Animals and their behaviors are an endless source of fascination. Understanding the intricacies of the behavior of even the simplest of organisms is a monumental undertaking. When the view is expanded to include the interactions between species, a whole new dimension of complexity is added to the ethologists’ world. One species-to-species interaction that is worth a closer look is that between anteaters and ants. Ants are known to leave pheromone trails for other ants to follow when a food source has been located. Do anteaters take advantage of this chemical signaling to locate and prey upon these ants?
That animals communicate with one another has been well documented in many species. Honey bees perform a waggle dance to communicate the location of nectar-rich flowers, male side-blotched lizards use head bobs in male-male competitions and mating rituals, and ants produce powerful chemical messages signifying everything from danger to the location of food. Given the character of natural selection, it is no surprise that predators have evolved ways to take advantage of the communication signals of their prey. A classic example of how such illegitimate receivers can intercept their prey’s communication can be found in the case of the fringe-lipped bat and the tungara frog. The mating call of the male tungara frog consists of a whine which may or may not be followed by a chuck. Males who end their call with a chuck are much more likely to attract a mate (Ryan, 1985). Unfortunately for the frog, this same chuck which females find so appealing is recognized by an opportunistic predator, the fringe-lipped bat (Ryan, 1983). The bat zeros in on the calling frog, scooping it from the water and devouring it. Are ants also victims of this evolutionary catch-22?
Ants, found in the order Hymenoptera, family Formicidae, are exemplary eusocial insects. They live in highly regimented, caste-containing communities in which helping behavior, including self-sacrifice, is integral. In order to effect this high level of colonial organization, ants have evolved a sophisticated system of communication via an extensive assemblage of pheromone-producing exocrine glands. Each pheromone signals a unique message depending upon the anatomical position of the gland from which it came. For example, in the leaf-cutting ant Acromyrmex subterraneus subterraneus, the venom gland in the gaster secretes trail pheromone (Do Nascimento et al., 1994). Crushing the head of a fire ant, Solenopsis invicta, will release an alarm pheromone from its mandibular gland, eliciting an immediate and vicious attack from the soldier caste. The foraging trail pheromone of the cryptobiotic ant, Prionopelta amabilis, is located in the basitarsal gland of its hind leg, which it drags across the ground to stimulate secretion (Hölldobler et al., 1992).
The subject of this proposed study is the leaf-cutter ant, Atta Cephalotes, which resides in the lowland tropical rain forests of Latin America. These ants build massive nest sites consisting of vast winding underground tunnels and chambers covered by a well-aerated roof which can be as large as 2700 square feet. Leaf-cutter society is divided into many castes including scouts, soldiers, and nurses. In leaf-cutters, as in all ant species, smell and taste are the most critical senses in communication: they can taste odors on their tongues and through odor receptors on their antennae (Hoyt, 1996).
The most amazing characteristic of leaf-cutter ants is their creation of and subsistence upon fungal gardens. A scout ant journeys from the nest in search of an appropriate source of leaves. Once located, she uses her serrated mandibles to deftly carve a circular portion of a leaf from its petiole. She then carries her treasure (which can weigh up to six times her body mass) back to the nest, secreting trail pheromone the entire way so that her fellow workers can easily locate the leaf source she has discovered. Back at the nest, the newly cut leaf is placed deep in the ground in a dark but well aerated chamber where it is cut into smaller pieces, fertilized with ant feces, rolled into pellets and “fed” to subterranean fungal mycelia. The fungus is carefully tended and provides the main source of food for the entire colony, and most especially for the larval queen to whom it is hand fed by nurse ants. Leaf cutters, then, are not herbivores since they do not eat the leaves they cut, but instead are true fungivores, subsisting entirely on the fungi they cultivate. The leaves, however, are crucial to the success of their fungal gardens, and locating a valuable leaf source is vital. It is equally vital that a chemical trail be laid to lead other workers to the leaf source so that the area may be efficiently stripped. The distance between leaf source and nest may be as far as several hundred feet and the pheromone odor can last 6-12 days (Hoyt, 1996). Considering these facts, it seems entirely possible that ant signals could be intercepted and exploited by illegitimate receivers such as the ant predator Tamandua mexicana, the banded anteater.
The banded anteater, or tamandua, inhabits much of Latin America, spending about half its time in trees of the tropical forest. The skull anatomy of these amazing mammals is highly specialized for ant and termite capture: they have a long snout with a pencil-sized mouth opening through which they can extend a rounded tongue coated with sticky saliva some 16 inches into a termite mound or ant nest. The characteristic of tamanduas most interesting to this study, however, is its excellent sense of smell, which it uses to detect prey (Dickman, 1984). This experiment will attempt to discern which scent or combination of scents produced by ants and their activities are exploited by tamanduas.
The experiment will be done in a laboratory setting in order to control the variables more carefully. If this trial proves conclusive in any significant way, it should also be tested in natural conditions.
Three specially-treated wooden boards will be required for the experiment:
1. One board will be trod upon by ants with no leaves available.A room with a floor built of many wooden boards will be constructed, of which most will be sterile controls. The three specially-treated boards will be placed within this construct, each one well-distanced from the other two.
2. Leaves will be placed on a second board, and ants freely allowed to traverse the board, retrieve the leaves, and take them back to their nest.
3. The third board will have been dabbed with fungal garden samples, but no ants will have been allowed access.
The first board will serve as a test of whether the smell of non-pheromone-producing ants, or ants producing non-trail pheromones, is an attractant to anteaters. The assumption is that the ants won’t produce trail pheromones in the absence of leaves, since they have no message to communicate. Because the ants traversing the second board locate a leaf source, the assumption is that they will produce trail pheromones communicating the location of the leaf source to their fellow ants. With the third board it can be determined if the anteaters are attracted to the fungal gardens rather than other ant-associated scents.
Next, one anteater at a time will be let loose in the room. Video cameras positioned on the ceiling will be used to observe and record the anteater’s movements. A time budget analysis will be formulated based on the following calculation: time spent with snout positioned over a given board divided by total time spent in room.
The null hypothesis of this experiment will be that anteaters show no particular interest in any one board. A number of alternative hypotheses are possible depending upon which specially treated boards spark the anteaters’ interest. The expected result, if indeed the anteaters are illegitimate receivers of trail pheromones, is that the anteaters will use the ants’ communication to track the ants, and thus will show a marked preference for the board containing trail pheromone (board 2). Alternatively, if the anteaters only show a marked preference for fungal gardens (board 3), it can be concluded that while these predators can sense ant nests, they are not using ants’ communication to further assist them in their foraging. In addition to these main hypotheses, several other results are possible depending on which combination of boards interest the anteaters.
It seems that the most logical and useful scent for anteaters to exploit would be trail pheromones, simply because they are widely spread between ant nests and leaf source. It has already been established that several other insects such as beetles and other ants can exploit leaf-cutter trails for their own purposes (Hoyt, 1996).
If the hypothesis that ant predators do follow pheromone trails proves valid, we can then look into which came first, the chicken or the egg? In other words, did ants develop pheromone-signaling first, and only later did ant predators evolve a mutation to take advantage of this communication, or was the ability to detect pheromone there all the time, only becoming useful later, when ants evolved pheromone signaling? This process could be studied similarly to the way the evolution of tungara frog chucks was studied by Ryan (1985). Ancestral anteater species from different geographical areas who are known to track ants through cues other than scent would be tested in the room. If they were found to show a preference for the trail pheromone-treated board, this would be evidence in support of an early mutation for pheromone detection.
When studying the interactions between species it is important to keep in mind that evolution is a dynamic process; the status quo is temporary. Just as anteaters have evolved techniques for efficient ant hunting, so have ants evolved complex and amazing anti-predation mechanisms. Many ants eat plants which contain toxins which do not bother the ants themselves but which do make them less palatable to predators. Also many ant species have poison glands which they can use alone or en masse to deliver painful stings to their attackers. Leaf-cutters, too, have evolved a defense strategy. As a scout carries her leaf prize back to the nest she is particularly vulnerable to attack, loaded down with a heavy burden and out in the open. To better her chances, a very small worker ant will ride shotgun at the top of the leaf, warning her of potential dangers so she may seek shelter.
One of the most basic principles of scientific methodology is to control all variables except the one being studied, in order to more precisely gauge its effect. While it is extremely useful to study single species in a laboratory setting, it is equally important to observe an animal in its natural habitat where all the selective pressures of its environment are in force. Communication within and between species, whether cooperative, competitive, or deadly, is a fascinating and worthwhile subject. In spite of the many practical and theoretical difficulties, studying the evolution of multiple species interaction is essential if we wish to glimpse the complexity of nature.
Dickman, Christopher R., 1984. Anteaters. The Encyclopedia of Mammals, ed. David Macdonald. Facts On File, New York.
Do Nascimento, Ruth R., Morgan, E. D., Moreira, Denise D. O., and Della Lucia, Terezinha M. C. 1994. Trail pheromone of leaf-cutting ant Acromyrmex subterraneus subterraneus (Forel). Journal of Chemical Ecology 20: 1719-1724.
Hölldobler, B., Obermayer, M., Wilson, E. O. 1992. Communication in the primitive cryptobiotic ant Prionopelta amabilis (Hymenoptera: Formicidae). Journal of Comparative Physiology A 170: 9-16.
Hoyt, Erich 1996. The Earth Dwellers. Simon & Schuster, New York. 319 pp.
Ryan, M. J. 1983. Sexual selection and communication in a neotropical frog, Physalaemus pustulosus. Evolution 37: 261-272. Cited in Alcock (1993).
Ryan, M. J. 1985. The Tungara Frog. University of Chicago Press, Chicago. 230 pp. Cited in Alcock (1993).