ABSTRACT
In order to insure the safety of the colony, termites use a vast array of strategies to defend their nests from threats. These threats include predators, resource competitors and natural events that could damage the colony.
It has been found that chemical communication plays a vital role in organizing and initializing alarm-defense behaviors in several species of termite. Chemicals such as semiochemicals and pheromones help the colony communicate alarm and initiate defense behaviors throughout the nest. These chemical cues can originate from the termites themselves or they can come from the potential threat. Termites will actually vary the number of soldiers who respond to an alarm depending on the semiochemicals secreted by the different species of predatory ants. This type of species-specific response allows the colony to deal with a threat without overreacting and leaving other areas of the nest unprotected. Soldiers can also be seen "escorting" foraging workers as they gather resources. This behavior allows the termites to forage further from the nest and also keep vital foraging tubes open longer in the face of attack by ants. The primary predators of termites are ants. Ants both raid the termite nests for food and compete with termites for resources. This situation has lead to many termite defense behaviors being aimed specifically at dealing with ant aggression.
Termites also have a variety of physiological defenses at their disposal. The soldiers of one species have evolved blunt heads which they use to plug access holes to the nest. Another species' soldiers have a nozzle shaped head which can spray a sticky fluid which incapacitates the threat. Some of these physiological adaptations are aimed not at predatory threats, but natural disasters such as flooding. It was discovered that, when faced with a flood situation, the termites do not even attempt to escape. They simply enter a quiescent state until the water is absorbed into the ground. These adaptations and behavioral patterns are what allow termites to defend their colonies from many different threats.
INTRODUCTION
Defensive behaviors in termites have evolved over time
to insure the survival of the colony in the face of several types of threat.
I will be focusing on defense behavior relating to threats that fall under
the categories of Predator Threats, Resource Competitors, and Natural Events.
Predatory threats to the colony consist (for the purposes of this paper)
mostly of attack by ants, although many other organisms rely upon the termite
as a food source. It is largely the responsibility of the soldier
caste to deal with predatory raids by ants, but even workers have a crucial
role in defending the colony from attack.
In order for a colony to thrive and grow, it requires
a continuous supply of raw materials and food. Foraging parties are
constantly scouting for new sources of raw materials for the colony to
use. To ensure a constant supply of resources, termites have evolved
a series of behaviors to minimize the threat of other competition for the
limited resources available in a given area. These resource competitors
are a major threat to the continued survival of the colony.
A less proactive but equally important form of defense
is that of response to natural events. Natural events like floods can potentially
be just as threatening to the safety of the colony as any ant attack.
Because of this, many termite species have evolved physiological adaptations
that allow them to survive if the nest floods as a result of heavy rains.
These three categories are the primary focus of this paper.
DISCUSSION
Natural Events:
Cataclysmic events such as floods have the potential to completely destroy an entire colony in a matter of minutes. Because of this, termites have evolved physiological adaptations that allow the colony survive events such as floods. These physiological adaptations that are not related to individual job function, i.e. forager, soldier, queen. While many physiological adaptations can be found in individual "castes", some adaptations can be found in every termite in the colony. Eastern subterranean termites, Reticulitermes flavipes, when faced with a flood of the nest, make no move escape to a higher location. They instead entered a quiescent state and simply wait for the dirt to soak up the water. At that point they rouse themselves and begin to repair the nest (Forschler and Henderson 1995). This experiment was performed with two other species of termite, R. virginicus, and C. formosanus Shiraki. Although each species exhibited the "flood quiescence" behavior, they were not all able to remain submerged for the same amount of time. R. flavipes was able remain in water for 19.6 hours. R. virginicus stayed submerged for 13.9 hours, and C. formosanus Shiraki for 11.1 hours. These results show that while this adaptation arose early in termite evolution, each species shows a different manifestation of the behavior based on its particular "need" for the defense. Specifically, it makes sense that C. formosanus Shiraki would have a lower fatality time than the others because as a species, C. formosanus Shiraki prefers to build above-ground nests so that it can more easily attach living trees (Forschler and Henderson 1995).
Resource Competition:
Resource foraging is the most important activity that the colony participates in. Behavioral adaptations that ensure the steady flow of food and other important materials are a necessity for the colony to survive. Soldier escort in foraging expeditions allows the foragers to move farther from the nest in search of resources. The escort protects the foragers from attack from ants. This can be seen in the manner in which the actual foraging column itself is organized. Foraging columns are essentially made up of three layers. The outermost layer is made up of soldiers whose job it is to raise the alarm and defend the column should ants attack. The second layer is made up of foragers moving from the nest to the foraging site. Logically, the most protected innermost part of the column is made up of foragers with food returning to the nest (Muira and Matsumoto 1998). Another benefit of soldier escort is protection of the foraging tunnel itself. Foraging groups with a significant soldier escort (20% of the foraging party) were able to defend a foraging tunnel from ant attack for three times as long as parties with minimal defense (5% soldier). This equates to three times the resources that were recovered in that time interval (Cornelius and Grace 1997). It was also found that the percentage of soldiers in the foraging group was as high as 100% when the group first leaves the nest. It then drops down to approximately 20% while the resources are brought into the nest. Then finally, it jumps back up to approximately 80% as the last foragers return. This makes sense because the soldiers, not the workers, are the first out of the nest to look for threats (Cornelius and Grace 1995), and the last back in to ensure the safety of the last foragers (Miura and Matsumoto 1998).
Predatory Defense:
The third and most interesting aspect of termite defense is that of predation defenses. This area of behavior is almost exclusively reserved for the soldiers of the colony. Soldiers are a morphologically distinct caste that are specialized to defend the colony from attack by enemies (Cornelius and Grace 1997).The exact method of defense that the soldiers use varies from species to species, but each is adapted to the environment and predators that the colony is exposed to. The specialized defensive traits of termite soldiers are thought to have evolved largely in response to attacks by ants (Traniello 1981, Deligne et al. 1981, Mill 1983).
A newly discovered South American species known as Cryptotermes abruptus exhibits a more passive but highly effective strategy known as phragmosis. The soldiers of C. abruptus have blunted heads that they use to plug entrances to the nest (Scheffrahn et al 1998). Nasute termite soldiers have nozzle-shaped heads which they use to squirt a sticky fluid that immobilizes the threat to the colony. Among the most interesting of these soldier defense behaviors is that of Globitermes sulphureus. Sometimes referred to as "a walking chemical bomb" (Oster and Wilson 1978), G. sulphureus soldiers defend their colonies by intentionally rupturing their bodies to secrete a sticky fluid that captures both the threat (ants) and the soldier itself. The soldier has large mandibles that it is very capable of using for defense without "exploding". In cases of extreme excitation, the soldier grabs the attacking ant in its mandibles and ruptures its integument thus immobilizing both ant and termite (Bordereau et. al. 1997).
However, the most effective method for termites to avoid ant predation is to construct physical barriers of soil, masticated wood, and salivary secretions (Deligne et al. 1981). In laboratory studies, it was found that when termites were confined to sand-filled containers where they could construct galleries in the sand and use these galleries as physical barriers to ant invasion, termite workers were able to survive in containers invaded by ants even in the absence of soldiers (Cornelius and Grace 1997).
Pheromones and semiochemicals also play a major role in defense organization. Workers of the species Skatitermes watti Coaton are capable of producing a defensive secretion in response to predatory threats that evokes an aggression response from other workers in the colony (Kistner 1975). C. formosanus Shiraki was found to have the ability to distinguish between several different species of ant based upon the semiochemicals secreted by them. This allowed the soldiers to determine the actual threat to the colony based on the specific ant species (Cornelius 1994). In the experiment, termite soldiers were found to be almost five times more likely to exhibit defense behaviors when exposed to the semiochemicals of the aggressive ant species Pheidole megacephala than other less aggressive species of ant. The advantage of chemical species identification is that some species of ants and other termite species actually help termite colonies by providing nitrogen-rich nutrients and improving the defense of the nest (Jaffe et al. 1995). Without species identification, the soldiers of the colony would attack any foreign insect to enter the nest. This would result in the loss of the above mentioned advantages of having other non-aggressive species within the nest.
This sensitivity to semiochemicals is also seen in feeding and tunneling behavior in foragers (Corneleus 1994). Sand treated with ant semiochemicals was avoided by termite workers when they were constructing new galleries (Cornelius et al. 1995). This avoidance behavior in the workers kept them from inadvertently tunneling into a neighboring ant colony accidentally.
CONCLUSION
Defensive behavioral responses in termites have evolved
to protect the colony from any threats to its survival. These threats
can be separated into the major categories of natural events, such as a
flood, predatory attack from raiding ants, and resource competition from
other organisms. The defensive responses that the termites have at their
disposal are as varied as the threats themselves. These responses
include physiological adaptations as well as behavioral modifications.
From flood-initiated quiescence to chemical identification of aggressive
ant species, termites have evolved defenses that will ensure the safety
of the colony regardless of the threat to the nest.
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