American alligator (Alligator mississippiensis) stick balancing

Tools are just objects, until they’re directly held and used by someone or something. Without that distinction about holding an object, we could end up classing not only the sailor’s sextant as a tool, but also the sun, stars and planets that are used for navigation via the sextant. And for practical purposes, it matters that the object is being held for a reason, that it has a purpose.

Today we’ll be wandering over to that line between object and tool, and poking around a bit. We’re looking at research from the past few years that asks the question: when an alligator or crocodile balances twigs and sticks on its snout, is that an accident, or a deliberate lure for unwary birds? Tool, or object?

On the surface

We begin near Chennai, India, at the Madras Crocodile Bank. In January 2007, Vladimir Dinets was studying mugger crocodile (Crocodylus palustris) behaviour in the large ponds at this research station. The station also housed an egret rookery, and during their breeding season these wading birds would regularly collect plant material for their nests from around the ponds. I’ll let Vladimir take up the tale, in which he:

repeatedly observed large muggers as they lay in shallow water along the edge of the pond with small sticks or twigs positioned across their snouts. The crocodiles remained perfectly still for hours, and if they did move to change position, they did it in such a way that the sticks remained balanced on their snouts. On one occasion, an intermediate egret (E. intermedia) approached one of the crocodiles and stretched its neck towards the stick; the crocodile lunged at the bird, but narrowly missed it.

The image at the start of this post is one of those mugger crocodiles. Dinets never saw a crocodile actually catch a bird, and neither did anyone else at the crocodile park. Whether the animals were accidentally surfacing beneath the sticks, or the ‘stick-displaying’ had a further purpose, wasn’t clear.

Half a world away, at the St Augustine Alligator Farm Zoological Park in Florida, USA, J.C. Brueggen and J.D. Brueggen had their own questions. They had seen American alligators (Alligator mississippiensis) floating in the farm ponds, also with sticks balanced across their snouts. They too had egrets wandering around collecting nest materials, and even had a record of an alligator snapping up one of those egrets after it wandered too close.

From the 2013 report co-authored by Dinets and the Brueggens, here’s what that looked like:

While these few observations were enough to hypothesise a link between stick balancing and egret snapping, it wasn’t clear whether that was really the case, at either site. So Dinets went to Louisiana, where he could systematically record alligator behaviour at places with local nesting birds, along with nearby bird-free areas.

The results of that work convinced Dinets and his co-authors that they were seeing a real phenomenon. Over the course of a year, he saw alligators displaying sticks 11 times, and only during the March-June breeding season of the local wading birds. Alligators balanced sticks at sites with and without birds, but more often at the nesting sites. However, none of the alligators actually captured a wading bird using those stick lures.

This would be among the first reports, anywhere, of reptiles using tools (not just crocodilians, reptiles in general). Nevertheless, their 11 systematic observations were enough for Dinets and his team to start speculating on how these animals were coordinating their stick-based hunting, noting:

It is also unknown to what extent stick displaying (and possibly other forms of bird baiting) by crocodilians is produced by individual insights, cultural transmission and/or previously evolved instinct.

These are big questions, even for very well known animal tool use. But let’s pause here for a moment and gather our thoughts. We have a claim that alligators (and related crocodiles) deliberately collect or surface beneath twigs as a means to trick nesting birds into coming close enough to get snatched. But even after a year of observation, the number of times that the stick balancing has gone from object interaction to tool use—meaning that the sticks are definitely employed to achieve a feathery meal—is extremely low. Is this tool use?

Going deeper

One great thing about global science is that if you’re asking a question, it’s likely someone else is too. In this case the someone else is actually two someones: Adam Rosenblatt and Alyssa Johnson of the University of North Florida.

Rosenblatt and Johnson weren’t as convinced as Vladimir’s original team that they were seeing genuine tool use. As they politely put it:

The observations by Dinets et al. (2013) are anecdotal, the systematic observations are correlative, and the behavior has never been documented leading to successful predation of birds in the wild.

Science doesn’t work just by doubting, though. So in 2018 Rosenblatt and Johnson set out to make their own observations. They went to four Florida ‘alligator-themed tourist attractions’, including the one at which the Brueggens had seen an alligator successfully grab an egret: St Augustine Alligator Farm. (Fun fact, that farm was founded in 1893 and is still going strong, showing just how keen people are to see apex predators do their thing.)

As in the earlier study, the new one compared sites with and without nesting birds, and focused on the breeding season when birds were actively gathering plant material. Rosenblatt and Johnson also went a step further, and observed alligator behaviour before and after the researchers added extra sticks into the ponds, in an attempt to prompt both alligator twig balancing and egret risk taking. They spent three days at each site.

Rosenblatt and Johnson found that naturally-occurring sticks were quite rare. The most at any site was 15, scattered across a pond measuring 9100 square metres, and none of the alligators balanced the natural twigs. Once extra sticks were added, alligators at all four sites did get some on their snouts, but again it was uncommon. For example, 240 of the 248 alligators at St Augustine didn’t balance sticks. And neither did 151 of the 153 alligators at Gatorland, the other site with nesting birds. No birds approached the alligators, and none were eaten.

Here’s one of the rare examples of the balancing behaviour at St Augustine—the white paint on the stick end shows it was added to the pond by the researchers:

Applying statistics to their data, Rosenblatt and Johnson found no difference in stick displays between the sites with and without nests. They did find that, when alligators balanced sticks, they did so for longer at the rookery sites, although because the birds didn’t go near the alligators the reason is unclear, as the researchers report:

We did repeatedly observe crocodilians exhibiting stick-displaying behavior, but our study design was unable to reveal the purpose of the behavior, if there is indeed a purpose at all.

Which brings us back to where we started. Tool use is not just object interaction: when I eat an apple using my hands the apple isn’t a tool, even if I manipulate it while munching. If I start cutting it up with a knife though, the knife is a tool—an object used for a goal. The same reasoning means that the apple could in fact be called a tool if, for some reason, instead of eating it I used it to strike something (an aggressive alligator maybe?), or to hold down the papers on my desk. The purpose matters.

So the jury is still out on crocodilian tool use, and reptilian tool use in general. The history of animal technology exploration tells us that it is usually premature or short-sighted to rule out any given class of animals as definitely not tool users, but in this instance we’re waiting for the definitive evidence. Maybe the alligators are signalling something to each other using those sticks, in which case they’d be social tools. Or maybe they’re using them as camouflage or as playthings, rather than bait, again making a case for tool use. Or maybe, in the equivalent of a falling leaf getting stuck in your hair, they just pop up out of the water under some sticks every now and then.

Sources: Dinets, V. et al. (2013) Crocodilians use tools for hunting. Ethology Ecology & Evolution 27: 74-78. || Rosenblatt, A. & A. Johnson (2019) An experimental test of crocodilian stick-displaying behavior. Ethology Ecology & Evolution 32: 218-226.

Main and second image credits: Dinets et al. (2013) || Third image credit: Rosenblatt & Johnson (2019)