Ant colonies have a striking division of labor: queens lay eggs, while workers, unable to reproduce, perform tasks such as brood care, foraging and defense. The task a worker performs depends on its size, age and experience. How is task performance in ants controlled by a tiny amount of nerve tissue, the brain? The goal of the project is to discover how differences in the brains of worker ants in a colony control task performance, and how the ecology of a species may influence worker brain structure and behavior. Ants may change tasks as they age, beginning as nurses and ending as foragers. Their brains need to be plastic so that they can generate age-appropriate behavior. In some species, workers show specialized behaviors, such as defending the colony, and are equipped with large and powerful jaws that make them effective soldiers. If workers are specialized for defense, their brains must respond to stimuli associated with threats to the colony and control aggressive behavior. Thus the brain of a nurse should respond only to the needs of brood and the brain of a defender should respond to threats from enemies. The project will analyze ant social behavior and examine age- and task-specific differences in the size of worker brains, brain compartments and individual nerve cells as well as the chemical composition of the brains of workers of different species in an extremely diverse group of ants. The expected subtle brain differences will help to understand how slight changes in brain structure and function can result in pronounced behavioral differences and how behavior develops during maturation. The project will train graduate and undergraduate students (including minority students) in behavioral, anatomical, neurobiological and molecular techniques and enrich the education of K-12 students by introducing them to behavior, neuroscience, ecology and evolution.
Social insects live in societies with division of labor that commonly goes along with differences in individual appearance. In general, a queen is responsible for reproduction and represents an egg-laying ‘machine’ while workers do not reproduce but perform all the required work (brood care, nest care, foraging, defense, etc.). In many ant species, different worker castes perform different tasks as they get older and in some species workers may be morphologically, physiologically and behaviorally specialized (e.g. large soldiers and small in-nest workers). In all animals it is the brain that generates and controls the behavior, and the project therefore focused on the brain of different ant workers. Is the brain of a young nest worker different from that of an older worker who leaves the nest to forage? Do soldiers with larger heads have larger or otherwise modified brains? To address these questions, we measured brain and brain component size, head muscle volumes, stained characteristic nerve cells and nervous pathways to compare them across individuals, and looked at learning behavior of certain ants, focusing on big-headed ants and other social insects. We found that young ants, which do not contribute to tasks that require forceful jaw bites (e.g. defense, chopping tough food items) are physically unable to perform such tasks - during the first week of adult life, their jaw muscles are thin and immature and cannot generate large forces. All the other muscles seem to mature earlier and allow newborn ants to almost immediately use their legs and feelers to contribute to nest duties. The same is true for ant brains - while all the nerve cells are present at birth, they are not as fully developed as they are in older, more experienced ants that have to face the outside environment during their foraging trips. This is similar in honey bees, whose brains develop over two weeks before they are ready to fly out. Interestingly, the comparative experiments showed that bumblebee brains are almost fully developed right after birth, which makes sense biologically as bumblebees, in contrast to honey bees, can fly out on their first day of life. Analyzing a particular nerve cell that can be identified in every individual and that is probably involved in modulating sensory information processing in a learning and memory center of the brain showed that this cell is more elaborate in soldier ants, perhaps modulating processing of sensory information (e.g. in advance of aggressive interactions). Measurements of brain composition suggested that the sense of smell is particularly prominent in ants as the parts of the brain involved in smell processing are very large and complex compared with other insects (and in contrast to brain regions processing visual information, which are small in ants). Behavioral experiments showed that ants can learn smells well and can be trained to discriminate different odors. This is reflected by the finding that a major learning center in the brain of ants is large compared with most other insects, and receives predominately input from the sense of smell. The project partly supported two graduate students and allowed 14 undergraduate students to get involved in hands-on research.
