Thursday, September 29, 2011

Fresh water stingray facts part III: Thinking rays.

Now you all might think that I shall rename the blog to stingray addict but this will be the last post about rays for a while :).

Rays and sharks in general is often perceived by aquarists as very smart creatures. Compared to many fishes sharks and rays have relatively large brains and in some studies, however very few, been shown to be rather smart. A large part of the brain of sharks and rays is dedicated to processing data from the Ampullae of Lorenzini which is the electro-sensory organ to locate prey. Freshwater stingrays of the family Potamotrygonidae generally have fewer of these ampullae which mean that their brains are not as large as in sea living stingrays. However their brains are still relatively large. The question is how smart are these rays really?

Tool use have been observed in many lineages of the animal kingdom, such as in primates (both human and non-human), in birds and in cephalopods to give some examples. However observations of this kind have been rare in the largest group of vertebrates, namely teleost fish, until now. In a few observations members of the wrasse  family (Labridae) have been seen using stones on the sea floor as anvils while trying to crush clams. This have been observed in three different genera of wrasses so it seem to be a common method used by these fish. The first observation of tool use in wrasses was described in 1995 for the yellow head wrasse.
So tool use have been observed in several species throughout the animal kingdom, and particularly in the bony fish lineage including tetrapods. The cartilaginous fish lineage on the other hand is less studied when it comes to tool use and memory studies. In 2005 a research group used five captive breed Potamotrygon castexi stingrays of the same litter to study cognitive abilities of these rays. They used a plastic tube with one white and one black painted side. On one side of the tube there was a mesh on the inside and the other side was open. Inside the tube they placed a piece of irresistible food for a stingray. The food was, due to the mesh, not accessible from one of the sides of the tube. What they observed was that the rays learned quite well that if they did the right thing they would get the food, they also learned the color on the side of the tube that was associated with the possibility to access food. The way the rays accessed the food was through blowing a jet of water into the tube to sort of flush the food out. This method, by blowing a jet of water, is used by many ray species to uncover pray hidden in the sediment. 

The authors of this paper suggest that this is an observation of tool use in a cartilaginous fish species, using water as a tool to access the food. However if this really is tool use is a matter of definition since its a behavior the rays are born with knowing what to do. One can however argue that in the setting with the test pipe it is in a way tool use since they need to learn to get the food out using the water. I would rather state that the paper is a good study on learning and memory in stingrays rather than an observation on tool use. In any case freshwater stingrays are very interesting and more studies on their cognition would be interesting.

Kuba MJ, Byrne RA, & Burghardt GM (2010). A new method for studying problem solving and tool use in stingrays (Potamotrygon castexi). Animal cognition, 13 (3), 507-13 PMID: 20020169