Behavioural and ecological factors affecting the trappability of two skink species in Nelson

Abstract

The syntopic skink species Oligosoma infrapunctatum and O. lineoocellatum appear to have different trappability estimates at Lake Station, Nelson. Long-term mark-recapture data has shown that O. lineoocellatum has a consistently lower trappability than O. infrapunctatum. In the present study several factors were investigated at Lake Station and in captivity to determine their effects on skink trappability. These factors included: (1) effect of bait type on captures, (2) occurrence of trap escapes, (3) activity patterns of skinks and (4) thermal sensitivity of sprint-running ability. The ecology of O. infrapunctatum and O. lineoocellatum was also compared, based on results from this study and others.

Two different bait types, pear and live invertebrates, were used in pitfall traps to determine their effectiveness for capturing skinks over two four-day periods at Lake Station. Bait type had a strong effect on capture success, with pear bait being superior to invertebrate bait in attracting both species to traps. Bait was not considered to be a factor influencing the trappability difference because both species were captured in high numbers using pear bait. The occurrence of escapes from traps was investigated in the field and in captivity. Skinks were left in traps for 9 h (in captivity) or 24 h (in the field) and monitored regularly. Although few escapes occurred for either species in the field and none occurred in captivity, O. lineoocellatum had a slightly higher escape rate than O. infrapunctatum and was significantly more often consumed by nocturnal predators. Higher predation rates on O. lineoocellatum may contribute to a low trappability estimate. At the field site, activity patterns of both populations were determined by monitoring skink captures at intervals throughout each day. Both species exhibited unimodal diel activity patterns. Skinks were taken into captivity to assess individual activity regimes using video surveillance. O. lineoocellatum spent significantly less time active than O. infrapunctatum and also emerged later and retreated earlier in the day. O. lineoocellatum may have a lower trappability because of its limited activity time. The effect of ambient temperature on sprint speed was tested in captivity. Skinks repeatedly ran along a track, following brief exposure to different ambient temperatures. O. lineoocellatum sprinted significantly faster than O. infrapunctatum across a wide range (6-32°C) of body temperatures. O. lineoocellatum appears to be well adapted physiologically to extreme temperatures. During this study it was also noted that O. lineoocellatum appeared to be more agile, wary and alert than O. infrapunctatum.

Results suggest that O. lineoocellatum is less trappable than O. infrapunctatum because it (1) spends less time active (especially during wet weather), (2) experiences higher rates of predation and may escape more often while in traps and (3) shows heightened wariness which may contribute to trap-shyness. In general, most of the behavioural and ecological differences between O. Infrapunctatum and O. lineoocellatum may be attributed to adaptations for survival in the different habitat types which they are associated with.