Three queen morphs with alternative nest-founding behaviors in the ant, Temnothorax longispinosus

In many ant species, multiple modes of founding new colonies occur in the same population. These modes include dependent founding, independent founding by haplometrosis (single queen), and independent founding by pleometrosis (multiple queens). In several cases, a dimorphism in queen size has been found, such that each morph specializes in a particular nest-founding behavior. I investigated queen size in the ant Temnothorax longispinosus in several southern Wisconsin populations and found three distinct queen morphs: small queens with very low fat content and short wings, large queens with low fat and long wings, and large queens with high fat and very long wings. Several traits associated with founding behavior correlated with these queen sizes. Small queens were produced in lower numbers, were more common in polygynous nests, and returned to the nest in higher proportions than both large queen morphs. The size ranges and fat levels of each queen morph were similar to those of other species that specialize in either haplometrosis (very large, high fat), pleometrosis (large, low fat), or dependent founding (small and low fat). However, there was extensive overlap in several of the founding behaviors, suggesting that the morphs in these populations have some flexibility in founding behavior. The queen morphs in these populations of T. longispinosus may resemble early stages in the evolution of more specialized dispersal polymorphisms found in other ant species. Received 11 January 2006; revised 15 September 2006; accepted 18 September 2006.     

Larval recognition by Temnothorax longispinosus and T. ambiguus hosts of the slave-making ant Protomognathus americanus revisited: colony-level referent ensures conspecific preference

Slave-making ants exploit the labour of their own or another species. Temnothorax ambiguus and T. longispinosus are both ant species that serve as hosts of the obligatory slave-making ant Protomognathus americanus and are facultative slave-makers themselves. We offered laboratory colonies of T. ambiguus and T. longispinosus a series of choices among different larval types to better understand their brood discrimination abilities. Workers of both species preferentially accepted nestmate over non-nestmate larvae. Both species preferentially retrieved unrelated conspecific larvae over congeneric allospecific larvae, and T. ambiguus workers consumed more allospecifics than conspecifics. When presented with conspecific versus P. americanus larvae, both species manifested a clear bias towards conspecific larvae in terms of earlier retrieval and reduced cannibalism. That workers did not prefer P. americanus larvae over conspecific larvae as documented in previous research likely reflects the fact that in the present study, subject workers had access to the entirety of their colony as a referent during rearing and at the time of testing, as they would in nature. Moreover, reciprocal contact between P. americanus and conspecific larvae increased acceptance of the slave-maker larvae, but did not appear to lessen the acceptability of conspecific larvae. This suggests that transfer of cues through contact may be sufficient to alter the recognition signature of P. americanus larvae increasing acceptability by their hosts.     

Worker dominance and policing in the ant Temnothorax unifasciatus

In many species of eusocial Hymenoptera, conflict about the production of males is resolved through “policing.” Recent studies in wasps and the ant Temnothorax unifasciatus suggest that in these species policing workers are dominant themselves and selfishly increase their own chances of later becoming fertile. Policing may therefore to some extent be associated with dominance and selfishness, and dominance and policing behaviour are indeed difficult to distinguish and often not mutually exclusive. Moreover, selfish policing requires that workers form rank orders already in the presence of the queen. Here, we try to allocate aggressiveness by workers towards policing and/or dominance behaviour and investigate whether hierarchies based on subtle, non-aggressive interactions exist in queenright colonies of the ant T. unifasciatus. We either split colonies into a queenright and queenless halve or temporarily removed the queen from complete colonies, which in both cases allows a few dominant workers to lay eggs in the queenless colony. Reunification of colony halves and return of the queen to orphaned colonies led to aggression against those workers that had become fertile during the absence of the queen. Dominant workers in reunited, split colonies were more severely attacked than those in orphaned colonies after return of the queen. Furthermore, we observed that workers, which later became dominant egg layers under queenless conditions, have more contact with the queen than other workers. Both results corroborate the existence of rank relationships among workers in queenright colonies and show that results from policing experiments may be affected by the disturbance of pre-existing hierarchies through colony splitting.     

Quorum sensing by encounter rates in the ant Temnothorax albipennis

Emigrating colonies of the ant Temnothorax (formerly Leptothorax)albipennis can choose the best of several nest sites, even whenthe active ants organizing the move do not compare sites. Thiscollective ability depends on a quorum rule used by ants assessinga candidate site. Only when the site's population has surpasseda threshold do they switch from slow recruitment of fellow activeants by tandem runs to rapid transport of the majority of thecolony. Here, I show that ants perceive the achievement of aquorum through their rate of direct encounters with nest matesat the site. When ants in a crowded site were prevented fromtactile contact with nest mates, they recruited by tandem runs,as though to an empty nest. Furthermore, when the encounterrate was raised independent of population, by reducing the sizeof the candidate nest, ants started to transport at a significantlylower population. The switch occurred at the same encounterrate regardless of nest size, whether the rate was measuredas the mean over the entire visit or as the inverse of the latencyuntil the first encounter. Because encounter rate reflects thedensity of nest mates and thus varies with nest size as wellas population, the ants' collective decision-making algorithmmay be robust to the exact population at which the switch totransport occurs. Ants cease monitoring quorum presence afterswitching to transport, coincident with an abrupt shorteningof visit duration by approximately 2 min, which may be interpretedas the time required for quorum detection.     

Colony size affects collective decision-making in the ant Temnothorax albipennis

Social insects are well-known for their ability to achieve robust collective behaviours even when individuals have limited information. It is often assumed that such behaviours rely on very large group sizes, but many insect colonies start out with only a few workers. Here we investigate the influence of colony size on collective decision-making in the house-hunting of the ant Temnothorax albipennis. In experiments where colony size was manipulated by splitting colonies, we show that worker number has an influence on the speed with which colonies discover new nest sites, but not on the time needed to make a decision (achieve a quorum threshold) or total emigration time. This occurred because split colonies adopted a lower quorum threshold, in fact they adopted the same threshold in proportion to their size as full-size colonies. This indicates that ants may be measuring relative quorum, i.e. population in the new nest relative to that of the old nest, rather than the absolute number. Experimentally reduced colonies also seemed to gain more from experience through repeated emigrations, as they could then reduce nest discovery times to those of larger colonies. In colonies of different sizes collected from the field, total emigration time was also not correlated with colony size. However, quorum threshold was not correlated with colony size, meaning that individuals in larger colonies adopted relatively lower quorum thresholds. Since this is a different result to that from size-manipulated colonies, it strongly suggests that the differences between natural small and large colonies were not caused by worker number alone. Individual ants may have adjusted their behaviour to their colony’s size, or other factors may correlate with colony size in the field. Our study thus shows the importance of experimentally manipulating colony size if the effect of worker number on the emergence of collective behaviour is to be studied. Received 13 December 2005; revised 9 May 2006; accepted 15 May 2006.     

The sensory arrays of the ant,Temnothorax rugatulus

Individual differences in response thresholds to task-related stimuli may be one mechanism driving task allocation among social insect workers. These differences may arise at various stages in the nervous system. We investigate variability in the peripheral nervous system as a simple mechanism that can introduce inter-individual differences in sensory information. In this study we describe size-dependent variation of the compound eyes and the antennae in the ant Temnothorax rugatulus. Head width in T. rugatulus varies between 0.4 and 0.7 mm (2.6–3.8 mm body length). But despite this limited range of worker sizes we find sensory array variability. We find that the number of ommatidia and of some, but not all, antennal sensilla types vary with head width.The antennal array of T. rugatulus displays the full complement of sensillum types observed in other species of ants, although at much lower quantities than other, larger, studied species. In addition, we describe what we believe to be a new type of sensillum in hymenoptera that occurs on the antennae and on all body segments. T. rugatulus has apposition compound eyes with 45–76 facets per eye, depending on head width, with average lens diameters of 16.5 μm, rhabdom diameters of 5.7 μm and inter-ommatidial angles of 16.8°. The optical system of T. rugatulus ommatidia is severely under focussed, but the absolute sensitivity of the eyes is unusually high.We discuss the functional significance of these findings and the extent to which the variability of sensory arrays may correlate with task allocation.     

Behavioral mechanisms of collective nest-site choice by the ant Temnothorax curvispinosus

This paper examines the individual behavior underlying collective choice among nest sites by the ant Temnothorax (formerly Leptothorax) curvispinosus. Colonies can actively compare options, rejecting a mediocre site when it is paired with a good one, but accepting the same mediocre design if it is instead paired with a worse site. This ability emerges from the behavior of an active minority of workers who organize emigrations. When one of these finds a promising site, she recruits nest mates to it, but only after a delay that varies inversely with site quality. Ants first recruit fellow active ants via slow tandem runs, but eventually switch to speedier transports of the colony’s passive majority. Later transports grow faster still, as ants improve their speed with experience. An ant’s choice of recruitment type is governed by a quorum rule, such that her likelihood of starting to transport increases with the population of the new site. The size of the quorum depends on experience, with ants demanding a larger population to launch immediately into transport than they do to switch to transport after first leading a few tandem runs. Perception of quorum attainment requires direct contact between ants. The ants’ behavior qualitatively matches that of T. albipennis, where models have shown that decentralized choice of the best site depends on quality-dependent recruitment delays, amplified by a quorum rule for initiating transport. Parameter estimates for an agent-based model show significant quantitative differences between the species, and suggest that T. albipennis may place relatively greater emphasis on emigration speed. Received 11 February 2005; revised 10 May 2005; accepted 20 May 2005.     

Clonal reproduction and genetic caste differences in a queen-polymorphic ant, Vollenhovia emeryi

Most social Hymenoptera are characterized by simple haploid sex determination and environment-based caste differentiation. This appears to be strikingly different in the queen-polymorphic ant Vollenhovia emeryi. Almost all long- and short-winged queens from a population in Central Japan were homozygous at three microsatellite loci, whereas workers were mostly heterozygous, suggesting either a complex system of genetic caste determination or, more likely, the production of female sexuals from unfertilized eggs by thelytokous parthenogenesis and of workers from fertilized eggs. Furthermore, male genotypes were not compatible with those of the queens and had exclusively the paternal allele found in the sterile, heterozygous workers, probably because males are produced from fertilized eggs after the exclusion of maternal nuclear DNA as recently reported for Wasmannia auropunctata. The genus Vollenhovia might provide an interesting model system to trace the evolution of unusual caste and sex determination systems.     

Effect of temperature and social environment on worker size in the ant Temnothorax nylanderi

Warm temperatures decrease insect developmental time and body size. Social life could buffer external environmental variations, especially in large social groups, either through behavioral regulation and compensation or through specific nest architecture. Mean worker size and distribution of worker sizes within colonies are important parameters affecting colony productivity as worker size is linked to division of labor in insect societies. In this paper, we investigate the effect of stressful warm temperatures and the role of social environment (colony size and size of nestmate workers) on the mean size and size variation of laboratory-born workers in the small European ant Temnothorax nylanderi. To do so, we reared field-collected colonies under medium or warm temperature treatments after having marked the field-born workers and removed the brood except for 30 first instar larvae. Warm temperature resulted in the production of fewer workers and a higher adult mortality, confirming that this regime was stressful for the ants. T. nylanderi ants followed the temperature size rule observed in insects, with a decreased developmental time and mean size under warm condition. Social environment appeared to play an important role as we observed that (i) larger colonies buffered the effect of temperature better than smaller ones (ii) colonies with larger workers produced larger workers whatever the rearing temperature and (iii) the coefficient of variation of worker size was similar in the field and under medium laboratory temperature. This suggests that worker size variation is not primarily due to seasonal environmental fluctuations in the field. Finally, we observed a higher coefficient of variation of worker size under warm temperature. We propose that this results from a disruption of social regulation, i.e. the control of nestmate workers over developing larvae and adult worker size, under stressful conditions.     

Impact of plant cover on the cavity‐nesting ant Temnothorax crassispinus

1. Plant communities influence the availability of important resources for ants, such as nest sites and food, as well as environmental conditions. Thus, plants affect the abundance and distribution of ants. 2. In a field experiment, the influence of plant cover on the settlement of nest sites and per‐capita productivity of sexual individuals by the ant Temnothorax crassispinus was analysed. In July 2014, in five areas with patches of alien balsam Impatiens parviflora, and another five of native balsam I. noli‐tangere, transects composed of artificial nests were established; the nest sites were situated inside patches of balsams, and outside of them. Four hundred and forty artificial nests were used. One year later, the nests were collected. 3. Colonies of the ants three times more often inhabited nest sites outside the patches of both balsams. Besides, colonies with queens were more frequently found in nest sites located away from balsams. The per‐capita productivity of sexual individuals was higher in nests collected in patches of balsam, and the colonies from patches of alien balsam produced a more female‐biased sex ratio. 4. In terms of the impact on the ant, no clear differences were found between the alien balsam and the native one. The most important factor affecting the fitness of ants in areas dominated by balsams is the presence of herbaceous plant cover rather than whether the plant is alien or native.     

Morphological variability of intercastes in the ant Temnothorax nylanderi: pattern of trait expression and modularity

Ants have distinct morphological castes (queens and workers), but aberrant queen-worker “intercastes” occasionally occur, both in wild and laboratory conditions. Intercastes are rare, however, such novel phenotypes may have evolutionary significance. Their morphology is highly variable in any given species, providing valuable information about the integration of queen traits (e.g. ocelli, wings, complex segmentation of thorax, large gaster and ovaries, spermatheca). Generally, these traits are all diminished or absent in workers. We used multivariate morphometry to analyze an exceptionally large sample of 101 intercastes of Temnothorax nylanderi. We determined distributions and correlations of traits, and confirmed the mosaic nature of intercastes. Queen-specific traits are not expressed coherently in intercastes, but the possible patterns of trait combination are limited. A large number of small-sized intercastes had disproportionately larger head, ocelli and gaster but smaller thorax. In contrast, queen-like growth of thorax and rudimentary wings only occurred in large-sized intercastes. This is the most comprehensive analysis of intercaste variability, and suggests the existence of constraints on recombination of caste-specific modular traits.     

The influence of hybridization on colony structure in the ant species Temnothorax nylanderi and T. crassispinus

The parapatric sibling ant species Temnothorax nylanderi and T. crassispinus hybridize in the contact zone in the Franconian Jura, Southern Germany. Aim of our study was to investigate the impact of hybridization on colony composition and fitness. We classified colonies as either ‘pure’ or containing hybrids by determining their allozyme pattern at GPI, an enzyme that is fixed for different alleles in the two parental species, and quantified their reproductive output. Most colonies with hybrid workers had a T. crassispinus queen. Colonies with heterozygous, hybrid workers produced more young workers than colonies of the parental species but similar numbers of male and female sexuals. Female sexuals from colonies with heterozygous workers had a significantly lower weight than female sexuals from pure colonies. Only a single reproductive queen was found to be heterozygous, suggesting reduced fitness of hybrid queens. As in the parental species, hybrid colonies appear to be frequently taken over by alien queens, which obscures the genetic colony structure. Received 6 April 2006; revised 10 June 2006; accepted 15 June 2006.     

Packaging of offspring by nests of the ant,Leptothorax longispinosus: parent-offspring conflict and queen-worker conflict

Models of the packaging of offspring predict that parental fitness is maximized by following a set of rules, including the rule to invest the minimal amount in each offspring. Offspring can maximize their fitness by demanding more resources than the parent is selected to give, leading to parent-offspring conflict over packaging. Social insect nests may also experience queen-worker conflict over packaging. Experiments were conducted, using two populations of the ant Leptothorax longispinosus, in order to determine the role of both parent-offspring conflict and queen-worker conflict in packaging. Parent-offspring conflict over packaging was detected towards males and workers, but not to females. This may be because both parental and offspring fitness are maximized by investing as much in possible in females so both parties benefit by cooperating over packaging of females. Queen-worker conflict over packaging was detected for females, males, and workers. The direction taken by the queen-worker conflict is best explained by asymmetries in genetic relatedness among nestmates.     

Starvation endurance in the ant Temnothorax nylanderi depends on group size,body size and access to larvae

Social interactions in animal groups can buffer environmental stress and may enhance survival under unfavourable conditions. In the present study, the impact on starvation endurance of social group, access to larvae and cold shock is studied in the ant Temnothorax nylanderi Förster. Resource sharing is expected to lead to grouped workers surviving longer than isolated ones. Access to larvae may increase longevity if larvae serve as food, or may interfere with survival if they induce caring behaviour in workers. Cold shock serves as a stress factor and a negative influence on survival is expected. The results show that isolated workers have a shorter lifespan than grouped workers, which in turn live for a shorter period than grouped workers with larvae. Beneficial ‘group effects’ contribute to group survival and the presence of larvae increases worker survival because the workers presumably feed on the larvae. Thus, improved starvation endurance may reflect an additional benefit of a social lifestyle. Moreover, variance in survival is lower for grouped workers than for isolated workers: group members not only demonstrate improved survival, but also smaller within‐group differences. Although a negative influence on survival is the expected outcome, this type of thermal stress is found to have no direct impact on starvation endurance other than moderating the differences between isolated and grouped workers.     

Developmental plasticity and evolutionary explanations

Developmental plasticity looks like a promising bridge between ecological and developmental perspectives on evolution. Yet, there is no consensus on whether plasticity is part of the explanation for adaptive evolution or an optional “add‐on” to genes and natural selection. Here, we suggest that these differences in opinion are caused by differences in the simplifying assumptions, and particular idealizations, that enable evolutionary explanation. We outline why idealizations designed to explain evolution through natural selection prevent an understanding of the role of development, and vice versa. We show that representing plasticity as a reaction norm conforms with the idealizations of selective explanations, which can give the false impression that plasticity has no explanatory power for adaptive evolution. Finally, we use examples to illustrate why evolutionary explanations that include developmental plasticity may in fact be more satisfactory than explanations that solely refer to genes and natural selection.     

Colony size does not predict foraging distance in the ant Temnothorax rugatulus: a puzzle for standard scaling models

Body size is often positively correlated with ecologically relevant traits such as fecundity, survival, resource requirements, and home range size. Ant colonies, in some respects, behave like organisms, and their colony size is thought to be a significant predictor of many behavioral and ecological traits similar to body size in unitary organisms. In this study, we test the relationship between colony size and field foraging distance in the ant species Temnothorax rugatulus. These ants forage in the leaf litter presumably for small arthropod prey. We found colonies did not differ significantly in their foraging distances, and colony size is not a significant predictor of foraging distance. This suggests that large colonies may not exhaust local resources or that foraging trips are not optimized for minimal distance, and thus that food may not be the limiting resource in this species. This study shows T. rugatulus are behaving in ways that differ from existing models of scaling.     

Developmental plasticity in schistosomes and other helminths

Developmental plasticity in helminth life cycles serves, in most cases, to increase the probability of transmission between hosts, suggesting that the necessity to achieve transmission is a prominent selective pressure in the evolution of this phenomenon. Some evidence suggests that digenean trematodes from the genus Schistosoma are also capable of limited developmental responses to host factors. Here we review the currently available data on this phenomenon and attempt to draw comparisons with similar processes in the life cycles of other helminths. At present the biological significance of developmental responses by schistosomes under laboratory conditions remains unclear. Further work is needed to determine whether developmental plasticity plays any role in increasing the probability of schistosome transmission and life cycle propagation under adverse conditions, as it does in other helminth life cycles.