Catchability in snakes: consequences for estimates of breeding frequency

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Abstract
Estimates of breeding frequencies in wild populations occupy a central place in life-history studies, but they remain hard to obtain in snakes. Capture–recapture procedures were carried out during 27 periods over 3 years in a large population of female asp vipers, Vipera aspis (317 marked individuals). Catchability was greater in reproductive females (2.64 ± 1.74 (SD) captures/year) than in nonreproductive females (1.76 ± 1.02). Three different methods were used to estimate breeding frequency: (i) the proportion of reproductive to nonreproductive females for each period (noncorrected estimates, NC) was measured; (ii) the ratio of the number of reproductive females to the total number of females was estimated using the CAPTURE program (catchability-corrected estimates, CE); (iii) breeding frequencies of marked individuals were observed in the field for 3 years (observed breeding frequency, OF). The results obtained with the three methods were different. The NC procedure indicated that 63% of females breed each year, suggesting an annual–biennial cycle. The CE (34% reproductive females) and OF (34% reproductive females) procedures both suggested a triennial cycle. The OF method was the most accurate; breeding frequencies were overestimated with the NC method. The NC method is the one most commonly used, and estimates of breeding frequency in snakes obtained in these studies may be imprecise. We conclude that differential catchability between reproductive and nonreproductive females should be taken into account in estimating breeding frequencies.