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
