An army ant that likes a bit of fruit

Army ants are big meat-eaters. In fact, it would be fair to consider them among the most important of the big predators in tropical regions.

Most of the 150+ species of New World army ants (Ecitoninae) seem to be specialist predators of other ants, using strength in numbers to overpower the defenses of prey colonies. A few species also prey on a range of large arthropods, including spiders and scorpions. It might come as a bit of surprise, then, that members of the army ant genus Labidus also like a bit of fruit in their diet, especially in the cerrado.

I had read a few accounts of Labidus species eating nuts and even used peanut butter to trap them in a collaborative project that I worked on while in Panama. But it wasn’t until I started working in cerrado that I saw a natrual example of this behavior.

The Pequi tree (Caryocar brasiliense) is a common cerrado species with seeds that are covered in bright orange, richly flavored, and oily flesh. During the fruiting season (usually October-January), the fruits drop and the fleshy seeds are collected by a variety of animals, including Labidus coecus.

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Labidus coecus workers simultaneously burying Pequi seeds and removing the flesh.

The ants simultaneously bury whole fruits (large green pods about the size of an orange) or individual seeds and harvest the flesh. The burying is achieved by removing small balls of soil from below the seed and piling it up the sides, ultimately forming a dome over the seed. It happens so quickly that you can actually see a seed sinking in real time. Burying behavior is quite common among Labidus species and it is presumably to protect large items while they are being harvested. These trees seem to be so attractive to L. coecus that they frequently move their nest (all army ants frequently shift nest sites) to the root structure below fruiting trees, taking up residence for a couple of months. Once the ants are done harvesting the  fruits and have moved on, the signs of their activity remain for some time, in the form seeds that are fully or partially buried with thousands of tiny balls of soil. The ants no doubt gain greatly from the oil-rich flesh, but what benefits or costs this has for the tree remains to be discovered.

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The telltale signs of Labidus activity. A Pequi fruit has been partially buried by small balls of soil. The seeds inside were buried deeper still, with the pulpy flesh stripped by the ants.

3 thoughts on “An army ant that likes a bit of fruit”

  1. James C. Trager

    It wouldn’t seem very good for the tree, in terms of dispersal, but some plants apparently “want” their seeds buried near the parent plant. I’m thinking here of the legume genus Astragalus and relatives.

    The pequi fruits seem to have the right size and nutrient content to be really attractive to the endangered cerrado parrot, the Hyacinth Macaw. (Damned pet trade — Grrr!!!)

  2. Labidus coecus may be good or bad in terms of Pequi seed survival, gemination, and dispersal. But more intriguing to me are the possible advantages of attracting a voracious arthropod predator into the root system. The benefits to the tree of soil turning, predation on root herbivores, and deposition of nitrogen-rich arthropod refuse (from the ‘normal’ army ant predator raids) could be huge.

  3. Maybe the trees can not self pollinate. At any rate it would benefit the ants to have several of these trees growing in a row.

    I’ve noticed Tetramorium species E doing the same thing with burying food but only when the ground is moist enough to do so.

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