Myrmecological News 18 1-11 Vienna, March 2013 Cooperative transport in ants (Hymenoptera: Formicidae) and elsewhere Tomer J. CZACZKES & Francis L.W. RATNIEKS Abstract Cooperative transport, defined as multiple individuals simultaneously moving an object, has arisen many times in ants, but is otherwise extremely rare in animals. Here we review the surprisingly sparse literature available on cooperative transport. Cooperative transport abilities in ants are a continuum, but three general syndromes are described: uncoordi- nated transport, in which transport is slow, poorly coordinated and characterised by frequent and long deadlocks; en- circling coordinated transport, in which transport is fast, well coordinated, and with few deadlocks; and forward-facing coordinated transport, carried out exclusively by army ants, in which one worker, usually of larger size, straddles an item at the front while one or more smaller workers help to lift at the back. In the two coordinated syndromes, the groups of ants involved constitute teams, and specialised recruitment to large items and adjustment of carrier number to match item size may occur. Some features of cooperative transport are specific adaptations, whilst others are already present in the behaviour of ants carrying items alone. One major benefit of cooperative transport appears to be that it allows a colony to utilize large food items in an environment with aggressive or dominant competitors by quickly removing the item to the nest rather than having to cut it up or consume it on the spot. In addition, compared to individual transport, cooperative transport may have other benefits such as increased transport speed or efficiency. The study of cooperative transport also includes computer simulations and robots. These provide biologists with new perspectives and also formalise questions for further study. Likewise, lessons learned from cooperative transport in ants can inform computer scientists and roboticists. Key words: Cooperative transport, organisation, foraging, group retrieval, ants, review, teams. Myrmecol. News 18: 1-11 (online 24 October 2012) ISSN 1994-4136 (print), ISSN 1997-3500 (online) Received 19 April 2012; revision received 20 June 2012; accepted 28 June 2012 Subject Editor: Alexander S. Mikheyev Tomer J. Czaczkes (contact author) & Francis L.W. Ratnieks, Laboratory of Apiculture & Social Insects, School of Life Sciences, University of Sussex, Falmer, United Kingdom BN19QG. E-mail: [email protected] Introduction Over the last hundred years the range of abilities consid- we address this deficiency by collating what is known about ered to be uniquely human has diminished. For example, a cooperative transport in ants. In doing this we also discuss sense of fairness or an aversion to inequality has been de- whether specific adaptations are used or required for co- monstrated in both monkeys (BROSNAN & DE WAAL 2003) operative transport, the ecology of cooperative transport, and dogs (RANGE & al. 2009). Tool use, another attribute and also introduce a simple terminology for the different once considered uniquely human, is now known in many syndromes of cooperative transport observed. Lastly, we taxa (e.g., chimpanzees, GOODALL 1964; crows, HUNT 1996; examine cooperative transport outside humans and ants in- fish, PAŚKO 2010; octopuses, FINN & al. 2009; and even cluding research on cooperative transport in other animals, insects such as ants, BANSCHBACH & al. 2006; and soli- robots and via computer simulations. tary wasps, BROCKMANN 1985). However, one behaviour Syndromes of cooperative transport in ants that is almost exclusively confined to humans is coopera- tive transport. Unlike tool use, our closest relatives the Cooperative transport, also referred to as group retrieval, great apes rarely seem to do this. Apart from humans, the group transport or cooperative carrying, is common but far only animals that regularly perform large scale cooperative from universal in ants. It is known in at least 40 genera in transport are ants. Cooperative transport can be defined as different subfamilies of the Formicidae (HÖLLDOBLER & two or more individuals simultaneously moving an item WILSON 1990, MOFFETT 1992, MOFFETT 2010). Although from one location to another. Although cooperative trans- no formal comparative analysis has been carried out, this port is a widely known behaviour of ants, and often fea- strongly suggests that cooperative transport has evolved tures in cartoons and the popular image of ants, it is sur- multiple times in ants. There is also much variation in appa- prisingly understudied and what information exists has nev- rent sophistication and effectiveness. The cooperative trans- er been comprehensively reviewed in the published litera- port abilities of particular ant species lie on a continuum ture, although MOFFETT (1987) surveys cooperative trans- from never occurring to highly specialised, efficient and port in his PhD thesis and later in MOFFETT (2010). Here rapid. For convenience, we categorize cooperative transport Box 1: Different definitions of "teams". Definitions range from the highly restrictive necessity for different castes, to the highly inclusive definition used in daily parlance. The highly restrictive definition includes very few natural examples, limited only to new and old-world army ants. The highly inclusive definition encompasses all cooperating groups, and so is perhaps too uninformative for scientific discourse. Name / proponent Description Requires Examples from sports Different castes work- "teams … can be defined as ● Multiple individuals wor- American Football, Rugby. ing concurrently members of different castes that king towards the same goal Different non-interchangeable "castes" (HÖLLDOBLER & come together for highly coordi- ● concurrently performed (fast runners, e.g., wingers in rugby, large WILSON 1990) nated activity in the performance subtasks tacklers, e.g., props in rugby) work con- of a particular task" ● members in non-inter- currently, performing different subtasks changeable roles (wingers receive passes and score tries, props tackle opposing teammates) Different sub-tasks be- "A team task requires different ● Multiple individuals wor- Basketball, Polo ing performed concur- subtasks to be performed concur- king towards the same goal Different subtasks (e.g., shoot guard or rently rently for successful completion." ● concurrently performed centre in Basketball) performed concur- (ANDERSON & FRANKS A team is a group of individuals subtasks rently, but no extreme physical differen- 2001) performing a team task. Individu- tiation of players. als not only have to work concur- rently, they must also coordinate their different contributions. Daily parlance Any group of individuals that work ● Multiple individuals wor- Tug-of-war, Bowling, Relay running (MOFFETT 2010) towards a single goal. Synonym- king towards the same goal In a tug-of-war players perform identical ous with "cooperating group". tasks concurrently. In bowling and relay running players perform identical roles, and do so singly. in ants into three general syndromes: uncoordinated trans- port, encircling coordinated transport, and forward-facing coordinated transport. In uncoordinated transport, item movement is charac- terised by frequent deadlocks in which ants pull in oppo- site directions resulting in no forward motion (SUDD 1965, MOFFETT 1986, PRATT 1989, MOFFETT 1992). These dead- locks are resolved by random changes in the composition, orientation or behaviour of the group members, which in- dicates lack of coordination (SUDD 1965). SUDD (1965), in an extensive study of cooperative prey transport by Myrmica rubra (LINNAEUS, 1758) and Formica lugubris ZETTER- STEDT, 1838, both of which perform uncoordinated trans- Fig. 1: Carebara simalurensis cooperatively transporting port, found three discrete stages to transport. Transport be- a large brood item. Notice how the ants lift the item us- gins when the first ants find the food item (stage one), but ing the underside of their heads and front legs. A similar then stops as more ants find the item and deadlock occurs behaviour is displayed by Pheidologeton diversus. In con- (stage two). Deadlock can last up to ten minutes, until ran- trast, ants carrying items individually grasp with their man- dom changes cause the deadlock to end. The third stage is dibles, as shown by the ant on the right which is trans- characterised by higher speed and path straightness than porting a small brood item. Image copyright Mark W. the first and second stages, implying better organisation of Moffett / Minden Pictures. the carriers who have by chance brought themselves into an effective alignment. However, no evidence was found of In the second syndrome, encircling coordinated trans- specific cooperative behaviour: Ants did not synchronise port, ants are recruited to a food item, encircle it, and quickly their pulling efforts and often pulled in opposite directions. transport the item back to the nest once a sufficient num- A burst of motion occurred when ants by chance attempted ber of ants have assembled to move the item (e.g., Pheido- to pull the item in the same direction. Whilst the ants logeton diversus (JERDON, 1851), M. Moffett, pers. comm.; "agreed" about the general direction the item is to be moved Leptogenys diminuta (SMITH, 1857), MASCHWITZ & STEG- in, they "disagreed" on how to achieve this. Nonetheless, HAUS-KOVAC 1991; Pheidole oxyops FOREL, 1908, see ants did not assemble randomly around the food