Giant tortoises are not as slow in the head as they are on the ground, new research suggests.

Not only can these 'living rocks' be taught simple goal-oriented tasks, some of them can still remember those teachings a whopping nine years later.

Upon training eight Galapagos tortoises (Chelonoidis cf. nigra) and nine Aldabra tortoises (Aldabrachelys gigantea) from two zoos, researchers found these giant creatures are fast learners, especially when they're taught alongside others.

"Thus," they write in a new paper, "we now provide the first evidence for social learning in Galapagos and Aldabra tortoises."

That might sound surprising, but it's true. Even today, we know very little about the tortoise family and next to nothing about giant land tortoises in particular.

Historically, giant tortoises have been considered solitary creatures with minimal intelligence, but in recent years it's become clear how wrong we were.

Some studies have shown that Aldabra tortoises can learn tasks with positive reinforcement. And research on red-footed tortoises proves these creatures can be taught to use a touch screen in return for a strawberry. In 2017, in fact, scientists discovered this particular species has a longer memory than we thought, holding onto visual cues for 18 months.

Still, neither Aldabra tortoises nor Galapagos tortoises are described as being particularly smart or social, even though in the wild these giants sleep, graze, and migrate in groups.

They also travel great distances, which suggests these creatures must have good directional memory; and Aldabras are known to 'nose' each other, probably as a form of social interaction, although the true reason is unknown.

"Even though giant tortoises may not form the complex social societies found in other species, they do, in the wild, share resources," the authors explain.

"It seems likely that they share information about those important resources, drawing attention to a location by directing a gaze and watching the consumption of food or water."

To test the animals' intelligence, the authors set up a simple experiment with food as a reward. First, the tortoises were trained to bite a coloured ball on the end of a stick.

Then, they were shown two coloured balls, and if they bit the correct colour, they were rewarded with food. Each tortoise was given a differently coloured target, and no reward was given if they failed to choose the right one.

The test was repeated 95 days later, and again, 9 years later. No difference existed between the two zoo locations, except at one of the zoos, the tortoises were trained individually, whereas at the other, the tortoises were trained in groups.

When choosing between two colours, all the animals readily learned to distinguish their target, the researchers found. But because each tortoise had a different colour assigned to them, the presence of the group didn't speed up their learning like it did in the first simple operant task, when they could look to their friends for help.

"In these circumstances, an observing tortoise would be attracted to the target by seeing a conspecific interacting with it," the authors explain.

"In addition, since observer animals saw their group-members consume their food rewards during training, it is likely that observational conditioning influenced the social learning process."

Whether or not these social cues were helping them learn, in the end all the tortoises performed just as fast on these tasks as other vertebrates.

And what they learned actually stuck with them, too. After three months had passed, most of the tortoises could relearn the discrimination task up to three times faster.

"Remarkably, animals that were tested 9 years after the initial training still retained the operant conditioning," the authors write.

"As animals remembered the operant task, but needed to relearn the discrimination task constitutes the first evidence for a differentiation between implicit and explicit memory in tortoises."

In other words, in the first task, where the tortoises just needed to go to a target, the animals are displaying implicit memory, which means they're not consciously recalling that information.

Whereas on the second task, when the tortoises are forced to choose between two colours, they're drawing on explicit memory, which is conscious and long-term.

Further research will be needed to confirm these results, as the sample size is still quite small and several of the tortoises were not able to progress to the second task - for complete lack of interest.

Nevertheless, as we begin to study the intelligence of these long-lived giants, it certainly seems as though we've been underestimating their abilities. Now, we can start to correct our perceptions.

The study was published in Animal Cognition.