Turtles capable of crossing the ocean have unexpectedly turned out to be “homebodies”

However, satellite tracking revealed a completely different picture. None of the nine tagged turtles left the Galápagos Archipelago whilst their transmitters were operational, and five of the animals spent most of their time near a single island. The longest period a single turtle was tracked was 418 days.

This does not mean that these tortoises never undertake long-distance migrations at all. On the contrary, researchers suggest that females in particular may set off for distant breeding beaches once every few years — it is simply that such a journey did not occur during the observation period.

Details

The researchers studied the so-called yellow morphotype of the green sea turtle, Chelonia mydas.

Despite the name, this is not a separate ‘yellow species’ of sea turtle. In the Galápagos, genetically distinct yellow and black morphotypes of the same species are found. The archipelago is considered the only known place where these two forms regularly occur together and utilise the same foraging areas.

In 2015, scientists captured nine adult or near-adult animals – seven males and two females – in the shallow coastal waters near four islands.

Each turtle was fitted with a transmitter that sent its coordinates via the Argos satellite system.

Tracking continued for between 45 and 418 days, until the transmitter stopped working or became detached from the shell.

This is precisely why it is incorrect to say that the scientists ‘monitored each turtle for three years’. The research project spanned a longer period, but no single satellite track lasted three years.

The results turned out to be much more localised than the scientists had expected.

Five of the nine animals remained near a single island for most of the observation period. They preferred shallow coastal waters and showed a marked preference for specific foraging areas.

The others travelled further.

The turtles, which the scientists named Bernardo and Sara, moved between Isabela and San Cristóbal. Leandro travelled between Isabela and Española.

The most active turned out to be the female, Ainoa. After being tagged near San Cristóbal, she headed for Santa Cruz and then on to Isabela.

However, even she did not undertake a true transoceanic journey.

Throughout the entire period that the satellite transmitters were operational, not a single one of the nine tortoises left the Galápagos Archipelago.

These ‘homebodies’ could still swim tens of kilometres

The term ‘homebodies’ is relative in this context.

We are not talking about animals that stayed in one small cove for months on end.

The tortoises sometimes ventured about 25 kilometres from the shore, and some regularly moved between the islands. The average estimate of the total area used by a single animal was approximately 1,498 km², although there were huge differences between individuals.

By human standards, this is by no means a small area.

But for sea turtles, capable of travelling thousands of kilometres across the open ocean, such behaviour does indeed appear very localised.

The researchers therefore speak primarily of a strong attachment to feeding grounds, rather than complete immobility.

The scientists observed another curious pattern.

All recorded movements between the islands took place during the cooler months — from April to October.

Why this is the case remains unknown.

Their movements may be influenced by water temperature, the distribution of algae and seagrass, currents or other seasonal changes in the environment.

However, the sample size is too small — just nine animals — to conclude at this stage that the scientists have identified a universal seasonal migration pattern for this group.

For now, this is more of an interesting indication that needs to be verified using a larger sample of turtles.

Scientists had expected them to swim much further

The unexpected nature of the result is linked to the origin of the yellow morphotype.

Females of this group are not known to nest regularly on the beaches of the Galápagos Islands. The researchers therefore suggest that, upon reaching reproductive age, they must leave the archipelago and head for breeding grounds in the western Pacific Ocean.

Such journeys are quite common among sea turtles.

The animals may feed in one area for years, before travelling vast distances to the beach associated with their place of origin.

It was precisely this kind of migration that the team expected to see in at least some of the tagged animals.

But it did not happen.

This led the researchers to focus on how long yellow-bellied sea turtles are able to remain within the Galápagos Marine Reserve.

But the scientists may simply not have waited long enough for the major migration to occur.

This is the main limitation of the study.

A total of nine tortoises were studied, of which only two were females. It is precisely the migration of females to their nesting sites that is one of the best-known examples of long-distance travel by marine tortoises.

Furthermore, the maximum operating time of a single transmitter was 418 days.

If a particular turtle breeds, for example, only once every few years, the likelihood of coincidentally encountering it during a long journey is quite low.

The authors themselves explicitly warn that the study provides only a snapshot of behaviour at a particular point in time, and that rare long-distance movements may have gone unnoticed.

Therefore, the conclusion is:

‘Galápagos yellow tortoises no longer migrate’

would be incorrect.

A much more accurate statement would be:

between possible rare reproductive migrations, they may show a very strong attachment to specific foraging areas in the Galápagos.

Why do the tortoises remain near the islands?

Adult green sea turtles differ from most other extant sea turtles in that their diet is predominantly plant-based.

They feed mainly on algae and seagrasses.

Consequently, a stable and abundant feeding ground is capable of keeping the animal in one area for a long time.

The Galápagos Marine Reserve covers a vast area of water with diverse coastal ecosystems. The turtles studied made particularly frequent use of the shallow waters near the islands.

This is consistent with the idea that the archipelago serves as a large and stable foraging area for the yellow morphotype.

However, the study does not prove that it is specifically the availability of food that explains the absence of long-distance migration in every individual.

The finding has proved important for the conservation of the Galápagos.

The study has a very practical implication.

If a sea turtle uses a single island, it might seem that protecting a specific coastal area is sufficient.

But some animals moved between two and even three islands.

This means that, to conserve the population, it is not only individual feeding grounds that are important, but also the marine corridors between the islands.

“The quality of the foraging habitat must be maintained around all the islands, rather than allowing it to deteriorate in specific locations,” emphasises the study’s lead author, John Rowe.

If pollution, fishing or other impacts destroy one of the areas they use, this could affect animals that regularly move between different parts of the archipelago.

The turtles may yet have a surprise in store for the scientists.

The most intriguing question now remains for the researchers.

If female yellow-footed tortoises really do not breed in the Galápagos, where do they go when it is time to lay their eggs?

The answer may require satellite transmitters capable of operating for much longer periods, and a larger sample of females.

It is quite possible that these seemingly ‘sedentary’ animals spend years within a few islands, and then, in a short space of time, undertake a journey across a significant part of the Pacific Ocean.

This is precisely why the current findings do not negate the well-known migratory abilities of sea turtles.

They reveal another aspect of their lives: an animal potentially capable of crossing the ocean can remain loyal to a territory that is small by oceanic standards for a very long time – even to a single island.

Source

Study:“Movements of yellow-morphotype green turtles (Chelonia mydas) in the Galápagos Marine Reserve”.

Authors: Juan Pablo Muñoz-Pérez, Daniela Estefania Alarcón-Ruales, Lyndsey K. Tanabe, Catherine E. Hart, Maike Heidemeyer, Todd Steiner, Randall Arauz, Carlos A. Valle, John W. Rowe.

Journal: *Frontiers in Amphibian and Reptile Science*, section *Behaviour and Ecology*, 2026.