Social Carrying

It is common to see an ant carrying another ant in her mandibles. In most cases these ants belong to separate species, with one of the ants having killed or scavenged the other, and then returning to her nest with the prey item. In cases where both ants belong to the same species and the ant being carried is dead, it is likely that the dead worker is being removed from her nest by one of her sisters, to be discarded away from the nest. Finally, there are cases where both ants belong to the same species and both are alive. Here, one ant transporting the other to a distant location, with this behaviour known as social carrying, and is a form of recruitment used to increase the number of workers at a site. It is used in two situations, one to bring additional workers to an existing or potential source of food, and the other to move workers from an old to a new nesting site, as part of the nest migration process.

It is interesting to note that any given species may show social carrying in relation to food recruitment or nest migration, but not necessarily both. This is especially true in species with solitary foraging behavior, where recruitment to food sources does not occur. These species may, however, use tandem running or social carrying during nest migration. This has been found in Diacamma rugosum (Fukumoto and Abe 1983), Neoponera apicalis (Fresneau 1985), Neoponera obscuricornis (Hölldobler and Traniello 1980), Brachyponera sennaarensis (Lachaud and Dejean 1994) and Ponera pennsylvanica (Pratt et al. 1994). Recruitment strategies are possible in these species but are never expressed for foraging, demonstrating that there is some flexibility in recruitment behaviour while there are ecological constraints in their expression to foraging (Lachaud and Dejean 1994).

For further details about social carrying see the classic 1974 paper by Möglich & Hölldobler.

Also note that social carrying is an uncommon behaviour as the vast majority of ants forage singly (less common) or use chemical trails to recruit nestmates to food sources (more commonly), and almost all nest relocation involves chemical trails (more commonly) or mass migration (less commonly). See Behavior and Life History, and particularly Tandem Running and Foraging behaviors in Poneroids and Ectatomminae, for additional details.

Carrying Positions Between Workers
In some ants, the pattern used by the carrying ant to transport the carried ant is highly variable, while in other species the pattern is very stereotypic and specific. These behaviours are generally taxon-specific, being similar and consistent within a taxonomic group and differing between groups. In all but the simplest cases, the carried individual folds its appendages tightly to its body during transport, and essentially all carried workers will be dropped if approached or disturbed.

In Myrmecia the carrying patterns are apparently not stereotyped, with the carrier grasping the carried by the mandibles or any other part of the body and dragging them over the ground.

A carrying behavior similar to Myrmecia was observed by Moglich & Hölldobler (1974) in Bothroponera tesseronoda. However, carrying occurs only rarely in this species with nestmates primarily recruited by tandem running (Maschwitz, Holldobler & Moglich, 1974).

Somewhat more complex is the transport behavior of Odontomachus. Here, the transporter ant simply grasps a nestmate by a leg, the petiole or some other part of the body, lifts it up and carries it away. Although there is no general, stereotyped response, the transported individual usually folds its appendages tightly to the body (Fig. I).

According to Rettenmeyer (1963), New World army ants (Eciton, Labidus, Neivamyrmex and close relatives) carry other adults in the same way they carry larvae and pupae, slung beneath the body and between the legs of the transporting worker.

Much more elaborate and stereo-typed behavior is displayed by Rhytidoponera metallica. When the transporting worker approaches a nestmate head on, it repeatedly grasps the ant at the head and jerks it slightly forward. The movement lasts only 1-2 seconds. The nestmate responds by turning its body axis slightly sideways, whereupon the transporting ant seizes the nestmate with a firm grip. After turning through an angle of approximately 180, the transportee is lifted and curled over the head of the carrying ant. During transportation the gaster is bent inwards and the appendages folded tightly to the body.

Pratt (1989) examined recruitment in Ectatomma ruidum (which belongs to the same subfamily as Rhyditoponera). He found that they lay chemical trails for mass recruitment to rich or difficult food sources while practicing stereotypic social carrying in the typical myrmicine mode (see below). He also found an additional social carrying posture previously unknown in ants.

Within the myrmicines, most species show stereotyped adult transport behavior which resembles that of Rhyditoponera. The transportee is either seized at the mandibles or at the "neck" or "cheeks" and curled over the head of the transporting worker (Fig. 2). But there are a few exceptions. For example, the harvester ants Pogonomyrmex badius, Pogonomyrmex rugosus and Pogonomyrmex barbatus employ a rather simple carrying technique. As in Odontomachus the transportee is grasped by any part of the body, lifted and carried away. This simple transporting behavior is especially remarkable, because in other Pogonomyrmex species, for example Pogonomyrmex maricopa, typical highly stereotyped adult transport behavior occurs, as seen in other myrmicines.

Another exception to the general myrmicine pattern is found in Crematogaster, where the transported ant is grasped at the petiole and carried with the head and legs pointed forward (Wilson, 1971).

And in Eurhopalothrix heliscata, Wilson & Brown (1984) found adult transport to be very uncommon, with their single observation being a well-colored (hence older) worker carrying another worker by the waist. But here, the transportee was waving its legs awkwardly and it did not respond in the typical fashion of assuming a passive pupal posture. This would suggest that true social carrying does not occur in this species.

The most uniform and stereotyped social carrying behavior can be observed in the subfamily Formicinae. In several species analyzed by Moglich & Hölldobler (1974) the behavioral patterns that initiate and guide carrying behavior are virtually invariant. They describe this behavior for Camponotus sericeus, a common species found in Sri Lanka. When a recruiting ant faces a nestmate head on, it conducts a jerking behavior for 2-3 seconds, grasping the nestmate at the mandibles and subsequently pulling it forward. Usually the recruiting ant responds by turning around for 180 degrees. The nestmate is thereby slightly lifted, and this elevation evidently causes it to fold its legs tightly to the body and to roll the gaster inward. In this "pupal" posture it is then carried to the target area.

Identical stereotyped adult transport behavior has been observed in many formicine ants and has been described in particular detail in Formica rufa (Zahn, 1957), Formica polyctena (Kneitz, 1964; Moglich, 1971), Cataglyphis sp. (Wehner & Lutz, 1969), Camponotus socius (Holldobler, 1974) and several other species.

Finally, there is a case where a single species shows multiple carrying strategies. Several species of Rhytidoponera are known to share their nests with Polyrhachis loweryi (Kohout, 2010). One of these species has been observed carrying other Rhytidoponera workers in the typical way during nest relocation, but used the formicinae pattern when carrying Polyrhachis loweryi workers to a new nest. This is likely an exceptional situation involving a host and its guest, but demonstrates an unexpected level of behavioural flexibility in these ants. It also suggests that the carried ant may be an active participant in determining the carrying behaviour rather than a passive bystander, influencing the method by which it is carried.

Note that these worker-worker carrying behaviours are identical independent of their use during foraging or nest migration.

Sex Specific Carrying Positions
Reproductive females (queens) are sometimes carried like workers (Fig. 4). But in cases where the size difference is too large, workers merely grasp the reproductives at the mandibles and pull them to the target area (Fig. 5). The first behavioral steps that lead to this pulling behavior are usually similar to those which initiate carrying behavior in workers.

The methods by which males are transported are notably different, however. These individuals are sometimes picked up at any part of the body and dragged or carried to the target area. In a few species workers apply specific stereotyped carrying methods for males. In several Camponotus species Moglich & Hölldobler (1974) observed that most of the males are grasped at the "neck" and lifted into an oblique position in which they are carried away (Fig. 6A). During transportation the males remain motionless with the antennae and legs folded to the body. As Fig. 6 B and C shows, there are occasionally exceptions of this carrying technique.

In some other species, as in Novomessor cockerelli and Aphaenogaster floridana, workers grasp the males between the mesosoma and gaster and carry them beneath their body between their legs (Fig. 7).

Food Recruitment
In general it seems that social carrying occurs primarily in species that show no evidence of pheromone involvement during foraging. In species which lay pheromone trails to food sources, there appears to be no need for social carrying as additional workers can be recruited using chemical trail markers alone. However, it should be noted that in most cases the mechanisms by which the scout is able to return to the food and the mechanism by which the carried worker finds the nest are unresolved, although visual orientation cues may be employed (e.g., Jaffe et al. 1990; Collett and Collett 2002) (Guénard & Silverman, 2011).

Guénard & Silverman (2011) examined carrying in response to food discovery in Brachyponera chinensis. They found that this species employs a unique yet relatively slow recruitment process, which they called tandem carrying, whereby foraging workers carry nestmates from the nest to a food source which is subsequently retrieved (Guenard & Silverman 2011). A successful tandem carry comprises several steps.
 * 1) A scout returns to the nest following the discovery of food too large to be moved by a single individual.
 * 2) Upon return to the nest, the scout solicits a nestmate worker by drumming it with its antennae.
 * 3) The antennated worker assumes a pharate (pupal)-like posture with legs appressed to the thorax.
 * 4) The scout, now referred to as the carrier, then picks it up.
 * 5) The carrier holds the recruited worker within its mandibles between the worker’s first and second pairs of legs of the mesometasternum.
 * 6) The carried worker’s head is positioned upwards while being transported to the food, after which it is released directly adjacent to or nearly within a 2-cm radius of the food.

Guénard & Silverman concluded that this behavior is characterized by a graded recruitment and by high spatial and temporal flexibility. First, the number of tandem carrying events is resource dependent, with more recruitment to large prey that cannot be carried by a single worker than smaller movable prey, even at high density. Second, the recruitment observed by tandem carrying can be adjusted quickly in space and within a time period of 5 to 10 min to maximize the exploitation of larger prey. The low recruitment efficiency of this behavior seems to be balanced by a strong flexibility.