Colony fusion and worker reproduction after queen loss in army ants

Abstract

Theory predicts that altruism is only evolutionarily stable if it is preferentially directed towards relatives, so that any such behaviour towards seemingly unrelated individuals requires scrutiny. Queenless army ant colonies, which have anecdotally been reported to fuse with queenright foreign colonies, are such an enigmatic case. Here we combine experimental queen removal with population genetics and cuticular chemistry analyses to show that colonies of the African army ant Dorylus molestus frequently merge with neighbouring colonies after queen loss. Merging colonies often have no direct co-ancestry, but are on average probably distantly related because of overall population viscosity. The alternative of male production by orphaned workers appears to be so inefficient that residual inclusive fitness of orphaned workers might be maximized by indiscriminately merging with neighbouring colonies to increase their reproductive success. We show that worker chemical recognition profiles remain similar after queen loss, but rapidly change into a mixed colony Gestalt odour after fusion, consistent with indiscriminate acceptance of alien workers that are no longer aggressive. We hypothesize that colony fusion after queen loss might be more widespread, especially in spatially structured populations of social insects where worker reproduction is not profitable.

Should I stay or should I go now? Patch use by African army ant colonies in relation to food availability and predation

“…In agricultural areas and savanna-like vegetation at 1100 m asl we found ants belonging to a D. (Typhlopone) species at four baits, whereas 51 of the 53 samples collected at the three other sites belonged to species in the subgenera Dorylus s.s. and Rhogmus and two belonged to D. molestus. Males of a D. (Typhlopone) species were collected at light sources just below the forest at around 1600 m asl but dispersal distances of the large Dorylus males are apparently very long (Kronauer et al, 2010). More extensive sampling is therefore required to determine whether colonies of this species occur in the agricultural area close to the forest.…”

Interspecific and conspecific colony mergers in the dwarf honey bees Apis andreniformis and A. florea

“…However, we suspected that queenless colonies may sometimes merge with nearby conspecific colonies, as has been observed in the Cape honey bee A. m. capensis (Neumann et al, 2001) and the African army ant Dorylus molestus (Kronauer et al, 2009). Furthermore, we have previously reported that A. florea workers are occasionally found in A. andreniformis nests in unmanipulated colonies (Wongvilas et al, 2010).…”

“…Two recent studies exclude the possibility of social parasitism and drift, causing the authors to postulate that colony mergers (Kronauer et al, 2009), and drifting (Sumner et al, 2007), occur because population viscosity leads to non-zero relatedness among nests. Thus, it is suggested that non-natal workers increase their inclusive fitness if they join and work for a nearby colony, for they will have at least some relatedness to it.…”

“…Thus, it is suggested that non-natal workers increase their inclusive fitness if they join and work for a nearby colony, for they will have at least some relatedness to it. In particular, a study of the African army ant Dorylus molestus (Kronauer et al, 2009) showed that queenless workers produce few if any viable males and are unable to raise a new queen. Therefore, the reproductive options of queenless workers are few, and are possibly maximized by merging with a nearby colony, which is likely to be related, even if marginally so (Kronauer et al, 2009).…”

“…In particular, a study of the African army ant Dorylus molestus (Kronauer et al, 2009) showed that queenless workers produce few if any viable males and are unable to raise a new queen. Therefore, the reproductive options of queenless workers are few, and are possibly maximized by merging with a nearby colony, which is likely to be related, even if marginally so (Kronauer et al, 2009). In a study of the paper wasp Polistes canadensis, 56% of females moved between nests and worked in the non-natal nests (Sumner et al, 2007).…”