Rafting event
Encyclopedia
Oceanic dispersal is a type of biological dispersal
that occurs when organisms transfer from one land mass to another by way of a sea crossing on large clumps of floating vegetation. Such matted clumps of vegetation are often seen floating down major rivers in the tropics and washing out to sea, occasionally with animals trapped on them. This is sometimes referred to as a "rafting event."
Rafting has played an important role in the colonization of isolated land masses, such as Madagascar
, which has been isolated for ~120 million years (Ma), and South America
, which was isolated for much of the Cenozoic
. Both land masses, for example, appear to have received their primate
s by this mechanism. According to genetic evidence, the common ancestor of the lemur
s of Madagascar
appears to have crossed the Mozambique Channel
by rafting between 50 and 60 Ma ago. Likewise, the New World monkey
s are thought to have originated in Africa
and rafted to South America
by the Oligocene
, when the continents were much closer than they are today. Madagascar also appears to have received its tenrecs
(25-42 Ma ago), nesomyid
rodents (20-24 Ma ago) and euplerid
carnivorans (19-26 Ma ago) by this route, and South America its caviomorph
rodents (over 30 Ma ago).
Among reptiles, several iguanid species
in the South Pacific have been hypothesized to be descended from iguanas that rafted over 8000 km from Central or South America (an alternative theory involves dispersal of a putative now-extinct iguana lineage from Australia or Asia). Similarly, skink
s of the related genera Mabuya
and Trachylepis
apparently both floated across the Atlantic from Africa to South America and Fernando de Noronha
, respectively, during the last 9 Ma. Skinks from the same group have also rafted from Africa to the Cape Verde islands, Madagascar, the Seychelles
, the Comoros
and Socotra
. (Among lizards, skinks and gecko
es seem especially capable of surviving long transoceanic journeys.)
Colonization of groups of islands can occur by an iterative
rafting process sometimes called island hopping
. Such a process appears to have played a role, for example, in the colonization of the Caribbean
by several groups of mammals of South American origin
.
However, oceanic dispersal of terrestrial species may not always take the form of rafting; in some cases, swimming or simply floating may suffice. Tortoises of the genus Chelonoidis
arrived in South America from Africa in the Oligocene; they were probably aided by their ability to float with their heads up, and to survive up to six months without food or fresh water.
Biological dispersal
Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...
that occurs when organisms transfer from one land mass to another by way of a sea crossing on large clumps of floating vegetation. Such matted clumps of vegetation are often seen floating down major rivers in the tropics and washing out to sea, occasionally with animals trapped on them. This is sometimes referred to as a "rafting event."
Rafting has played an important role in the colonization of isolated land masses, such as Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
, which has been isolated for ~120 million years (Ma), and South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, which was isolated for much of the Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
. Both land masses, for example, appear to have received their primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
s by this mechanism. According to genetic evidence, the common ancestor of the lemur
Lemur
Lemurs are a clade of strepsirrhine primates endemic to the island of Madagascar. They are named after the lemures of Roman mythology due to the ghostly vocalizations, reflective eyes, and the nocturnal habits of some species...
s of Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
appears to have crossed the Mozambique Channel
Mozambique Channel
The Mozambique Channel is a portion of the Indian Ocean located between the island nation of Madagascar and southeast Africa, primarily the country of Mozambique. It was a World War II clashpoint during the Battle of Madagascar...
by rafting between 50 and 60 Ma ago. Likewise, the New World monkey
New World monkey
New World monkeys are the five families of primates that are found in Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Platyrrhini parvorder and the Ceboidea superfamily, which are essentially synonymous since...
s are thought to have originated in Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
and rafted to South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
by the Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
, when the continents were much closer than they are today. Madagascar also appears to have received its tenrecs
Tenrecidae
Tenrecidae is a family of mammals found on Madagascar and parts of Africa. Tenrecs are widely diverse, resembling hedgehogs, shrews, opossums, mice and even otters, as a result of convergent evolution. They occupy aquatic, arboreal, terrestrial and fossorial environments...
(25-42 Ma ago), nesomyid
Nesomyidae
Nesomyidae is a family of African rodents in the large and complex superfamily Muroidea. It includes several subfamilies, all of which are native to either continental Africa or to Madagascar...
rodents (20-24 Ma ago) and euplerid
Eupleridae
The family Eupleridae is a group of carnivorans endemic to Madagascar and comprising 10 known species in seven genera. Probably the best known species is the Fossa , in the subfamily Euplerinae...
carnivorans (19-26 Ma ago) by this route, and South America its caviomorph
Caviomorpha
Caviomorpha is the rodent infraorder or parvorder that unites all South American hystricognaths. It is supported by both fossil and molecular evidence.-Origin:...
rodents (over 30 Ma ago).
Among reptiles, several iguanid species
Brachylophus
The genus Brachylophus consists of three extant iguanid species native to the islands of Fiji and a giant extinct species from Tonga in the South Pacific. One of the extant species, B...
in the South Pacific have been hypothesized to be descended from iguanas that rafted over 8000 km from Central or South America (an alternative theory involves dispersal of a putative now-extinct iguana lineage from Australia or Asia). Similarly, skink
Skink
Skinks are lizards belonging to the family Scincidae. Together with several other lizard families, including Lacertidae , they comprise the superfamily or infraorder Scincomorpha...
s of the related genera Mabuya
Mabuya
Mabuya is a genus of long-tailed skinks nowadays restricted to species from the Americas. The American mabuyas are primarily carnivorous, though many are omnivorous. Formerly, many Old World species were placed here, as Mabuya was a kind of "wastebasket taxon"...
and Trachylepis
Trachylepis
Trachylepis is a skink genus in the subfamily Lygosominae found mainly in Africa. Its members were formerly included in the "wastebin taxon" Mabuya, and for some time in Euprepis. As defined today, Trachylepis contains the clade of Afro-Malagasy mabuyas. The genus also contains a species from the...
apparently both floated across the Atlantic from Africa to South America and Fernando de Noronha
Fernando de Noronha
Fernando de Noronha is an archipelago of 21 islands and islets in the Atlantic Ocean, offshore from the Brazilian coast. The main island has an area of and had a population of 3,012 in the year 2010...
, respectively, during the last 9 Ma. Skinks from the same group have also rafted from Africa to the Cape Verde islands, Madagascar, the Seychelles
Seychelles
Seychelles , officially the Republic of Seychelles , is an island country spanning an archipelago of 115 islands in the Indian Ocean, some east of mainland Africa, northeast of the island of Madagascar....
, the Comoros
Comoros
The Comoros , officially the Union of the Comoros is an archipelago island nation in the Indian Ocean, located off the eastern coast of Africa, on the northern end of the Mozambique Channel, between northeastern Mozambique and northwestern Madagascar...
and Socotra
Socotra
Socotra , also spelt Soqotra, is a small archipelago of four islands in the Indian Ocean. The largest island, also called Socotra, is about 95% of the landmass of the archipelago. It lies some east of the Horn of Africa and south of the Arabian Peninsula. The island is very isolated and through...
. (Among lizards, skinks and gecko
Gecko
Geckos are lizards belonging to the infraorder Gekkota, found in warm climates throughout the world. They range from 1.6 cm to 60 cm....
es seem especially capable of surviving long transoceanic journeys.)
Colonization of groups of islands can occur by an iterative
Iteration
Iteration means the act of repeating a process usually with the aim of approaching a desired goal or target or result. Each repetition of the process is also called an "iteration," and the results of one iteration are used as the starting point for the next iteration.-Mathematics:Iteration in...
rafting process sometimes called island hopping
Island hopping
Island hopping is a term that refers to the means of crossing an ocean by a series of shorter journeys between islands, as opposed to a single journey directly across the ocean to the destination.- Forms :...
. Such a process appears to have played a role, for example, in the colonization of the Caribbean
Caribbean
The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...
by several groups of mammals of South American origin
Mammals of the Caribbean
A unique and diverse albeit phylogenetically restricted mammal fauna is known from the Caribbean region. The region—specifically, all islands in the Caribbean Sea and the Bahamas, Turks and Caicos Islands, and Barbados, which are not in the Caribbean Sea but biogeographically belong to the same...
.
However, oceanic dispersal of terrestrial species may not always take the form of rafting; in some cases, swimming or simply floating may suffice. Tortoises of the genus Chelonoidis
Chelonoidis
Chelonoidis is a genus of turtles in the tortoise family. They are found in South America and the Galápagos Islands. They were formerly assigned to Geochelone, but a recent comparative genetic analysis has indicated that they are actually most closely related to African hingeback tortoises. Their...
arrived in South America from Africa in the Oligocene; they were probably aided by their ability to float with their heads up, and to survive up to six months without food or fresh water.