Red harvester ant

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. Red harvester ant nests are characterized by a lack of plant growth and small pebbles surrounding the entrance to the tunnel, which usually descends at a pronounced angle. Hulls of seeds may be found scattered around the nest. In grassland areas, such as ranches, the lack of plant life makes red harvester ant colonies very easy to spot, and where they are very plentiful, they may make serious inroads into the grazing available to livestock. The mounds are typically flat and broad, 0 to 100 mm (0.0 to 3.9 in) high, and 300 to 1,200 mm (12 to 47 in) in diameter. Even larger denuded areas have been reported, on the order of 10 m2 (110 sq ft). Three to eight trails typically lead away from the mound, like 'arms'. These trails are used by ants to collect and bring food back to the mound. 'Scout' ants are the first ones out of the mound every morning. They seek food, and mark their path as they return to the mound to alert the worker ants. The worker ants follow the scent trail and collect the food. Other worker ants clean, extend, and generally tend to the mound, the queen, and the brood. All the ants in the colonies are females apart from the winged males produced in the breeding season. The reproductive unit of ant populations is the colony. A single virgin queen first mates with several males at a reproductive aggregation site formed by male harvester ants. She then flies to a new site to produce an offspring colony. 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.