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Survival probability of a population of Madagascar spider tortoises (Pyxis arachnoides)

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The spider tortoise (Pyxis arachnoides) is endemic to the coastal, dry forests of southwest Madagascar, one of the country’s most threatened habitats. Very little is known of the biology of this Critically Endangered species. We devised a three year capture-mark-recapture study to assess mean annual survival for the following cohorts; juvenile, adult male, adult female and adult of both sexes, for a population of spider tortoises using a Cormack-Jolly-Seber model. Low recapture probabilities prevented the modelling of all but the adult of both sexes cohort. Mean annual survival was 0.823 ( SE ± = 0 . 15 ; 95 % CI = 0 . 565 - 1 . 0 ). We hypothesise that habitat loss could be impacting the survival of this cohort; however establishing control data to test for this is difficult because of the widespread habitat loss, even within protected areas. Therefore we suggest extending the study across a number of gradients of impacted habitat and increase the duration of the study to assess this risk to the population and improve model robustness.

Affiliations: 1: 2Department of Environment Earth and Ecosystems, The Open University, Milton Keynes, MK7 6AA, United Kingdom ; 2: 1Nautilus Ecology, Oak House, Pond Lane, Greetham, Rutland, LE15 7NW, United Kingdom ; 3: 3Centro di Studi Ambientali Demetra s.r.l., via Olona 7, I-00198 Rome, Italy ; 4: 5Madagascar Biodiversity Partnership, Omaha’s Henry Doorly Zoo, Grewcock’s Center for Conservation and Research, 3701 South 10th Street, Omaha, NE 68107, USA ; 5: 4Département de Biologie Animale, Université d’Antananarivo, BP 906, Antananarivo 101, Madagascar

*Coresponding author; e-mail:

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