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New research uncovers the predatory behavior of Florida's skull-collecting ant

North Carolina Museum of Natural Sciences

"Add 'skull-collecting ant' to the list of strange creatures in Florida," says Adrian Smith a scientist at the North Carolina Museum of Natural Sciences and North Carolina State University. His new research describes the behavioral and chemical strategies of a Florida ant, Formica archboldi, that decorates its nest with the dismembered body parts of other ant species.

A video about these ants and this research, for media use, can be found here: https://youtu.be/heuYzC12etg

"In 1958, shortly after this ant was described as a species, scientists reported something weird about it," says Smith, the author of this study. Its nests were home to a collection of decapitated heads of trap-jaw ants. Trap-jaw ants are known as fierce insect predators, not easy prey for other ants. Since then, researchers have speculated that F. archboldi either inherits old trap-jaw ant nesting sites or is somehow a specialized predator. However, scientists had yet to study the biology of this ant in detail.

"This was a study that grew out of reading a peculiar observation in a 60-year-old research paper," says Smith. "Odds were that these ant heads weren't in Formica nests by chance and that there was some interesting biology behind this natural history note." In researching this entomological oddity, Smith was surprised to find that F. archboldi chemically mimic their trap-jaw ant prey and use what is normally a chemical defense, a spray of formic acid, as a deadly weapon against trap-jaw ants.

Using high-speed video recordings Smith found that F. archboldi attacks involve a targeted spray of formic acid that quickly leads to an immobilized trap-jaw ant. Time-lapse video observation of the interior nest chambers of laboratory colonies found that freshly-killed trap-jaw ants are dragged into the nest like food items and dismembered. Leading to nests filled with trap-jaw ant body parts, as is found in natural colonies.

"The scientifically surprising finding of this study was that these ants chemically match or mimic the chemical profiles of two species of trap-jaw ant," says Smith. The ants match in their cuticular hydrocarbons, a complex layer of waxes that coat the outer surface of an ant. Ants typically use these compounds as nestmate and species-specific signals. "It's really unusual for an ant species to display this much variation in chemical signature. Also, chemical mimicry is usually a tactic used by social parasites, but there's no evidence that F. archboldi are a parasitic species."

"In aiming to figure out one unusual aspect of this ant's biology, this research has turned up another in the chemical data," says Smith. Though this study was unable to find a direct link between chemical mimicry and predatory behavior, this chemical mimicry likely hints at a long evolutionary history between these ant species. "Now Formica archboldi is the most chemically diverse ant species we know of. Before this work, it was just a species with a weird head-collecting habit. Now we have what might be a model species for understanding the evolution of chemical diversification and mimicry", says Smith.

The paper, "Prey specialization and chemical mimicry between Formica archboldi and Odontomachus ants," is published in the journal Insectes Sociaux.

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Note to Editors: The study abstract follows.

"Prey specialization and chemical mimicry between Formica archboldi and Odontomachus ants"

Author: Adrian A. Smith, North Carolina Museum of Natural Sciences & North Carolina State University

Published: 11-16-2018, Insectes Sociaux

The article is available at: https://doi.org/10.1007/s00040-018-0675-y

Abstract: Formica ants are not known to be prey specialists on other ant species, however, for the past 60 years, field observations on Formica archboldi in the southeastern United States have noted that their nests are scattered with body parts of Odontomachus trap-jaw ants. This study investigates the relationship between F. archboldi and Odontomachus. Through a series of behavioral experiments and a descriptive study of their chemical ecology, I find: (1) behavioral evidence that F. archboldi are more capable predators of Odontomachus in comparison to other Formica. (2) F. archboldi match the cuticular hydrocarbon profiles of the native species of Odontomachus that they occur with. This includes O. brunneus and O. relictus and the intraspecific variation found across Florida populations of O. brunneus. (3) F. archboldi do not display a prey retrieval preference towards hydrocarbon-matching Odontomachus as compared to mismatching. (4) F. archboldi that match Odontomachus hydrocarbon profiles do not receive lower levels of aggression than mismatching F. archboldi. Beyond providing natural history insights into the relationship between these species, this study expands our knowledge of an important insect chemical phenotype. The intraspecific variability in F. archboldi cuticular hydrocarbon profiles is among the greatest reported for social insects and provides a unique case of how non-parasitic species can generate parasite-like chemical-mimic phenotypes.

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