Seasonal Variation in Habitat Use in Sympatric Afrotropical Semi-Aquatic Snakes, Grayia Smythii and Afronatrix Anoscopus (Colubridae)

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Seasonal variation in habitat use in sympatric Afrotropical semi-aquatic snakes, Grayia smythii and Afronatrix anoscopus (Colubridae)

Luca Luiselli1, Godfrey C. Akani2, Francesco M. Angelici1, Linda Ude1, Sunday M. Wariboko1

Identifying habitat shifts by snakes and deter- Grayia smythii (or smithii) and Afronatrix mining the potential causes of such shifts are anoscopus (Serpentes: Colubridae) are among important to understanding snake ecology and the most abundant snakes in the aquatic habi- evolution (Gibbons and Semlitsch, 1987). Habi- tats of rainforest zones in West Africa (Chip- tat shifts between seasons have been demon- paux, 1999), including southern Nigeria (Butler strated for a number of species, in both tem- and Reid, 1990). Nonetheless, until recently, lit- perate (e.g., Reinert, 1984) and tropical re- tle data were available on the ecology of these gions (e.g., Shine and Lambeck, 1985). Sea- species (e.g., see Butler and Reid, 1986, 1990). sonal shifts in habitat use are most likely in Much recent field research has been conducted regions with ephemeral habitats or in regions on these semi-aquatic colubrids in southern where the climatic variability between seasons Nigeria, and there are now detailed data on their is high. Considering that many zones in south- diets (Akani and Luiselli, 2001; Luiselli et al., ern Nigeria are characterized by a mosaic of 1998, 2003), reproductive biology (Luiselli and aquatic habitats (ponds, lakes, lagoons, rivers, Akani, 2002a), thermal ecology (Luiselli and creeks, marshes), and that several of these habi- Akani, 2002a), and population biology (Akani tats are strongly seasonal (i.e. they are flooded and Luiselli, 2001). during the wet months and almost dry during In this paper we report some data on the the dry months; see climatic data in fig. 1), it habitat use and season in sympatric populations is likely that habitat use of some species may of G. smythii and A. anoscopus from a swamp- exhibit seasonal variation in response to pro- rainforest in southern Nigeria. nounced seasonality. This may be especially This study was conducted between September 2000 and June 2003 in a swamp-rainforest (about 44.5 ha) around true for the aquatic species, which are often ◦ ◦ Eket, Akwa-Ibom State (04 50 N, 07 59 E), Nigeria, char- found in temporary ponds or in water basins acterized by a moist rainforest patch along the banks of subjected to strong seasonal variations in water- the River Quo-Ibo (Kwa-Ibo). The area is partially inun- depth (Luiselli and Akani, 2002a, b). dated during the wet season, when several ponds form. The swamps, which are dominated by the trees of the genus Elaeis, are abundantly populated by snakes of both species. The study area has a tropical climate with a wet season 1 - Centre of Environmental Studies “Demetra” and from May to October (fig. 1), and with relatively constant F.I.Z.V., Correspondence address: via Olona 7, I-00198 monthly temperature throughout the year (mean monthly Rome, Italy range between 29 and 32◦C). e-mail: [email protected] Three main habitat types for these semi-aquatic snakes 2 - Department of Applied and Environmental Biology, were identified in the study area: (i) freshwater river (in- Rivers State University of Science and Technology, cluding the banks); (ii) brackish-water river (including the P.M.B. 5080, Nkpolu, Port Harcourt, Rivers State, Nige- banks) (mainly with mangroves, Rhizophora racemosa); ria (iii) freshwater ponds not connected with the main course

Figure 1. Frequency of rainy days per month (bars) and mean monthly relative humidity (line) at a locality of southern Nigeria, about 80 km of distance to our study area (Calabar; data from the Department of Geography, University of Calabar, Nigeria). of the river, and about 15 to 130 m of linear distance from in the study area. To avoid pseudo-replication of data when the river itself. Freshwater river occupied 22.7 ha of surface, establishing relative habitat use, habitat type was recorded brackish-water river, 4.4 ha, and ponds, on average 17.3 ha only once from each individual. during the wet months and 3.8 ha during the dry months. The relative surfaces occupied by the various habitats were Although specimens were sexed, no inter- calculated by satellite imaging of the study area provided by sexual difference was found in the behav- Aquater s.p.a. (San Lorenzo in Campo, Italy). iour analysed here. We recorded 426 speci- Fieldwork was conducted during the wet and dry seasons. On average, 10 searching days per month were spent in mens (222 males, 204 females) of G. smythii, the field throughout the whole study period, and each day in and recaptured 53 males and 81 females. We the field was about 6-9 hours long. We searched for snakes recorded 460 specimens (213 males, 247 fe- along defined transects in the various appropriate habitats (freshwater river, brackish-water river, and ponds). Snakes males) of A. anoscopus and recaptured 91 males were captured by hand, often while they were hiding under and 97 females. Assuming that the proportion cover, and by using pitfall traps with drift fences, and fish of specimens captured in the various habitats traps. Locality and the habitat data were recorded for each snake. Although it was impossible to standardize the num- are indicative of relative habitat use, then it re- ber of traps in relation to the habitat type, every effort was sulted that: G. smythii (total n = 426) used made to minimize eventual biases among habitat types by most intensively the freshwater river (54.7%) using a similar density of pitfalls and fish traps in the vari- and the ponds (32.4%), and more rarely the ous habitat types surveyed for snakes. Indeed, we placed a fish-trap every 30 m at the border of the water basins (both brackish-water river (12.9%); A. anoscopus (to- ponds and river tract, in both wet and dry seasons), and so, tal n = 460) used most intensively the ponds although the number of fish traps varied between seasons (48.7%) and the freshwater river (44.1%), and depending on the extension of the flooded areas, however their density in relation to water surface remained constant. more rarely the brackish-water river (7.2%). Pitfall traps (1000 mm deep; 290 mm top internal diameter; Only specimens captured in two consecutive 220 mm bottom internal diameter; without handles) were seasons (i.e. one wet season followed by the sunk into the ground, 30 m apart, along a drift fence (50 cm high, 600 m long) on either side of the River Quo-Ibo. Pit- next dry season and vice versa) were consid- fall traps with fences crossed for approximately 360 m (on ered in our analyses for detecting interseasonal each banks) the freshwater river habitat, for approximately habitat changes: we excluded all specimens that 80 m (on each banks) the brackish-water river habitat, and for approximately 160 m the pond habitat, i.e. grossly fol- were recaptured at longer time intervals in or- lowing the relative availability of the various habitat types der to have a similar length period between cap-