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Major evolutionary transitions (METs) across Earth's biological history describe fusions of lower-level entities into higher-level individuals (evolutionary transitions in individuality: ETIs) as well as novel forms of information storage and transmission (Information Leaps). Obligate eusociality is frequently listed as a MET—most often in the context of being an ETI and with an extrapolation that the ETI requires inclusive fitness maximization for all parties. However, obligate eusociality neither fundamentally alters how information is stored and transmitted nor meets the various criteria proposed for an ETI. We argue that rather than representing a higher-level individual, the evolution of non-reproductive worker castes is more analogous to a novel 'organ' that maintains homeostasis and nurtures the gonadal tissue of mated queens. Worker castes benefit queens by performing dangerous but necessary functions such as foraging, while enabling the gamete-producing functions to be kept relatively safe. This is an ecologically successful and significant evolutionary innovation, which can be thought of as a major competitive transition (MCT). In this context, we hypothesize that worker castes are most likely to evolve through parental manipulation. Employing such a MCT perspective generates a broad series of predictions about eusocial life histories. [ABSTRACT FROM AUTHOR]

Ant colonies are likely able to access food locations by multiple paths that can vary predictably in length or mortality risk. To favor one path over another requires ants to perceive, communicate and act upon important differences in length and risk between paths. Here, we present replicate Linepithema humile colonies with four equal-length paths to a sugar source. The paths vary in probabilities of encountering a risk cue that range from 0 to 100%. The risk cues were either live workers of an aggressive competitor (Liometopum occidentale: LO) or only formic acid (FA), a defensive chemical common in many formicine species. Both the probabilities of encounter and type of cue affected path preferences. Although across both cues the zero-risk path was most used, the use was not exclusive and patterns of response to the cues differed significantly. More ants were on all the paths when the cue was LO rather than FA, leading to a higher success rate at finding food. Worker numbers on paths with LO did decline over time as consistent with recognizing neighbors to be 'dear enemies' that are a reduced threat to the colony. Similarly, changes in path usage suggested that the FA cue also became viewed as less threatening over time when competitors were never simultaneously present. The results are consistent with L. humile exhibiting the behavioral plasticity and communicative ability needed to categorize and predict risk to efficiently collect food while defending against other ant species. [ABSTRACT FROM AUTHOR]

When ants encounter a previously unexplored area, a variety of search patterns are possible. Previous studies have found that subsequent individuals tend to follow those that have gone before. This suggests that ants leave signature marks as they walk, independent of having encountered food, and that others recognize these and follow. This experiment tested if Argentine ants (Linepithema humile) also exhibit this pattern of behavior. Series of workers individually navigated through a grid consisting of two consecutive right-versus-left decisions. The resulting pattern does not significantly differ from random choice, as workers are neither more nor less likely to choose the path taken by a previous ant. The time between decisions also does not predict the path chosen. Either L. humile does not mark as they walk or workers do not use such marks in guiding their exploratory behavior. Under natural conditions, this suggests that L. humile is more expansive in initially distributing its workers across novel ground and less likely to have them clump together than species that do follow each other. This has implications for how the relative effectiveness in finding versus exploiting and defending food sites might vary across species. [ABSTRACT FROM AUTHOR]

Linepithema humile (Mayr, 1868) ants self-organize mass recruitment to food sources by laying pheromone trails to direct foragers. Initial exploration may occur randomly, but selection of the shortest route is often derived from positive feedback mechanisms rather than individual intelligence or decision-making. In this study, we examined whether L. humile foraging routes minimize travel distance and maximize linearity (i.e., have the fewest turns). Colonies of L. humile foraged in an artificial grid containing a number of possible routes to a long-lasting food source. In the design, there were three possible shortest routes that varied only in the number of turns that the ants had to make. The majority of foraging ants preferred one of the three shortest routes to the food, and significantly favored the path with the fewest turns. Selection of the shortest, straightest route may maximize mass recruitment to the food source by decreasing the chance of foraging ants losing the trail. [ABSTRACT FROM AUTHOR]

In eusocial nests, colony task are divided among queens and workers, but how this division of labor develops is unknown for most species. We compared division of labor and aggressive behavior among queens and workers in the facultatively eusocial bee, Megalopta genalis, using nests with established queen-worker pairs and nests in which the incipient worker had recently emerged. We find that the majority of aggression is directed from queens toward workers in both incipient and established relationships. Established workers forage and perform trophallaxis as donors more frequently than queens, but both queens and workers perform trophallaxis as donors when workers are young. Queens spend significantly more time nest guarding than incipient and established workers, perhaps because older workers spend more time foraging and incipient workers spend significantly more time in cells than do queens. Our results show that the development of worker behavior involves dynamic temporal changes in task performance among queens and workers during the 10 days after worker emergence. During this establishment period, queens engage in maternal care by feeding their daughters, but are also aggressive toward them. This may be a mechanism by which queens coerce their daughters into becoming non-reproductive workers. [ABSTRACT FROM AUTHOR]

Ant queens often cooperate in starting colonies (pleometrosis), but not all foundresses are likely to achieve equal reproductive success. Therefore, joining decisions may be influenced by queens' perceptions of a partner's likelihood to be of mutualistic benefit or to be a successful competitor in eventually controlling reproduction. Large queen size (as measured by weight) was assumed to be a desired characteristic in a mutualistic partner, but to be avoided in a potential competitor. With respect to this variable, Lasius pallitarsis queens appeared to join others in a manner consistent with increasing their competitive advantage. When given a choice between joining another queen or nesting alone, only queens with a large weight advantage were significantly likely to join. When given a choice between joining either a light or a heavy queen, queens of all weights preferentially joined the lighter resident. Moreover, when queen condition was improved by feeding, changes in joining behavior were consistent with predictions of improved competitive ability. Finally, lighter queens were more likely to leave nest sites when joined by others. However, queens significantly aggregated when in high densities, which may be consistent with gaining mutualistic benefits. Pleometrosis appears to have an evolutionary dynamic between mutualistic group benefits and individual competition to monopolize those benefits. [ABSTRACT FROM AUTHOR]

Lasius pallitarsis ants were placed in situations where they encountered unfamiliar areas and had to choose between one of two directions for further exploration. Workers advancing onto new ground apparently leave behind some chemical signature to which later ants orient. This orientation occurred under two types of experimental conditions. First, ants show a significant tendency to follow each other as they advance out from their colony into unexplored areas. The same ants transferred into an entirely novel situation, in which there is no obvious 'homeward' direction, show similar behavior. When ants are coming from familiar ground, following tends to increase as the number of ants passing the decision point increases. However, following decreases when ants are on entirely novel ground. The results are discussed in relation to models describing foraging responses. [ABSTRACT FROM AUTHOR]

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