Ryohei Yamaoka

Professor, Kyoto Institute of Technology

Ants: Creatures That Rule the Earth

Suppose adventurers from an unknown planet came to Earth and observed the surface from their spaceship; they counted the number of animals that were larger than 1 mm in body length. The results: the total reached 10 raised to the 18th power, that is, 100 million times 10 billion. Ants turned out to be predominant over others, totaling 100 million times 100 million, or 1% of the total. On the other hand, human beings merely amounted to 100 million times 60. Judging from these numbers, Earth is a planet of ants.

Personally, I also think that ants are the predominant animals on Earth, and hence have great interest in the reasons why they have come to assume such a predominant position. One of the contributing factors may be their consolidated community, which is based on highly advanced chemical communications.

There are as many as 12,000 species of ants on Earth, about 260 species of which have been identified in Japan. They exist across the globe, from the tropics around the equator to the tundra. Moreover, their ecology varies substantially among species: agricultural ants that cultivate mushrooms; slave-making ants that exploit slaves; harvester ants that collect seeds; guest ants; thief ants, and so forth. Every behavior that is inherent to our community can be found in the community of ants.

Ants branched off from the family of bees in the course of their evolution. The major difference between these two species of insects is the fact that: bees are diurnal, and collect food from a wide range of areas, making use of their wings and advanced compound eyes; while ants, which have lost their wings, must crawl on the ground. In exchange for this degeneration, however, ants have obtained chemical sensors on their antennas that can sense various chemical signals ? a feature that enables them to work day and night in collecting food.

Why Do Ants Advance in Line to Collect Food?

The compound eyes of ants are not very developed, but they can be active even at night and in the darkness of their nests. Ants work, recognizing various chemical signals with their antennas, which can sense volatile chemical substances (odors) in the air, as well as nonvolatile chemical substances (contact chemicals) on contact. Contact chemicals may be similar to ¡Ètastes¡É we sense with our tongues. Observing ants crawling on the ground, we notice that they are constantly making contact with the ground and anything they come across, using their antennas. Fig. 1 (The World of Ants Seen Through Their Antennas) shows the typical behaviors of ants induced by contact chemicals.

The community of ants can be referred to as a ¡Èsuper organism¡É that is made up of numerous ants living in the same nest (colony). Specifically, a queen ant is equivalent to the reproductive cell of other animals, and a worker ant is a piece of somatic cell. Work sharing among worker ants can be compared to the division of somatic cells.

Ants belonging to the same colony communicate through the medium of a variety of ¡Èpheromones¡É in and outside their nest. Pheromones can be defined as ¡Èchemical substances that a given individual secretes outside the body to induce specific behaviors or physiological changes among other individuals belonging to the same species.¡É Ants have quite a few exocrine glands that outnumber those of any other animal on Earth. Fig. 2 shows part of those exocrine glands. A long trail of ants is a familiar sight, but this is all attributable to pheromones. That is, ants that have found food leave a trail marking on the ground on their way back to the nest. These pheromones are relatively volatile and last merely 30 minutes or so. Likewise, when they attack a large live insect, they spray it with formic acid and alarm pheromones to summon reinforcements.

Thin alarm pheromones have attracting action, while thick ones have repellent action. These pheromones are also volatile, however, effective for individuals apart from one another. By contrast, identification and orientation pheromones, as described later, are both nonvolatile contact chemicals that can be sensed only through direct contact with the antennas of ants.

Using Pheromones to Identify Mates

The surface of the bodies of all insects are covered with oils collectively called ¡Ècuticle wax,¡É the main ingredients of which are nonvolatile higher hydrocarbons of high molecular weight. Each species has its own composition of hydrocarbons. An ant takes the offensive against other ants or insects upon touching them with its antennas because it senses the differences in the composition of the hydrocarbons.

Among others, the most mysterious ecology of ants as social insects is that ants belonging to the same species but different colonies fight with one another. This habit was attributed to odors unique to each colony (colony odors), though their identity remained unraveled until recently. Through a series of our studies, we were able to shed some light on these odors.

First, we compared the composition of hydrocarbons among the same species of ants belonging to different colonies. Being the same species, the composition turned out to be consistent. But the proportion of each component varied substantially with colonies (See Fig. 3), while they were almost the same among individuals of the same colony. In addition, it turned out that the proportion of each component, a feature that is unique to each colony, was not hereditary determined and that the biosynthetic proportion of each component varied even among individuals of the same colony. We also found that the proportion of each hydrocarbon was unified among individuals of the same colony by means of at least the following three methods:

1. A dense aggregation induced by a queen ant that diffuses the aggregation pheromone augments direct contact among individuals, thereby mixing and homogenizing their external secretion.

2. Ants are constantly grooming their body surface and other individuals. As a result, ingested external secretion is stored in an organ called ¡Èrear pharyngeal glands¡É in the head; the stored external secretion is then homogenized and applied to themselves or other individuals again through further grooming (See Fig. 4).

3. The components stored in rear pharyngeal glands are also passed between individuals through the exchange of nutrients ? a common practice among ants (or one of the characteristics of social insects).

Based on these results, we found that the identity of colony odors lies in the difference in the proportion of each hydrocarbon contained in external secretion ? a feature that is unique to each colony.

The Strategy of Insects Living Symbiotically with Ants

We studied the strategy of symbiotic insects living together with ants in order to track down the method by which ants identify their mates from aliens (i.e., the same species of ants belonging to other colonies).

¥¨¥¤¥³¥¢¥Ö¥é¥Ð¥Á are parasitic insects that deposit their eggs on ¥Ê¥·¥Þ¥ë¥¢¥Ö¥é¥à¥·, which swarm around the roots of mugworts. And ¥«¥ï¥é¥È¥Ó¥¤¥í¥±¥¢¥ê live symbiotically with ¥¨¥¤¥³¥¢¥Ö¥é¥Ð¥Á, while protecting them. These bees approach aphids to deposit their eggs ? an attempt that is constantly hampered by the attack of ants. But their countermeasure against this attack is marvelous: taking advantage of unguarded moments, they jump onto the back of ants; the ants begin to rampage, but the bees start rubbing the abdomens of the ants, using a pair of their hind legs; the ants are calmed down, as the bees touch and lick the bodies and antennas of the ants, using their antennas and mouths; they jump off the ants after about 30 minutes; and they eventually bite off their wings to approach the ants, bending their abdomens inside. As a result of this series of behaviors, the ants stop attacking the bees. Instead, they begin to feed the bees mouth to mouth. This particular behavior is called ¡Èthe exchange of nutrients among mates,¡É and indicates that the ants have accepted the bees as their mates.

Fig. 5 shows changes in the proportion of each hydrocarbon contained in the external secretion of the bees in the course of the behaviors mentioned above. The secretion, which had contained only n-alkane before jumping onto the ants, became identical with that of the ants (even the proportion of each component) after the completion of the behaviors. In other words, the components of the external secretion of the ants can be easily transferred to the bees through close contact among their bodies and grooming for about 30 minutes. The bees establish ¡Èchemical mimicry¡É by obtaining the same components of the external secretion of the ants, thereby becoming their mates. It is indeed a skillful strategy that takes advantage of the ability of ants that are superior in recognizing information chemical substances.

The world ¡Èseen¡É through the antennas of ants must be a wonderful chemical world. We hope to see with our own eyes the world of ants, in which various molecules fly about, and to enjoy conversations with them, making full use of their pheromones.