Jul
8
2009

Navigation and spatial cognition in mammals is thought to be related to the hippocampus, which helps animals form a spatial map. But reptiles lack this seahorse-shaped brain structure, so how do they navigate?

A tortoise will actually use different methods of navigation depending on the presence or absence of visual cues in its environment, according to a new study. The study, “Visual and response-based navigation in the tortoise (Geochelone carbonaria),” was designed to investigate whether the reptilian medial cortex plays a similar role to the mammalian hippocampus in navigation and spatial cognition.

For the study, a red-footed tortoise was placed in an eight-armed radial maze, with food at the end of each arm. In the first part of the experiment, a black curtain was placed around the maze to obstruct the tortoise’s view of the room. Four large geometrical shapes of different colors were placed on the curtain to act as visual cues. Lead researcher Anna Wilkinson of the Department of Neurobiology and Cognition, University of Vienna, said the tortoise, Moses, appeared not to use these cues, and instead adopted an interesting navigational method, called the “turn-by-one-arm strategy.”

“During this training phase the tortoise didn’t use the cues, but instead learned to choose the arm next to the one that she had just left (the turn-by-one-arm strategy),” Wilkinson said. “She turned consistently in one direction within a trial but would vary direction between trials.”

Removal of the geometrical shapes on the curtain did not significantly affect Moses’ use of the turn-by-one-arm strategy, the study says.

In the second part of the experiment, the curtain was removed, and Moses was able to see the whole room, including multiple visual cues such as shelves, computers, the experimenter, and posters. The presence of these cues seemed to activate another navigational mechanism in the reptile’s brain, and it appeared to start using the room cues to navigate.

“The change in behavior has not been observed in mammals and birds and suggests the presence of two processes that can control navigation in our tortoise,” Wilkinson said. ”One appears to be based on visual cues. ... The second mechanism involves a simple response-based strategy of a sort not usually observed in mammals, but it does appear to be present in at least some fish. This pattern of findings suggest that when tortoises (or at least this tortoise) navigate in a situation with poor environmental cues they use a simple, but efficient response-based strategy; but when more cues are available they switch away from this, and apparently navigate using the surrounding visual cues.”

The study concludes that while the reptilian medial cortex is thought to serve a similar purpose to the mammalian hippocampus, the difference in navigation methods suggests that the two brain structures do not operate in exactly the same way. Wilkinson said she and her colleagues are working on another set of experiments to look into the subject further. These experiments will look at whether the behavioral flexibility the tortoise displayed is present in other chelonians or even reptiles and fish in general, she said.

Aside from the intriguing findings on reptile cognition, another discovery from the study came as a shock to Wilkinson.

“We found out that our subject, Moses, was a girl,” she said. “Not scientifically interesting, but surprising for me (I have owned her since she was a hatchling).There is no sexual dimorphism in this species until they reach around 5 years of age.”

The study was published in the journal Animal Cognition in May.

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