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Quantity assessment in animals

When I first joined Zoo Atlanta, almost 20 years ago, one of the things I was excited about was the possibility of working with monitor lizards.  These are the sleek, active lizards in the family Varanidae that includes the famous Komodo dragon (Varanus komodoensis), which is the largest of the dozens of species in Asia, Africa, and Australia. I had done a month of field work with several species in northern Australia some years earlier, and I was fascinated by them and their evident intelligence and alertness. A few years ago, I did some really fun and interesting work on learning and memory in monitor lizards, in collaboration with some Herpetology Care Team members and students.  

When I got here and expressed interest in monitors, my Zoo colleagues invariably enthusiastically pointed out that they were so smart that they could count!  That really intrigued me, but I could find no mention of such in the published literature. I eventually found out that a researcher at San Diego Zoo had done some work that involved presenting monitors with different numbers of food reinforcers and observing their reactions. When I spoke with that person, I learned the unfortunate story of how all of their video tapes—their raw data—were erased and the work was never completed or published. It turns out that the original study was about assessing quantities, and not about counting numbers. So, the rumor mill had twisted it a bit. But, in any case, the work was never completed or recreated. That would be interesting!  Hmmmmm ………I’ll be thinking about that.  

But the concept of being able to determine the larger of two quantities has endless applications for animals.  Big = More is a valuable piece of information when assessing size of a retreat, amounts of food available, or the amount of open space through which to escape a surprise predator attack.  But can they determine such things? 

  • Can animals distinguish that 1 pound of apples is less than 1.5 pounds of apples?   
  • Can they assess that 9 pellets is fewer than 5 pellets? 

These are simple and very interesting questions. Unsurprisingly, behaviorists have established that animals with eyes can essentially do so just fine, and a related body of work has documented how these abilities develop in babies, including humans, as they grow. 

With that basis, the questions start to get more complicated and even more interesting. 

  • Do they distinguish between equal amounts of food, presented in 5 vs. 9 piles? 
  • Without seeing, can they determine if 1 pound of apples has less smell than 2 pounds? 

Zoo Atlanta and our various research collaborators have done some of this work, including work involving our superstar elephants Tara and Kelly. It turns out that elephants consider quantities in essentially the same way that we do so. Animals that can distinguish between quantities typically are better at doing so when the two quantities are very different; more similar quantities are difficult for all of us to distinguish. You already know this from your daily life: it is very easy for us to determine that 2 cans of soda is less than 4 cans of soda (a difference of two, ratio of 0.5).  But will you be able to determine if 4,560 cans of soda is less than 4,558 (a difference of two, ratio of 0.99)?  I don’t think any of us can get that one. How about 1 oz of sugar vs. 11/18 oz of sugar?  Nope.  You get the idea.  

These two extreme examples illustrate the Ratio Effect, which states that the ability to choose the larger amount decreases as the ratio between the two samples. Our colleague at Agnes Scott College, Dr. Bonnie Perdue, led a study some years ago showing that African savanna elephants (= Tara and Kelly!) follow this principle perfectly. Her work was especially important because it showed that an earlier study had claimed that elephants could not do so. So that early study turned out to be flawed. However, until Tara and Kelly helped Dr. Perdue sort it all out, we were really confused regarding what we thought we knew about mammal cognition.  

My students and I recently completed a study of quantitative assessment in eastern box turtles (Terrapene carolina). We are completing the final statistics and writing the paper now. We asked the basic questions comparing two different quantities or two different numbers of pieces. A previous paper had shown that a species of tortoise did not quite match the Ratio Effect, so we also tested that in the box turtles. And we presented them with the complicated question of different both amount and number, in different ways. That’s hard to visualize, so here is a cartoon of some of the different combinations using sticks of turtle chow: 

Picture1 1 

So, I’ll leave you with that cliffhanger! You should see our work announced here sometime in 2024, hopefully. Thanks for reading! I hope this short post helps you appreciate the more mathematical side of animals and their inherent diversity in a new light on your next visit to the Zoo! 

 Joe Mendelson, PhD 
Director of Research  


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