Abstract
How workers within an ant colony perceive and enforce colony boundaries is a defining biological feature of an ant species. Ants fall along a spectrum of social organizations ranging from single-queen, single nest societies to species with multi-queen societies in which workers exhibit colony-specific, altruistic behaviors towards non-nestmate workers from distant locations. Defining where an ant species falls along this spectrum is critical for understanding its basic ecology. Herein we quantify queen numbers, describe intraspecific aggression, and characterize the distribution of colony sizes for tawny crazy ant (Nylanderia fulva) populations in native range areas in South America as well as in their introduced range in the Southeastern United States. In both ranges, multi-queen nests are common. In the introduced range, aggressive behaviors are absent at all spatial scales tested, indicating that within the population in the Southeastern United States N. fulva is unicolonial. However, this contrasts strongly with intraspecific aggression in its South American native range. In the native range, intraspecific aggression between ants from different nests is common and ritualized. Aggression is typically one-sided and follows a stereotyped sequence of escalating behaviors that stops before actual fighting occurs. Spatial patterns of non-aggressive nest aggregation and the transitivity of non-aggressive interactions demonstrate that results of neutral arena assays usefully delineate colony boundaries. In the native range, both the spatial extent of colonies and the average number of queens encountered per nest differ between sites. This intercontinental comparison presents the first description of intraspecific aggressive behavior for this invasive ant and characterizes the variation in colony organization in the native-range, a pre-requisite to a full understanding of the origins of unicoloniality in its introduced range.
Highlights
How social insect colonies define the boundaries of their societies is fundamental to their biology
Sites in the native range differed in the number of queens found per nest (Wilcoxon: Χ2 = 10.6, DF = 3, N = 115, P < 0.02) with more queens per nest found in the Uruguay roadside survey (1.23±2.70) than in Argentina Site 2 (0.05±0.23) or Argentina Site 3 (0±0), while Argentina Site 1 was intermediate (1.18±5.2)
Within the introduced range, intraspecific aggression is absent at all spatial scales tested, supporting the inference that this ant is unicolonial within North America
Summary
How social insect colonies define the boundaries of their societies is fundamental to their biology. Defining where a species falls along this spectrum and quantifying the variation that colonies exhibit in this trait is critical for understanding the basic ecology of a species Defining these traits for an invasive ant species takes on additional importance and complexity as a complete picture requires characterizing the nature of intraspecific aggression and using that knowledge to define the limits and organization of colonies in both the introduced and native range of that species. Through human-mediated, jump dispersal local-populations of N. fulva have established in every Gulf coast state and in counties throughout Florida and the Gulf region of Texas [10] Both red imported fire ants and Argentine ants originate in South America and have spread from established populations in the Southeastern United States to global distributions [11,12,13]. With their current North American bridgehead, tawny crazy ants are poised to follow that same path to introduction into other parts of the world
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