Chemicals Reveal Foes

Biological Strategy

Carpenter ant

AskNature Team

Sense Chemicals (Odor, Taste, etc.) From the Environment

Chemicals are important for signaling and communication among living systems, either intentionally (such as when two living systems try to find one another) or unintentionally (such as when a plant emits a chemical signal that an herbivore can use to find a tasty bite). They are also important for other uses, such as navigating or finding sources for minerals. But chemical signals are often relatively weak and disperse when moving through water or gases. Therefore, detecting them requires specialized abilities, including a way to determine where they are coming from. A well known example of sensing chemicals can be seen in ants following a pheromone trail laid down by others in their colony to locate a quality and abundant food source.

Search this Function

Respond to Signals

To interact with its environment, a living system must not only sense a variety of signals, but also respond to them. To be energy- and material-efficient, those responses must be appropriate to the signal. This generally requires sensing thresholds to trigger an appropriate level of response (for example, hiding under a shrub versus running away to avoid a predator). Response strategies are tied to a specific signal and often have a response threshold, which determines how strong a signal must be to warrant expending energy to respond. One example is a plant that lives in arid regions in South Africa. Its seed capsules remain closed until rainfall triggers them to open to release the seeds. But the plant only responds to a second rainfall, thus protecting against releasing its seeds before there is enough sustained water for them to grow.

Search this Function

Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

Search this Living System

Carpenter ants identify intruders based on the scent of cuticular chemicals not present on nest-mates.

For colony-based social insects, distinguishing nest-mates from non-nest-mates is important for maintaining the evolutionary fitness of the colony. Colony members must help only nest-mates and ensure non-nest-mates aren’t allowed to threaten the resources of the colony. Carpenter ants were long thought to distinguish nest-mates from non-nest-mates by comparing the chemical signature of suspect ants with their own. However, the biochemical mechanism is actually much simpler than that. Carpenter ants secrete a variety of hydrocarbons on their cuticles. The hydrocarbons include many complexly branched molecules that can form an almost limitless number of unique structures. The specific hydrocarbon structures and relative abundances thereof are unique to each colony (due to shared diet) and act as a shared chemical fingerprint for nest-mates. Ants detect the hydrocarbon signature of other ants using their antennae and use it to determine if they are friends or foes. Instead of both checking for the presence of friendly-type hydrocarbons and checking for the presence of foreign-type hydrocarbons, the ants simply respond to chemical signatures that contain new elements not previously habituated to. In other words, they do not identify nest-mates; they only identify non-nest-mates. Since nest-mates consume the same specific diet and share food with each other, they develop the same cuticular hydrocarbon fingerprint.

In a simple form of learning, the antennal lobes become desensitized by near-constant exposure to nest-mate fingerprints and response to it is reduced. However, the reception of a signature that contains additional compounds not previously habituated to (e.g., that of a non-nest-mate) initiates the olfactory receptor response fully and causes aggresive behavior. This mechanism explains why insects with practically no hydrocarbon fingerprint (i.e., certain colony parasites and young workers) do not elicit a response in guard ants. To put it simply, carpenter ants don’t distinguish nest-mates by comparing fingerprint similarity, they do so instead by determining whether or not a fingerprint fits within the pattern of the habituated signature. The neurological basis is perhaps best understood by human analogy. When constantly exposed to an odor, one becomes accustomed to it and may not be able to detect its presence. Nonetheless, even after being habituated to that specific odor, it is possible to notice the presence of a new one.

AskNature