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Herpetofaunal observations on Eagle Island, Middle Brother, North
Brother and Diego Garcia, with an overview of previous records in
the Chagos Archipelago.
Nik Cole
School of Biological Sciences, University of Bristol,
Woodland Road, Bristol BS8 1UG, UK
&
Durrell Wildlife Conservation Trust, Les Augrès Manor, La Profonde Rue,
Trinity, Jersey JE3 5BP, Channel Islands, UK
[nikccole@hotmail.com]
Abstract: Eagle Island of the Chagos Archipelago is the second largest island of the largest coral atoll
structure in the World and yet nothing is known of the island’s terrestrial herpetofauna. Eagle
Island was extensively surveyed for terrestrial reptiles and amphibians over a ten week period
during the Chagos Ecological Restoration Project 2006. Nesting attempts of marine turtles
were also recorded over the duration of the project. Herpetofaunal observations were also made
on the neighbouring islands, Middle Brother and North Brother and the largest island of the
archipelago Diego Garcia. Five species were encountered: green turtle Chelonia mydas, hawksbill
turtle Eretmochelys imbricata, common house gecko Hemidactylus frenatus, mourning gecko
Lepidodactylus lugubris and the marine toad Bufo marinus. These findings are presented with an
overview of previous records of herpetofauna in the Chagos Archipelago.
Key Words: Chagos, Diego Garcia, Gecko, Bufo, Turtle.
Introduction
The Chagos Archipelago is comprised of five emergent atolls including more
than 60 small islands and represents the World’s largest atoll structure. The archipelago is
positioned approximately 500km to the South of the Maldives in the middle of the Indian
Ocean. Herpetofaunal records do exist for Diego Garcia, the Peros Banhos and Salomon
atolls, and two islands in the Great Chagos Bank. However, the terrestrial herpetofauna
has not previously been surveyed on Eagle Island of the Great Chagos Bank. At 243ha
Eagle Island is the second largest island in the archipelago and significantly larger than
most other islands, the majority of which are below 100ha in size.
Eagle Island was extensively surveyed over a ten week period, between
February and April 2006, during the Chagos Ecological Restoration Project to eradicate
the island’s introduced population of black rats Rattus rattus, Linnaeus 1758 (Hillman
2006, Meier 2006). Limited time was also spent on Middle Brother and North Brother
of the Great Chagos Bank and Diego Garcia. Like Eagle Island, terrestrial herpetofauna
has not previously been recorded for North Brother. Five species were recorded during
this project; the green turtle Chelonia mydas, Linnaeus 1758, the hawksbill turtle
Eretmochelys imbricata, Linnaeus 1766, the common house gecko Hemidactylus
frenatus, Schlegel 1836, the mourning gecko Lepidodactylus lugubris, Duméril &
Bibron 1836 and the marine toad Bufo marinus, Linnaeus 1758.
Phelsuma 17 (2009); 40-48
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A history of observations
All five species observed throughout the duration of the project have been
previously recorded within the Chagos Archipelago. Bourne (1886a,b) recorded the
presence of the green turtle and hawksbill turtle, the Mauritian gecko Platydactylus
mauritanicus, Linnaeus 1758 and the mud tortoise Geoemyda trijuga thermalis,
Schweigger 1812. Platydactylus mauritanicus is the former name for the Moorish
gecko Tarentola mauritanica, which is found throughout the Mediterranean region.
Bourne (1886a) suggests that the gecko probably arrived from Mauritius, although it
has never been recorded there (Cole 2005, Cheke & Hume 2008). Bourne was most
likely referring to the common house gecko, which is the most abundant non-native
lizard in Mauritius and at a glance, is somewhat similar to the Moorish gecko in being
a small greyish scansorial lizard. The mud tortoise described as G. t. thermalis, is the
former name of the Sri Lanka black turtle or Ceylon terrapin Melanochelys trijuga
thermalis, which was thought to have been introduced to Diego Garcia (Boulenger 1889,
Deraniyagala 1939). The African mud turtle Pelusios subniger, Lacépède 1788 was
recorded on Diego Garcia in the Percey Sladen Trust expedition to the Indian Ocean in
1905 by Gardiner & Cooper (1907) who suggested that the species was introduced from
either Zanzibar or Madagascar. Both turtle species were thought to have died out by the
1940s, although turtles were reported in the 1970s it is unknown whether it was one
or both species (see Lever 2003). From the Percey Sladen Trust expedition Boulenger
(1908) identified specimens of the common house gecko collected from Diego Garcia,
the Peros Banhos and Salomon atolls and specimens of the mourning gecko collected
from Diego Garcia.
More recent herpetological surveys have been conducted in the Chagos
Archipelago, which include the 1978/79 Joint Services Chagos Research Expedition
(Dutton 1981) and the 1996 Friends of the Chagos Expedition (Barnett & Emms 1998).
Only two reptile species were recorded in the 1970s expedition; the common house
gecko and the hawksbill turtle (Dutton 1981). The 1990s expedition recorded three
terrestrial species; the common house gecko, the mourning gecko and the marine toad
(Barnett & Emms 1998). The common house gecko was recorded on Ile Diamant, Ile du
Coin, Ile Yeye, Moresby Island and Petite Soeur of the Peros Banhos atoll; Ile Anglais,
Ile Boddam and Ile Poule of the Salomon atoll; Danger Island and Middle Brother of
the Great Chagos Bank; and Diego Garcia. The mourning geckos were recorded on
Ile Takamaka of the Salomon atoll; Ile du Coin of the Peros Banhos atoll; and Diego
Garcia. Barnett & Emms (1998) also record the marine toad as being very common
on Diego Garcia. Nests of the hawksbill turtle were recorded on all 11 islands of the
Salomon atoll and 11 of the 17 islands searched of the Peros Banhos atoll (Dutton
1981). However, it is thought that Dutton may have confused green turtle tracks and
nests for those of hawksbills as significant numbers of both species were found nesting
in a survey of 67 islands throughout the archipelago (Mortimer & Day 1999).
Recent introductions to Diego Garcia include the marine toad and the brown
tree snake Boiga irregularis, Merrem 1982; fortunately the snake has been prevented
from establishing, but the marine toad is common on the island (Fritts 1993, Barnett &
Emms 1998, Cheke 2008, pers. obs.). In 2006 there were unsubstantiated accounts of
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the mutilating gecko Gehyra mutilata, Wiegmann 1836 and the agamid lizard Calotes
versicolor, Daudin 1802 on Diego Garcia. These two reptiles were not seen during
the 2006 project, but an agamid lizard was observed on the island near Simpson Point
in March 2007 by Jenny Daltry, Chris Hillman and Guntram Meier during a follow-
up expedition to the 2006 Chagos Ecological Restoration Project (C. Hillman pers.
comm.).
Observations on Diego Garcia
Searches on Diego Garcia were limited between the harbour and Simpson Point
in north-western arm of the island between the 1st and 3rd Feb and on the 25th April 2006.
Nevertheless, three species were found, the common house gecko, mourning gecko and
marine toad. The common house gecko (snout to vent length [SVL]±SD = 51.6±4.0mm,
mass±SD = 2.9±0.6g, N = 6) were the most frequently encountered species and were
found in abundance on all buildings and coastal vegetation (predominantly the trunks of
coconut palms Cocos nucifera, L., and branches of Scaevola taccada, Gaertner-Roxb.)
at night, but could also be heard calling at all times day or night in most locations.
The mourning gecko (SVL±SD = 42.8±2.1mm, mass±SD = 1.6±0.4g, N = 6) were
occasionally found on buildings occupied by the common house gecko, but were most
abundant on roadside telegraph poles in the absence of the common house gecko.
Marine toads were also encountered after heavy rainfall along the road verge leading to
Simpson Point from the airport.
Observations on Middle and North Brother
Middle Brother and North Brother were both visited on the 7th Feb 2006,
between 1000hrs and 1230hrs and 1400hrs and 1430hrs, respectively. To prevent
disturbance to the nesting seabird colonies only the coastal perimeter was surveyed on
Middle Brother and a 100m stretch of coast on North Brother. Three common house
geckos (SVL±SD = 49.1±2.6mm, mass±SD = 2.7±0.4g, N = 3) and two eggs from the
same species were found beneath loose bark of fallen coconut palms and Tournefortia
argentea, L.f., on Middle Brother. Nothing was heard or seen on North Brother, despite
suitable habitat. However, owing to the restricted amount of time spent and limited
areas surveyed little can be said about the absence of geckos.
Observations on Eagle Island
Searches were made over the entire island, whilst creating more than 80km of
tracks to set and service bait stations on a daily basis by team members of the Chagos
Ecological Restoration Project (Hillman 2006, Meier 2006). Numerous additional
searches were made in the day and night across the island and around the coastline.
Four species were encountered; the green turtle, hawksbill turtle, common house gecko
and mourning gecko.
Adult house geckos (SVL±SD = 50.7±3.3mm, mass±SD = 2.8±0.6g, N = 11)
were mostly encountered at night on the trunks of coconut palms and the trunks and
branches of Guettada speciosa, L., Hibiscus tiliaceus, L., Morinda citrifolia, L., S.
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taccada and T. argentea at almost all locations across the island. House gecko calls
could also be heard at all locations on the island, but individuals were rarely seen in areas
of closed canopy palm forest or wetland mangrove dominated by Lumnitzera racemosa,
Willd. (see Hillman 2006). All house geckos observed in the closed canopy palm forest
were high within the palm fronds. Sub-adult and juvenile individuals were most often
seen amongst the herbaceous ground layer in relatively open areas. In contrast to the
house gecko on Eagle Island the abundance and distribution of the mourning gecko was
respectively lower and more restricted. Mourning geckos (SVL±SD = 43.7±1.8mm,
mass±SD = 1.9±0.2g, N = 5) were frequently found close to and on the remains of
buildings, on small boulders along the shoreline and also within the mangrove area on the
western side of the island (see Hillman 2006). Mourning geckos were also encountered,
albeit rarely, at other locations across the island on the trunks of G. speciosa and the
fronds of coconut palms. Eggs of both gecko species were found in the areas where
individuals were known to be present. Mourning geckos are ‘egg gluers’ attaching pairs
of eggs directly to the substrate and often communally with several pairs of eggs glued
at the same site. Mourning gecko eggs were mostly found within cavities of L. racemosa
trunks and associated entanglements of Cassytha filiformis, L., above high-tide mark in
the mangrove. Mourning gecko eggs were also found within crevices of coralline stone
walls and upon palm fronds. House geckos deposit their eggs loosely in pairs within
substrates and were mostly found behind loose bark, within litter collected at the base
of palm fronds, and crevices and cavities within tree trunks.
All turtle observations, tracks and nests were recorded during daily walks
along the coastline of Eagle Island by members of the Chagos Ecological Restoration
Project. Twenty six individual turtle tracks were recorded along the sandy beach that ran
clockwise from the northeast to the southeast coast of the island. The width of all tracks
were measured and assigned by their size to either the green or hawksbill turtle (see
Hillman 2006). After each track had been measured and the location recorded it was
erased from the beach to prevent it being recorded again. Green turtles accounted for 23
of the tracks recorded and were mostly found along the narrow beach on the central east
coast of the island (Hillman 2006). Green turtles were also the most common of the two
species seen within the surrounding lagoon. Two of the tracks were made by hawksbill
turtles, one of which was observed nesting early one morning on the northeast coast of
the island (Hillman 2006). Of the two species seen within the lagoon, green turtles were
the most common. These observations support the previous findings of Mortimer & Day
(1999) that green turtle nest more frequently within the Great Chagos Bank area than
hawksbill turtles.
Introductions of the terrestrial herpetofauna
The terrestrial herpetofauna recorded over the past 120 years are known or
at least thought to have been introduced by the early inhabitants. The marine toad is
certainly a recent introduction that has established on Diego Garcia, arriving sometime
in the 1980s (Lever 2003). The introduction of the cane toad, the agamid lizard and
probable arrival mutilating gecko have most likely occurred accidentally or purposefully
as a result of military activities and occupation of Diego Garcia (Cheke 2008). This is
44
known to be true at least for the brown tree snake. Both of the predominantly nocturnal
gecko species are highly commensal with people and have been introduced to numerous
locations outside of their natural range throughout the tropics and sub-tropics. However,
there is some evidence to suggest that these two species may have arrived to the Chagos
Archipelago naturally. The common house gecko is native throughout Asia and the
Indo Pacific and has been introduced to at least nine locations within the Indian Ocean,
although it is thought to be native to the Seychelles (Cheke 1984). The species is
thought to have arrived from Sumatra by self dispersal, rafting on debris washed out to
sea and carried by oceanic currents (Cheke 1984). If this is the case then the geckos are
likely to have arrived in the Chagos by the same means, as the archipelago is positioned
directly between Sumatra and the Seychelles. Likewise the mourning gecko, with a
similar native distribution to the house gecko, is thought to have reached the island of
Rodrigues within the Mascarenes by self dispersal across the Indian Ocean (EN. Arnold
pers. comm.). The origins of the gecko species and how they arrived to the Chagos
Archipelago remains unclear. Tail tips were collected from both gecko species from
each population discovered on the islands. These samples have been deposited at the
Natural History Museum (London) where subsequent genetic work may help toward
determining the natural and unnatural spread and origin of these anthropogenic species
throughout the Indian Ocean.
Acknowledgements
These observations would not have been possible without the assistance of the
project members of the Chagos Ecological Restoration Project; Darryl Birch, Elaine
Fraser, Pete Haverson, Chris Hillman, Nicole Lohrmann, Elli McManus, Guntram
Meier, Fieke Molenaar, Alex Page and Andrew Sheppard. Trips to Middle and North
Brother were organised and led by Charles Sheppard. This work was carried out under
the auspices of the Chagos Ecological Restoration Project managed by Fauna & Flora
International with funding from the Overseas Territories Environmental Programme
(FCO & DFID), the Flagship Species Fund (DEFRA) and the Chagos Conservation
Trust and support from the British Indian Ocean Territory Administration.
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