Long-term memory for food quality in tortoises

The red-footed tortoise, Chelonoidis carbonaria. Photo by Paul Jones.

I am fascinated by how animal cognition can shape ecological communities. While my research is generally focused on learning, memory is another (inherently connected) aspect of cognition. Memory allows animals to retain information about food sources, roost or burrow locations, and social partners across time periods when they may not encounter those resources or individuals. If the environment changes in a consistent way, such as seasonally available food, memory may allow an animal to more quickly take advantage of a resource as it becomes available again. This week's paper is lead-authored by Francesca Soldati at the University of Lincoln in the UK. Soldati and colleagues examined long-term (18 months) memory in captive red-footed tortoises, Chelonoidis carbonaria. 

Red-footed tortoises are native to the northern South America, from Panamá through northern Argentina, and common as pets worldwide. Wikipedia says in reference to the coloration on their legs "the species name carbonaria means 'coal-like' referring to a dark coal with glowing patches" which is just lovely. They are omnivorous, but in particular consume large amounts of fruit. Red-footed tortoises are becoming a model system for studying reptile learning, as they quickly learn associations between colors and food, socially learn from other tortoises, and can be trained to use touchscreens to solve spatial tasks. On an irrelevant side note, they do not, however, exhibit contagious yawning. 

So what food to red-footed tortoises particularly care about the quantity and quality of? Fruit jelly of course. And in case it wasn't obvious, mango is better than apple. Soldati and colleagues trained tortoises to associate different colored cards with mango vs. apple jelly cubes (quality) and with different sizes mango jelly cubes (quantity). 18 months later the tortoises were shown the same colored cards, and exhibited the same preferences for colors paired with mango jelly and larger jelly quantities. [As an aside, I am a bit concerned about the Clever Hans phenomenon in these experiments. It would be nice to know that experimenters were blind to what the tortoises were trained to when they conducted the tests.] 

18 months is longer than the time between fruiting for most plants in the native tortoise habitat. The authors propose that memory for high quality or large fruits may be retained by tortoises across seasons when those fruit are not available, potentially increasing the frequency or speed with which tortoises visit fruiting trees as they come into season. The amount of frugivory by tortoises may in turn have consequences for seed dispersal of the plants. This postulation raises a lot of further questions: how far does a tortoise range over 18 months? what is the density of fruiting trees of various qualities? do they remember the locations of fruiting trees across seasons in order to revisit them when they begin to fruit? do they associate the fruit quality with other seasonal cues enabling them to revisit high quality fruiting trees at the right time of year? It is known that red-footed tortoises exhibit spatial memory in captivity (memory for the location of food in a maze). Studying the ways in which tortoises use memory in the wild is important (albeit difficult) research to understand the effects memory may have on seed dispersal.