AFC a écrit :Très intéressant, mais je me demande si c'est le cas de toutes les espèces, notamment de celles que l'on a l'habitude de voir en France, ou si c'est un comportement qui est apparue chez la fourmi matabele, habituée à combattre.
C'est ce qui est expliqué dans le compte-rendu de l'étude proposé en lien.
Des conclusions exclusives apparaissent, mais des hypothèses restent ouvertes pour d'autres espèces.
Je recopie quelques extraits :
We consider that this behavior could only emerge in species that forage or hunt in groups and in a limited spatial domain so that injured individuals are likely detected by other nestmates.
We were able to show that this behavior is triggered through the chemical compounds DMDS and DMTS harbored in the mandibular gland reservoir ; thus, this suite of compounds is a pheromone that seems to be specifically released when the individual is injured, in order to induce rescue behavior.
The only other known species harboring this pheromone is the solitary hunting ponerine ant Paltothyreus tarsatus, in which the pheromone triggers digging behavior, most likely to rescue trapped nestmates (25). This species is in the same genus group as Megaponera (8), but being a solitary forager, P. tarsatus probably has not evolved the same kind of cooperative foraging-injury rescue behavior as in M. analis.
This type of rescue behavior, focused specifically on injured and handicapped individuals after hunting, is unique in social insects.
Although the benefits seem obvious, there are several reasons as to why this has not yet been discovered in other species.
First, because this behavior can only evolve in group-hunting species, where an injured ant can be detected by its nestmates, it excludes all solitary hunting species as potential candidates.
Second, it is also essential that hunting occurs in isolated events, thus creating the risk for the injured ant to be separated from the group during the return journey.
In a constantly occupied trail between ants and food source, the increased risk carried by an injured ant would be marginal, because it is constantly surrounded by nestmates warding off potential predators.
In M. analis, the outward and return journeys are conducted as a discrete column with all ants marching together.
The fact that the ants wait after the fight so that all ants may gather before returning to the nest (11, 15) exemplifies the importance of returning as a group.
Third, the prey species must be able to inflict a high amount of nonlethal injuries from which the ants can recover.
With their large soldiers, termites fulfill this criterion as injury-inducing prey.
Group foraging ant species that focus on leaf cutting, nectar, seeds, or scavenging are less likely to develop this rescue behavior of termite-raiding ants.
Fourth, the benefit to the colony by the rescued ant has to outweigh the cost of help.
In M. analis, the majors carry back termites and injured individuals; because minors sustain the most injuries (Fig. 4), the additional task for the majors to carry them back seems minimal from an energetic point of view (fig. S1A). Moreover, because on an average raid only 30% of the ants carry back prey (11), a large part of the workforce is available to help the injured individuals without decreasing the profits of the raid.
Because the cost of helping an injured ant is therefore likely to be marginal in M. analis, it is thus ignored in our model.
Last, the value of an individual for the colony plays an important role. This can be approximately quantified through the mean mortality rate in a colony.
For a colony to be in equilibrium, the number of ants being born has to match the mortality rate, and in equilibrium, the population turnover is directly related to the life span of the individual.
In M. analis, the population turnover is relatively low, with a birth rate of only 13 ants per day, demonstrating again the importance of rescuing the injured.
Species with a very high turnover, such as army ants, seem likely to derive less benefit from saving one injured ant, although this hypothesis remains to be tested.
The specific biology of M. analis therefore provides the right circumstances where the benefit of saving the injured is especially large. We thus argue that this behavior evolved as part of an evolutionary arms race against termites, as a means of minimizing losses during raids and therefore foraging costs.
Rescue behavior has been previously observed in ants (1) but in very different contexts.
Excavating trapped nestmates after a cave-in and rescuing an ant that fell in an antlion trap are both situations in which the individuals are confronted with an imminent danger, that is, suffocation or being eaten (1, 3, 5).
This is not the case in our situation: Not only are the injured ants in many cases handicapped for life through the loss of extremities, but the immediate danger toward these ants is far less obvious.
There is no direct threat to the injured ant but rather an abstract increased predation risk if these ants were to return alone.
This study demonstrates that complex rescue behavior can evolve in very unique situations if the necessary drivers are present, even in species that are very likely unable to recognize the increased risks to which they are exposed to.
Our model also helps us to identify other candidate species in which this behavior might be found.
Other ponerine genera, such as Leptogenys, also focus on hunting termites, with some of them hunting in groups (27); examining their raiding behavior in more detail could be promising.
Slave-making ants could potentially also fulfill the criteria, if their prey can inflict a significant amount of nonlethal injuries.
Infos complémentaires, ici : http://advances.sciencemag.org/content/ ... 02187.full
