A Deadication to Tidy Graveyards

Our lab keeps two types of ants: turtle ants (Cephalotes varians), and slender twig ants (Pseudomyrmex gracilis). My labmates and I were surprised to notice that both species like to move all of their colony’s dead into small piles– graveyards, if you will! This behavior is called necrophoresis, and it’s actually pretty common amongst social insects like ants, termites, and bees.

One of our Pseudomyrmex gracilis ants carrying a corpse.

Two alive Pseudomyrmex ants (outlined in green).  

All of the other dark spots in this photo are parts of dead ants.

I’ve been particularly interested in the Pseudomyrmex graveyards. They seem to have more corpses and take up more space than the graveyards made by the turtle ants. Here are the main questions I’ve had about Pseudomyrmex necrophoresis:

1. Why do both of our (separate) Pseudomyrmex colonies have their graveyards on the same side of their respective boxes? 
2. Why do they make one large pile of corpses, instead of multiple smaller piles?

I started exploring the first question by performing my own mini experiment during ant feedings. Our ant colonies are kept in clear plastic boxes on a shelving unit. Both the Pseudomyrmex colonies make their graveyards on the sides of their boxes that are facing out of the shelf. My first thought was because the colony boxes are rectangular, and the side of the box that’s facing outwards is one of the short ends, that maybe the ants are trying to get their graveyards as far away from their nests as possible. So I rotated one of the colony boxes by 90º, so the long edge of the box was now facing outwards. To my surprise, within two days, the entire corpse pile had been moved to this long edge! Curious, I decided to take “before” photos of both Pseudomyrmex colonies, rotate both of their boxes by 90º, and see what would happen to their corpse piles. Here are the photos:

Photos of two Pseudomyrmex gracilis colonies before and after 90º rotation.

Colony 18-7 Before rotation	Colony 18-7 2 days after rotation	Colony 18-7  7 days after rotation
Colony 18-8 Before rotation	Colony 18-8 2 days after rotation	Colony 18-8 7 days after rotation

These photos show how the two colonies moved their graveyards (seen as piles of yellow and black specks) in response to their container being rotated 90º. In all of these images, the end of the box facing out of the shelf (receiving the most light) is on the right. The images have been edited to reduce skew and shadows.

It seems that, regardless of the orientation of the box, the ants still prefer to place their corpses on the side facing out. It also looks like after the initial move of the graveyard to its new location, it starts to become more spread out. When I mentioned my observations to Prof Donaldson, she suggested that because there is more light coming through the side of the box facing out of the shelf, the ants might feel like it’s more “outside” or more removed from the nest than the other sides of the box. 

After doing a little bit of digging into ant necrophoresis literature, it seems like this might be the case! Although I couldn’t find any papers specific to any kind of Pseudomyrmex ant, I did find an interesting thesis from the University of Maine about the graveyard placement preferences of the European red ant (Myrmica rubra L.). In a lab setting, the ants placed significantly more corpses on surfaces that were in direct light (as opposed to shadow). Similarly, in the field, significantly more corpses were carried to plots receiving direct sunlight than ones in shade. Even though this study was examining a different species of ants, a tendency to locate graveyards in areas that receive direct sunlight mirrors what we see with our Pseudomyrmex colonies.

This brings us to my second question- why do Pseudomyrmex form one big pile of corpses instead of multiple smaller ones? Unfortunately, I haven’t been able to find an entirely clear answer. It seems that there’s some variation across species: a paper by Banik et al. examining two species of ants in India found that in a lab setting, one species (Camponotus compressus) formed a graveyard (which they eventually abandoned, opting to start another one in a separate location). On the other hand, another species (Diacamma vagans) removed corpses from their nest, but spread them out instead of forming a pile. The authors note that their experimental setup may have restricted the usual necrophoresis behavior of the ants, as they were given less than a meter of space to move in. However, the fact that two different ant species in the same type of experimental setup did not create similar graveyards signals that necrophoresis behaviors likely differ by species.

Originally, I was under the impression that our turtle ants were just bad at making graveyards because they were not nearly as visible, dense, or consistent in location as the Pseudomyrmex ones. Having learned different species of ants seem to form different types of graveyards, I feel like I owe the turtle ants an apology. Instead of being disappointed in our turtle ants, I’m now very impressed with our Pseudomyrmex ants and their graveyards!


Further Reading

Banik, Sarmisth, Sutopa Biswas, Ranajit Karmakar, and Ratanlal Brahmachary. 2010. “Necrophoresis in two Indian ant species, Camponotus compressus (Fabricius) and Diacamma vagans (Smith) (Insecta: Hymenoptera: Formicidae).” Proceedings of the Zoological Society 63 (December): 87-91. doi:10.1007/s12595-010-0012-6.

• Two species of ants found in India had different ways of managing the dead when observed in the lab: one did not form any graveyards, whereas the other formed a graveyard, abandoned it, and formed a new one in a different location.

Diez, Lise, Jean-Louis Deneubourg, Lucie Hoebeke, and Claire Detrain. 2011. “Orientation in corpse-carrying ants: memory or chemical cues?” Animal Behaviour 81 (June): 1171–1176. doi:10.1016/j.anbehav.2011.02.026.

• For Myrmica rubra colonies with low death rates, corpses were often dispersed throughout the environment instead of accumulated in graveyard piles.

Diez, Lise, Laura Moquet, and Claire Detrain. 2013. “Post-mortem Changes in Chemical Profile and their Influence on Corpse Removal in Ants.” Journal of Chemical Ecology 39 (December): 1424–1432. doi:10.1007/s10886-013-0365-1.

• Ants tell if another ant is dead by smelling them! Oleic and linoleic acids are produced after an ant dies, and signals to the colony that it’s a corpse and should be removed. 

Diez, Lise, Philippe Lejeune, and Claire Detrain. 2014. “Keep the nest clean: survival advantages of corpse removal in ants.” Animal Behaviour 10 (July). doi:10.1098/rsbl.2014.0306.

• Adults in colonies have higher survival rates if the colony is allowed to remove its corpses from the nest.

Graham, Carrie E. 2007. "Necrophoresis and the Behavioral Responses of the European Red Ant, Myrmica rubra L. (Hymenoptera: Formicidae) to the Fungel Entomopathogen Beauveria bassiana (Bals.) Vuill." University of Maine Electronic Theses and Dissertations. 1229.

• European red ants placed significantly more corpses in directly lit areas than ones in shadow. This result held for both laboratory and field settings!