Social isolation causes mortality by disrupting energy homeostasis in ants

The indirect influence of potential mates on survival and reproduction of Tyrophagus curvipenis (Acari: Acaridae)

The social-sexual environment is well known for its influence on the survival of organisms by modulating their reproductive output. However, whether it affects survival indirectly through a variety of cues without physical contact and its influence relative to direct interaction remain largely unknown. In this study, we investigated both the indirect and direct influences of the social-sexual environment on the survival and reproduction of the mite Tyrophagus curvipenis (Acari: Acaridae). The results demonstrated no apparent influence of conspecific cues on the survival of mites, but the survival and reproduction of mated female mites significantly changed, with the females mated with males having a significantly shortened lifespan and increased lifetime fecundity. For males, no significant difference was observed across treatments in their survival and lifespan. These findings indicate that direct interaction with the opposite sex has a much more profound influence on mites than indirect interaction and highlight the urgent need to expand research on how conspecific cues modulate the performance of organisms with more species to clarify their impacts across taxa.

Differential Neuroanatomical, Neurochemical, and Behavioral Impacts of Early-Age Isolation in a Eusocial Insect

Social experience early in life appears to be necessary for the development of species-typical behavior. Although isolation during critical periods of maturation has been shown to impact behavior by altering gene expression and brain development in invertebrates and vertebrates, workers of some ant species appear resilient to social deprivation and other neurobiological challenges that occur during senescence or due to loss of sensory input. It is unclear if and to what degree neuroanatomy, neurochemistry, and behavior will show deficiencies if social experience in the early adult life of worker ants is compromised. We reared newly-eclosed adult workers of Camponotus floridanus under conditions of social isolation for 2 to 53 days, quantified brain compartment volumes, recorded biogenic amine levels in individual brains, and evaluated movement and behavioral performance to compare the neuroanatomy, neurochemistry, brood-care behavior, and foraging (predatory behavior) of isolated workers with that of workers experiencing natural social contact after adult eclosion. We found that the volume of the antennal lobe, which processes olfactory inputs, was significantly reduced in workers isolated for an average of 40 days, whereas the size of the mushroom bodies, centers of higher-order sensory processing, increased after eclosion and was not significantly different from controls. Titers of the neuromodulators serotonin, dopamine, and octopamine remained stable and were not significantly different in isolation treatments and controls. Brood care, predation, and overall movement were reduced in workers lacking social contact early in life. These results suggest that the behavioral development of isolated workers of C. floridanus is specifically impacted by a reduction in the size of the antennal lobe. Task performance and locomotor ability therefore appear to be sensitive to a loss of social contact through a reduction of olfactory processing ability rather than change in the size of the mushroom bodies, which serve important functions in learning and memory, or the central complex, which controls movement.

Early life stress and altered social behaviors: A perspective across species

Childhood and adolescent affiliations guide how individuals engage in social relationships throughout their lifetime and adverse experiences can promote biological alterations that facilitate behavioral maladaptation. Indeed, childhood victims of abuse are more likely to be diagnosed with conduct or mood disorders which are both characterized by altered social engagement. A key domain particularly deserving of attention is aggressive behavior, a hallmark of many disorders characterized by deficits in reward processing. Animal models have been integral in identifying both the short- and long-term consequences of stress exposure and suggest that whether it is disruption to parental care or social isolation, chronic exposure to early life stress increases corticosterone, changes the expression of neurotransmitters and neuromodulators, and facilitates structural alterations to the hypothalamus, hippocampus, and amygdala, influencing how these brain regions communicate with other reward-related substrates. Herein, we describe how adverse early life experiences influence social behavioral outcomes across a wide range of species and highlight the long-term biological mechanisms that are most relevant to maladaptive aggressive behavior.

Social isolation shortens lifespan through oxidative stress in ants

Social isolation negatively affects health, induces detrimental behaviors, and shortens lifespan in social species. Little is known about the mechanisms underpinning these effects because model species are typically short-lived and non-social. Using colonies of the carpenter ant Camponotus fellah, we show that social isolation induces hyperactivity, alters space-use, and reduces lifespan via changes in the expression of genes with key roles in oxidation-reduction and an associated accumulation of reactive oxygen species. These physiological effects are localized to the fat body and oenocytes, which perform liver-like functions in insects. We use pharmacological manipulations to demonstrate that the oxidation-reduction pathway causally underpins the detrimental effects of social isolation on behavior and lifespan. These findings have important implications for our understanding of how social isolation affects behavior and lifespan in general.

Homeostatic Regulation of Energetic Arousal During Acute Social Isolation: Evidence From the Lab and the Field

Recent evidence suggests that social contact is a basic need governed by a social homeostatic system. Little is known, however, about how conditions of altered social homeostasis affect human psychology and physiology. Here, we investigated the effects of 8 hr of social isolation on psychological and physiological variables and compared this with 8 hr of food deprivation in a lab experiment (N = 30 adult women). Social isolation led to lowered self-reported energetic arousal and heightened fatigue, comparable with food deprivation. To test whether these findings would extend to a real-life setting, we conducted a preregistered field study during a COVID-19 lockdown (N = 87 adults; 47 women). The drop in energetic arousal after social isolation observed in the lab replicated in the field study for participants who lived alone or reported high sociability, suggesting that lowered energy could be part of a homeostatic response to the lack of social contact.

Body mass and cuticular hydrocarbon profiles, but not queen number, underlie worker desiccation resistance in a facultatively polygynous harvester ant (Pogonomyrmex californicus)

As small-bodied terrestrial organisms, insects face severe desiccation risks in arid environments, and these risks are increasing under climate change. Here, we investigate the physiological, chemical, and behavioral mechanisms by which harvester ants, one of the most abundant arid-adapted insect groups, cope with desiccating environmental conditions. We aimed to understand how body size, cuticular hydrocarbon profiles, and queen number impact worker desiccation resistance in the facultatively polygynous harvester ant, Pogonomyrmex californicus. We measured survival at 0% humidity of field-collected worker ants sourced from three closely situated populations within a semi-arid region of southern California. These populations vary in queen number, with one population dominated by multi-queen colonies (primary polygyny), one population dominated by single-queen colonies, and one containing an even mix of single- and multi-queen colonies. We found no effect of population on worker survival in desiccation assays, suggesting that queen number does not influence colony desiccation resistance. Across populations, however, body mass and cuticular hydrocarbon profiles significantly predicted desiccation resistance. Larger-bodied workers survived longer in desiccation assays, emphasizing the importance of reduced surface area-to-volume ratios in maintaining water balance. Additionally, we observed a positive relationship between desiccation resistance and the abundance of n-alkanes, supporting previous work that has linked these high-melting point compounds to improved body water conservation. Together, these results contribute to an emerging model explaining the physiological mechanisms of desiccation resistance in insects.

Group Size Buffers against Energetic Stress in Honeybee Workers (Apis mellifera)

Individuals of the Western honeybee species, Apis mellifera, live in large groups of thousands of worker bees, a queen, and a few drones. Workers interact frequently with related individuals while performing various tasks. Although it is well understood why and under which conditions sociality can evolve, the consequences for individuals living in permanent groups are less well understood. As individuals of solitary species become stressed when kept at high density, it might be the opposite in obligate social species. Here, I use an experimental laboratory set-up to study the effect of varying group sizes on the magnitude and within-group variance of stress responses towards energetic and heat stress. While only a weak difference was found in the magnitude of an energetic stress response as a function of group size, the within-group variance showed a statistically significant positive relationship with group size for the glucose/trehalose ratio, a marker for energetic stress. The heat stress marker, hsp70AB gene expression, did not show any relationship to group size. Individuals of obligate social species seem to benefit from adaptations to permanent group living, e.g., buffering against stress, especially at a higher density of individuals. The consequences of infections and immune system activation in isolated individuals are discussed.

Increased Stress Levels in Caged Honeybee (Apis mellifera) (Hymenoptera: Apidae) Workers

Honeybees, Apis mellifera, usually live in large colonies consisting of thousands of individuals. Within the colony, workers interact with their social environment frequently. The large workforce, division of labour, and other features may promote the ecological success of honeybees. For decades, artificial mini colonies in cages within the laboratory have become the gold standard, especially in experiments related to toxicology, effects of pesticides and pathogens. Experiments using caged bees and full-sized colonies yielded contradictory results. Here, the effect of cage experiments on the stress level of individual bees is analysed. Two different stress response were targeted, the heat shock response and the mobilization of energetic resources. While no differences were found for varying group sizes of bees, very strong effects emerged by comparing caged workers with bees from natural colonies. Caged workers showed increased levels of hsp expression and reduced haemolymph titres for trehalose, the energy storage sugar. These results reveal that the lack of the social environment (e.g., lack of queen, lack of sufficient group size) induce stress in caged bees, which might act synergistically when bees are challenged by additional stressors (e.g., pesticides, pathogens) resulting in higher mortality than observed under field conditions.

Isolation disrupts social interactions and destabilizes brain development in bumblebees

Social isolation, particularly in early life, leads to deleterious physiological and behavioral outcomes. Here, we leverage new high-throughput tools to comprehensively investigate the impact of isolation in the bumblebee, Bombus impatiens, from behavioral, molecular, and neuroanatomical perspectives. We reared newly emerged bumblebees in complete isolation, in small groups, or in their natal colony, and then analyzed their behaviors while alone or paired with another bee. We find that when alone, individuals of each rearing condition show distinct behavioral signatures. When paired with a conspecific, bees reared in small groups or in the natal colony express similar behavioral profiles. Isolated bees, however, showed increased social interactions. To identify the neurobiological correlates of these differences, we quantified brain gene expression and measured the volumes of key brain regions for a subset of individuals from each rearing condition. Overall, we find that isolation increases social interactions and disrupts gene expression and brain development. Limited social experience in small groups is sufficient to preserve typical patterns of brain development and social behavior.

A genetic screen for Drosophila social isolation mutants and analysis of sex pistol

Prolonged periods of forced social isolation is detrimental to well-being, yet we know little about which genes regulate susceptibility to its effects. In the fruit fly, Drosophila melanogaster, social isolation induces stark changes in behavior including increased aggression, locomotor activity, and resistance to ethanol sedation. To identify genes regulating sensitivity to isolation, I screened a collection of sixteen hundred P-element insertion lines for mutants with abnormal levels of all three isolation-induced behaviors. The screen identified three mutants whose affected genes are likely central to regulating the effects of isolation in flies. One mutant, sex pistol (sxp), became extremely aggressive and resistant to ethanol sedation when socially isolated. sxp also had a high level of male–male courtship. The mutation in sxp reduced the expression of two minor isoforms of the actin regulator hts (adducin), as well as mildly reducing expression of CalpA, a calcium-dependent protease. As a consequence, sxp also had increased expression of the insulin-like peptide, dILP5. Analysis of the social behavior of sxp suggests that these minor hts isoforms function to limit isolation-induced aggression, while chronically high levels of dILP5 increase male–male courtship.

Social isolation interaction with the feeding regime differentially affects survival and results in a hump-shaped pattern in movement activity

Eusocial insects depend on their colonies, and it is therefore clear why isolation triggers many negative effects on isolated individuals. Here, we examined the effect of social isolation on the desert ant Cataglyphis niger, asking whether isolation, either with access to food or under starvation, impairs survival, and whether isolation modifies movement activity and digging to bypass an obstacle. Social isolation led to shorter survival but only when food was provided. This effect might be due to food not being digested correctly under isolation. Although isolated ant workers were more active immediately post isolation than 2-24 hours later, their movement moderately increased two days post isolation. We suggest that the changes in movement activity are adaptive: first, the worker increases activity intended to reunite it with the lost colony. Then, when the colony is not found, it reduces activity to conserve energy. It later increases activity as a final attempt to detect the colony. We expected isolated workers to dig faster to bypass an obstacle, but we did not detect any effect on digging behavior. We demonstrate here the complex effects of isolation on survival and movement activity, in interaction with additional factors - feeding and isolation duration.

Influence of Labor Conditions and Interaction Among Individuals on Circadian Activity Rhythms in the Ant Camponotus Japonicus

In this study, we investigated the relationship between the activity rhythms of Camponotus japonicus worker ants and their interactions. Specifically, one or two workers collected from either inside or outside the nest in a breeding colony were placed in a measurement system under a constant dark condition, and their activity rhythms were measured for 14 days. We thereby examined the relationship between the activity rhythm in the system and the experimental conditions, which consisted of four different combinations of working locations during breeding (in/outside the nest) and single/double workers (one ant / two ants) in the measurement system, over a total of 96 samples. A large number of the sampled ants (about 90% of the total) showed circadian activity rhythms. The proportion of circadian activity rhythm was lower and the dispersion of the period was larger in the circadian activity rhythm observed in single workers collected from within the nest than in the other three experimental conditions. In all four experimental conditions, the amplitude of the circadian activity rhythm decayed on an approximate 5-day scale. These results provide quantitative evidence that the activity rhythm of ants is determined by the location of labor and individual interactions during breeding.

The plasticity of lifespan in social insects

One of the central questions of ageing research is why lifespans of organisms differ so tremendously among related taxa and, even more surprising, among members of the same species. Social insects provide a particularly pronounced example for this. Here, we review previously published information on lifespan plasticity in social insects and provide new data on worker lifespan in the ant Cardiocondyla obscurior, which because of its relatively short lifespan is a convenient model to study ageing. We show that individual lifespan may vary within species with several reproductive and social traits, such as egg-laying rate, queen number, task, colony size and colony composition. For example, in Cardiocondyla, highly fecund queens live longer than reproductively less active queens, and workers tend to live longer when transferred into a novel social environment or, as we show with new data, into small colonies. We hypothesize that this plasticity of lifespan serves to maximize the reproductive output of the colony as a whole and thus the inclusive fitness of all individuals. The underlying mechanisms that link the social environment or reproductive status with lifespan are currently unresolved. Several studies in honeybees and ants indicate an involvement of nutrient-sensing pathways, but the details appear to differ among species. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

Division of labor in work shifts by leaf-cutting ants

Foraging rhythms in eusocial insects are determined by the colony´s overall pattern. However, in leaf-cutting ant workers, individual rhythms are not fully synchronized with the colonies' rhythm. The colony as a whole is nocturnal, since most worker activity takes place at night; however some workers forage during the day. Previous studies in individualized ants suggest nocturnal and diurnal workers coexistence. Here observations within the colony, in leaf-cutting ants, showed that workers have differential foraging time preference, which interestingly is associated to body size and differential leaf transportation engagement. Nocturnal ants are smaller and less engaged in leaf transportation whereas diurnal ants are bigger and more engaged in leaf carriage. Mechanisms underlying division of labor in work shifts in ants are still unknown but much can be extrapolated from honeybees; another social system bearing a similar pattern. A collective organization like this favors constant exploitation of food sources while preserving natural individual rhythm patterns, which arise from individual differences, and thermal tolerance, given by the size polymorphism presented by this species.

Social isolation causes downregulation of immune and stress response genes and behavioural changes in a social insect

Humans and other social mammals experience isolation from their group as stressful, triggering behavioural and physiological anomalies that reduce fitness. While social isolation has been intensely studied in social mammals, it is less clear how social insects, which evolved sociality independently, respond to isolation. Here we examined whether the typical mammalian responses to social isolation, e.g., an impaired ability to interact socially and immune suppression are also found in social insects. We studied the consequences of social isolation on behaviour and brain gene expression in the ant Temnothorax nylanderi. Following isolation, workers interacted moderately less with adult nestmates, increased the duration of brood contact, and reduced the time spent self-grooming, an important sanitary behaviour. Our brain transcriptome analysis revealed that only a few behaviour-related genes had altered their expression with isolation time. Rather, many genes linked to immune system functioning and stress response had been downregulated. This probably sensitizes isolated individuals to various stressors, in particular because isolated workers exhibit reduced sanitary behaviour. We provide evidence of the diverse consequences of social isolation in social insects, some of which resemble those found in social mammals, suggesting a general link between social well-being, stress tolerance, and immune competence in social animals.

Silencing of the phosphofructokinase gene impairs glycolysis and causes abnormal locomotion in the subterranean termite Reticulitermes chinensis Snyder

Phosphofructokinase (PFK) is a rate-limiting enzyme in glycolysis, but its linkage with locomotion in termites is not well understood, despite the demonstrated involvement of this gene in the locomotion of different animals. Here, we investigated the effect of the pfk gene on locomotion in the subterranean termite Reticulitermes chinensis Snyder through RNA interference and the use of an Ethovision XT tracking system. The knockdown of pfk resulted in significantly decreased expression of the pfk gene in different castes of termites. The pfk-silenced workers displayed higher levels of glucose but lower levels of nicotinamide adenine dinucleotide (NADH) and adenosine triphosphate (ATP) production and decreased activity of the PFK enzyme. Furthermore, abnormal locomotion (decreased distance travelled, velocity and acceleration but increased turn angle, angular velocity and meander) was observed in different castes of pfk-silenced termites. We found caste-specific locomotion among workers, soldiers and dealates. Additionally, soldiers and dealates showed higher velocity in the inner zone than in the wall zone, which is considered an effective behaviour to avoid predation. These findings reveal the close linkage between the pfk gene and locomotion in termites, which helps us to better understand the regulatory mechanism and caste specificity of social behaviours in social insects.

Formicine ants swallow their highly acidic poison for gut microbial selection and control

Animals continuously encounter microorganisms that are essential for health or cause disease. They are thus challenged to control harmful microbes while allowing the acquisition of beneficial microbes. This challenge is likely especially important for social insects with respect to microbes in food, as they often store food and exchange food among colony members. Here we show that formicine ants actively swallow their antimicrobial, highly acidic poison gland secretion. The ensuing acidic environment in the stomach, the crop, can limit the establishment of pathogenic and opportunistic microbes ingested with food and improve the survival of ants when faced with pathogen contaminated food. At the same time, crop acidity selectively allows acquisition and colonization by Acetobacteraceae, known bacterial gut associates of formicine ants. This suggests that swallowing of the poison in formicine ants acts as a microbial filter and that antimicrobials have a potentially widespread but so far underappreciated dual role in host-microbe interactions.

Network analyses reveal structure in insect social groups

Animals, from flies to humans, interact with each other, forming complex relationships and structured social interaction networks. These networks describe patterns of interactions that occur within a group. Social network analysis (SNA) is the statistical analysis of nodes, which represent individuals within a network who are connected by social ties, often called edges, that represent interactions between individuals. Here, we review recent studies on social interaction networks in insects with an emphasis on flies. In flies and other insects, SNA has revealed the contribution of group structure to disease transmission, feeding strategy, fighting, mating, and oviposition. The literature shows that SNAs are useful to understand mechanisms underlying group behavior as well as the evolution of social structure.

Oxytocin/vasopressin-like peptide inotocin regulates cuticular hydrocarbon synthesis and water balancing in ants

Inotocin, the oxytocin/vasopressin-like peptide, is widely conserved in arthropods; however, little is known about its molecular function. Here, we show that, in ants, the expression levels of inotocin and its receptor are correlated with the age of workers and their behavior. We also demonstrate that inotocin signaling is involved in desiccation resistance by regulating the synthesis of cuticular hydrocarbons. We propose that the up-regulation of inotocin and its receptor as workers age and switch tasks from nursing to foraging is a key physiological adaption to survive drier environments outside of the nest.

Oxytocin/vasopressin-like peptides are important regulators of physiology and social behavior in vertebrates. However, the function of inotocin, the homologous peptide in arthropods, remains largely unknown. Here, we show that the level of expression of inotocin and inotocin receptor are correlated with task allocation in the ant Camponotus fellah. Both genes are up-regulated when workers age and switch tasks from nursing to foraging. in situ hybridization revealed that inotocin receptor is specifically expressed in oenocytes, which are specialized cells synthesizing cuticular hydrocarbons which function as desiccation barriers in insects and for social recognition in ants. dsRNA injection targeting inotocin receptor, together with pharmacological treatments using three identified antagonists blocking inotocin signaling, revealed that inotocin signaling regulates the expression of cytochrome P450 4G1 (CYP4G1) and the synthesis of cuticular hydrocarbons, which play an important role in desiccation resistance once workers initiate foraging.

Social environment affects the transcriptomic response to bacteria in ant queens

Social insects have evolved enormous capacities to collectively build nests and defend their colonies against both predators and pathogens. The latter is achieved by a combination of individual immune responses and sophisticated collective behavioral and organizational disease defenses, that is, social immunity. We investigated how the presence or absence of these social defense lines affects individual-level immunity in ant queens after bacterial infection. To this end, we injected queens of the ant Linepithema humile with a mix of gram+ and gram- bacteria or a control solution, reared them either with workers or alone and analyzed their gene expression patterns at 2, 4, 8, and 12 hr post-injection, using RNA-seq. This allowed us to test for the effect of bacterial infection, social context, as well as the interaction between the two over the course of infection and raising of an immune response. We found that social isolation per se affected queen gene expression for metabolism genes, but not for immune genes. When infected, queens reared with and without workers up-regulated similar numbers of innate immune genes revealing activation of Toll and Imd signaling pathways and melanization. Interestingly, however, they mostly regulated different genes along the pathways and showed a different pattern of overall gene up-regulation or down-regulation. Hence, we can conclude that the absence of workers does not compromise the onset of an individual immune response by the queens, but that the social environment impacts the route of the individual innate immune responses.

Oxytocin-like signaling in ants influences metabolic gene expression and locomotor activity

Ants are emerging model systems to study cellular signaling because distinct castes possess different physiologic phenotypes within the same colony. Here we studied the functionality of inotocin signaling, an insect ortholog of mammalian oxytocin (OT), which was recently discovered in ants. In Lasius ants, we determined that specialization within the colony, seasonal factors, and physiologic conditions down-regulated the expression of the OT-like signaling system. Given this natural variation, we interrogated its function using RNAi knockdowns. Next-generation RNA sequencing of OT-like precursor knock-down ants highlighted its role in the regulation of genes involved in metabolism. Knock-down ants exhibited higher walking activity and increased self-grooming in the brood chamber. We propose that OT-like signaling in ants is important for regulating metabolic processes and locomotion.-Liutkevičiūtė, Z., Gil-Mansilla, E., Eder, T., Casillas-Pérez, B., Di Giglio, M. G., Muratspahić, E., Grebien, F., Rattei, T., Muttenthaler, M., Cremer, S., Gruber, C. W. Oxytocin-like signaling in ants influences metabolic gene expression and locomotor activity.

Honey bee (Apis mellifera) sociability and nestmate affiliation are dependent on the social environment experienced post-eclosion

Underpinning the formation of a social group is the motivation of individuals to aggregate and interact with conspecifics, termed sociability. Here, we developed an assay, inspired by vertebrate approaches to evaluate social behaviours, to simultaneously examine the development of honey bee (Apis mellifera) sociability and nestmate affiliation. Focal bees were placed in a testing chamber which was separated from groups of nestmates and conspecific non-nestmates by single-layer mesh screens. Assessing how much time bees spent contacting the two mesh screens allowed us to quantify simultaneously how much bees sought proximity and interaction with other bees and their preference for nestmates over non-nestmates. Both sociability and nestmate affiliation could be detected soon after emergence as an adult. Isolation early in adult life impaired honey bee sociability but there was no evidence for a critical period for the development of the trait, as isolated bees exposed to their hive for 24 h when as old as 6 days still recovered high levels of sociability. Our data show that, even for advanced social insects, sociability is a developmental phenomenon and experience dependent.

Recurrence analysis of ant activity patterns

In this study, we used recurrence quantification analysis (RQA) and recurrence plots (RPs) to compare the movement activity of individual workers of three ant species, as well as a gregarious beetle species. RQA and RPs quantify the number and duration of recurrences of a dynamical system, including a detailed quantification of signals that could be stochastic, deterministic, or both. First, we found substantial differences between the activity dynamics of beetles and ants, with the results suggesting that the beetles have quasi-periodic dynamics and the ants do not. Second, workers from different ant species varied with respect to their dynamics, presenting degrees of predictability as well as stochastic signals. Finally, differences were found among minor and major caste of the same (dimorphic) ant species. Our results underscore the potential of RQA and RPs in the analysis of complex behavioral patterns, as well as in general inferences on animal behavior and other biological phenomena.

orco Mutagenesis Causes Loss of Antennal Lobe Glomeruli and Impaired Social Behavior in Ants

Life inside ant colonies is orchestrated with diverse pheromones, but it is not clear how ants perceive these social signals. It has been proposed that pheromone perception in ants evolved via expansions in the numbers of odorant receptors (ORs) and antennal lobe glomeruli. Here, we generate the first mutant lines in the clonal raider ant, Ooceraea biroi, by disrupting orco, a gene required for the function of all ORs. We find that orco mutants exhibit severe deficiencies in social behavior and fitness, suggesting they are unable to perceive pheromones. Surprisingly, unlike in Drosophila melanogaster, orco mutant ants also lack most of the ∼500 antennal lobe glomeruli found in wild-type ants. These results illustrate that ORs are essential for ant social organization and raise the possibility that, similar to mammals, receptor function is required for the development and/or maintenance of the highly complex olfactory processing areas in the ant brain. VIDEO ABSTRACT.

Early social isolation impairs development, mate choice and grouping behaviour of predatory mites

The social environment early in life is a key determinant of developmental, physiological and behavioural trajectories across vertebrate and invertebrate animals. One crucial variable is the presence/absence of conspecifics. For animals usually reared in groups, social isolation after birth or hatching can be a highly stressful circumstance, with potentially long-lasting consequences. Here, we assessed the effects of social deprivation (isolation) early in life, that is, absence of conspecifics, versus social enrichment, that is, presence of conspecifics, on developmental time, body size at maturity, mating behaviour and group-living in the plant-inhabiting predatory mite Phytoseiulus persimilis. Socially deprived protonymphs developed more slowly and were less socially competent in grouping behaviour than socially enriched protonymphs. Compromised social competence in grouping behaviour was evident in decreased activity, fewer mutual encounters and larger interindividual distances, all of which may entail severe fitness costs. In female choice/male competition, socially deprived males mated earlier than socially enriched males; in male choice/female competition, socially deprived females were more likely to mate than socially enriched females. In neither mate choice situation did mating duration or body size at maturity differ between socially deprived and enriched mating opponents. Social isolation-induced shifts in mating behaviour may be interpreted as increased attractiveness or competitiveness or, more likely, as hastiness and reduced ability to assess mate quality. Overall, many of the social isolation-induced behavioural changes in P. persimilis are analogous to those observed in other animals such as cockroaches, fruit flies, fishes or rodents. We argue that, due to their profound and persistent effects, early social deprivation or enrichment may be important determinants in shaping animal personalities.

Plasticity of Daily Behavioral Rhythms in Foragers and Nurses of the Ant Camponotus rufipes: Influence of Social Context and Feeding Times

Daily activities within an ant colony need precise temporal organization, and an endogenous clock appears to be essential for such timing processes. A clock drives locomotor rhythms in isolated workers in a number of ant species, but its involvement in activities displayed in the social context is unknown. We compared locomotor rhythms in isolated individuals and behavioral rhythms in the social context of workers of the ant Camponotus rufipes. Both forager and nurse workers exhibited circadian rhythms in locomotor activity under constant conditions, indicating the involvement of an endogenous clock. Activity was mostly nocturnal and synchronized with the 12:12h light-dark-cycle. To evaluate whether rhythmicity was maintained in the social context and could be synchronized with non-photic zeitgebers such as feeding times, daily behavioral activities of single workers inside and outside the nest were quantified continuously over 24 hours in 1656 hours of video recordings. Food availability was limited to a short time window either at day or at night, thus mimicking natural conditions of temporally restricted food access. Most foragers showed circadian foraging behavior synchronized with food availability, either at day or nighttime. When isolated thereafter in single locomotor activity monitors, foragers mainly displayed arrhythmicity. Here, high mortality suggested potential stressful effects of the former restriction of food availability. In contrast, nurse workers showed high overall activity levels in the social context and performed their tasks all around the clock with no circadian pattern, likely to meet the needs of the brood. In isolation, the same individuals exhibited in turn strong rhythmic activity and nocturnality. Thus, endogenous activity rhythms were inhibited in the social context, and timing of daily behaviors was flexibly adapted to cope with task demands. As a similar socially-mediated plasticity in circadian rhythms was already shown in honey bees, the temporal organization in C. rufipes and honey bees appear to share similar basic features.

Lifespan behavioural and neural resilience in a social insect

Analyses of senescence in social species are important to understanding how group living influences the evolution of ageing in society members. Social insects exhibit remarkable lifespan polyphenisms and division of labour, presenting excellent opportunities to test hypotheses concerning ageing and behaviour. Senescence patterns in other taxa suggest that behavioural performance in ageing workers would decrease in association with declining brain functions. Using the ant Pheidole dentata as a model, we found that 120-day-old minor workers, having completed 86% of their laboratory lifespan, showed no decrease in sensorimotor functions underscoring complex tasks such as alloparenting and foraging. Collaterally, we found no age-associated increases in apoptosis in functionally specialized brain compartments or decreases in synaptic densities in the mushroom bodies, regions associated with integrative processing. Furthermore, brain titres of serotonin and dopamine--neuromodulators that could negatively impact behaviour through age-related declines--increased in old workers. Unimpaired task performance appears to be based on the maintenance of brain functions supporting olfaction and motor coordination independent of age. Our study is the first to comprehensively assess lifespan task performance and its neurobiological correlates and identify constancy in behavioural performance and the absence of significant age-related neural declines.

Survival after pathogen exposure in group-living insects: don't forget the stress of social isolation!

A major cost of group-living is its inherent risk of pathogen infection. To limit this risk, many group-living animals have developed the capability to prophylactically boost their immune system in the presence of group members and/or to mount collective defences against pathogens. These two phenomena, called density-dependent prophylaxis and social immunity, respectively, are often used to explain why, in group-living species, individuals survive better in groups than in isolation. However, this survival difference may also reflect an alternative and often overlooked process: a cost of social isolation on individuals' capability to fight against infections. Here, we disentangled the effects of density-dependent prophylaxis, social immunity and stress of social isolation on the survival after pathogen exposure in group-living adults of the European earwig Forficula auricularia. By manipulating the presence of group members both before and after pathogen exposure, we demonstrated that the cost of being isolated after infection, but not the benefits of social immunity or density-dependent prophylaxis, explained the survival of females. Specifically, females kept constantly in groups or constantly isolated had higher survival rates than females that were first in groups and then isolated after infection. Our results also showed that this cost of social isolation was absent in males and that social isolation did not reduce the survival of noninfected individuals. Overall, this study gives a new perspective on the role of pathogens in social evolution, as it suggests that an apparently nonadaptive, personal immune process may promote the maintenance of group-living under pathogenic environments.

Moribund Ants Do Not Call for Help

When an antlion captures a foraging ant, the victim's nestmates may display rescue behaviour. This study tested the hypothesis that the expression of rescue behaviour depends on the life expectancy of the captured ant. This hypothesis predicts that the expression of rescue behaviour will be less frequent when the captured ant has a lower life expectancy than when it has a higher life expectancy because such a response would be adaptive at the colony level. Indeed, significant differences were found in the frequency of rescue behaviours in response to antlion victims with differing life expectancies. In agreement with prediction, victims with lower life expectancies were rescued less frequently, and those rescues had a longer latency and shorter duration. There was also a qualitative difference in the behaviour of rescuers to victims from the low and high life expectancy groups. Several explanations for these findings are proposed.