Age-dependent release of and response to alarm pheromone in a ponerine ant.

Social insect societies are characterized by division of labour and communication within the colony. The most frequent mode of communication is by chemical signals. In general, pheromones elicit specific responses in the receiver, although reactions may vary depending on the receiving individual's physiological or motivational state. For example, it has been shown that pheromones can elicit different responses in morphological worker castes. However, comparably little is known about such effects in worker castes of monomorphic species. Here, we comprehensively studied a monomorphic species showing age polyethism, the thelytokous ant Platythyrea punctata Our analyses revealed that the species' alarm pheromone consists of (S)-(-)-citronellal and (S)-(-)-actinidine, and is produced in the mandibular glands. Ants responded with increased movement activity and increasing ant density towards the pheromone source in whole-colony bioassays, confirming the alarming effect of these compounds. We found age classes to differ in their absolute pheromone content, in the propensity to release alarm pheromone upon disturbance and in their reaction towards the pheromone. Absolute amounts of pheromone content may differ simply because the biosynthesis of the pheromone begins only after adult eclosion. Nonetheless, our results indicate that this clonal species exhibits age-related polyethism in the emission of as well as in the response to its alarm pheromone.

Lipid content influences division of labour in a clonal ant.

The fat body, a major metabolic hub in insects, is involved in many functions, e.g. energy storage, nutrient sensing and immune response. In social insects, fat appears to play an additional role in division of labour between egg layers and workers, which specialize in non-reproductive tasks inside and outside their nest. For instance, reproductives are more resistant to starvation, and changes in fat content have been associated with the transition from inside to outside work or reproductive activities. However, most studies have been correlative and we still need to unravel the causal interrelationships between fat content and division of both reproductive and non-reproductive labour. Clonal ants, e.g. Platythyrea punctata, are ideal models for studying task partitioning without confounding variation in genotype and morphology. In this study, we examined the range of variation and flexibility of fat content throughout the lifespan of workers, the threshold of corpulence associated with foraging or reproduction and whether low fat content is a cause rather than a consequence of the transition to foraging. We found that lipid stores change with division of labour from corpulent to lean and, in reverted nurses, back to corpulent. In addition, our data show the presence of fat content thresholds that trigger the onset of foraging or egg-laying behaviour. Our study supports the view that mechanisms that regulate reproduction and foraging in solitary insects, in particular the nutritional status of individuals, have been co-opted to regulate division of labour in colonies of social insects.

Stress and early experience underlie dominance status and division of labour in a clonal insect.

Cooperation and division of labour are fundamental in the 'major transitions' in evolution. While the factors regulating cell differentiation in multi-cellular organisms are quite well understood, we are just beginning to unveil the mechanisms underlying individual specialization in cooperative groups of animals. Clonal ants allow the study of which factors influence task allocation without confounding variation in genotype and morphology. Here, we subjected larvae and freshly hatched workers of the clonal ant Platythyrea punctata to different rearing conditions and investigated how these manipulations affected division of labour among pairs of oppositely treated, same-aged clonemates. High rearing temperature, physical stress, injury and malnutrition increased the propensity of individuals to become subordinate foragers rather than dominant reproductives. This is reflected in changed gene regulation: early stages of division of labour were associated with different expression of genes involved in nutrient signalling pathways, metabolism and the phenotypic response to environmental stimuli. Many of these genes appear to be capable of responding to a broad range of stressors. They might link environmental stimuli to behavioural and phenotypic changes and could therefore be more broadly involved in caste differentiation in social insects. Our experiments also shed light on the causes of behavioural variation among genetically identical individuals.

Ergonomics of load transport in the seed harvesting ant Messor barbarus: morphology influences transportation method and efficiency.

We studied in the field the load transport behavior of workers of the polymorphic Mediterranean seed harvester ant Messor barbarus Individual ants used two different methods to transport food items: carrying and dragging. The probability of dragging instead of carrying varied significantly with both the mass of the item transported and its linear dimension. Moreover, the values of item mass and length at which dragging began to occur increased with increasing size of the workers. However, larger ants began dragging at decreasing values of the relative mass represented by the items transported, which reflects different biomechanical constraints resulting from allometric relationships between the different parts of their body. Transport rate was significantly higher in large ants but varied in the same way for workers of different sizes with the relative mass of the item transported. Nevertheless, although large ants were individually more efficient than small ants in transporting food items, the relative transport rate, defined as the ratio of transport rate to the mass of the ant, was higher for small ants than for large ants. Colonies should thus have a greater benefit in investing in small ants than in large ants for the transport of food items. This may explain why the proportion of large ants is so small on the foraging columns of M. barbarus and why large ants are most often employed in colonies for tasks other than transporting food items.

Non-lethal fungal infection could reduce aggression towards strangers in ants.

Many parasites interfere with the behaviour of their hosts. In social animals, such as ants, parasitic interference can cause changes on the level of the individual and also on the level of the society. The ant-parasitic fungus Rickia wasmannii influences the behaviour of Myrmica ants by expanding the host's nestmate recognition template, thereby increasing the chance of the colony accepting infected non-nestmates. Infected ants consistently show an increase of the alkane tricosane (n-C23) in their cuticular hydrocarbon profiles. Although experimental application of single compounds often elicits aggression towards manipulated ants, we hypothesized that the increase of n-C23 might underlie the facilitated acceptance of infected non-nestmates. To test this, we mimicked fungal infection in M. scabrinodis by applying synthetic n-C23 to fresh ant corpses and observed the reaction of infected and uninfected workers to control and manipulated corpses. Infected ants appeared to be more peaceful towards infected but not uninfected non-nestmates. Adding n-C23 to uninfected corpses resulted in reduced aggression in uninfected ants. This supports the hypothesis that n-C23 acts as a 'pacifying' signal. Our study indicates that parasitic interference with the nestmate discrimination of host ants might eventually change colony structure by increasing genetic heterogeneity in infected colonies.

Absence of genetic isolation across highly fragmented landscape in the ant Temnothorax nigriceps.

BACKGROUND: Human activities, including changes in agricultural landscapes, often impact biodiversity through habitat fragmentation. This potentially reduces genetic exchange between previously connected populations. Using a combination of nuclear and mitochondrial markers, we investigated (i) genetic diversity and population structure at multiple spatial scales and (ii) colony genetic structure and queen mating frequency in the ant species Temnothorax nigriceps in a highly anthropized environment. RESULTS: Although the results highlighted genetic structure on a European spatial scale, they did not reveal an impact of fragmentation on a regional scale, and we did not observe any genetic population structure on a regional scale. Across all populations, regardless of their geographical location, colony structure suggested monogyny (a single queen per colony) and monandry (single mating). However, nestmates were more related than expected, indicating that large-scale dispersal does not fully prevent genetic isolation. CONCLUSIONS: Despite living in fragmented patches of habitat, populations of Temnothorax nigriceps are apparently genetically not isolated at a regional scale. However, large-scale dispersal alone does not prevent genetic isolation. The ecological requirements of T. nigriceps may explain their resilience to habitat fragmentation by allowing them to survive in very small patches of suitable habitat. The deeper investigation of the diversity of functional habitats for this species should allow to appreciate better the mechanisms permitting this species to overcome the negative impacts of fragmentation.

Resource sharing is sufficient for the emergence of division of labour.

Division of labour occurs in a broad range of organisms. Yet, how division of labour can emerge in the absence of pre-existing interindividual differences is poorly understood. Using a simple but realistic model, we show that in a group of initially identical individuals, division of labour emerges spontaneously if returning foragers share part of their resources with other group members. In the absence of resource sharing, individuals follow an activity schedule of alternating between foraging and other tasks. If non-foraging individuals are fed by other individuals, their alternating activity schedule becomes interrupted, leading to task specialisation and the emergence of division of labour. Furthermore, nutritional differences between individuals reinforce division of labour. Such differences can be caused by increased metabolic rates during foraging or by dominance interactions during resource sharing. Our model proposes a plausible mechanism for the self-organised emergence of division of labour in animal groups of initially identical individuals. This mechanism could also play a role for the emergence of division of labour during the major evolutionary transitions to eusociality and multicellularity.

Oxidative stress and senescence in social insects: a significant but inconsistent link?

The life-prolonging effects of antioxidants have long entered popular culture, but the scientific community still debates whether free radicals and the resulting oxidative stress negatively affect longevity. Social insects are intriguing models for analysing the relationship between oxidative stress and senescence because life histories differ vastly between long-lived reproductives and the genetically similar but short-lived workers. Here, we present the results of an experiment on the accumulation of oxidative damage to proteins, and a comparative analysis of the expression of 20 selected genes commonly involved in managing oxidative damage, across four species of social insects: a termite, two bees and an ant. Although the source of analysed tissue varied across the four species, our results suggest that oxidative stress is a significant factor in senescence and that its manifestation and antioxidant defenses differ among species, making it difficult to find general patterns. More detailed and controlled investigations on why responses to oxidative stress may differ across social species may lead to a better understanding of the relations between oxidative stress, antioxidants, social life history and senescence. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

Comparative transcriptomic analysis of the mechanisms underpinning ageing and fecundity in social insects.

The exceptional longevity of social insect queens despite their lifelong high fecundity remains poorly understood in ageing biology. To gain insights into the mechanisms that might underlie ageing in social insects, we compared gene expression patterns between young and old castes (both queens and workers) across different lineages of social insects (two termite, two bee and two ant species). After global analyses, we paid particular attention to genes of the insulin/insulin-like growth factor 1 signalling (IIS)/target of rapamycin (TOR)/juvenile hormone (JH) network, which is well known to regulate lifespan and the trade-off between reproduction and somatic maintenance in solitary insects. Our results reveal a major role of the downstream components and target genes of this network (e.g. JH signalling, vitellogenins, major royal jelly proteins and immune genes) in affecting ageing and the caste-specific physiology of social insects, but an apparently lesser role of the upstream IIS/TOR signalling components. Together with a growing appreciation of the importance of such downstream targets, this leads us to propose the TI-J-LiFe (TOR/IIS-JH-Lifespan and Fecundity) network as a conceptual framework for understanding the mechanisms of ageing and fecundity in social insects and beyond. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

When invasive ants meet: effects of outbreeding on queen performance in the tramp ant Cardiocondyla itsukii.

Most disturbed habitats in the tropics and subtropics harbor numerous species of invasive ants, and occasionally the same species has been introduced repeatedly from multiple geographical sources. We examined how experimental crossbreeding between sexuals from different populations affects the fitness of queens of the tramp ant Cardiocondyla itsukii, which is widely distributed in Asia and the Pacific Islands. Eggs laid by queens that mated with nestmate males had a higher hatching rate than eggs laid by queens mated to males from neighboring (Hawaii × Kauai) or distant introduced populations (Hawaii/Kauai × Okinawa). Furthermore, inbreeding queens had a longer lifespan and produced a less female-biased offspring sex ratio than queens from allopatric mating. This suggests that the genetic divergence between different source populations may already be so large that in case of multiple invasions eventual crossbreeding might negatively affect the fitness of tramp ants.

Age-dependent changes in cuticular color and pteridine levels in a clonal ant.

Social insects are emerging models for studying aging and the longevity/fecundity trade-off. Research on the demography of colonies and populations are hampered by the lack of reliable age markers. Here we investigate the suitability of cuticular pigmentation and pteridine fluorescence for age grading individuals of the clonal ant Platythyrea punctata. We found that both traits varied with age. Cuticular color darkened with individual's age until 25-30 days after hatching. For pteridine fluorescence, we found that P. punctata workers show a decrease in head pteridine levels over time until 70-80 days of age. Together with other markers, such as age-based behavior, cuticular coloration and pteridine fluorescence may help to estimate the age structure of colonies.

Does substrate coarseness matter for foraging ants? An experiment with Lasius niger (Hymenoptera; Formicidae).

We investigated whether workers of the ant species Lasius niger are able to sense and discriminate the coarseness of the substrate on which they walk. First, we studied the way in which substrate coarseness affects the ants' locomotory behaviour. Second, we investigated the spontaneous preference of ants for substrates of different coarseness. And third, we tested with a differential conditioning procedure the ants' capacity to learn to associate a given coarseness with a food reward. The locomotory behaviour of ants differed according to substrate coarseness: ants moved significantly faster and had more sinuous trajectories on a fine than on a coarse substrate. No spontaneous preference for a substrate of a given coarseness was observed and, even after 20 successive conditioning trials, there was little evidence of the effect of experience on substrate coarseness discrimination. Overall however, ants trained on fine sand made significantly more correct choice than those trained on coarse sand. We discuss these results and argue that in L. niger substrate coarseness may be more important at the collective level, by interacting with the chemical properties of the pheromone trail used in mass recruitment to food source, than at the individual level.

The effects of fungal infection and physiological condition on the locomotory behaviour of the ant Myrmica scabrinodis.

Parasite infection often results in alterations in host behaviour. These changes vary greatly in their magnitude, from slight shifts in the time spent by the host performing a given activity to the appearance of novel behaviours. The effects of parasites can differ with the age and the physiological condition of the host. Rickia wasmannii is an ectoparasitic fungal symbiont in Myrmica ants that covers the whole body surface of the host and reduces its lifespan. The fungus is present in both young and old individuals, making it an optimal subject for the study of age-related parasitic effects. We tested the effect of fungal infection on the locomotory activity of the Myrmica scabrinodis ant in different age categories. The fat content of workers was measured as a proxy for their physiological status. Based on our findings, old workers bore more thalli and were leaner than young individuals, while they tended to move at higher speeds and with a lower degree of meandering. Young individuals covered smaller distances, at slower speeds and with a higher degree of meandering. Contrary to our expectations, the infection intensity of R. wasmannii affected neither the fat content nor the locomotory activity of ant workers. However, the two age classes seem to have different strategies with regards to the relationship between fat content and distance covered. Our results suggest that characteristics of locomotory activity differ between the age classes in many respects, and are also influenced by their physiological status, but parasitism by R. wasmannii does not seem to have a straightforward effect on any of the variables studied.

Facultative social insects can provide insights into the reversal of the longevity/fecundity trade-off across the eusocial insects.

In eusocial insects, reversal of the fecundity/longevity trade-off and extreme differences in life histories between castes of the same species garner scientific and public interest. Facultative social species at the threshold of sociality, in which individuals are socially plastic, provide an excellent opportunity to understand the causes and mechanisms underlying this reversal in life history trade-off associated with eusociality. We briefly present the ultimate factors favoring sociality and the association between fecundity and longevity in facultative eusocial insects, including kin selection and disposable soma, as well as proximate mechanisms observed in such species, such as differences in hormone titers and functions. Potential genetic underpinnings of lifespan and fecundity differences between castes are discussed and future research directions are proposed.

The Effect of Trail Pheromone and Path Confinement on Learning of Complex Routes in the Ant Lasius niger.

Route learning is key to the survival of many central place foragers, such as bees and many ants. For ants which lay pheromone trails, the presence of a trail may act as an important source of information about whether an error has been made. The presence of trail pheromone has been demonstrated to support route learning, and the effect of pheromones on route choice have been reported to persist even after the pheromones have been removed. This could be explained in two ways: the pheromone may constrain the ants onto the correct route, thus preventing errors and aiding learning. Alternatively, the pheromones may act as a 'reassurance', signalling that the learner is on the right path and that learning the path is worthwhile. Here, we disentangle pheromone presence from route confinement in order to test these hypotheses, using the ant Lasius niger as a model. Unexpectedly, we did not find any evidence that pheromones support route learning. Indeed, there was no evidence that ants confined to the correct route learned at all. Thus, while we cannot support the 'reassurance' hypothesis, we can rule out the 'confinement' hypothesis. Other findings, such as a reduction in pheromone deposition in the presence of trail pheromones, are remarkably consistent with previous experiments. As previously reported, ants which make errors on their outward journey upregulate pheromone deposition on their return. Surprisingly, ants which would go on to make an error down-regulate pheromone deposition on their outward journey, hinting at a capacity for ants to gauge the quality of their own memories.

Avoid mistakes when choosing a new home: Nest choice and adoption of Leptothorax ant queens.

In ants, mating and colony founding are critical steps in the life of ant queens. Outside of their nests, young queens are exposed to intense predation. Therefore, they are expected to have evolved behavior to accurately and quickly locate a nesting place. However, data on the early life history of female reproductives are still lacking. Leptothorax gredleri is a suitable model organism to study the behavior of young queens. Reproductives can be reared under artificial conditions and readily mate in the laboratory. After mating, L. gredleri queens have the options to found solitarily, seek adoption into another colony, or return into their natal nest. In this study, we investigated the decision-making processes of female sexuals before and after mating. In particular, we tested whether female sexuals use chemical cues to find their way back to the nest, studied if they prefer their own nest over other nesting sites and followed the adoption dynamics of mated queens over 8 weeks (plus hibernation and spring). We showed that female sexuals and freshly mated queens spent more time on substrate previously used by workers from their own colony and from another colony than on a blank substrate. This discriminatory capability of queens appears to be lost in old, reproductive queens. Nest choice experiments showed that female sexuals and freshly mated queens can distinguish their own nest while old mated queens do not. When reintroduced in their maternal colony, young queens were readily adopted, but a few weeks later aggression against young queens led to their emigration from the maternal nest and eventually also death.

A preliminary checklist of the ants (Hymenoptera, Formicidae) of Andorra.

Within the last decade, checklists of the ant fauna of several European countries have been published or updated. Nevertheless, no ant checklists have hitherto been published for the principality of Andorra, a small landlocked country located in the eastern part of the Pyrenees. This work presents a critical list of the ant species of Andorra based on a review of the literature and on the biological material we collected during several field campaigns conducted in Andorra since the year 2005. Seventy-five species belonging to 21 genera of Formicidae were recorded. Nine species were recorded for the first time in Andorra: Aphaenogaster gibbosa (Latreille, 1798), Camponotus lateralis (Olivier, 1792), Camponotus piceus (Leach, 1825), Formica exsecta Nylander, 1846, Lasius piliferus Seifert, 1992, Tapinoma madeirense Forel, 1895, Temnothorax lichtensteini (Bondroit, 1918), Temnothorax niger (Forel, 1894), Temnothorax nigriceps (Mayr, 1855). The most speciose genera were Formica Linnaeus, 1758 and Temnothorax Forel, 1890 with 14 and 12 species, respectively. The ant fauna of Andorra is mostly dominated by Central European species (some are typical cold climate specialists); however species belonging to the Mediterranean ant fauna were also found. This can be explained by the particular geographic situation of Andorra which is characterized by a high mountain Mediterranean climate.

Permanent genetic resources added to Molecular Ecology Resources Database 1 December 2011-31 January 2012.

This article documents the addition of 473 microsatellite marker loci and 71 pairs of single-nucleotide polymorphism (SNP) sequencing primers to the Molecular Ecology Resources Database. Loci were developed for the following species: Barteria fistulosa, Bombus morio, Galaxias platei, Hematodinium perezi, Macrocentrus cingulum Brischke (a.k.a. M. abdominalis Fab., M. grandii Goidanich or M. gifuensis Ashmead), Micropogonias furnieri, Nerita melanotragus, Nilaparvata lugens Stål, Sciaenops ocellatus, Scomber scombrus, Spodoptera frugiperda and Turdus lherminieri. These loci were cross-tested on the following species: Barteria dewevrei, Barteria nigritana, Barteria solida, Cynoscion acoupa, Cynoscion jamaicensis, Cynoscion leiarchus, Cynoscion nebulosus, Cynoscion striatus, Cynoscion virescens, Macrodon ancylodon, Menticirrhus americanus, Nilaparvata muiri and Umbrina canosai. This article also documents the addition of 116 sequencing primer pairs for Dicentrarchus labrax.