Duncan E. Jackson

Trail geometry gives polarity to ant foraging networks

Pheromone trails are used by many ants to guide foragers between nest and food. But how does a forager that has become displaced from a trail know which way to go on rejoining the trail? A laden forager, for example, should walk towards the nest. Polarized trails would enable ants to choose the appropriate direction, thereby saving time and reducing predation risk. However, previous research has found no evidence that ants can detect polarity from the pheromone trail alone. Pharaohs ants (Monomorium pharaonis) produce elaborate trail networks throughout their foraging environment. Here we show that by using information from the geometry of trail bifurcations within this network, foragers joining a trail can adaptively reorientate themselves if they initially walk in the wrong direction. The frequency of correct reorientations is maximized when the trail bifurcation angle is approximately 60 degrees, as found in natural networks. These are the first data to demonstrate how ant trails can themselves provide polarity information. They also demonstrate previously unsuspected sophistication in the organization and information content of networks in insect societies.

Communication in ants

Research shows that ants, and also honeybees and other species of social insects, use several pheromones or other signals in organizing their foraging system. Two important and connected questions, therefore, are to determine why multiple signals are needed and how they work together. Some progress has been made in the honeybee, where four dances and a pheromone are known to be involved. The role of most of these signals is known. The waggle dance directs foragers to food and recruits them to foraging, the vibratory dance prepares foragers for work by causing them to move into the dance floor area where returning foragers make waggle dances, and the tremble dance recruits additional workers to the task of unloading nectar foragers. But what is not known is precisely how they work together and why five signals (and maybe more) are needed. Why not four?One possibility is that some of these signals are fine tuning. Multiple signals may be needed because of inherent limitations in the signals used. For example, the short-lived attractive and repellent trail pheromones used by Pharaohs ants can direct foragers to the rewarding branch at a trail bifurcation but a single one of these pheromones can only direct about 75% to the rewarding branch. Perhaps the presence of two pheromones can increase this to 90%.We also need to understand communication mechanisms in relation to the foraging method. The solid substrate upon which ants walk from nest to food is suitable for depositing trail pheromones to guide nestmates. A trail pheromone is obviously less useful for flying social insects, such as honeybees. However, some stingless bees do mark routes to food with pheromone, which they deposit on vegetation as discrete beacons rather than a continuous trail. The use of a trail pheromone means channels of communication may be continuously open for ants, because they are capable of a continual, reactive exchange of information with nestmates whilst foraging. This is in marked contrast with the honeybee, where the signals used to organize foraging are communicated in the nest. Foraging is a dynamic process and an important role of communication is to recruit or direct nestmates rapidly to a food source. But the need to do more than this, for example to retain a longer term memory, may require additional pheromones or signals.The multiple signals used in social insect communication provide shared information and enable the colony (or system) to be more responsive or better regulated, so that it functions better. Similar complexity is found at other biological levels, such as cell signalling pathways, where positive and negative feedback provide the capacity for control at multiple levels and enable greater flexibility in system responsiveness. A major interdisciplinary challenge in modern biology is to understand how complex adaptive systems function, and how they function robustly yet flexibly. One goal in this research is to determine if there are any general principles underlying adaptive biological systems. The focus of this research, and the funding, is usually directed at the organismal level or below, particularly cells in a multicellular organism, or molecules within cells. Insect societies provide another level of organization for comparison.

Insect communication: ‘No entry’ signal in ant foraging

Forager ants lay attractive trail pheromones to guide nestmates to food, but the effectiveness of foraging networks might be improved if pheromones could also be used to repel foragers from unrewarding routes. Here we present empirical evidence for such a negative trail pheromone, deployed by Pharaohs ants (Monomorium pharaonis) as a ‘no entry’ signal to mark an unrewarding foraging path. This finding constitutes another example of the sophisticated control mechanisms used in self-organized ant colonies.

Longevity and detection of persistent foraging trails in Pharaoh's ants Monomorium Pharaonis (L.)

Pheromone trails provide a positive feedback mechanism for many animal species, and facilitate the sharing of information about food, nest or mate location. How long pheromone trails persist determines how long these environmental memories are accessible to conspecifics. We determined the time frame over which Pharaohs ant colonies can re-establish a long-lived trail and how the behaviour of individual workers contributes to trail re-establishment. We used artificially constrained pheromone trails on paper to investigate trail longevity and individual responses. Trails formed by traffic of 1000?2000 ant passages could be re-established after 24 h, and after 48 h for 4000?8000 ant passages. Only 27.5% of individual foragers were highly successful in a bioassay testing the ability to detect trails established 24 h earlier. Trail-finding ability was significantly correlated with a low antennal position. Long-lived trail detection scores increased significantly in 57% of foragers after 5 h of food deprivation and isolation, but declined again after feeding. In a control study, only 9% of foragers changed their scores, when isolated with food present. A high degree of conservatism was found such that, regardless of treatment, 21% always failed the bioassay and 17% always succeeded. Our demonstration of long-lived components in Pharaohs ant trails and of a behavioural specialization in `pathfinding? shows that pheromone trails are more complex at the individual level than is generally recognized.

U-turns on ant pheromone trails

Many ant species use branching networks of pheromone trails for orientation between nest and resources [1,2,3]. Ants on trails make adaptive U-turns for correcting their course using visual cues [4, 5] or trail geometry information [2]. However, the role of seemingly non-corrective U-turns on trails is poorly understood. We found that a minority of ants consistently make frequent and seemingly inappropriate U-turns during foraging bouts. These frequent U-turners were also highly likely to lay pheromone trail, whilst non-turners rarely did so. Our data suggest that U-turning ants make a greater contribution to trail persistence than do non-turners.

Weak patriline effects are present in the cuticular hydrocarbon profiles of isolated Formica exsecta ants but they disappear in the colony environment.

Chemical recognition cues are used to discriminate among species, con-specifics, and potentially between patrilines in social insect colonies. There is an ongoing debate about the possible persistence of patriline cues despite evidence for the mixing of colony odors via a “gestalt” mechanism in social insects, because patriline recognition could lead to nepotism. We analyzed the variation in recognition cues (cuticular hydrocarbons) with different mating frequencies or queen numbers in 688 Formica exsecta ants from 76 colonies. We found no increase in the profile variance as genetic diversity increased, indicating that patriline effects were absent or possibly obscured by a gestalt mechanism. We then demonstrated that an isolated individuals profile changed considerably relative to their colony profile, before stabilizing after 5 days. We used these isolated individuals to eliminate the masking effects of the gestalt mechanism, and we detected a weak but statistically significant patriline effect in isolated adult workers and also in newly emerged callow workers. Thus, our evidence suggests that genetic variation in the cuticular hydrocarbon profile of F. exsecta ants (n-alkanes and alkenes) resulted in differences among patrilines, but they were obscured in the colony environment, thereby avoiding costly nepotistic behaviors.

Decentralized communication, trail connectivity and emergent benefits of ant pheromone trail networks

Communication improves decision-making for group-living animals, especially during foraging, facilitating exploitation of resources. Here we model the trail-based foraging strategy of Pharaoh’s ants to understand the limits and constraints of a specific group foraging strategy. To minimise assumptions we used model parameters acquired through behavioural study. Pharaoh’s ants (Monomorium pharaonis) exploit the geometry of trail networks bifurcations to make U-turns, if they are walking the wrong way. However, 7% of foragers perform apparently incorrect U-turns. These seemingly maladaptive U-turns are performed by a consistent minority of specialist U-turners that make frequent U-turns on trails and lay trail pheromones much more frequently compared to the rest of the colony. Our study shows a key role for U-turning ants in maintaining the connectivity of pheromone trails. We produced an agent-based model of a heterogeneous ant community where 7% of agents were specialised frequent U-turners whilst the remaining 93% rarely U-turned. Simulations showed that heterogeneous colonies enjoyed significantly greater success at foraging for distant food resources compared to behaviourally homogeneous colonies. The presence of a cohort of specialised trail-layers maintains a well-connected network of trails which ensures that food discoveries are rapidly linked back to the nest. This decentralised information transfer might ensure that foragers can respond to dynamic changes in food distribution, thereby allowing more individuals in a group to benefit by successfully locating food finds.

Social Evolution: Pathways to Ant Unicoloniality

Unicolonial ant species live in interlinked populations known as super-colonies, where workers and queens move freely. New research suggests that low intra-specific resource competition leads to an absence of inter-colony aggression.

Kin Recognition: Knowing Who's Boss in Wasp Colonies

Paper wasps recognise the dominant individual in their colony and surrender reproduction to this alpha individual. Contrary to expectations her dominance status is not signalled by a chemical indicator of fertility.

An Agent-Based Behavioural Model of Monomorium Pharaonis Colonies

In this study X-machines and hierarchical organized X-machines will be used to model different aspects of the behaviour of social insect communities. The model is organized as a community of complex agents showing similarities to networks of P systems.