Puzzle of ants' suicide mission to protect the nest

Sep 24, 2008

(PhysOrg.com) -- Scientists studying social insect behaviour have discovered a remarkable example of self-sacrifice in a species of ant found in Brazil.

The Brazilian ant Forelius pusillus routinely seals the entrance to its nest each night. The entrance is first closed from the inside. One or a few ants remain outside and continue closing the entrance, carrying and kicking sand and soil into the hole in the ground.

Those ants that stay outside to complete the task are therefore unable to return to the nest and die before the nest is re-opened from the inside each morning. Social insects such as bees, ants and wasps are known for giving their lives to defend their hive or nest (ie a honey bee worker dies after stinging).

This is the first observed example, however, of defensive self-sacrifice in which the sacrifice is pre-emptive rather than directed at a current threat such as a predator already at the nest.

The researchers who made the discovery are now left puzzling what the reason might be behind the evolution of such self-sacrificing behaviour on a nightly basis, with no apparent threat in sight.

The findings are published in the November issue of The American Naturalist journal. The project was carried out by an international team led by Dr. Adam Tofilski of the Agricultural University of Krakow, Poland, and Professor Francis Ratnieks of the University of Sussex, UK.

The ants left outside spend at least 15 minutes carrying and kicking sand into the hole until it is invisible. This showed that they were involved in deliberate entrance-closing activities, not merely trying to find a way in.

Showing that the ants left outside died was challenging. It was not possible to find the dead bodies, as the ants were practically invisible, being 2mm long and sand-coloured. Two lines of evidence showed that many or all of the ants left outside died. First, when the colonies re-opened their nest entrances in mid-morning there were never any ants waiting to be let in. Second, the researchers set up nest entrances in plastic bowls so that they could find the ants the next morning, alive or dead.

The ants left outside probably die due to the extreme heat at the soil surface, which they could not survive for long.

Professor Ratnieks, who is Professor of Apiculture at the University of Sussex, where he also heads the new laboratory of social insects, says: "A few workers sacrificed per day would be a small cost for a large colony to pay for improved nest defence."

But one big puzzle remains. What they are defending against by closing the entrance? Professor Ratnieks says: "It may be ants of the same species, parasites or predators. Or it may be to prevent water from the sudden heavy rainstorms from flooding the nest."

Studying the kind of behaviour exhibited by the Brazilian ants, and the possible reasons behind it, could help scientists to understand the evolutionary importance of altruistic behaviour.

Citation: 'Pre-emptive defensive self-sacrifice by ant workers' The American Naturalist, by Adam Tofilski (Agricultural University, Krakow), Margaret J. Couvillon (University of Arizona), Sophie E. F. Evison (University of Sheffield), Heikki Helanterä (Helsinki University and University of Sussex), Elva J. H. Robinson (University of Bristol), and Francis L. W. Ratnieks (University of Sussex), November 2008. See American Naturalist.

Provided by University of Sussex

Explore further: Seychelles poachers go nutty for erotic shaped seed

add to favorites email to friend print save as pdf

Related Stories

Florida harvester ants regularly relocate

Nov 19, 2014

Florida harvester ants move and construct a similar subterranean nest about once a year, according to a study published November 19, 2014 in the open-access journal PLOS ONE by Walter Tschinkel from Florid ...

Why some butterflies sound like ants

Oct 29, 2014

Ant nests can offer a lot to organisms other than just ants. They are well-protected, environmentally-stable and resource-rich spaces—in many ways everything a tiny creature could ask for in a home. So ...

52-million-year-old amber preserves 'ant-loving' beetle

Oct 02, 2014

Scientists have uncovered the fossil of a 52-million-year old beetle that likely was able to live alongside ants—preying on their eggs and usurping resources—within the comfort of their nest. The fossil, ...

Recommended for you

Scanning robot helps put insect collection online

19 minutes ago

A robot capable of scanning a tray of insect specimens in a few minutes will help make the virtual images and tagging information available to the public online, according to South Dakota State University ...

New method for quickly determining antibiotic resistance

43 minutes ago

Scientists from Uppsala University, the Science for Life Laboratory (SciLifeLab) in Stockholm and Uppsala University Hospital have developed a new method of rapidly identifying which bacteria are causing an infection and ...

New mushroom discovered on campus is the first since 1985

59 minutes ago

Two researchers who recently named the first new species of mushroom from the UC Berkeley campus in more than 30 years are emphasizing the need for continued green and open space on campus, as well as a full-fledged ...

Model evaluates where bioenergy crops grow best

1 hour ago

Farmers interested in bioenergy crops now have a resource to help them determine which kind of bioenergy crop would grow best in their regions and what kind of harvest to expect.

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

superhuman
4 / 5 (1) Sep 24, 2008
So did they find the bodies or not? Maybe those ants somehow dig through or go to some other entrance. The bodies are a required evidence if sacrifice claim is made.

If they really sacrifice maybe they do so by producing some toxic substance so as to poison any predator who comes looking for ant meal.

The predator which lead to this behavior could be long extinct, the fact that there is none there now doesn't mean there wasn't some serious threat in the past.

Or those ants could simply believe in some evil ant god who demands sacrifices every night in exchange for colony prosperity.
biohead
not rated yet Sep 25, 2008
I'd like to know if they found the ants as well. What is the purpose if not. I don't assume because there is no count, etc.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.