title: Chemically armed mercenary ants protect fungus-farming societies.
abstract: The ants are extraordinary in having evolved many lineages that exploit closely related ant societies as social parasites, but social parasitism by distantly related ants is rare. Here we document the interaction dynamics among a Sericomyrmex fungus-growing ant host, a permanently associated parasitic guest ant of the genus Megalomyrmex, and a raiding agro-predator of the genus Gnamptogenys. We show experimentally that the guest ants protect their host colonies against agro-predator raids using alkaloid venom that is much more potent than the biting defenses of the host ants. Relatively few guest ants are sufficient to kill raiders that invariably exterminate host nests without a cohabiting guest ant colony. We also show that the odor of guest ants discourages raider scouts from recruiting nestmates to host colonies. Our results imply that Sericomyrmex fungus-growers obtain a net benefit from their costly guest ants behaving as a functional soldier caste to meet lethal threats from agro-predator raiders. The fundamentally different life histories of the agro-predators and guest ants appear to facilitate their coexistence in a negative frequency-dependent manner. Because a guest ant colony is committed for life to a single host colony, the guests would harm their own interests by not defending the host that they continue to exploit. This conditional mutualism is analogous to chronic sickle cell anemia enhancing the resistance to malaria and to episodes in human history when mercenary city defenders offered either net benefits or imposed net costs, depending on the level of threat from invading armies.
authors.affiliation: Centre for Social Evolution, Department of Biology, University of Copenhagen, DK-2100 Copenhagen, Denmark.
authors.forename: Rachelle M M
authors.initials: RM
authors.lastname: Adams
authors.forename: Joanito
authors.initials: J
authors.lastname: Liberti
authors.forename: Anders A
authors.initials: AA
authors.lastname: Illum
authors.forename: Tappey H
authors.initials: TH
authors.lastname: Jones
authors.forename: David R
authors.initials: DR
authors.lastname: Nash
authors.forename: Jacobus J
authors.initials: JJ
authors.lastname: Boomsma
mesh_terms: D000818:Animals; D001000:Ants / Q000382:microbiology*; D005658:Fungi / Q000502:physiology*; D006790:Host-Parasite Interactions; D010802:Phylogeny; D011235:Predatory Behavior; D013559:Symbiosis; D055549:Volatile Organic Compounds / Q000378:metabolism*
chemical_list: D055549:Volatile Organic Compounds
keywords: Attini; Solenopsidini; evolutionary transition; symbiosis
references.citation: Q Rev Biol. 1996 Mar;71(1):37-78
references.pmid: 8919665
references.citation: Curr Biol. 2012 Sep 11;22(17):R671-3
references.pmid: 22974989
references.citation: Nature. 2002 May 30;417(6888):505-6
references.pmid: 12037556
references.citation: Nature. 2004 Oct 14;431(7010):841-4
references.pmid: 15483611
references.citation: Science. 2010 Jul 9;329(5988):212-5
references.pmid: 20616278
references.citation: J Chem Ecol. 1991 Sep;17(9):1897-908
references.pmid: 24257928
references.citation: Naturwissenschaften. 2003 Dec;90(12):568-71
references.pmid: 14676954
references.citation: Br Med J. 1954 Feb 6;1(4857):290-4
references.pmid: 13115700
references.citation: Annu Rev Entomol. 2001;46:573-99
references.pmid: 11112180
references.citation: Proc Biol Sci. 2007 Nov 7;274(1626):2717-21
references.pmid: 17711838
references.citation: Am Nat. 2012 May;179(5):595-605
references.pmid: 22504542
references.citation: Proc Natl Acad Sci U S A. 2008 Apr 8;105(14):5435-40
references.pmid: 18362345
references.citation: Trends Ecol Evol. 1986 Dec;1(6):155-60
references.pmid: 21227804
references.citation: Proc Natl Acad Sci U S A. 2006 Nov 28;103(48):18172-7
references.pmid: 17079492
references.citation: Proc Biol Sci. 2009 Jul 7;276(1666):2461-8
references.pmid: 19364750
references.citation: Trends Ecol Evol. 1999 Feb;14(2):49-53
references.pmid: 10234251
references.citation: Naturwissenschaften. 2000 Dec;87(12):549-54
references.pmid: 11198197