Individual study: Use of tranquillisation and acclimatisation as means of improving translocation success of wild rabbits Oryctolagus cuniculus from Parc-du-Sausset to Héric, France
Letty J., Marchandeau S., Clobert J. & Aubineau J. (2000) Improving translocation success: an experimental study of anti-stress treatment and release method for wild rabbits. Animal Conservation, 3, 211-219
Translocation is an important tool in conservation biology. However, translocation success is generally low for numerous animal species due to factors such as habitat unsuitability, handling stress and environment novelty. High mortality is often observed just after introduction and it is thought that induced stress is likely to be a major cause. Such stress could be caused by either handling or environmental novelty. Using a common species in translocation experiments provides a means of testing methods of improving success, without the need to cause mortality to rare species. The results of a study in which tranquillisers administration and pre-release acclimatisation were used as translocation anti-stress treatments for wild rabbits Oryctolagus cuniculus are described.
Capture & translocation procedures: A total of 109 wild rabbits Oryctolagus cuniculus were captured by ferreting warrens in Parc-du-Sausset, France. These were vaccinated against RHVD and myxomotosis, and then transported by car to Héric, 400 km to the southwest. Between capture and release, rabbits were kept individually in partitioned wooden boxes. 104 Rabbits were released (5 died during transportation) into recently restructured farmland comprising of a patchwork of cultivated fields and pasture. Of these, approximately half were tranquillised by two intra-muscular injections (0.1 mg/kg) of carazolol just after capture. Approximately half the tranquilised and half the non-tranquilised rabbits were acclimatised in 100 m² enclosures for three days prior to release (see Table 1, attached). Rabbits used for acclimatisation were captured on 13 January 1997 (three days later for the other rabbits to allow simultaneous release into the wild).
Survival rates: All rabbits were released on 17 January. After release, survival rates were estimated using the Cormack-Jolly-Seber method, in which survival is estimated from the number of re-sightings during search periods subsequent to release. Nocturnal spotlight re-sighting sessions were conducted every evening during the first week following release and thereafter monitoring was reduced to twice a week for 7½ weeks.
During the period immediately after release, tranquilised individuals tended to be re-sighted more often. After controlling for the possibility that tranquilisation affects re-sighting rates, it was shown that tranquilisation did not increase survival. Females survived better when acclimatised, but males showed the opposite tendency. The sex-dependent benefits of acclimatisation were thought to be due to differences in social behaviour. Females tend to disperse over shorter distances than males and might be expected to undergo fewer prospecting movements when acclimatised. Consequently they may have been less prone to early mortality, particularly as a result of predation. By contrast, acclimatised males may have experienced an extra stress, due to aggression with unfamiliar males within the acclimatisation pens with no possibility of escape.
There was a strong interaction between the effects of tranquilisation and acclimatisation on early sighting rates. Acclimatised and tranquilised individuals were re-sighted less often than tranquilised and unacclimatised individuals, but the converse was true for untranquilised individuals. However, there appeared to be no particular interaction between the two effects on survival rates, suggesting that it is behaviour rather than mortality that is affected.
Conclusions: Evidence suggests that environmental stress overrides handling stress in determining the level of early survival of translocated wild rabbits. Tranquilisation, which is thought to reduce handling stress, appeared to have no effect on survival. The extent to which pre-release enclosures reduce environmental stress appears to depend on social behaviour. Amongst aggressive males, enclosures potentially increase stress by reducing evasion from aggressive encounters. Non-aggressive females, potentially benefit from acclimatisation as they undergo fewer prospective movements upon release and as a consequence their risk of predation is lower. The results demonstrate the need to consider behaviour in the design of translocation methods.
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