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The first substantiated case of trans‐oceanic tortoise dispersal

DOI:10.1080/00222930601058290
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Justin Gerlach at University of Cambridge
Justin Gerlach
Catharine Muir
Catharine Muir
Catharine Muir
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Matthew Richmond at RSK Environment
Matthew Richmond
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Abstract and Figures

In December 2004 an Aldabra giant tortoise Dipsochelys dussumieri was washed ashore on the coast of east Africa, probably having been carried off the shore of Aldabra atoll, 740 km away. Although trans‐oceanic dispersal is assumed to be the mechanism by which tortoises and many other animals became established on islands throughout the world, this is the first direct evidence of a tortoise surviving such a sea‐crossing.
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SHORT COMMUNICATION
The first substantiated case of trans-oceanic tortoise
dispersal
JUSTIN GERLACH
1
, CATHARINE MUIR
2
&
MATTHEW D. RICHMOND
3
1
Nature Protection Trust of Seychelles, Mahe, Seychelles,
2
Sea Sense, Dar es Salaam, Tanzania, and
3
Samaki Consultants Ltd, Dar es Salaam, Tanzania
(Accepted 9 October 2006)
Abstract
In December 2004 an Aldabra giant tortoise Dipsochelys dussumieri was washed ashore on the coast of
east Africa, probably having been carried off the shore of Aldabra atoll, 740 km away. Although trans-
oceanic dispersal is assumed to be the mechanism by which tortoises and many other animals became
established on islands throughout the world, this is the first direct evidence of a tortoise surviving such
a sea-crossing.
Keywords: Dipsochelys dussumieri, tortoise, trans-oceanic dispersal, Aldabra, Tanzania
Introduction
Land tortoises are present on several island groups, most notably the Galapagos and the
Seychelles islands. These island populations are assumed to be descended from ancestors
that rafted or drifted from mainland populations. There is some experimental and
anecdotal support for the idea of tortoises floating for extended periods of time (Townsend
1936; Gerlach 2005) but no direct evidence of successful sea crossing. Here we report on a
case of an Aldabra giant tortoise, Dipsochelys dussumieri (Gray, 1831), crossing several
hundred kilometres of ocean and summarize past reports of others being found in the sea.
The Kimbiji tortoise
On 14 December 2004, an Aldabra giant tortoise was found walking out of the sea at
06:00 h at Kimbiji, 35 km south of Dar es Salaam, Tanzania. It was in an emaciated
condition and with an extensive growth of goose barnacles (Lepadidae) (Figure 1). The
animal was female and weighed 25 kg with a carapace measuring 77 cm long and 74 cm
Correspondence: Nature Protection Trust of Seychelles, PO Box 207, Mahe, Seychelles. Email: jstgerlach@aol.com
Published 28 December 2006
Journal of Natural History, 2006; 40(41–43): 2403–2408
ISSN 0022-2933 print/ISSN 1464-5262 online # 2006 Taylor & Francis
DOI: 10.1080/00222930601058290
Downloaded By: [Gerlach, Justin] At: 14:14 9 April 2007
wide (curved carapace measurements) and was taken to a breeding centre in Dar es Salaam
(measurements were taken in December 2004). After 3 months, the tortoise had gained
2 kg.
There are three possible sources for this animal: the small Changuu Island in the
Zanzibar Channel, a few miles west of Zanzibar Town; one of the introduced populations
of the Seychelles islands (there being free-range or wild tortoises on several islands, most
importantly Curieuse and Fregate islands); or Aldabra atoll (Figure 2), one of a few places
where giant tortoise are still found in the wild.
Details of the spacing between shell scutes helps identify different populations and is
related to diet. Tortoises from low-density populations (Changuu and the introduced
Seychelles populations) have rapid growth, with pronounced and widely separated growth
annuli on the scutes. On Aldabra this pattern is found in low-density populations such as
on Picard and Malabar islands, but the high-density population on Grande Terre
comprises relatively small tortoises with only weakly developed annuli. The Kimbiji tortoise
has a smoother shell than tortoises from any of the introduced populations and resembles
animals from Aldabra, specifically those from Grande Terre. Kimbiji and Aldabra are some
740 km apart.
The barnacles covering the fore-limbs of the tortoise were not measured in 2004 but
comparing the photograph from 2004 with the size of the tortoise scales on the fore-legs,
the white shell plates of the largest specimens can be estimated to have been up to 2 cm long
and 0.8–1 cm wide. The barnacles strongly resemble Lepas anserifera Linnaeus, 1767 and
are thickest in density on the lower legs and carapace. These will have settled and begun
growth soon after the tortoise started its floating journey. Monitoring of marine fouling on
offshore floats of Fish Aggregation Devices (FADs) off Tanzania during 2005 (Richmond
and Mohamed 2006) has shown that this species of barnacle settles soon after immersion,
with visible 2 mm long plates after 9 days, and 3 cm long plates after 11 weeks. These data,
though meagre, suggest the Kimbiji tortoise barnacles to be about 6–7 weeks old.
The prevailing ocean current along this portion of the east African coast is the north-
flowing East Africa Coastal Current. Between Aldabra and Kimbiji the prevailing current is
the South Equatorial Current (SEC) which would have carried the tortoise westward at
Figure 1. The Aldabra tortoise at Kimbiji, shortly after its discovery in December 2004. Photograph: C. Muir.
2404 J. Gerlach et al.
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speeds of 1–3 knots (Admiralty Chart 4701 Maputo to Muqdisho). This current speed
equates to a drifting time of between 6 and 17 days to reach Africa. The northeast monsoon
would be blowing during this period which probably slows the SEC, hence the 3-week
extreme seems more likely. A floating tortoise will only have a very low windage, however, it
may have at some point been actively swimming either with, or against the current,
potentially further slowing its progress westward. Off Changuu Island further north, in
contrast, the prevailing currents are northerly, away from Kimbiji, thus discounting this
alternative origin. Ocean currents from Alphonse would also carry the tortoise to Africa,
taking at least twice as long. The above evidence, which in conjunction includes shell annuli
details, ocean currents, and suggested time at sea from barnacle growth, strongly support a
sea-crossing from Aldabra atoll west to Africa that lasted several weeks, or months.
This long period of uncertain drifting presents four potential threats to a floating
tortoise—desiccation from the tropical sun, drowning during storms and attack from sharks
Figure 2. Map showing probable dispersal route.
Trans-oceanic tortoise dispersal 2405
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or collision with shipping. The ability for tortoises to survive in starved condition (e.g. in
the hulls of ships) is well known with records for many months. Crossing a reef during low
tide with heavy sea may be dangerous for a tortoise, but at high tide it would easily wash
over the shallow rocky grounds and on to the beach.
About a year and half after its arrival in Africa, after being kept and well fed in a breeding
centre in Dar es Salaam, on 8 May 2006 the tortoise measured 85.5 cm curved length,
83.0 cm curved width, 55.0 cm plastron length, and 42.5 kg in weight. In a simple manner,
these measurements reflect an increase in body length of about 10% but a more significant
increase in body weight of about 60%. It appears the tortoise continued to add weight but
at a faster rate than over the first 3 months, perhaps indicating that recovery from the
floating journey requires many months.
Other records
There are several records of tortoises from the Aldabra population entering the atoll’s
lagoon (Grubb 1971). These may drift to other islands in the atoll, as indicated by the
movement of marked individuals (Gibson and Hamilton 1984) and the colonization of
Esprit island between 1975 and 2000 (Blackmore 2001). There is one record of a live
tortoise being found in the open ocean. This was found by a passing ship, examined, and
returned to the ocean (Gerlach 2005). The eventual fate of this animal is unknown.
On 20 December 2005 a giant tortoise was seen in the sea off Alphonse island at the
south of the Amirantes group (Figure 3). Fortunately, the tortoise was spotted by a boat
arriving at the island, this was able to stop and rescue the tortoise. It was lifted into a net,
transferred to a launch, and returned to the island’s tortoise enclosure. The tortoise was
exhausted by its ordeal and spent several hours resting before making any movement but
appeared to recover with no long-term effects. When found, this tortoise was swimming
strongly but as it was over 1 km from the island it stood little chance of saving itself. It
would have been unable to see any land from its low position in the water and had probably
scant indication of a direction in which to head.
It seems that the tortoise had escaped from the enclosure some time before. In its
wanderings it must have ended up on the beach or on the reef flat at low tide and
Figure 3. Aldabra tortoise at sea off Alphonse in December 2005. Photograph: J. Gerlach.
2406 J. Gerlach et al.
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then been unable to return to land as the tide rose, sweeping it out of the lagoon and out
to sea.
Discussion
In addition to the Aldabra tortoise records summarized above there are also records of
Galapagos giant tortoises (Chelonoidis nigra (Quoy and Gaimard, 1824)5Geochelone
elephantopus (Harlan, 1827)) being swept 32 km into the sea by hurricanes (Townsend
1936). Despite these observations, there have been no observations of a giant tortoise
making a successful crossing between islands.
Rafting or drifting between isolated land masses is the only mechanism of dispersal open
to many animals. There is a record of a tree trunk carrying a boa constrictor to St Vincent
island in 1827 (Thornton 1971) but the details of this are not clear. The first direct
evidence in support of the practicality of rafting was the sighting of green iguanas (Iguana
iguana Linnaeus, 1758) landing on a vegetation raft on Anguilla in 1995 (Censky et al.
1998). This appears to have resulted in colonization, at least temporarily, following a
month-long journey of some 230 km (assuming the source population to have been on
Guadeloupe, although this has been questioned; Breuil 1999). The 740 km journey covered
by the Kimbiji tortoise is remarkable in that this was covered by a land animal floating
without the aid of a raft, normally assumed for trans-oceanic colonization. In the case of
giant tortoises their large size would preclude the use of all but the very largest of rafts, but
they have long been recognized as being well adapted to dispersal. In the present case the
restriction of barnacles to the lower half of the carapace supports previous assumptions that
the large lungs underlying the carapace would provide buoyancy, allowing tortoises to float
for extended periods of time, with the head reaching above the surface at least periodically
(Pritchard 1996).
This colonization ability and their morphological variability led to their central influence
on Darwin’s development of his ideas on evolution. It is thus particularly significant that a
giant tortoise can be demonstrated to have made the crossing between a continent and an
oceanic island. It is ironic that the first documented trans-oceanic movement of a tortoise
occurred from an island to a continent, rather than the reverse direction that is so important
to island biogeography.
Acknowledgements
The authors wish to thank the Turtle Conservation Officers from Kimbiji village, Jumanne
Juma and Saidi Jumbe, who discovered the tortoise and reported the find.
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2408 J. Gerlach et al.
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The Growth, Ecology and Population Structure of Giant Tortoises on Aldabra
Article
The giant tortoise of Aldabra, Geochelone gigantea, shows quite marked changes in proportions with age, although during growth the relations between the length of the carapace and various measurements of the plastron and scutes involve not only strong but also weak allometry. Certain scutes show a predisposition to split during growth. Accidental damage to the carapace is frequent. Males reach over 100 cm in carapace length but females are smaller, up to 80 cm. There is no segregation between the sexes in any single measurement investigated, except among the very largest animals. A general appraisal of carapace and tail shape is sufficient to sex only animals above 60 cm in carapace length. The number of annuli on each scute corresponds to the number of years of age at least up to the formation of the tenth to fifteenth annulus. A general assessment of the pattern of growth is made by plotting body measurements against number of annuli. Growth curves of individual tortoises are reconstructed by relating measurements of successively formed annuli to age. Growth rate is recorded by plotting the difference between successive pairs of annulus measurements against age. The growth rate of ageable tortoises varies between local populations on South Island and between populations of South and Middle Island. Growth rate declines with age, reaching asymptotes at mean values of between 20 and 30 years. Some individuals exhibit sudden increases in growth rate after several years of very slow growth. There is a well-marked daily cycle of activity, feeding being limited to the early morning and late evening. Agonistic behaviour is virtually absent. Breeding is seasonal and the males select partners from within a limited size range of tortoises. Most mating attempts are unsuccessful. On Aldabra, tortoises occur in a wide variety of habitats, in each of which they depend on a different plant species or vegetational association for food. On coastal plains the chief source of food is Sporobolus virginicus. A variety of small herbs is consumed on the barren stretches of coastal champignon. Distribution in these areas is profoundly affected by the availability of shade. Further inland, the tortoises browse heavily on Guettarda speciosa in woods dominated by this tree. They take advantage of seasonal successions in the vegetation associated with freshwater pools, feeding on each community as it develops. Most of the woody plants near the pools are ignored. On the platin, browsing is selective and the regeneration of some trees is held in check. A very important food source here is the 'tortoise turf' (a sward in which Panicum sp. is often dominant) developed under conditions of heavy grazing and susceptible to erosion by wind and the tortoises themselves. On Middle Island, where the population is small, the tortoises exert very little effect on the vegetation. Associations with other animals are mostly casual, but along the south coast dunes Coenobita rugosus is dependent on tortoise faeces for food. Fossilized tortoise bones have been discovered at many points on Aldabra, deposited in brown limestone. They probably date from before the interstadial of about 30 000 years ago. Some adult tortoises range over 7 km or more, across a variety of habitats, but many individuals appear to be sedentary. The population of South Island is enormous-of the order of 100 000 animals-with a density of about 30 hm-2 on the platin. Higher densities are reached in Guettarda woodland. Local variation in numbers, size range and age structure depend on habitat preferences, differential movement of age classes and regional differences in growth rate. Attempts at assessing age class distribution are affected particularly by undersampling of the younger age classes, and the difficulty of counting the worn growth rings in animals with more than about 14. In the census sample, which may itself be an imperfect sample of the whole South Island population, at least 35% of the animals are below 20 years of age and only about 20% can have reached sexual maturity. More than 50 age classes may be present, but this and similar deductions are still speculative.
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Population processes in a large herbivorous reptile: the giant tortoise of Aldabra Atoll
Article
  • Feb 1984
Physical barriers divide the population of giant tortoises (Geochelone gigantea Schweigger) on Aldabra into several sub-populations of different density, which nevertheless are similar genetically. We measured individual growth rates in each sub-population. Mortality was estimated using data from Bourn and Coe (1979). Reproduction and recruitment were studied using data from previous work (Swingland and Coe 1979) and our own estimates of clutch size, egg weight, and laying frequency from 1975 to 1981.Individual growth rates were strongly dependent only on individual size and sub-population density and not on age or sex. Within a sub-population, the relationship between specific growth rate and size (linear measure) was best fitted by a Gompertz model, except for very young tortoises which grew faster in volume, though not in weight, than expected. Animals at high densities grow slowly to a small size whereas those at low densities grow fast to a large size. At very high density many juveniles remain at a small size without growing or maturing.Mortality of larger (> ca. 5 years old) animals was independent of density, but did depend on size in the highest-density sub-population, as predicted by the Gompertz growth model.Reproduction and recruitment were negatively density-dependent over the whole density range (5 to 35 animals ha-1) studied. Clutch size and laying frequency were strongly influenced by sub-population density, but egg weight was not. Laying frequency varied within sub-populations according to rainfall (presumably via annual food supply).All except one sub-population are seen as stages in the development of the same interactive system. Competition between individuals is nearly, but not purely, of scramble type. The remaining sub-population is either a distinct interactive system in which food supply for very young animals is important, or it is a non-interactive system controlled by the effect of natural enemies on very young animals. This suggests that the equilibrium density and/or dynamics of giant tortoise populations are highly sensitive to mortality factors affecting very young animals.In low density sub-populations the animals are large, have many young, low relative reproductive effort, and a short generation time. In high density sub-populations they are small, have few young, high relative reproductive effort, and a long generation time. This variation is largely phenotypic. It is anomalous with respect to r-K life history theory but is a logical consequence of indeterminate growth combined with size-determined risk and benefit functions and may have contributed to the giant tortoises' success as island colonisers.
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Two giant tortoises were swept twenty miles by hurricane
  • Jan 1936
  • 119-120
  • C H Townsend
Townsend CH. 1936. Two giant tortoises were swept twenty miles by hurricane. Bulletin of the NewYork Zoological Society 39:119-120.
  • Jan 2000
  • 1-36
  • S Blackmore
Blackmore S, editors, 2001. Proceedings of the Aldabra Science and Conservation Workshop held on Aldabra 8-18th December 2000. Phelsuma 9B:1-36.
The trial of deepwater Fish Aggregation Devices (FADs) in Tanzania. Dar es Salaam: Samaki Consultants Ltd for MRAG(FMSP)/DfID/Project partners
  • Jan 2006
  • M D Richmond
Richmond MD, Mohamed A. 2006. The trial of deepwater Fish Aggregation Devices (FADs) in Tanzania. Dar es Salaam: Samaki Consultants Ltd for MRAG(FMSP)/DfID/Project partners, 57 p.
Darwin's islands: a natural history of the Galápagos
  • Jan 1971
  • 322
  • I Thornton
Thornton I. 1971. Darwin's islands: a natural history of the Galápagos. New York: Natural History Press. p 322.
  • Jan 1999
  • M Breuil
Breuil M. 1999. Editorial. West Indian Iguana Specialist Group Newsletter 2(1) [online]. http://www.iucn-isg.org/ newsletters/s1999v2n1.php#taxon.
57 Dar es Salaam: Samaki Consultants Ltd for MRAG(FMSP)/DfID/Project partners
  • M D Richmond
  • A Mohamed