Abstract

The first major portion of this paper considers the behavior of queen ants with regard to colony founding, fighting and foraging.  Obviously, queens do not generally forage or fight after the colony has been founded.  However, a queen often must fight and forage in the initial stages of colony founding.  Moreover, she must have enough stored energy to produce offspring and meet the energy demands of her own body until the first generation of workers has hatched and developed.  At which time these workers can assume the role of feeding and caring for the queen.  Queens of some species of ants may rely on cannibalism of other foundress queens, their eggs and their larva to survive and enhance there own reproductive fitness.  Still others may actively forage before entering and sealing the founding chamber.

Secondly, foraging behavior varies widely throughout the family Formicidae.  This paper examines the relationship between colony size and foraging behavior.  Specifically, the attributes of foraging by smaller colonies which are supposedly more primitive as compared to those colonies which are larger and considered more modern.  Furthermore, the communication systems and information exchanged between scouts and recruits is examined.  How do ants deal with prey items that are to large for individual retrieval to the nest? What mechanisms govern group retrieval?

Lastly, the fighting strategies of various ant species are discussed.  This has to do with raiding behavior and colony defense as well as resource defense.  These strategies vary between species.  In addition, individual specialization and colonial division of labor come into play.  Specialized chemicals serve as toxic weapons and communication pheromones.  To further understand conflict among ants it is crucial to consider territorial behavior.

Examining the topic in the above fashion allows us to study foraging throughout the colony life cycle of different ant species.  In doing so, we find that there are a few general theories and assumptions that can be made.  However, there are also many aspects of foraging behavior that differ greatly throughout the family Formicidae.

Introduction

In examining the foraging behavior of ants it is probably best to approach the task in chronological order.  That is to say we should probably begin with queen behavior and colony foundation.  Not surprisingly, both of these aspects of behavior are driven by energy and thus foraging is involved.

Colonies can be separated into two groups: single foundress (monogyny) and multiple foundress (polygyny).  Moreover, some queens are accompanied in nest founding by workers from the mother colony, while others do it on their own but later rely on their first offspring to help out (Tschinkel 209).  Monogyny is often associated with queens who found a colony without the help of workers from the mother colony (Tsuji 84).  Conversely, polygyny is often associated with queens that require the help of workers from the mother colony (Tsuji 84).  This begs the question: what factors favor each of these different strategies (Tsuji 84)? Which system is more efficient? Which system yields a larger number of offspring?

One could conclude that polygyny is less advantageous than monogyny for individual queens; due to the fact that the degree of genetic relatedness of a queen to a worker is less in colonies that exhibit polygyny (Tsuji 83).  Obviously, queens want their offspring to be as genetically similar to themselves as possible.  Another disadvantage to polygyny is that queens may be aggressive toward one another after the first workers have hatched and some if not all less dominant queens may be killed by the workers (Sommer 287).  This is the case in the ant Lasius niger (Sommer 287).  Furthermore, polygynous queens reproductive period is shorter than that of monogynous queens (Tsuji 84).  Monogynous queens produce more progeny over the course of their life span; which is also longer than that of polygynous queens (Tsuji 84).

On the other hand, polygyny must offer some advantage over monogyny or ants would not exhibit this behavior.  For instance: "Polygyny is widespread in ants...(Ito 83)." In fact: "Sometimes polygynous and monogynous biotypes coexist, even within the same species...  (Tsuji 84)."

As a general rule, in the family Formicidae, colonies that develop quickly in the early stages are better off than those which are slower to develop and grow in size (Tschinkel 210).  For example, in the fire ant, Solenopsis invicta, multiple queens often found a colony together (Tschinkel 209).  This may give such a colony an advantage because larger numbers of workers can be produced quickly (Tschinkel 209).  Also: "The major advantage that does accrue to multiple founding queens is that they produce larger initial broods and worker forces in less time than the solitary founding queens (Sommer 287)." This makes these colonies less susceptible to brood raiding by nearby colonies (Tschinkel 209).  Tschinkel also reports that size of workers is less important than numbers of workers; when it comes to resisting brood raiding and attempting to raid other broods (209-210).

Another advantage to polygyny maybe the ability to eat other queens that have died or been killed.  "Cannibal queens lost much less weight to produce a given number of minims than unfed control queens, and these minims were heavier (Tschinkel 209)." This simple experiment shows that cannibalism can help to insure greater reproductive success because cannibal queens can produce more progeny.

Still other ants such as those of the genus Harpegnathos exhibit a vastly different approach to nest building and colony foundation.  These ants are considered primitive because they form relatively small colonies, they have a relatively simple nest structure and exhibit little or no worker specialization (Crosland 18).  Indeed, these factors can be related to queen behavior.  As one might expect, nests of these ants were found to contain only one or in a few cases two queens (Crosland 19).  Not surprisingly, the queens varied little in morphology from the workers (Crosland 19).  These ants have not been studied in detail (Crosland 18).  Consequently, little is know about their methods of reproduction and colony foundation (Crosland 18).  However, a recent study has shown that ants of the genus and species Harpegnathos saltator do in fact exhibit monogyny (Liebig 133).  "Queens disperse by flight and found colonies alone (Liebig 133)." Strangely, workers are capable of producing offspring and do so in the event that the queen dies but they do not found colonies in the wild (Liebig 133).

Finally, some of the most primitive ants of the genus Pachycondyla have no distinct queens (Heinze 441).  Instead, reproduction is controlled by a social hierarchy (Heinze 441).  Many of the workers are fertile and lay eggs but all except the eggs of the most dominant individuals are eaten (Heinze 441, 442, 445).

Next, it is important to study the Foraging behavior of workers after the colony has been founded and established.  Clearly, foraging behavior is different for different species of ants.  However, there are some general trends which can be observed.

Foraging in ants is often the responsibility of scout ants.  These scouts locate prey and transport it back to the colony.  For those prey items that are too heavy or unwieldy for a single scout to transport, the scout goes back to the nest and recruits nest mates (other workers) to help (Robson 3).  Recruits are carried back to the food source or they follow a pheromone trail left by the scout.  Once enough workers have assembled, group transport occurs.

As is the case for many species of ants, "Workers of the ant Formica schaufussi forage as individuals and cooperate in groups to retrieve arthropod prey (Traniello 284)." This was also stated by Robson and Traniello: "Foragers of the ant Formica schaufussi recruit nestmates to large arthropod prey and cooperatively transport the prey to the nest (1)." Experimentally, Traniello and Beshers determined that: "...the average loading ratios of groups were greater than the loading ratios of single foragers (284)." In addition, group foraging reduced competition from other ants and helped to insure that large prey items made it back to the nest (Traniello 284).  These factors seem to corroborate and lead to the conclusion that group prey retrieval is more efficient than single ant prey retrieval (Traniello 284).

Army ants of genus Eciton are known to exhibit group foraging in specialized groups that overwhelm large prey; eventually breaking it down into manageable pieces which are then collectively transported back to the nest (Robson 3).

Not only does foraging behavior differ between different species of ants, it also differs with change in prey size (Detrain 537).  These responses by workers may be induced and governed by the behavior of a scout.  Specifically, the intensity of the pheromone trail it chooses to emit (Detrain 537).  For example, in the ant Pheidole pallidula: "...for a large prey item (a cockroach), strong recruitment was induced by ants that dashed back to the nest laying a more continuous chemical trail (Detrain 537)." Furthermore, "...the trail-laying behaviour to small food items was always weaker and more discontinuous, making recruitment slower (Detrain 544)." Hence, the scout determines the quality and size of a prey item and dictates the response of other workers accordingly.  The scout makes its decision based on, "...  a simple key criterion: the prey tractive resistance (Detrain)."

The previous scenario is understandable and commonly observed but it is based upon arthropod prey items.  What happens when a different kind of prey item is found? Let us examine the ant response to a different food source, such as a drop of sugar water (Detrain 545).  According to Detrain and Deneubourg, the size of the drop determines the degree of recruiting behavior by the scout (545).  More specifically, "The ability to fill its crop seems to be another simple criterion used by the forager to 'decide' whether to recruit (Detrain 545)."

Lastly, scout foraging behavior is directly related to colony survival and this means that the colony depends on the individual performance of its scouts (Herbers 141).

The reason I have chosen to include fighting behavior is that it can be closely associated with foraging behavior and it often occurs as a result of resource (food) defense.  For Brown and Gordon, "...results showed that all ants were equally likely to interact with non-nestmates (Adams 1127)." Therefore, it is advantageous for all workers to be able to fight and participate in resource defense.  Thus, all workers are capable of fighting.

When considering defense and fighting behavior in ants it is important to pay attention to what kind of enemy presents the threat.  Clearly, other ants of the same species and ants of different species often threaten the livelihood and food resources of the colony (Whitehouse 1207).  Although, many ant species are polymorphic and therefore have soldier castes, these soldiers don't seem to play as big a role as expected in wars with other colonies of ants (Whitehouse 1207).  Instead, the ant Atta laevigata, "...responded to conspecific and interspecific ant threats by recruiting mainly small ants (Whitehouse 1207)." This trend can be justified according to "the square law (Whitehouse 1207)." It states that "...if all individuals are equally vulnerable to attack, then individuals are killed at a rate proportional to the number of opponents...(Whitehouse 1207)." In basic terms, the larger group of ants wins the war and this seems to be the major factor in winning large scale ant to ant conflicts (Whitehouse 1207).  If this is true, it seems to eliminate the need for a soldier caste.  Therefore, the soldier caste must have a different function.  Naturally, ants have to ward off other threats like vertebrates who might disturb the nest.  This is where soldiers play a vital role (Whitehouse 1207).  Soldier ants are larger more developed worker ants which often have enlarged jaws and are specialized for fighting and irritating vertebrates (Whitehouse).  "Thus the soldier caste may be used to deter vertebrate threats (Whitehouse 1208)." In addition, when pitted against a single foreign ant, the soldier ant is more likely to win the fight than another worker ant.  Indeed, they prove to be more effective than large groups of worker ants in certain conflict situations (Whitehouse 1207).  This is summarized by the linear law, which states: "If the battle is a series of one-to-one conflicts, however, with excess individuals waiting for a free opponent,...a few good fighters will be better than many poor fighters (Whitehouse 1207)."

Ants mark their territory with pheromone trails.  If these territorial markers are traversed then aggressive behavior ensues.  Individual fighting ability may dictate an ant colony's territory size (Adams 1127).  Adams states: "Animals with greater fighting ability can exert greater pressure, which tends to push boundaries away from stronger residents towards their weaker neighbors.  (Adams 1127)." In light of this, one would expect to find larger territory sizes in more aggressive ant species.

Discussion

To recap, the first topic of this review was queen behavior.  It is possible to classify ants according to functional reproductive groups.  These groups are those species whose queens exhibit monogyny and those whose queens exhibit polygyny (Tschinkel, (Tsuji).  Furthermore, in monogynous species, workers from the mother colony seldom assist the new queen in colony foundation (Tsuji).  Whereas, in polygynous species, workers from the mother colony often assist new queens in colony foundation (Tsuji).  The advantages to monogyny include: greater numbers of offspring over the course of a full life span, greater longevity after the early stages of colony foundation, longer reproductive period and close genetic relatedness to all offspring (Tsuji).  The advantages to polygyny include: much greater survival rates in the early stages of colony foundation, higher success rates in colony foundation, faster colony foundation and the ability to cannibalize other dead queens and their offspring in order to provide extra fat reserves (Tsuji), (Crosland), (Sommer).  Of these two strategies, both are efficient in different ways.  Although polygyny yields more in total reproductive output, monogyny could be considered more efficient because it requires less investment in number of queens and their is a greater reproductive yield per queen (Tsuji).  As a general rule in the family Formicidae, the faster a colony is founded the more likely it is to be successful (Tschinkel), (Tsuji).  This is true because in the early stages of a colony queens are more vulnerable to predators, parasites and brood raiding (Tschinkel), (Tsuji).  Of course, this rule favors polygyny (Tschinkel), (Tsuji).  Colonies of primitive ant species have smaller nests, smaller colony populations, less specialized individuals and fewer queens per colony (Crosland).  Queens in these species are often more like workers and sometimes workers can take on the role of reproduction (Crosland).  In fact, in ants of the genus Pachycondyla, there are no true queens and workers are capable of founding colonies (Heinze).  These colonies consist only of workers which aggressively form a dominance hierarchy; in which only the most dominant worker's offspring survive without being eaten by other workers (Heinze).

Once the colony has been established, foraging becomes essential for survival and workers are in charge of this task.  Different species of ants exhibit different foraging strategies.  In most ant species the foraging is done by single scouts which locate prey and bring it back to the nest if they can (Traniello).  If the prey item is too large for the scout to transport by itself, then the scout relies on help from other workers (Traniello).  Scouts recruit other workers to help them by using chemical pheromone trails and Carrying recruits back to the food source (Detrain).  When larger prey items are present, scouts secrete thicker and more continuous pheromone trails (Detrain).  Scouts assess prey size and the need for help based on "prey tractive resistance (Detrain)." In the case of other kinds of food, such as sugar water, the decision of whether to recruit help is governed by whether or not the scout's crop is filled by the food source (Detrain).  Moreover, group prey retrieval is more efficient than single ant prey retrieval (Traniello).  Army ants of genus Eciton employ a remarkable group foraging strategy in which several workers hunt down and overpower large prey items (Robson).  In any case, ant colonies depend on the individual performance of their scouts in order to attain food and survive.

Finally, the food reserves gathered by scouts and workers must be defended from various threats.  Workers and soldier castes are responsible for fighting and defending colony resources (Adams).  Soldier ants are primarily used in defending the colony against vertebrate threats but they are also superior warriors in small scale, one-on-one confrontations with other ants (Whitehouse).  Surprisingly, large numbers of low quality fighters prove more useful in large scale ant wars against other ant colonies (Whitehouse).  Territorial boundaries are marked by using pheromones which are emitted from the abdomen of ants (Adams).  Violation of these territories often leads to conflict (Adams).

In conclusion, queen behavior with regard to nest founding is linked to foraging behavior and fighting behavior which are also linked.  These aspects of ant behavior are essential to colony survival.
 
 

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