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The effects of decreased body temperature on the onset, duration and action of medetomidine and its antagonist atipamezole in juvenile farmed estuarine crocodiles (Crocodylus porosus)


  • Annabelle Olsson,

    Corresponding author
    1. Hartleys Crocodile Adventures, Wangetti via Cairns, Qld, Australia
    2. Wildlife Conservancy of Tropical Queensland, Cairns, Qld, Australia
    • Wildlife Health and Conservation Centre, University of Sydney, Camden, NSW, Australia
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  • David Phalen

    1. Wildlife Health and Conservation Centre, University of Sydney, Camden, NSW, Australia
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Annabelle Olsson, Hartleys Crocodile Adventures, c/o 22 Junction St, Edge Hill, Qld 4870, Australia. E-mail



To determine the efficacy of medetomidine for immobilisation of captive juvenile crocodiles over a range of temperatures, and its reversibility with atipamezole.

Study design

Prospective experimental study.


Forty male estuarine crocodiles (body weight 2.0 to 4.8 kg).


Each crocodile was randomly assigned to one of four temperature groups: Group 1:32 °C; Group 2:27 °C; Group 3:22 °C; and Group 4:17 °C (= 10 for each group). Medetomidine (0.5 mg kg−1) was administered intramuscularly (IM) into the thoracic limb of all crocodiles. After 50 minutes, all animals from each group received 2.5 mg kg−1 atipamezole IM in the opposite thoracic limb and time to recovery was documented. Heart and respiratory rates and the degree of immobilisation were monitored every 5 minutes until recovery, and behaviour monitored for 7 subsequent days.


Onset of immobilisation occurred at 15 ± 10 minutes in Group 1, and at 30 ± 10 minutes in Groups 2 and 3. In Group 4, animals were not immobilised. Recovery following atipamezole was 10 ± 5 minutes at all temperatures. One-way analysis of variance (anova) demonstrated a significant difference in induction times between groups (< 0.01) but not in recovery times following atipamezole administration (< 0.25). Heart and respiratory rates decreased markedly following medetomidine administration and increased markedly following atipamezole reversal.

Conclusions and clinical relevance

Medetomidine administered in the thoracic limb of juvenile captive estuarine crocodiles provides profound sedation or immobilisation at temperatures of 22 °C and above. Atipamezole administered in the contralateral thoracic limb results in consistent reversal of the effects of medetomidine and a return to normal behaviour within 15–20 minutes regardless of temperature. Even though immobilisation is not induced at 17 °C, profound reversible sedation does occur reliably and repeatably.