Arabian babbler (Argya squamiceps) caterpillar preparing

Earlier this year, in a piece for the anthropology magazine Sapiens, Abby Desmond and I introduced the idea of fixels. We defined a fixel as something used by an animal in a similar way to a tool, but which is not held or moved around—think a fence post that a cow might use to scratch itself, or a tree branch against which a monkey pounds a handheld nut.

(The etymology of our new term is straightforward, emphasising the fixed nature of the object with the suffix -el or -le commonly used to denote a tool of some sort, as seen in handle, thimble and girdle.)

We decided we needed a word for fixels because people studying animal tool use have often downplayed or overlooked non-held objects. They might be called proto-tools, as if they were a precursor to full fledged technologies, or borderline tools, as if their users were standing outside of the tool party, peering over the fence. The implication is that using fixed items is somehow lesser behaviour requiring lesser cognitive effort, not ‘true’ tool use. The poor fixel users are shunted aside as false, dull, wannabe tool users that never made it to the big time.

But fixels don’t necessarily take less effort or less planning or less coordination to use. The gulls I’ve watched hovering as they repeatedly drop clams on the rocks of the Thames shore weren’t behaving randomly, or lazily. They were pursuing the goal of opening up a tasty meal, and they did so by targeting the same spots day after day. The shoreline rocks that cracked the clams weren’t held by the gulls at any point, but they were enabling the gulls to break into their prey just as surely and deliberately as any chimpanzee bashing away with a handheld stone at a forest nut.

Today’s post, then, is a closer look at fixel use. Our hero is the Arabian babbler, a fairly large perching bird from southwestern Asia. And it’s got a caterpillar problem.

Aye, there’s the rub

Guiding us in this bird v food showdown are Yitzchak Ben Mocha and his colleagues in Germany and Israel. Earlier this year they published a report on how wild Arabian babblers in the Israeli Shezaf Nature Reserve deal with prey. Over more than a decade of watching and filming these wild birds, beginning in 2010 and continuing up to December 2023, Ben Mocha concentrated on 20 birds that processed food rather than just gobbling it straight up.

Those 20 birds came from 5 different groups, and ranged in age from youngsters of a few months to a veteran aged 10 years. What united them all in this study was the way that they used the ground as an ally in their food preparation.

You see, there are two types of caterpillar. One has hairs, the other does not. For each, the babblers may want to remove the potentially toxic internal organs before gulping down the external, hollowed-out body, or at least scraping away those irritating hairs. This is where fixel use comes in, and it’s clever.

If a bird found some food that didn’t need processing at all, it was usually just eaten where it was found. But for a hairless caterpillar (say, Hyles livornica), the babblers preferred to carry it to a hard, smooth ground surface. There, they smashed it repeatedly against the substrate until it broke apart and was ready to eat. But if the caterpillar had long hairs or setae (e.g., Casama innotata), then the birds carried their meal to a rough sandy area. At that point there was some pounding still, but also a lot more rubbing, dragging the caterpillar back and forth across the sand in a very rigorous hair removal process.

This video accompanying the paper by Ben Mocha et al. has a series of babbler bouts showing the different food processing behaviours. Note that the last activity (Video S5) is a particularly comprehensive takedown of a hairless caterpillar via pounding. Caterpillar fans be warned:

Fixation

The Arabian babbler activities recorded in Israel are not those of mindless eating machines grabbing what they can and gulping it down. Nor are we seeing an indiscriminate reflex that ends with every caterpillar smashed to a pulp no matter what species it is. We are seeing thoughtful decisions that respond not only to the type of prey, but to the fixel needed to best process it. The whole babbler decision and action process is summed up by the researchers in this neat flowchart:

Of course, creating a new category like fixels also raises questions. Most obviously, what exactly are the fixels in cases like those of the babblers? Tools are easier, because they are separate from the world: when a Galapagos woodpecker finch probes a tree hole using a cactus spine it’s clear that the spine is the tool. But how do we delineate a fixel from general background noise?

One option is to just say the the Earth itself is the fixel. That’s true, but it’s not helpful. Rather the fixel is best thought of as the object that has the attention of the fixel user. For the babblers, that’s relatively small patch of ground that has the right texture for the task at hand/beak: either a clear hard surface or a sandy zone. In the video above you can see what happens when a hairy caterpillar accidentally falls outside of the rough sand that the babbler prefers, as the bird recognises the problem and carefully nudges the prey back into the rub zone. This is not just semantics either. By paying attention to what animals pay attention to, we can begin to look at questions of re-use (are some sandy spots better than others, and worth flying further to use?) and spatial patterning (do hairless caterpillars get eaten more when they happen to be near an area of open hard ground?).

Once we accept fixel use as different and related but not inferior to tool use, we can look for additional links to other behaviour. In the case of the Arabian babblers, Yitzchak Ben Mocha’s work is already helping us start making connections. For example, in 2019 he co-authored a report with Simone Pika on the same population of babblers as in the caterpillar study. It turns out these birds also regularly present objects to other specific babblers by holding them up in their beak, with presented items ranging from insect prey to sticks, seeds, flowers and eggshells.

Here are some examples from that 2019 study:

Ben Mocha and Pika carefully reviewed several hypotheses that might explain this behaviour. It was done by both male and female birds (although mainly males) and wasn’t restricted by social rank, ruling out sex or rank based explanations. The objects were clearly an intentional signal to another specific babbler, which the scientists deduced by watching how the recipient of the message responded and where the attention of both birds was directed. And the outcome of the object display was consistent: the most common response from the watching bird was to move toward somewhere hidden from the sight of other nearby babblers, and get ready for sex.

Birds that simply held an object in their beak, for example when trying to feed another bird, didn’t get the same response. The held objects were common in the area and typically only one was displayed, so it wasn’t a matter of doing something difficult or unusual to gain attention. It also didn’t matter whether the object was edible or not: this wasn’t a case of offering food for sex. The researchers concluded that the logical explanation was that it was a deliberate and controlled signal from one bird to another, using cues that didn’t draw attention from other nearby birds. Specifically, holding up an object and giving a subtle head shake was bird-talk for ‘how about we find a secluded spot and mate?’.

Together, the fixel study and the one on object-based signalling show Arabian babblers to be highly aware of their environment and the various opportunities it affords. The signal sticks and other objects are in fact tools, used for social rather than feeding purposes. The fact that the same animals use fixels as a solution for preparing prey doesn’t mean that they are incapable of ‘true’ tool use, in fact the opposite might be a better conclusion. Even more accurately, we could conclude that tool and fixel use are two approaches to the same problem of how to make use of external objects, and at different times different animals will make use of both, either or neither.

Ben Mocha et al. make a similar point in their 2024 paper, concluding that the flexibility and intention shown by these birds:

support the hypothesis that the distinction between proto-tool use and tool use is semantic rather than entailing cognitive difference.

It may be that our fixel term doesn’t catch on, even though something like it is sorely needed. Time will tell. Still, the next time you see a report of an animal showing borderline or proto-tool use, just remember these birds, and ask yourself what a writer might be thinking or implying by using those terms. If someone’s using them without a well-justified reason, particularly in a dismissive sense, then it may not just be the birds that are babbling.

Sources: Ben Mocha, Y., Frisoni, F., Keynan, O., & Griesser, M. (2024) Proto-tool use for food processing in wild Arabian babblers: matching processing methods, substrates and prey types. Animal Cognition 27:35. || Ben Mocha, Y., & Pika, S. (2019). Intentional Presentation of Objects in Cooperatively Breeding Arabian Babblers (Turdoides squamiceps). Frontiers in Ecology and Evolution 7:87.

Main image credit: Artemy Voikhansky; https://animalia.bio/arabian-babbler || Video credit: Ben Mocha et al. (2024) || Second image credit: Ben Mocha et al. (2024) Fig. 3 || Third image credit: Ben Mocha & Pika (2019) Fig. 1