ArticlePDF Available

The first substantiated case of trans‐oceanic tortoise dispersal

Authors:

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.
Content may be subject to copyright.
Downloaded By: [Gerlach, Justin] At: 14:14 9 April 2007
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.
Downloaded By: [Gerlach, Justin] At: 14:14 9 April 2007
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
Downloaded By: [Gerlach, Justin] At: 14:14 9 April 2007
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.
Downloaded By: [Gerlach, Justin] At: 14:14 9 April 2007
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.
References
Blackmore S, editors, 2001. Proceedings of the Aldabra Science and Conservation Workshop held on Aldabra 8–
18th December 2000. Phelsuma 9B:1–36.
Breuil M. 1999. Editorial. West Indian Iguana Specialist Group Newsletter 2(1) [online]. http://www.iucn-isg.org/
newsletters/s1999v2n1.php#taxon.
Censky EJ, Hodge K, Dudley J. 1998. Over-water dispersal of lizards due to hurricanes. Nature 395:556.
Gerlach R. 2005. Editorial. Birdwatch 55:1–3.
Gibson CWD, Hamilton J. 1984. Population processes in a large herbivorous reptile: the giant tortoise of Aldabra
Atoll. Oecologia 61:230–240.
Grubb P. 1971. The growth, ecology and population structure of giant tortoises on Aldabra. Philosophical
Transactions of the Royal Society of London B 260:327–372.
Trans-oceanic tortoise dispersal 2407
Downloaded By: [Gerlach, Justin] At: 14:14 9 April 2007
Pritchard PCH. 1996. The Gala´pagos tortoises: nomenclatural and survival status. Lunenberg, MA: Chelonian
Research Foundation. p 85.
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.
Thornton I. 1971. Darwin’s islands: a natural history of the Gala´pagos. New York: Natural History Press. p 322.
Townsend CH. 1936. Two giant tortoises were swept twenty miles by hurricane. Bulletin of the NewYork
Zoological Society 39:119–120.
2408 J. Gerlach et al.
... As a minor note, Hennessy states that it occurred in 2006 (p. 61), which is instead the year it was published (Gerlach et al. 2006). Moreover, I thought the accounts that centered on the network of contingent factors that ultimately led to the Galápagos being established as a national park were really fascinating and well laid out (Chapter 5). ...
Article
Full-text available
“Struggles over conservation are not between those who destroy and those who conserve nature...,but among populations who understand the value of nature very differently.”
... In considering the origins of Galápagos' landvertebrate clades, the present-day view is that the colonizers dispersed to the archipelago mainly by passively rafting on uprooted trees or vegetation mats. The tortoises, though, probably floated over (see Gerlach et al., 2006). However, in the 19 th and early 20 th centuries, prior to the detailed surveying of the ocean basins, as well as the formulation of a dynamic Earth model, there was a broadly even split between proponents invoking land-bridges (e.g. ...
Article
Full-text available
Based on a synthesis of new molecular phylogenetic data, a detailed review is presented for the origins of the Galápagos’ native land-locked vertebrates [42 species; 11 clades: geckos (3), lava lizards (2), giant tortoises (1), iguanas (1), racer snakes (1) and oryzomyine rodents (3)]. Nine groups have roots in coastal Ecuador and Peru and would have been transported to the archipelago on rafts, many on the Humboldt Current. Inferring the sources of the giant tortoises, which probably floated over unaided, and the iguanas is more challenging because their closest living relatives occupy ground remote from the Pacific. Acknowledging uncertainties with the age-dating of both the phylogenetic tree nodes and the landmass emergences, seven, probably eight, of the colonizations likely involved beachings on the modern-day islands within the last 4 Myr. Three, possibly four, of the earlier arrivals may have been on now-submerged landmasses that were created by the Galápagos volcanic hotspot. Alternatively, the true sister taxa of the Galápagos species could be extinct and these colonizations, too, are more recent. This is likely for the giant tortoises. The assembled data set hints at the oldest/youngest clades showing the highest/lowest levels of diversification, although other factors also exert an influence.
... In any case, fossil record, molecular data, as well as direct observations of extant taxa, attest that Mauremys and, especially, large testudinids are well capable of dispersing throughout large marine distances (Gerlach et al., 2006;Vamberger et al., 2014;Kehlmaier et al., 2019;Georgalis et al., 2020). As such, a direct terrestrial corridor would not be strictly necessary for these turtle taxa to reach Scontrone. ...
Article
Full-text available
We here describe a small turtle assemblage originating from the early Tortonian (late Miocene) palaeoisland of Scontrone, central Italy, a locality previously known mostly for its endemic mammals and giant birds, which were otherwise shared only with the Gargano localities, another fossiferous area belonging to the same palaeobioprovince. The fossil turtle remains from Scontrone are referred to the geoemydid Mauremys sp. and a so far unidentified large-sized testudinid. The biogeographic origins of the Scontrone insular chelonians are discussed. The Scontrone geoemydid adds to the known occurrences of Mauremys in the late Miocene of the Mediterranean. The Scontrone large tortoise represents the oldest known Mediterranean insular testudinid, predating significantly the well-known Quaternary endemic island tortoises of the area.
... Faunal interchange between Asia and Australia evokes long distance oceanic dispersal, which at face value seems unlikely. However, recent analyses of terrestrial vertebrates demonstrates that long-distance dispersal over oceans is possible and can occur upwards of 100s to 1000s of kilometers between landmasses (Blom et al., 2019;de Queiroz, 2005;Gerlach, Muir & Richmond, 2006;Hawlitschek, Ramirez Garrido & Glaw, 2017). In addition to these modern examples of faunal oceanic interchange, dinosaurs, including titanosaurians, have recently been proposed to have dispersed across oceanic barriers (Longrich et al., 2021). ...
Article
Full-text available
A new giant sauropod, Australotitan cooperensis gen. et sp. nov., represents the first record of dinosaurs from the southern-central Winton Formation of the Eromanga Basin, Australia. We estimate the type locality to be 270–300 m from the base of the Winton Formation and compare this to the semi-contemporaneous sauropod taxa, Diamantinasaurus matildae Hocknull et al., 2009, Wintonotitan wattsi Hocknull et al., 2009 and Savannasaurus elliottorum Poropat et al., 2016. The new titanosaurian is the largest dinosaur from Australia as represented by osteological remains and based on limb-size comparisons it reached a size similar to that of the giant titanosaurians from South America. Using 3-D surface scan models we compare features of the appendicular skeleton that differentiate Australotitan cooperensis gen. et sp. nov. as a new taxon. A key limitation to the study of sauropods is the inability to easily and directly compare specimens. Therefore, 3-D cybertypes have become a more standard way to undertake direct comparative assessments. Uncoloured, low resolution, and uncharacterized 3-D surface models can lead to misinterpretations, in particular identification of pre-, syn- and post-depositional distortion. We propose a method for identifying, documenting and illustrating these distortions directly onto the 3-D geometric surface of the models using a colour reference scheme. This new method is repeatable for researchers when observing and documenting specimens including taphonomic alterations and geometric differences. A detailed comparative and preliminary computational phylogenetic assessment supports a shared ancestry for all four Winton Formation taxa, albeit with limited statistical support. Palaeobiogeographical interpretations from these resultant phylogenetic hypotheses remain equivocal due to contrary Asian and South American relationships with the Australian taxa. Temporal and palaeoenvironmental differences between the northern and southern-central sauropod locations are considered to explain the taxonomic and morphological diversity of sauropods from the Winton Formation. Interpretations for this diversity are explored, including an eco-morphocline and/or chronocline across newly developed terrestrial environments as the basin fills.
... elephants, hippopotamuses;van der Geer et al. 2010van der Geer et al. , 2015 or have restricted agility (e.g. tortoises; Gerlach et al. 2006). As a consequence, the faunal assemblages on the two sorts of islands are markedly different. ...
Article
Full-text available
Further to the debate associated with the viability of land-bound mammals being able to colonize remote frontiers by way of long-distance over-water dispersal, observations are documented of monkeys (red howlers, Alouatta seniculus) occupying free-standing trees within a large floating island on the Magdalena River in north-west Colombia. Also, we contribute to the discussion related to the 15.0–12.5 Ma (Middle Miocene) arrival of the howler monkeys in Central America, which is well before the Panama Isthmus had fully emerged 3.0–2.8 Ma (Late Pliocene). We speculate it was by way of a raft similar to the ones reported here, possibly from a river entering the sea from northern Colombia.
... Direct observations are powerful evidence to validate transoceanic dispersal mechanisms and processes, however, such data is very rare, and the paucity of direct observations remains among the primary obstacles to the development of dispersal biogeography as a mature scientific discipline (Ran 2013). While tangible observations remain scarce or contested in the literature, mounting evidence indicates that tropical storms and hurricanes may be responsible for enabling the over-water dispersal of various species to new locations; examples include plants and seeds (Bhattarai and Cronin 2014), mammals (Shiels et al. 2020), amphibians (Vences et al. 2004;Measey et al. 2007), reptiles (Censky et al. 1998;Calsbeek and Smith 2003;Gerlach et al. 2006), fish (Johnston and Purkis 2015), and invertebrates (Torres 1988;Andraca-Gómez et al. 2015). ...
Article
Full-text available
The biogeographic origins of flora and fauna in the Caribbean have remained a hotly de-bated topic in island biogeography, with evidence to suggest that both vicariance (tectonic activity) and oversea dispersal mechanisms are at play in isolating species distributions and/or expanding them. Here we document the first evidence of the Central American Snapping Turtle (Chelydra rossignonii) on Utila Island, part of the Bay Islands, Honduras. Information is presented on two adult female individuals that were encountered washed ashore following separate extreme weather events in the region; most recently a single specimen was retrieved following Hurricane ETA in November 2020. Though dispersal in this case has not led to establishment, our novel observations may support previous inferences that long-distance overseas dispersal is a valid mechanism for explaining the occurrence of a potentially wide variety of terrestrial and freshwater biodiversity on the Bay Islands, as well as in the wider Caribbean.
... These two endemic genera have no documented fossil record, although preliminary mentions do exist in the literature, but without any kind of description or figures (e.g., Pyxis; Burney et al. 2020). In any case, Aldabrachelys is well known to be capable of large trans-oceanic travelling (Gerlach et al. 2006) and this fact can explain at least certain Late Pleistocene and Holocene occurrences on different islands. As far as the Mascarenes radiation is concerned, molecular data and ancient DNA have recently demonstrated that the lineage of Cylindraspis is likely of African or Eurasian origin, diverged sometime around the late Eocene, and subsequently dispersed to the area of the Mascarenes, likely by island-hopping across the now-sunken islands of the Réunion hotspot (Kehlmaier et al. 2019). ...
Article
Turtles of the clade Testudinoidea have a rather scarce fossil record in Afro-Arabia, ranging from the late Eocene up to the Quaternary. The vast majority of testudinoid fossils from Afro-Arabia are ascribed to Testudinidae, which has had a continuous presence in the area since the late Eocene. Geoemydidae is poorly documented by fragments found throughout the Neogene across northern Africa and the Middle East. Emydidae is absent from the fossil record of this area. All valid named taxa pertain to testudinids. Within Testudinidae, the majority of known fossil species are members of the clade Geochelona, while a few others belong to the clade Testudona. Four fossil taxa are members of now-extinct genera, five are members of extant genera, and seven cannot be assigned to a known genus with certainty. The fossil record also documents that several extant genera had a much broader distribution during the Neogene and Quaternary. Endemic insular lineages were formerly present on the Canary Islands, Cape Verde islands, and on several islands in the Western Indian Ocean. The highest known diversity of testudinoids seems to have existed during the Neogene; however, definitive conclusions are hampered by the extremely poor Paleogene record and large, unsampled areas of Afro-Arabia. A taxonomic review of the 22 named Afro-Arabian taxa finds 16 nomina valida, 1 nomen invalidum, and 5 nomina dubia.
... We also ignore the mechanisms that enable terrestrial snakes to survive during long periods at sea and how long they can do so. Terrestrial tortoises travel hundreds of kilometres, can float for weeks in ocean currents, and can sometimes survive in hostile conditions without food and with limited access to freshwater (Gerlach et al., 2006). Such events are rare, at least for human observers, and remain largely undocumented in terrestrial snakes. ...
... For most reptiles, there are two different ontogenetic stages to ocean crossing: the individual stage and the egg stage. Currently, ocean crossing at the individual stage has been reported abundantly through swimming, floating, or rafting [1,[31][32][33]. Compared with large species which can swim for a long distance, floating and rafting are the more likely ways for small reptiles to cross oceans [1,34]. ...
Article
Full-text available
Dehydration and hypersalinity challenge non-marine organisms crossing the ocean. The rate of water loss and saltwater tolerance thus determine the ability to disperse over sea and further influence species distribution. Surprisingly, this association between physiology and ecology is rarely investigated in terrestrial vertebrates. Here we conducted immersion experiments to individuals and eggs of six lizard species differently distributed across Taiwan and the adjacent islands to understand if the physiological responses reflect the geographical distribution. We found that Plestiodon elegans had the highest rate of water loss and the lowest saltwater tolerance, whereas Eutropis longicaudata and E . multifasciata showed the lowest rate of water loss and the highest saltwater tolerance. Diploderma swinhonis , Hemidactylus frenatus , and Anolis sagrei had medium measurements. For the eggs, only the rigid-shelled eggs of H . frenatus were incubated successfully after treatments. While, the parchment-shelled eggs of E . longicaudata and D . swinhonis lost or gained water dramatically in the immersions without any successful incubation. Combined with the historical geology of the islands and the origin areas of each species, the inferences of the results largely explain the current distribution of these lizards across Taiwan and the adjacent islands, pioneerly showing the association between physiological capability and species distribution.
Preprint
Full-text available
Oceanic archipelagos are known to host a variety of endemic plant species. The genetic diversity and structure of these species is an important indicator of their evolutionary history and can inform appropriate conservation strategies that mitigate the risks to which they’re exposed, including invasive species and environmental disturbances. A comprehensive consideration of the role of their natural history, as well as the landscape features and the geological history of the islands themselves is required to adequately understand any emerging patterns. Such is the case for the guayabillo ( Psidium galapageium ), an understudied endemic plant from the Galapagos Islands with important ecological and economic roles. In this study we designed and evaluated 13 informative SSR markers and used them to investigate the genetic diversity, population structure and connectivity of the guayabillo populations from San Cristobal, Isabela and Santa Cruz islands. A total of 208 guayabillo individuals were analyzed, revealing a strong population structure between islands and two distinct genetic lineages for the Santa Cruz population. Overall, the guayabillo genetic diversity is relatively high, an unusual pattern for an insular endemic species which is possibly explained by its polyploidy and the geographical features of the islands. These include their broad altitudinal ranges and habitat heterogeneity. For populations displaying a lower genetic diversity such as San Cristobal, the history of human disturbance could be an important factor explaining these observations. Some similarities between individuals in Santa Cruz and the San Cristobal population could be explained by population differentiation or distinct natural histories of separate lineages. Our findings highlight the complex population dynamics that shape the genetic diversity of species like the guayabillo and emphasize the need to explore the currently unresolved questions about this Galapagos endemic plant.
Article
Full-text available
The possibility and probability of over-water dispersal as a mechanism to explain the distribution of terrestrial animal species in the Caribbean has been hotly debated since the early part of this century1,2. Each theory that has been proposed — including land bridges and over-water dispersal — has involved over-water dispersal to some extent in the distribution of animals. Yet many people remain sceptical of over-water dispersal, believing that the use of rafts is improbable, unobservable and consequently untenable. Here we present evidence to support over-water dispersal as the mechanism by which green iguanas colonized Anguilla.
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.
Article
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.
Two giant tortoises were swept twenty miles by hurricane
  • C H Townsend
Townsend CH. 1936. Two giant tortoises were swept twenty miles by hurricane. Bulletin of the NewYork Zoological Society 39:119-120.
  • 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
  • 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
  • I Thornton
Thornton I. 1971. Darwin's islands: a natural history of the Galápagos. New York: Natural History Press. p 322.
  • 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