Population study on Astrochelys radiata SALAMANDRA 45 4 219-232 Rheinbach, 20 November 2009 ISSN 0036-3375 Population study on Astrochelys radiata (Shaw, 1802) in the Tsimanampetsotsa National Park, southwest Madagascar Jutta M. Hammer & Olga Ramilijaona Abstract. During three months of studying a population of Astrochelys radiata in the Tsimanampet- sotsa National Park, data for 32 tortoises were recorded. On two parallel transects, carapace length and weight of each animal were carried out and the exact location of each animal discovered during surveys recorded. Population densities of 32 to 47 animals per km² were estimated. A four-day survey to the east- ern boundary of the park was carried out to gain data from a less human influenced region. Even though the study took place during the dry season the tortoises were found in abundance. During this study a great number of large and therefore old tortoises were found on the transects. In contrast, the compari- son study near the eastern edge of the Park exclusively yielded large tortoises. This fact could not be suit- ably analysed within this study due to time constraints but should be taken into consideration in future studies. Transect T2 showed greater population densities of Astrochelys radiata compared to transect T. Even though low numbers of tortoises were recorded during the transect study a viable population of A. radiata at Tsimanampetsotsa can be inferred. Keywords. Testudinidae, Astrochelys radiata, Madagascar, Tsimanampetsotsa, morphology, ecology, population structure. Introduction diata have been found in the region of Mo- rondava, about 300 km north of its current The island of Madagascar is well known for range area (Pedrono & Smith 2003). Today its species richness and high endemism, espe- the distribution of the radiated tortoise is re- cially among its herpetofauna. Species diver- stricted to a narrow band of about 00 km sity reaches an endemism rate of 99% in rep- along the south-western coast of the island of tiles and 93% among amphibians (Nussbaum Madagascar (O’Brien 2002, Fig. ). North of & Raxworthy 2000, Glaw & Vences 2007). Toliara and to the east of Ambondro the spe- The family Testudinidae is represented by five cies is almost extinct nowadays (Juvik 975). tortoise species in Madagascar, four of which Lewis (995) reported a decline of the tor- are endemic to the Indian Ocean island (Leu- toises’ range and population densities with- teritz 2005, Glaw & Vences 2007). Where- in the last ten years. The International Union as tortoise species in the north (Kinixys belli- for the Conservation of Nature and Natural ana), in the northwest (Astrochelys yniphora) Resources (IUCN) classified the radiated tor- and in the western part of Madagascar (Pyxis toise as a vulnerable species in 982. Only planicauda) each show a restricted distribu- after a recent tortoise specialist workshop tion area, both southern tortoise species (As- hosted by the IUCN, Wildlife Conservation trochelys radiata, Pyxis arachnoides) distribu- Society and Conservation International, the tion ranges stretch along the south-western status of A. radiata was upgraded to critically and southern coast of Madagascar. The lat- endangered (IUCN 2008). ter species has been divided into three sub- Typical habitat where A. radiata can be species inhabiting adjacent regions (Glaw found comprises the dry deciduous forest in & Vences 2007). Subfossil remains of A. ra- the Mahafaly Plateau south of Toliara (Nuss- © 2009 Deutsche Gesellschaft für Herpetologie und Terrarienkunde e.V. (DGHT) http://www.salamandra-journal.com 219 Jutta M. Hammer et al. Fig. 1. Schematic map of southern Madagascar showing estimated ranges of Astrochelys radiata in the years 1865, 1975 and 2000 (data from O’Brien 2002). baum & Raxworthy 2000, Seddon et al. recommend putting additional areas of the 2000). Adult specimens may reach up to 38 Mahafaly Plateau under protection. The ra- cm carapace length and weigh up to 3 kilo- diated tortoise is amongst the most common grams. Sexual dimorphism in adult tortoises reptiles within the national park even though is indicated by a concave plastron and a long- its population density was calculated to be er tail in male specimens (Pedrono & Smith only 27 animals per km² (Leuteritz 2002), 2003). Carapace colour varies from bright whereas Lewis (995) found densities of 262 yellow to nearly black animals displaying a animals per km² in Tsimanampetsotsa. Tsi- pattern of stripes radiating from the centre manampetsotsa first received protected sta- of each carapace scute. In old specimens the tus in 927 when a nature reserve was in- colouration may change to a more uniform stalled. In 966 the protection status was con- colour. verted to that of a national park embracing Lying within the core distribution area of 43,200 ha of forest and wetland (Goodman A. radiata, the Tsimanampetsotsa Nation- et al. 2002). The residents of surrounding vil- al Park draws scientific attention due to its lages consider the park, and especially Lake unique deciduous dry forest. As this forest Tsimanampetsotsa, sacred. Within its con- type belongs among the most endangered fines they both collect medicinal plants and habitats, Du Puy & Moat (998) strongly bury their dead below the Mahafaly Plateau. 220 Population study on Astrochelys radiata The flora of the national park is dominated geological and floral structure three differ- by xerophytic plants, of the Didieraceae and ent types of habitat are discernible within Euphorbiaceae families, and contains a high the study area. On the northern banks of percentage of endemic plant species (AN- the Manampetse salt lake, a forest forma- GAP et al. 999). The management of the Na- tion consists of a thick thornbush scrub and tional Park is being carried out by Madagas- trees of up to seven meters in height. Main- car National Parks (formerly called Associa- ly on sandy ground, the most common spe- tion Nationale pour la Gestion des Aires Pro- cies within this forest formation are Salva- tégées or ANGAP), which is represented by dora angustifolia, Euphorbia laro, Gyrocarpus an office in the nearby village of Efoetse. americanus and Grewia grevei. Aligning with The removal of animals seems to remain the eastern shore of the salt lake a limestone the greatest threat to wild populations (Dur- cliff stretches in a northwestern to southeast- rell et al. 989, Behler 2002, Lewis 995, ern direction. On this limestone cliff and the O’Brien et al. 2003, Glaw & Vences 2007). adjacent plateau Alluaudia comosa, Diospyros ANGAP (200) reports harvesting of A. ra- manampetse and Commiphora mahafalen- diata in the western part of Tsimanampet- sis are the most common plants. The thick sotsa, both outside and inside the protected thornbush reaches only four meters high and area. Within reach of the seashore tortoises forms a dense, nearly impenetrable wall of can be easily shipped to Toliara. Collections vegetation. East of the calcite plateau a sink seem to supply both national food markets formation containing red sands predomi- and the international pet trade (O’Brien nates. Large trees are rarely found in this type et al. 2003). Therefore, a decline of tortoise of habitat which is dominated by Didiera populations in the national park is at severe madagascariensis. Other common plants are risk, and the protection of A. radiata is a spe- Delonix andansonioïdes, Euphorbia plagian- cific declared aim of the programme of the tha and Gyrocarpus americanus. Tsimanampetsotsa National Park (ANGAP Two former zebu-cart tracks were used 200). Still, a national park can only help pro- as transects in the western study area and tect an existing population by preserving its henceforth are referred to as transect (T) natural habitat. Therefore this study aims to and transect 2 (T2). Running approximately determine more information about the sta- parallel, both transects cross all three habitat tus of A. radiata within the National Park of types. They are of 2.9 (T) and 2.4 (T2) kilom- Tsimanampetsotsa. Does a viable population eters in length, separated by an average dis- of radiated tortoises exist in Tsimanampet- tance of .5 kilometres (Fig. 2), which allows sotsa? How can this population be character- us to assume independent samples in tortoise ised? Is this national park of importance for data acquisition. Both transects were marked tortoise protection, and does it contain a vi- using flagging tape, and waypoints along the able reserve (or assurance) population of A. route were recorded using GPS-technology radiata? Which human activities within the (geko 30, Garmin). national park influence tortoise habitat or be- The survey was carried out during the dry haviour? What preservation measures should season between 23 May and 9 August 2006. be enacted to ensure the survival of A. radia- Transects were walked twice daily from 8:00 ta in Tsimanampetsotsa? to 2:00 h and from 4:00 to 8:00 h in four- day periods starting on 29 May 2006, 5 June 2006, 24 June 2006, 0 July 2006, 24 July Material and methods 2006 and 09 August 2006. Both transects were connected and could be walked in a cir- The main study area (which will be referred cle. During the survey all four possibilities of to as the western study area) lies in the north- starting positions on the transects were var- western part of Tsimanampetsotsa. Due to ied to allow equal daytime research during 221 Jutta M. Hammer et al. Fig. 2. Location of the permanent transects and survey squares within Tsimanampetsotsa National Park compiled from GPS data mapped in the study area. the survey. With one exception these four- same number was notched into the marginal day-periods of transect walking were evenly scutes using a saw blade (Cagle 939, Leute- distributed during the study period. Find- ritz 2002). Juveniles were marked only with ing a tortoise the perpendicular distance of the marker pen.