deep ponds. herons, were observed to prey even on adult Nile monitors cushions of dry leaves are common due to increased specimens from the water surface. leaf abscission in the dry season. The surface of (Varanus niloticus ornatus) were also observed covered with float- other of the ponds at this time is green preying on adult Grayia smythii in areas Lemna Azolla and ing macrophytes, including sp.,. sp., the Niger Delta, cobras (Naja nigricollis and lotus, these snakes Ceratophyllum demersum, Nymphaea etc. Naja melanoleuca) also may prey on dissolved and Total in and known to Water temperature, pH, oxygen, as they can forage water are eat Dissolved Solid (TDS), and Alkalinity, measured occasionally on snakes (Luiselli and Angelici, in in situ, are presented Table 1. 2000). Downloaded from Brill.com10/08/2021 09:46:15AM via free access Contributions to Zoology, 70 (3) - 2001 141 Methods Samples of Grayia smythii were collected both by traps and by capture of specimens encountered during standardized routes throughout the study area. Specimens trapped were captured by means of 12 basket traps with no return valves (fishing gears commonly used in the area), set amongst barricades (approximately I m high) that entirely surrounded the As swamp perimeter. already ex- the varied plained, swamp perimeter considerably between seasons. These barricades were made of palm fronds obtained from the Oil palm tree, Elaeis Fig. I. Correlation between body length (SVL, cm) and tail guineensis and reeds. The traps were set for 65 in length Grayia smythii from the study area.For statistical details, days in the dry season, and for 57 days in the wet see text. season in both shallow and deeper waters. They were set by 8.00 am each day and inspected by the same time the following day. Other sources of the specimens included fishermen’s catch, both when the inundated the swamp was and when ponds were bailed in dry season. All captured individuals were taken to the research laboratory at Department of Biological Sciences, Rivers State University of and Science Technology (Port Harcourt), not more than two hours after collection. Morphometric measurements (snout-vent length (SVL), tail length (tL), head length (HD), head width (HW), and body mass were recorded from all the captured speci- mens. The specimens that had died were already Fig. 2.
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