Animals need to be able to find resources in order to survive and reproduce. However, wondering around randomly is inefficient and can’t provide any sure means of survival. For this reason, most animals have evolved strategies to orient themselves towards goals in their environment. When this orientation is towards where the animal lives, it’s referred to as homing. If an animal is displaced far enough from it’s home, the new goal becomes to find a habitat. Although the goal has changed, the principle remains the same: find a place to live or die.
The suggest mechanisms for turtles to locate the nearest habitat included geotaxis (movement either up or down an incline), olfaction, use of light characteristics (movement towards or away from light, orientation using gradients of light polarity or wavelength), use of local topographic landmarks, use of a sun compass, and use of a magnetic compass. The first three (geotaxis, olfaction, and use of light characteristics) can be considered water-finding mechanisms. The final three mechanisms (use of topographic landmarks, a sun compass, and a magnetic compass) can be used to locate water, but only if the animal has a knowledge of its surroundings (ex: mental map). Each of the orientation mechanisms operates within a certain distance from the goal. Beyond these distances, the direction of the goal is unknown and the animal must begin a search for the resources it needs to survive. Presumably, if turtles are placed where they can no longer detect water, they will begin searching and use a straight-line search strategy.
There were two objectives in this study. The first was to figure out whether the eastern painted turtles used an orientation mechanism when placed in an unfamiliar territory. The second was to determine if turtles navigate towards the nearest body of water in an unfamiliar area. These both connected to the key casual question- What causes turtles to find their way in unfamiliar territory?
For the experiment, turtles were captured and brought to foreign lakes, far enough away from their own habitats so that homing would not be possible. Each turtle was released individually in remote areas so that human and other turtles interference was minimized. Each turtle was attached to thread (tied to a stake at the release point) so that the turtle’s path could be tracked later. Approximately 24 hours after release, the turtles were recaptured, and returned to their home lake. Upon returning to collect data, the thread paths were followed with a GPS (global positioning system) to record them.
In total, 60 turtles were captured and released. Of the 60 turtles released, 10 did not move further than 10 meters so their trails were excluded from further analysis. Of the 10 who didn’t move more than 10 meters, exactly half were from each of the two sites, indicating that the release habitat was likely not the reason for their lack of movement. Overall, turtles traveled in relatively straight paths for hundreds of meters, well beyond the distance they could see in the dense forests where they were released.
An unexpected result, however, was that although the turtles walked in straight paths, they didn’t orient towards water or in any specific direction. Overall path tortuosity didn’t differ among distances. This precludes that either the turtles wanted to find water and couldn’t or that they could perceive the water but it wasn’t their goal. Assuming the former is correct, it may mean that turtles can’t find water in unfamiliar territory or, if they can, they can only do so at distances less than 100 meters. If this is all true, it begs the question whether or not the need for water is actually a driving motivation for turtles’ movement in this study.
The eastern painted turtles’ inability to find water suggests that they can only find water using homing. So from the previous predicted hypotheses, the turtles use the mechanisms of using local landmarks, a sun compass, or a magnetic compass.
However, other species can find water in unfamiliar habitats. Yellow-bellied slider turtles released in unknown habitats can successfully orient themselves towards water from 300 meters away. Both the yellow-bellied slider turtles and the eastern painted turtles are in the same family, so now a new question is apparent: why has the ability to locate water in unfamiliar territory evolved in some species and not others? This is because the eastern painted turtle does not live in such conditions where they must relocate themselves and thus they have not evolved this ability.
In conclusion of the study, a compass mechanism is the only proposed that could explain why painted turtles, in an unfamiliar habitat, moved in such straight paths yet could not find water. It would allow turtles to maintain a direction but in an unfamiliar territory they lack a map.
The main idea in this article is centered around the search for what causes turtles to have an orientation (towards water specifically) in unfamiliar territory. This was thoroughly evaluated, up until the turn when it was discovered that perhaps turtles don’t have the driving motivation to find water. The authors were still able to strongly support their arguments. They completed the experiment and provided thorough logic and tortuosity calculations. It was strongly supported throughout the entire article and very persuasively presented to the readers.
Smaller, minor ideas were periodically introduced through the article. One that caught my eye in particular was the idea that perhaps, through evolution, the eastern painted turtles could no longer locate water unless using homing, in which case they would have to be in familiar territory. The authors though, only spent two additional lines as a means of an explanation for this wondrous phenomenon. I, as a reader, don’t feel this is sufficient to meet the burden of the question. I think this idea, while presented as a minor one, could potentially lead up to a much greater meaning. It should have been focused on, at least give it some more ink, because if this were true, then in fact the whole study would be disproved and this would be the true conclusion. As a reader, I was disappointed with such a short ending that includes little explanation or background support.
I believe that the article was fairly convincing and informing as far as the ways turtles orient direction. For the type of article, however, it was poorly set up to be in a scientific method. Besides the experiment, which had bold label, the rest of the information had to be dug up after some thought. I think this is partially excusable because the conclusion ended up being unique to the article. Instead of simply just one of the six mechanisms suggested at the beginning, while one was chosen in the end, it seemed that the more appropriate conclusion was that eastern painted turtles don’t have the ability (from their earlier lineage) to find water in unfamiliar territory because of the previous lack of need to do so.
Caldwell, I.R. and V.O. Nams. (2006). A compass without a map: tortuosity and orientation of eastern painted turtles (Chrysemys picta picta) released in unfamiliar territory. Canadian Journal of Zoology, 84, 1129-1137.