Pogonomyrmex barbatus is a species of harvester ant from the genus Pogonomyrmex. Its common names include red ant and red harvester ant. These large (5– to 7-mm) ants prefer arid chaparral habitats and are native to the Southwestern United States. Nests are made underground (up to 2.5 m deep) in exposed areas. Their diets consist primarily of seeds, and they consequently participate in myrmecochory, an ant-plant interaction through which the ants gain nutrients and the plants benefit through seed dispersal. Red harvester ants are often mistaken for fire ants, but are not closely related to any fire ant species, native or introduced.
The main food source for red harvester ants usually consists of seeds, which they hoard in great numbers. The food is first ground to a bread-like consistency using the ants' large mandibles, and is then stored in a granary, assuring the colony access to food year-round. Seed collection on behalf of the red harvester ants benefits their ecosystem through the process of myrmecochory, in which ants aid in the dispersal of seeds while foraging for food. Both plants and ants benefit from this relationship: the plants increase their dispersal range and density, while the ants benefit from acquiring nutrients and ensuring a more secure food supply in future harvests. This is typically understood as a mutualistic interaction. Dead insects are also collected during foraging.
Much research has been done on the foraging behavior of the red harvester ant. Three types of workers are most involved in the foraging process: nest patrollers, trail patrollers, and foragers. On a given day, nest patrollers emerge first from the nest to assess the safety and profitability of foraging. The colony gets the majority of its water from the metabolism of the fats in seeds. If food is scarce, or if it is a particularly hot day, the energy and water benefits of foraging may be outweighed by its energy and water costs. In this case, the colony may be forced to rely on its extensive food stores. Seeds may be stored in the nest for months or even several years.
The colony is able to communicate through momentary antennal contact involving the transfer of cuticular hydrocarbons. Other visual, temporal, and olfactory cues may also be involved. Based on the nest patrollers’ reports, trail patrollers may leave the nest to determine the best possible foraging direction. This decision is based upon various economic factors such as food availability and neighboring nests’ foraging behavior. As both types of patrollers return, foragers assess their rates of return to decide whether to leave the nest to find food. In an experiment involving patroller mimics, a return rate of one patroller every 10 seconds stimulated the highest level of foraging activity. This return rate indicates high availability of food and good foraging conditions, therefore a favorable cost-benefit ratio for the foragers exists. If the patroller return rate is too high, it may be a warning of danger, such as the sighting of a predatory lizard. A lower return rate could indicate lack of available food, or heavy competition.
Ant foraging is guided by chemical signals that lead the ants up to 50–60 m from the nest at times. Once an ant has decided to forage, it will almost always continue until it has found food to take back to the nest; 90% of returning foragers are “successful”. Therefore, forager return rate is a good indicator of food availability. Logically, overall foraging activity is influenced by the rate of returning foragers. When food is plentiful, foragers quickly find it and immediately return to the nest. Foragers still in the nest interpret this to mean food searches will be profitable: low energy input with a high chance of a seed reward. Foraging activity consequently is increased. Likewise, a decline in food availability, indicated by a decrease in forager return rate, causes the colony to decrease its foraging activity.
The ability for a colony to quickly regulate its foraging behavior is quite important since their food source is variable and scattered. This regulatory ability is variable in itself, influenced by food availability, current need to eat, and colony health. Colonies are more likely to adapt to varying forager return rates when the rate of foraging is high, which may be because variances are simply harder to detect during low rates. Overall, the red harvester ant demonstrates a remarkable ability in social cognition, cost-benefit analysis, and behavioral economics.
Mating is perennial and species-specific between subspecies of the red harvester ant. Males distinguish reproductive females of their species from females of other species using pheromones. This occurs as a result of sexually stimulating pheromones secreted from the poison glands of reproductive P. barbatus females. These pheromones are perfectly detected by males of the species upon physical contact, resulting in the stimulation of sexual behavior.
Mating occurs at reproductive aggregation sites. Secretions from the mandibular glands of males initially mark these sites. The secretions attract females, as well as additional males, resulting in strong male competition for mates. The system of mating within P. barbatus is comparable to lek behavior performed by many vertebrate species. The male ants congregate and collectively give off a pheromone that attracts reproductive females. The more males present to give off the pheromone, the stronger the attraction for the females. Although this practice strongly favors certain males and leaves many others without mates, the congregation of males attracts more females on the whole as opposed to pheromone release on behalf of a solitary male. Consequently, it is more desirable for the less attractive males to remain in the lek than to attempt to attract females on their own.
