Raiders from the sky: slavemaker founding queens select for aggressive host colonies

Ant behaviour and brain gene expression of defending hosts depend on the ecological success of the intruding social parasite

The geographical mosaic theory of coevolution predicts that species interactions vary between locales. Depending on who leads the coevolutionary arms race, the effectivity of parasite attack or host defence strategies will explain parasite prevalence. Here, we compare behaviour and brain transcriptomes of Temnothorax longispinosus ant workers when defending their nest against an invading social parasite, the slavemaking ant Temnothorax americanus. A full-factorial design allowed us to test whether behaviour and gene expression are linked to parasite pressure on host populations or to the ecological success of parasite populations. Albeit host defences had been shown before to covary with local parasite pressure, we found parasite success to be much more important. Our chemical and behavioural analyses revealed that parasites from high prevalence sites carry lower concentrations of recognition cues and are less often attacked by hosts. This link was further supported by gene expression analysis. Our study reveals that host-parasite interactions are strongly influenced by social parasite strategies, so that variation in parasite prevalence is determined by parasite traits rather than the efficacy of host defence. Gene functions associated with parasite success indicated strong neuronal responses in hosts, including long-term changes in gene regulation, indicating an enduring impact of parasites on host behaviour. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

Egg discrimination is mediated by individual differences in queen olfactory responsiveness and boldness

Nest parasites attempt to shift the cost of rearing young from themselves to others. Despite strong selection to avoid this exploitation, there is considerable variation among-individuals in susceptibility to nest parasites. We evaluated the effects of individual variation in boldness, aggressiveness, and olfactory responsiveness on egg discrimination in wasps: Polistes metricus, which founds nests as singleton individuals, and P. dominula, which founds nests in small coalitions. Aggressiveness and boldness were evaluated using individuals’ response to mechanical disturbance, and olfactory responsiveness was evaluated using individuals’ tendency to respond to a novel rewarding stimulus. Egg discrimination was evaluated by presenting each queen with a variety of foreign eggs: 1) unaltered eggs laid by the resident (negative control group), 2) eggs produced by the resident female that were removed and replaced (procedural control), 3) eggs of foreign conspecifics (conspecific egg), and 4) eggs of a heterospecific congener (heterospecific egg). Females of both species never rejected untampered eggs and rejected procedural controls in only 35% of cases. Both species were twice as likely (70% rejection rate) to reject eggs of foreign conspecifics or heterospecifics. In P. dominula, bolder individuals and those with low olfactory responsiveness were more likely to reject foreign eggs. In P. metricus, boldness was not associated with egg rejection, but individuals with heightened olfactory responsiveness were more likely to reject foreign eggs. Thus, there are contrasting associations between behavioral phenotypes and egg rejection across species. These results are discussed in light of differences in the colony founding behavior of these species.

The influence of slavemaking lifestyle, caste and sex on chemical profiles in Temnothorax ants: insights into the evolution of cuticular hydrocarbons

Chemical communication is central for the formation and maintenance of insect societies. Generally, social insects only allow nest-mates into their colony, which are recognized by their cuticular hydrocarbons (CHCs). Social parasites, which exploit insect societies, are selected to circumvent host recognition. Here, we studied whether chemical strategies to reduce recognition evolved convergently in slavemaking ants, and whether they extend to workers, queens and males alike. We studied CHCs of three social parasites and their related hosts to investigate whether the parasitic lifestyle selects for specific chemical traits that reduce host recognition. Slavemaker profiles were characterized by shorter-chained hydrocarbons and a shift from methyl-branched alkanes to n-alkanes, presumably to reduce recognition cue quantity. These shifts were consistent across independent origins of slavery and were found in isolated ants and those emerging in their mother colony. Lifestyle influenced profiles of workers most profoundly, with little effect on virgin queen profiles. We detected an across-species caste signal, with workers, for which nest-mate recognition is particularly important, carrying more and longer-chained hydrocarbons and males exhibiting a larger fraction of n-alkanes. This comprehensive study of CHCs across castes and species reveals how lifestyle-specific selection can result in convergent evolution of chemical phenotypes.

The influence of space and time on the evolution of altruistic defence: the case of ant slave rebellion

How can antiparasite defence traits evolve even if they do not directly benefit their carriers? An example of such an indirect defence is rebellion of enslaved Temnothorax longispinosus ant workers against their social parasite Temnothorax americanus, a slavemaking ant. Ant slaves have been observed to kill their oppressors' offspring, a behaviour from which the sterile slaves cannot profit directly. Parasite brood killing could, however, reduce raiding pressure on related host colonies nearby. We analyse with extensive computer simulations for the Temnothorax slavemaker system under what conditions a hypothetical rebel allele could invade a host population, and in particular, how host-parasite dynamics and population structure influence the rebel allele's success. Exploring a wide range of model parameters, we only found a small number of parameter combinations for which kin selection or multilevel selection could allow a slave rebellion allele to spread in the host population. Furthermore, we did not detect any cases in which the reduction of raiding pressure in the close vicinity of the slavemaker nest would substantially contribute to the inclusive fitness of rebels. This suggests that slave rebellion is not costly and perhaps a side-effect of some other beneficial trait. In some of our simulations, however, even a costly rebellion allele could spread in the population. This was possible when host-parasite interactions led to a metapopulation dynamic with frequent local extinctions and recolonizations of demes by the offspring of few immigrants.

The Effect of Social Parasitism by Polyergus breviceps on the Nestmate Recognition System of Its Host, Formica altipetens

Highly social ants, bees and wasps employ sophisticated recognition systems to identify colony members and deny foreign individuals access to their nest. For ants, cuticular hydrocarbons serve as the labels used to ascertain nest membership. Social parasites, however, are capable of breaking the recognition code so that they can thrive unopposed within the colonies of their hosts. Here we examine the influence of the socially parasitic slave-making ant, Polyergus breviceps on the nestmate recognition system of its slaves, Formica altipetens. We compared the chemical, genetic, and behavioral characteristics of colonies of enslaved and free-living F. altipetens. We found that enslaved Formica colonies were more genetically and chemically diverse than their free-living counterparts. These differences are likely caused by the hallmark of slave-making ant ecology: seasonal raids in which pupa are stolen from several adjacent host colonies. The different social environments of enslaved and free-living Formica appear to affect their recognition behaviors: enslaved Formica workers were less aggressive towards non-nestmates than were free-living Formica. Our findings indicate that parasitism by P. breviceps dramatically alters both the chemical and genetic context in which their kidnapped hosts develop, leading to changes in how they recognize nestmates.

Be meek or be bold? A colony-level behavioural syndrome in ants

Consistent individual variation in animal behaviour is nearly ubiquitous and has important ecological and evolutionary implications. Additionally, suites of behavioural traits are often correlated, forming behavioural syndromes in both humans and other species. Such syndromes are often described by testing for variation in traits across commonly described dimensions (e.g. aggression and neophobia), independent of whether this variation is ecologically relevant to the focal species. Here, we use a variety of ecologically relevant behavioural traits to test for a colony-level behavioural syndrome in rock ants (Temnothorax rugatulus). Specifically, we combine field and laboratory assays to measure foraging effort, how colonies respond to different types of resources, activity level, response to threat and aggression level. We find evidence for a colony level syndrome that suggests colonies consistently differ in coping style--some are more risk-prone, whereas others are more risk-averse. Additionally, by collecting data across the North American range of this species, we show that environmental variation may affect how different populations maintain consistent variation in colony behaviour.

The ecological success of a social parasite increases with manipulation of collective host behaviour

Many parasites alter the behaviour of their host to their own advantage, yet hosts often vary in their susceptibility to manipulation. The ecological and evolutionary implications of such variation can be profound, as resistant host populations may suffer lower parasite pressures than those susceptible to manipulation. To test this prediction, we assessed parasite-induced aggressive behaviours across 16 populations of two Temnothorax ant species, many of which harbour the slavemaker ant Protomognathus americanus. This social parasite uses its Dufour's gland secretions to manipulate its hosts into attacking nestmates, which may deter defenders away from itself during invasion. We indeed find that colonies that were manipulated into attacking their Dufour-treated nestmates were less aggressive towards the slavemaker than those that did not show slavemaker-induced nestmate attack. Slavemakers benefited from altering their hosts' aggression, as both the likelihood that slavemakers survived host encounters and slavemaker prevalence in ant communities increased with slavemaker-induced nestmate attack. Finally, we show that Temnothorax longispinosus colonies were more susceptible to manipulation than Temnothorax curvispinosus colonies. This explains why T. curvispinosus colonies responded with more aggression towards invading slavemakers, why they were less likely to let slavemakers escape and why they were less frequently parasitized by the slavemaker than T. longispinosus. Our findings highlight that large-scale geographic variation in resistance to manipulation can have important implications for the prevalence and host preference of parasites.

Warring arthropod societies: Social spider colonies can delay annihilation by predatory ants via reduced apparency and increased group size

Sociality provides individuals with benefits via collective foraging and anti-predator defense. One of the costs of living in large groups, however, is increased apparency to natural enemies. Here, we test how the individual-level and collective traits of spider societies can increase the risk of discovery and death by predatory ants. We transplanted colonies of the social spider Stegodyphus dumicola into a habitat dense with one of their top predators, the pugnacious ant Anoplolepis custodiens. With three different experiments, we test how colony-wide survivorship in a predator-dense habitat can be altered by colony apparency (i.e., the presence of a capture web), group size, and group composition (i.e., the proportion of bold and shy personality types present). We also test how spiders' social context (i.e., living solitarily vs. among conspecifics) modifies their behaviour toward ants in their capture web. Colonies with capture webs intact were discovered by predatory ants on average 25% faster than colonies with the capture web removed, and all discovered colonies eventually collapsed and succumbed to predation. However, the lag time from discovery by ants to colony collapse was greater for colonies containing more individuals. The composition of individual personality types in the group had no influence on survivorship. Spiders in a social group were more likely to approach ants caught in their web than were isolated spiders. Isolated spiders were more likely to attack a safe prey item (a moth) than they were to attack ants and were more likely to retreat from ants after contact than they were after contact with moths. Together, our data suggest that the physical structures produced by large animal societies can increase their apparency to natural enemies, though larger groups can facilitate a longer lag time between discovery and demise. Lastly, the interaction between spiders and predatory ants seems to depend on the social context in which spiders reside.

Integrating animal personality into insect population and community ecology

Despite the recent surge of interest in the concept of animal personalities, that is, temporally consistent individual differences in behavior, few studies have integrated intraspecific behavioral variation in population or community ecology. Insects and other arthropods provide ideal model systems to study how intraspecific behavioral variation affects phenomena in ecology. This is due to the fact that arthropods not only are highly amenable to experimental manipulation, but they also allow us to answer general ecological questions on multiple scales of biological organization. Herein, we review recent developments and views on how the framework of animal personality could provide a deeper understanding of classic issues in (1) population ecology (e.g., local adaptation, dispersal, and invasion), (2) community ecology (e.g., food webs and ecosystem engineering), and (3) more insect-focused topics such as metamorphosis and pollination biology.

Oh sister, where art thou? Spatial population structure and the evolution of an altruistic defence trait

The evolution of parasite virulence and host defences is affected by population structure. This effect has been confirmed in studies focusing on large spatial scales, whereas the importance of local structure is not well understood. Slavemaking ants are social parasites that exploit workers of another species to rear their offspring. Enslaved workers of the host species Temnothorax longispinosus have been found to exhibit an effective post-enslavement defence behaviour: enslaved workers were observed killing a large proportion of the parasites' offspring. As enslaved workers do not reproduce, they gain no direct fitness benefit from this 'rebellion' behaviour. However, there may be an indirect benefit: neighbouring host nests that are related to 'rebel' nests can benefit from a reduced raiding pressure, as a result of the reduction in parasite nest size due to the enslaved workers' killing behaviour. We use a simple mathematical model to examine whether the small-scale population structure of the host species could explain the evolution of this potentially altruistic defence trait against slavemaking ants. We find that this is the case if enslaved host workers are related to nearby host nests. In a population genetic study, we confirm that enslaved workers are, indeed, more closely related to host nests within the raiding range of their resident slavemaker nest, than to host nests outside the raiding range. This small-scale population structure seems to be a result of polydomy (e.g. the occupation of several nests in close proximity by a single colony) and could have enabled the evolution of 'rebellion' by kin selection.