Materials and methods
The following vouchers, all from Nigeria or, in additional cases, from other countries of West Africa, were measured for snout-vent length (SVL, to the nearest 1 mm) and head length (HL, to the nearest 1 mm): CAS 104584, 16956, 136118, 103119, 147664, 103276, 121002, 154799, 103278, 103277, 16981, 16982, 7298, ZMUC 6529, 6527, 6528, 6526, 655, 6541, 65511, 6539. In addition, the same measurements were taken from 13 unnumbered specimens stored at the Institute Demetra (Roma, Italy), at the Cross River National Park (Akampka and Butatong, Nigeria), at the University of Calabar (Nigeria), at the University of Uyo (Nigeria), at the University of Abraka (Nigeria), and at the Department of Biological Sciences, Rivers State University of Port Harcourt (Nigeria). When possible, we recorded the same measures from free-ranging specimens, captured between September 1996 and September 2000 in southern Nigeria (for the surveyed areas, see Luiselli & Angelici, 2000).
Cobras were captured by hands with the aid of sticks, or by pitfall traps with drift fences, or by traps used by local people to capture frogs and fish. The captured snakes were sexed, measured, and palpated in the abdomen until regurgitation of ingested food or defecation occurred. Specimens found already dead (road-killed or macheted by farmers, or preserved in local collections; for a list of them see Luiselli and Angelici 2000) were dissected in order to obtain diet data. No specimens were killed or damaged for the purposes of this study. Prey items were identified to the lowest taxon possible. The mass of the prey item at the time of its ingestion was estimated, when possible, by comparing the item to intact conspecifics of various sizes from the authors’ personal collection, or measuring the fresh biomass in perfectly preserved items.
Statistical analyses were performed with all tests two-tailed and a set at 5%. Data on raw body masses of both snakes and their prey were natural log-transformed to meet the assumptions of linearity. In the text, we present mean values ± 1 standard deviation.