Of slavery in human society ants and the defense against it

"Slavery is founded in the selfishness of man's nature - opposition to it, is his love of justice.”

                                                                                                                                                                                                 -Abraham Lincoln

If you had thought slavery is a concept only in humans, you are mistaken. Slavery is seen even in ants!*

Before you find out more about the ant slavery, you need to know the social structure of any ant colony. Generally, each colony has a Queen which is the sole reproductive individual of the colony. There can be multiple queens in a colony too. There is a division of labour in the colony. That means, work is divided between many individuals: soldiers protect the colony, foragers search for food, maiden workers clean their nest, nurse brood and the queen and so on.

Coming to slavery—some species of ants are known to invade colonies of closely related ant species (host species) and steal their brood. Once the brood matures to become workers, they work as slaves in the slave maker colony. The slaves forage for food, maintain the nest and are involved even in slave making raids on other ant colonies. Some species of slave makers have gone to the extent that they have lost the ability to forage and thus are completely dependent on slaves¹.  

The process of slave making begins with a few scout ants checking out the potential slave colonies. It is followed by a raid in which many slave maker ants involve in a fierce fight with the potential slaves. Many ants from both the colonies get killed in the process. By the end of it, if the slave makers succeed, they steal the brood and carry them to their nest.

Photo: Alex Wild

Slave maker=red ant, Host=black ant

At this point, you might be wondering—‘Don’t the host species remember that the slave maker scouting is followed by the raids and thus prepare to defend more fiercely? ’

Yes, they do. In a paper published in the journal Behavioral Ecology, Pamminger and others tested if there is any increase in host ants’ aggression as an induced defense (after a scouting event) against slave makers. They also checked if the host ants’ response to non-nestmate conspecifics/slaves is context dependent.

Optional reading--[To answer the above questions, they used 3 treatment groups and a control group consisting different combinations of dead slave maker ants and conspecific ants. They stimulated scouting events by 4 consecutive cycles of encounters. First cycle consisted of conspecifics in all groups. Second cycle had slave makers in treatment 1 and 2 while treatment 3 had species of same genus that of the host. In third cycle treatment 2 had slave maker while all other groups had conspecifics. Fourth cycle had all conspecifics except in treatment 3 in which no ant was presented. Control group had conspecific dead ants.  They noted the responses of host ants for each of the 4 cycles of encounters in the 3 different treatments.]

It is revealed that host ants remember the slave makers and show an increased aggression for at least 3 days. After an encounter, host ants react equally aggressively to slave makers and non-nest mate conspecific/slaves. So the response to conspecifics is context dependent--higher after a slave maker encounter than in normal encounter with the conspecifics. Host ants also show a seasonal variation in response to the conspecifics (higher aggression in summer as there are few slave raids during this time). The response to slave makers is always high. All this makes sense knowing that, while conspecifics are competitors for resources, slave makers are a threat to the survival of the colony.

In a broad picture of animal behavior, this study adds a pixel of environment dependent behavior response. It showed that, in an animal, different behavioral changes are induced by competitors/predators/slave makers in different conditions.

Throughout the study, dead conspecifics and slave makers were used. This would mean that the behavior of these individuals during the slave raid and the corresponding responses of host ants were completely ignored. So, the findings need to be validated with experiments using live ants.  Costs and benefits of the induced responses need to be quantified.

*Note: Even though the term ‘slavery’ is anthropomorphized, one should not think of considering slavery as normal (i.e. nature chose such a concept— so it’s okay to support it). One should not base morals strictly based on what is seen in nature.

References:

1.        ‘The Ants’ by E O Wilson and Bert Holldobler

2.   Pamminger et al, “Increased host aggression as an induced defense against slave-making ants”,Behavioral Ecology, 2011

3.     http://www.nature.com/scitable/knowledge/library/social-parasitism-in-ants-13256421