Waste not, want not

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What do arboreal ants eat? This is not such an easy question to answer as one might think. Nitrogen, vital for building proteins, is typically in short supply in the tops of trees. Ants as a group are often viewed as scavengers, getting nitrogen from dead arthropods that they find in the environment. But dead arthropods in the canopy will tend to fall, or be blown, to the ground.

So where do arboreal ants get the nitrogen they need to build their colonies?  One important source is the trees themselves. Arboreal ants drink nitrogen-poor but extremely abundant plant sap by tending sap-sucking insects. In essence, the ants use the bugs as drinking straws, and some groups can be seen as herbivores in terms of the source of much of the nitrogen they acquire.

But are there other nitrogen-rich snacks in the canopy that ants rely on? The answer, at least for Cephalotes, would seem to be yes. Cephalotes just love to eat bird droppings. In fact, a nice fresh dropping will elicit stronger recruitment than any other food source. They also love leaves that have freshly deposited urine on them, from arboreal mammals. Although these sources of vertebrate waste most likely have less than ideal forms of nitrogen, Cephalotes have a gut packed full of microorganims that may well help is converting it to something useful.

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Cephalotes eduarduli (large) and Cephalotes maculatus (small) feeding on a large bird dropping in the cerrado canopy.
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Cephalotes persimilis feeding on bird droppings on a long blade of grass coming into contact with their home tree

The fun part of Cephalotes’ affinity for this unusual resource, however, is that it makes for a very convenient and highly effective “natural” bait. I have now switched exclusively to urine baits harvested from, er, a bipedal cerrado primate in my work with Cephalotes. Aside from being free and unlimited (as long as the primate(s) in question have a good water supply), it is more effective than traditional baits, like sardines. For those interested in behavior, like me, it is also a natural food resource for this group and therefore invaluable in studying natural foraging and recruitment behaviors (ants don’t tend to find many naturally occurring sources of fish protein on the canopy). Moreover, it makes for good photographs, by pulling a large number of ants from nearby nests and becoming invisible once the water has evaporated off. It turns out that this food is also highly attractive to many other groups of arboreal ants. This is intriguing from a  scientific standpoint, but also very useful for any myrmecologist interested in finding and photographing some nice arboreal ants. Something to keep in mind next time you are all in the field and nature calls.

Cephalotes depressus in a fun-for-all feeding frenzy
Cephalotes depressus in a fun-for-all feeding frenzy at a urine bait

9 thoughts on “Waste not, want not”

  1. James C. Trager

    Hi Scott:

    I seem to recall reading something once about Cephalotes eating pollen (found in their crops) caught on plant surfaces. I have observed them spending a lot of time “grooming” hairy Tillandsia plants, known to catch a lot of such small particles. But they could have been after urine or bird poo, I suppose.

  2. Hey Scott,

    I am a rove beetle systematics student and I am particularly interested in myrmecophilous staphylinids, and naturally I am fascinated by ants as well.

    I’m curious, have you ever seen myrmecophiles (especially beetles) with these arboreal ants? I would be interested to know whether they have been able to penetrate this niche as well? Especially with these urine traps, I wonder if you ever see guests running with them or preying on the foraging columns?

    Hope to hear.

  3. James, yes you are quite right. Pollen in another unusual (albeit slightly less entertaining) food resource taken by Cephalotes. The reference is as follows: Urbani and De Andrade. Pollen Eating, Storing, and Spitting by Ants. Naturwissenschaften (1997) vol. 84 (6) pp. 256-258. The gut symbionts might help with the pollen too.

    In my experience, the pollen collecting behavior is typically done by solitary foragers. As you said, they move over the surface of a leaf slowly scrapping or grooming the surface. I have never seen them harvest pollen directly from flowers, even when open flowers are right next to the leaves they are foraging on. In contrast, I only ever saw large numbers of workers on leaves with a distinct urine odor (dropping are obviously much easier to spot). With that said, recruitment might occur if a leaf is found with a large pollen dump on it (say, a flower hitting a leaf in heavy wind), I just never saw it.

  4. Taro, that’s an interesting question. Unfortunately, I haven’t seen any staphylinid guests inside nest cavities (I have opened a good number of nest cavities from over 20 Cephalotes species) or running with foragers. I hope that I would spot them too, as I am quite familiar with staphylinid guests from my army ant work. In fact, I got my first opportunity to work with army ants by helping Carl Rettenmeyer with his guest work (which I am sure you are familiar with). If I ever find a Cephalotes staphylinid guest, I’ll be sure to let you know!

  5. James,
    Sure, the salt is no doubt a general attractant to arboreal ants. The water content (before it evaporates) is probably a big deal too in the cerrado dry season. For me, though, the big part of this story is the relationship between this usual food and the nitrogen-fixing symbionts in Cephalotes and (possibly) others. We will be seeing more about this soon in the literature.

  6. James C. Trager

    To Taro:

    There are arboreal ant crickets, genus Myrmecophila, in Asia, but these are in more humid environs than the cerrado. And here’s a funny little report (with several notable typos and misnomers in the insect taxonomy) from the US National Park Service, that mentions Myrmecophila in cucujid burrows in sequoia trees!: http://www.nps.gov/history/history/online_books/science/12/chap6.htm

    Also, I found this http://www.cercyon.eu/Publications_PDF/BilyEtAl_BuprestidMyrmecophily_abstract.pdf.

    On the other hand, I recently broke open a large batch of hollow stems in Ecuador in quest of ants, and didn’t see any other artropods, except pseudococcids, with the ants.

  7. Thanks Scott and James for your replies.

    Yes, please keep your eyes open, I’d really be interested if beetles have penetrated this niche, since it must be safer for the ants to live in trees.

    As far as Myrmecophila go, there seems to be some variation in their integrative status. I’ve seen some people keep them alive without the ants and so they must be able to wander from nests to disperse.

    The myrmecophilous buprestid sure seems interesting, but I’m gong to keep my doubts until this behavior is observed more times.

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