Sophistication in the foraging trail networks of Pharaoh’s ants

Francis L. W. Ratnieks

Department of Biological & Environmental Science

University of Sussex

Laboratory of Apiculture & Social Insects

Social Insects: C1139

Aims 1. To describe experiments investigating foraging trail networks in Pharaoh’s ants 2. To present hypotheses for the existence of multiple trail pheromones in Pharaoh’s ants

Objectives 1 Understand what the experiments found out, why they were designed the way they were, and the implications of the results 2. Understand the broader picture and be able to compare Pharaoh’s ants to honey bees in how foragers are directed to where the food is.

Aims & Objectives

Self Organisation of Ant Trails In a previous lecture we saw how an ant colony could establish pheromone trails leading to food via self organization, if the workers followed some simple rules.

Individual foragers react to local conditions foragers are walking around outside nest if a forager finds food, she lays trail back to the nest if a forager finds a trail, she follows it to the food

Global pattern arises from agent behaviour trails leading to food sources are established this is an adaptive pattern

However, real ants seem to have greater sophistication that this. This lectures will describe recent research carried out at Sheffield on the Pharaoh’s ant, Monomorium pharaonis, which uses at least three different trail pheromones combined

“The idea that molecular signalling cascades share fundamental properties with colonies of ants and Internet communication systems is adding new meaning to the idea of interdisciplinary science.”

Jasny B. R. & Ray, L. B. 2003. Life and the art of networks. Science 301: 1863

Insight from Insect Societies

Not All Ants Use Trail Pheromones

Cataglyphis fortis

Temnothorax: Tandem Running The experiments described in this lecture show remarkable sophistication in the foraging trail network of Pharaoh’s ants and their use of pheromones, and also in Lasius niger ants. However, not all ants use trail pheromones and in some species there is no communication at all among foragers. Cataglyphis desert ants forage for dead insects killed by the heat. The ants forage individually and have remarkably sophisticated navigational abilities. A worker wanders around looking for food. When it finds food it can walk back directly to the nest entrance. It knows where it is at all times by summing up the distances it walks at different angles to the sun. From this it can determine how to walk straight back home. These ants forage individually and do not share information as their food does not occur in patches worth recruiting to. In many ants a successful forager will directly lead nestmates to the food. When one forager leads one other ant this is often called tandem running.

Not All Ants Use Trail Pheromones

Pheromone Trails to Food in Pharaoh’s Ants

Pharaoh’s Ants, Monomorium pharaonis

Pharaoh’s Ants As a Study Species The Pharaoh’s ant, Monomorium pharaonis, is a good species to study pheromone trails and foraging. The worker ants only 2mm long, and even short foraging distances, such as 50cm, are realistic. In this way foraging can be studied in the lab. An ant colony is kept in a small wooden box or plastic tube within a larger plastic box which acts as a foraging territory. Workers do not seem to be guided by their own memory or landmarks when foraging. Most social insects are very hard to breed in the lab. Not Pharaoh’s ants. Nestmate males and young queens mate readily. In addition, colonies have multiple queens. To make two colonies you just split a colony. Pharaoh’s ants are “unicolonial” meaning that colonies are not well defined. Ants from different nests can be combined without fighting. It is easy to make up experimental colonies to whatever size you want simply by adding or subtracting ants. Pharaoh’s ants are found in the UK where they are an introduced pest. It is thought that they come from Africa.

Trail Visible on Smoked Glass

Measuring Longevity of Pharaoh’s Ant Trails

Measuring Longevity of Pharaoh’s Ant Trails

Short-lived Attractive Pheromone

nest box

foraging box bridge syrup feeder

test-trail

Bioassay for Short-Lived Trails Train to syrup feeder on one branch, 20 minutes

nest

Trail Decay: Short-Lived

masking

Remove feeder and masking, 2 minutes

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Trail Decay: Short-Lived Count ants turning left of right, 30 minutes

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Trail Decay: Short-Lived

remove remove

Two Paths Branched …

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Trail Decay: Short-Lived Jeanson, Deneubourg, Ratnieks. 2003. Physiological Entomology

This experiment used one colony per trial. The colony was given a Y-shaped foraging trail with a syrup feeder at the end of one branch. The other branch had no food. The branch with no food had a piece of paper over the substrate. After c. 20-30 minutes a trail had been established. The ants were brushed off and the masking paper on the non-feeder side was removed. Ants which reached the bifurcation were brushed off after they had made their choice between branch that had led to food and the other branch. The proportion that walked towards the food side was initially c. 80% when the trail had been laid on polycarbonate plastic and 70% when laid on newspaper. The proportion decayed to 50% within 10-15 minutes. This shows that the attractive pheromone decays rapidly. In addition, the fact that only 70-80% initially chose the rewarding branch shows that the attractive pheromone alone, versus a branch with no attractive pheromone, is not enough to eliminate errors in trail choice. Jeanson, Ratnieks, Deneubourg. 2003. Physiological Entomology

Longevity of Short-Lived Attractive Pheromone

Measuring Longevity of Pharaoh’s Ant Trails

Long-lived Attractive Pheromone

Bioassay for Long-Lived Trails

nest box

foraging box bridge

syrup feeder paper

corridor

1. Establish Trail (500-8000 ants)

2. Store trail 2-96 hours away from ants

3. Can colony re-establish trail?

In this experiment a colony was allowed to form a straight trail on a piece of photocopier paper to a syrup feeder by using two strips of plastic to confine the ants to a narrow corridor. The paper was then stored away from any ants for up to 4 days. The paper was then given to another colony, with a syrup feeder in the same position as before. The ants were observed to see if they formed a trail to the feeder in the same path as before. The results show that a colony could re-establish a trail in the same location as before even two days after the original trail had been established. The strength of the original trail depended on the number of ants that had walked the trail when establishing it. These results are strikingly different to the previous experiment, which showed that an attractive trail can decay in 10-15 minutes. Jackson, Martin, Holcombe, Ratnieks. 2006. Animal Behaviour

Longevity of Non-Volatile Attractive Pheromone

Trail Re-establishment

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Trail Decay: Long-Lived

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Individual Level Behaviour: Trail Joining

The previous experiment worked at the “global” level to determine if the colony could re-establish a trail. This study works at the local level by studying the behaviour of single ants. A piece of trail is cut out and placed on a larger sheet of paper. A forager ant is placed on the paper in such a way as to cross the trail at right angles. When it does this it may carry straight over the trail or turn 90 degrees to join the trail. It turns out that only ants walking with their antennae touching the substrate, who comprise about 20% of the foragers, can detect the trail. This result indicates that the trail is marked with a non-volatile chemical.

Antennae Down

Antennae Up

“No Entry”

Discovery of Repellent Trail Pheromone

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Discovery of “No Entry” Pheromone feeder no feeder

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but not on trail.

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feeder feeder Colony 1 Colony 2

Of ants that proceed, 72% chose control

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This experiment used two colonies. The first colony was given a Y-shaped foraging trail with a syrup feeder at the end of one branch. The other branch had no food. The second colony had a similar Y-shaped trail but with feeders at both ends so that the ants would visit both branches. After c. 20-30 minutes trails had been established. Paper substrate on the non-feeder side was then cut out from the trail of colony 1 and placed on the trail of colony 2. The other side received a piece of control substrate from colony 1. This had been in the colony’s foraging territory so that ants could walk all over it, but had not been part of a trail. In the modified trail system of colony 2, 69% of ants reaching the branch continued on; of these 72% chose the control side. These data show that the substrate from the non-rewarding branch of the trail in colony 1 repelled ants, because it was chosen less than a neutral control. The results could not be explained by differing amounts of attractive pheromone. Robinson, Jackson, Holcombe, Ratnieks. 2005. Nature

Discovery of “No Entry” Pheromone

Why So Many Trail Pheromones?

The experiments described previously indicate the existence of three trail pheromones. Why so many? What is their purpose. We don’t know but what follows is one idea based on complementary roles.

1. Attractive: volatile (decay 10 mins) 2. Attractive: non-volatile (decay 2 days) 3. Repellent: volatile

Why So Many Trail Pheromones? Hypothesized Pheromone Roles. 1

nest foraging territory

non-volatile attractive pheromone

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Hypothesized Pheromone Roles. 2

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Hypothesized Pheromone Roles. 4 Slide 1. The trail network is marked by non-volatile pheromone. This acts as a long term memory, c. 2 days, of where the colony has been foraging and walking. It leads to all locations where food has been collected in the past few days and where food may perhaps occur again. Slide 2. If food is found at a particular location the path is marked with the volatile attractive pheromone. This guides ants along the correct path and helps them chose the correct branch to take at trail bifurcations. Slide 3. At trail bifurcations the non-rewarding branch is marked with the no-entry pheromone. This helps ants chose the correct branch at a trail bifurcation in which only one branch leads to food. Slide 4. When the food at one location is all gone, this branch becomes less attractive by the decay of the volatile attractive pheromone and the laying of the no entry pheromone. These ideas are just a hypothesis for the moment. They make sense, but must be tested by experiments. Ratnieks, F. L. W. 2008, chapter in book by Lio et al.; Robinson et al. 2008. Insects Sociaux.

Hypothesized Pheromone Roles

Recruitment to Foraging

The experiments described in this lecture investigate how Pharaoh’s ant foragers are directed to food in their environment but not how foragers are caused to leave the nest. In the honey bee the waggle dance and the shaking signal can both function to increase the amount of foragers. Pharaoh’s ants do have mechanisms to send foragers out of the nest, which were studied by Sudd.

Recruitment to Foraging

Sudd (1960) claims that a trail does not recruit unless activation of workers in the nest also occurs. Sudd (1957) describes four types of foraging communication in the nest: 1. sharing of food brought in 2. activation by disturbance leading to a propagation wave 3. direct stimulation from „excited ants‰ 4. scent of food may also contribute Once foraging is in action, the number of foragers recruiting to a trail stays high, even if no fed workers return to recruit more.

Sudd, J. H. (1957). "Communication and recruitment in Pharaoh's ant, Monomorium pharaonis (L.)." British Journal of Animal Behaviour 5: 104-109 Sudd, J. H. (1960). "The foraging method of Pharaoh's ant, Monomorium pharaonis (L.)." Animal Behaviour 8: 67-75

Recruitment to Foraging Honey Bee Waggle Dance: 2 Functions Dancer (forager)

Dance Followers (unemployed foragers)

The waggle dance both recruits more workers to foraging and directs them to food. But it is not essential that a single signal have these two roles. Honey bee colonies also have another “dance”, the vibratory signal, that is involved in recruitment to foraging, but it does not direct recruits to food. It causes them to move to the dance area of the nest.

Comparing Species: Lasius niger (the

common garden ant)

Studying Lasius niger foraging

Lasius niger Lasius niger Lasius niger: memory versus trail pheromone

At a trail with a T bifurcation with food at the end of one branch, individual L. niger foragers rapidly learn which branch leads to food choosing correctly 95% of the time after 3 previous visits (a). At a T with trail pheromone deposited by 20 successful foragers on the food branch versus an unmarked branch only 70% naïve ants choose the side with food (b). Data from Grueter, Czaczkes, Ratnieks (submitted).