Colony Structure of a Slavemaking Ant. Ii. Frequency of Slave Raids and Impact on the Host Population

Abstract

Abstract. The parasite pressure exerted by the slavemaker ant Protomognathus americanus on its host species Leptothorax longispinosus was analyzed demographically and genetically. The origin of slaves found in colonies of the obligate slavemaker was examined with nuclear and mitochondrial DNA markers to make inferences about the frequency and severity of slave raids. Relatedness of enslaved L. longispinosus workers in the same nest was very low, and our data suggest that, on average, each slavemaker nest raids six host colonies per season. Therefore, the influence of slavemaker species on their hosts is much stronger than simple numerical ratios suggest. We also found that slave relatedness was higher in small than in large slavemaker nests; thus, larger nests wield a much stronger influence on the host. We estimated that in the study population, on average, a host nest has a 50% chance of being attacked by a slavemaker colony per year. Free-living Leptothorax colonies in the vicinity of slavemaker nests did not represent the source of slaves working in P. americanus colonies, which suggests that raided nests either do not survive or migrate after being raided. Colony composition and intranest relatedness of free-living L. longispinosus colonies differed markedly between areas with slavemakers and those that are parasite-free. In the presence of slavemakers, host colonies were less likely to be polygynous and had fewer workers and a higher relatedness among worker brood. Host nests with slavemaker neighbors allocated more resources into sexuals, possibly caused by these shifts in nest demography. Finally, enslaved Leptothorax workers in P. americanus nests appeared to be less efficient than their counterparts in free-living colonies. Thus, slavemakers exert a much stronger impact on their hosts than had previously been suspected and represent an unique system to study parasite-host coevolution.Key words. Coevolution, microsatellites, parasite-host interaction, raiding behavior, social insects, social parasitism.Received December 21, 1999. Accepted September 18, 2000.Slavemaking ants have intrigued biologists because of their highly specialized behavior and life history, and the species Protomognathus americanus exemplifies that specialization. A P. americanus slavemaking colony is initiated when a newly mated queen invades a host colony, which in this case is one of three Leptothorax species; the slavemaker queen then proceeds either to drive out or to kill the adult workers and resident queen(s) of the invaded colony (Wesson 1939). Invasions occur in mid to late summer, when host colonies contain worker pupae. After invading, the slavemaker queen waits a few weeks until the pupae emerge. These ants as adults start to forage, care for brood, and engage in other tasks necessary for colony maintenance. Shortly after a takeover, then, Leptothorax workers are available to help the P. americanus queen raise her own offspring. Typically, the slavemaker queen produces one or two daughter workers in her first year. Thes...

Increased host aggression as an induced defense against slave-making ants

“…Due to the small number of slave-making workers present in a nest (Foitzik and Herbers 2001), slave-making workers belonging to 25 nests were pooled and randomly chosen for the experiment. Each opponent was used 3 times.…”

“…This parasite exerts especially strong selection pressures on its main host species Temnothorax longispinosus . Its high prevalence and frequent slave raids (Foitzik and Herbers 2001), often lead to the destruction of attacked colonies, thus greatly reducing host fitness (Foitzik et al 2009). Protomognathus americanus colonies are patchily distributed within host populations, resulting in an unequal parasitism risk for host colonies (Herbers and Foitzik 2002).…”

“…Therefore, raiding parties include workers of both ant species, that is, slavemaker and host workers (Figure 1). Enslaved host workers that participate in raids often harm attack host colonies more than slavemakers because these Temnothorax slaves frequently attack and sting defending host workers (Foitzik and Herbers 2001). In contrast, Protomognathus workers only try to drive away defenders and use glandular secretions to cause confusion among host workers but never sting (Foitzik and Herbers 2001).…”

“…Enslaved host workers that participate in raids often harm attack host colonies more than slavemakers because these Temnothorax slaves frequently attack and sting defending host workers (Foitzik and Herbers 2001). In contrast, Protomognathus workers only try to drive away defenders and use glandular secretions to cause confusion among host workers but never sting (Foitzik and Herbers 2001). Scouting events invariably precede slave raids, so the presence of a slavemaker worker within or close to the host nest provides a reliable indicator of a slavemaker attack on the host colony in the near future.…”

Spatial structure and nest demography reveal the influence of competition, parasitism and habitat quality on slavemaking ants and their hosts

“…On a finer scale, slavemaking colonies are not closer to their host colonies than randomly expected, a pattern found to be valid on a coarser scale [WV in the current study; NY in a previous study [20]]. The maximal distance of slave-raids in natural populations is unclear, but polydomous nest parts of a single parasite colony were found between 0.26 and 5.56 m apart [41,42]. Specifically in this case a too fine scale may be of less importance, because slavemakers can reach all parts of the study plots.…”