Pogonomyrmex Occidentalis Research Articles

Rodent competition and fire alter patterns of mound and disk formation of western harvester ants.

Consumers exert top-down controls on dryland ecosystem function, but recent increases in fire activity may alter consumer communities in post-fire environments. Native consumers, including ants and rodents, likely have critical roles in defining post-fire plant community assembly and resilience to biological invasions. This study aimed to understand how western harvester ants (Pogonomyrmex occidentalis) that form mounds and large vegetation-free disks that significantly influence plant community structure in the Great Basin Desert respond to fire and rodent community abundance. We tested this by installing treatment plots that excluded or allowed rodents and were burned or unburned in a full factorial design. We measured ant disk and mound size and density in each experimental plot. Fire increased ant mound density by 126% compared to unburned plots. Rodent presence decreased mound density by 59%, mound diameter by 13%, and mound height by 166%. We also show an interaction where the adverse effects of rodents on ant disk density were greater in burned than in unburned plots. The results suggest that booms in rodent populations are likely to have suppressive effects on ant mound and disk formation in native shrublands but that harvester ants may be released from rodent competition with the emergence of invasive grass-fire cycles.

The ontogeny of selection on genetic diversity in harvester ants.

Selection may favour traits throughout an individual's lifetime or at a particular life stage. In many species of social insects, established colonies that are more genetically diverse outperform less diverse colonies with respect to a variety of traits that contribute to fitness, but whether selection favours high diversity in small colonies is unknown. We tested the hypothesis that selection favours genetically diverse colonies during the juvenile period using a multi-year field experiment with the harvester ant, Pogonomyrmex occidentalis. We used controlled matings to generate colonies that varied in genetic diversity and transplanted them into the field. We monitored their survival for seven (the 2015 cohort, n = 149) and six (the 2016 cohort, n = 157) years. Genetically more diverse colonies had greater survival, resulting in significant viability selection. However, in both cohorts survival was not influenced by genetic diversity until colonies were three years old. We suggest that changes in their internal organization enabled colonies to use the benefits of multiple genotypes, and discuss possible mechanisms that can generate this pattern.

The benefits of being big and diverse: early colony survival in harvester ants.

In sessile organisms such as plants and benthic invertebrates, founding propagules typically suffer extremely high rates of mortality due to both extrinsic and intrinsic factors. Many social insect species share similarities with these groups, but factors influencing early colony survival are relatively unstudied. We used a field experiment to measure the importance of environmental quality relative to intrinsic colony properties in the harvester ant, Pogonomyrmex occidentalis, by monitoring the survival of 584 experimental colonies. We measured survival of transplanted colonies over four months in each of three years (2014-2016) at a site in western Colorado. Colony survival was primarily determined by colony features. Multiple mating by the queen and larger colony size at the time of transplant increased survival, but queen size, maternal lineage and the composition of plant species in the vicinity of the colony did not. Food supplementation increased survival significantly when natural food was scarce, but was not consistently beneficial, in contrast to predictions. Our results emphasize the general importance of rapid growth and early attainment of large size in the survival of sessile species. However, attributes specific to ants that are a consequence of their sociality also strongly affected survival. Colonies with multiply-mated queens were more likely to survive over a wide range of circumstances, highlighting the importance of this trait even at the early stages of colony life.

Fire and plant invasion, but not rodents, alter ant community abundance and diversity in a semi‐arid desert

AbstractHuman activities are increasing the size, frequency, and severity of disturbance across earth's ecosystems including deserts. Ants are important drivers of ecosystem function and are good bioindicators of ecosystem sensitivity to disturbance and change. Rodents also play an important role in ecosystem response to disturbance and often compete with ants for resources. The purpose of our study was to test the main and interactive effects of fire, rodent activity, and time on ant forager abundance, species richness, and diversity, as well as changes in ant mound density and disk area in the Great Basin Desert. We experimentally applied burn and rodent exclusion treatments and used pitfall traps to collect ants each month from April through October from 2014 to 2016. Over the three‐year period, burned areas had lower richness and diversity than unburned areas. Rodent exclusion had minimal effects on the ant community, and there was not a significant rodent exclusion interaction with fire. Treatment effects varied by month and year. The western harvester ant, Pogonomyrmex occidentalis, was the most abundant ant species, comprising about 70% of the total ants captured. Shifts in ant diversity following fire were driven by positive responses of harvester ants to burned habitat conditions. In contrast, all other ant species when analyzed together had lower forager abundance in burned plots, which drove lower ant diversity in burned plots. Ant forager abundance, richness, and diversity increased each year of the study in all plots; however, richness and diversity remained lower in burned areas than in unburned areas each year. Structural equation modeling shows that the effects of fire on ant community diversity are partially mediated through the plant community. While rodents affected the plant community, those effects do not seem to transfer over to the ant community. Pogonomyrmex occidentalis mound density was higher in burned areas, but disk area was smaller. Our results suggest that fire has adverse effects on ant community diversity. This could have long‐lasting effects on ecosystem function in the face of a changing fire regime in deserts of North America caused by invasive annual plants.

Spatial Aspects of Corpse Removal in the Western Harvester Ant, Pogonomyrmex occidentalis

Eusocial insects actively combat pathogen proliferation with a myriad of tactics, one of which is the removal of corpses from the nest, a behavior known as necrophoresis. Spatial patterns of corpse depositions by colonies of the western harvester ant, Pogonomyrmex occidentalis, were examined. Colonies were presented with nestmate and non-nestmate corpses to discern if each type of waste was handled differently. Specialized areas for corpse disposal were not observed. Non-nestmate corpses were carried farther from the nest than were nestmate corpses, perhaps reducing the chance of introduction of pathogens new to the colony that may be harbored by a non-nestmate. Factors external to the nest mound, such as slope and neighboring colonies, had no perceptible effect on these depositions and did not change the uniformity of dispersal of this particular waste. These findings add to our knowledge of the intricacies of corpse removal in eusocial insects and suggest that this is a more dynamic activity than previously thought.

Pavement Ant Workers (Tetramorium caespitum) Assess Cues Coded in Cuticular Hydrocarbons to Recognize Conspecific and Heterospecific Non-Nestmate Ants

Most ants live in closed societies from which non-members are excluded through fighting or ritualized displays to protect colony resources. Nestmate recognition is the process by which ants discriminate nestmate from non-nestmate ants. Ants use cues coded in mixtures of long-chain hydrocarbon compounds on the cuticle as nestmate recognition cues. Pavement ants (Tetramorium caespitum) form conspicuous wars between neighboring colonies that are organized after workers meet and make the decision to fight after assessing nestmate recognition cues. These wars involve thousands of individuals. Fighting is ritualized and few ants die in the process. We identified 24 cuticular hydrocarbon compounds, above 1% in relative abundance, in the profile of pavement ants with chain lengths ranging from 15 to 31 carbon atoms. Cuticular lipids contained, in order of abundance: mono-methyl alkanes (45–56%), n-alkanes (range: 16–40% relative abundance), and alkenes (10–20%), with small or trace amounts of di-methyl, tri-methyl alkanes and fatty acids. Results from behavioral tests show that pavement ants assess information in cuticular hydrocarbon profiles to recognize both conspecific and heterospecfic (Pogonomyrmex occidentalis and Camponotus modoc) non-nestmate ants and that the relative abundance of methyl-branched alkanes and alkenes codes for nestmate status, at least for conspecific interactions. Our data add to a growing body of knowledge about how ants use cuticular hydrocarbon based nestmate recognition cues to prevent the intrusion of non-nestmates in to colony space.

AN INVERTEBRATE ECOSYSTEM ENGINEER UNDER THE UMBRELLA OF SAGE-GROUSE CONSERVATION.

Conservation practitioners often rely on areas designed to protect species of greatest conservation priority to also conserve co-occurring species (i.e., the umbrella species concept). The extent to which vertebrate species may serve as suitable umbrellas for invertebrate species, however, has rarely been explored. Sage-grouse (Centrocercus spp.) have high conservation priority throughout much of the rangelands of western North America and are considered an umbrella species through which the conservation of entire rangeland ecosystems can be accomplished. Harvester ants are ecosystem engineers and play important roles in the maintenance and function of rangeland ecosystems. We compared indices of the abundance of western harvester ants (Pogonomyrmex occidentalis) and Greater Sage-Grouse (Centrocercus urophasianus) at 72 sites in central Wyoming, USA in 2012. The abundance of harvester ant mounds was best predicted by a regression model that included a combination of local habitat characteristics and the abundance of sage-grouse. When controlling for habitat-related factors, areas with higher abundances of sage-grouse pellets (an index of sage-grouse abundance and/or habitat use) had higher abundances of ant mounds than areas with lower abundances of sage-grouse pellets. The causal mechanism underlying this positive relationship between sage-grouse and ant mound abundance at the fine scale could be indirect (e.g., both species prefer similar environmental conditions) or direct (e.g., sage-grouse prefer areas with a high abundance of ant mounds because ants are an important prey item during certain life stages). We observed no relationship between a broad-scale index of breeding sage-grouse density and the abundance of ant mounds. We suspect that consideration of the non-breeding habitat of sage-grouse and finer-scale measures of sage-grouse abundance are critical to the utility of sage-grouse as an umbrella species for the conservation of harvester ants and their important role in rangeland ecosystems.

Homeward navigation in Pogonomyrmex occidentalis harvester ants

This study determined the relative importance of sources of navigational information in Pogonomyrmex occidentalis harvester ants. Displacement experiments, polarized light manipulations, and view manipulations tested the use of path integration, polarized light patterns, and visual guidance in the ability of Pogonomyrmex ants to orient homeward during foraging. In the displacement experiments, worker ants that were displaced from their original locations to new locations equidistant from the nest initially walked in the direction of the home vector that would be determined by a path integrator (PI). Sham treatments showed that handling and returning ants to their original location had no effect on homeward navigation. Neither a polarizing filter nor changing nearby landmarks affected orientation. However, placement of a wall behind the nest while an ant was navigating homeward caused it to become disoriented. Ants viewing the wall walked in random directions and took more time to find their nest. These results suggest that P. occidentalis workers rely on PI to navigate homeward after foraging, but do not follow the home vector indicated by the PI if the panorama has been significantly altered and, therefore, conflicts with the available PI information.

Assessing Microhabitat Cover and Distance Effects on Harvester Ant Intraspecific Seed Preference

Harvester ants have long been known to exhibit interspecific seed preference and this preference has been thought to be associated with distance in a manner analogous with optimal foraging theory. However, little attention has been given to how intraspecific seed preference changes or how microhabitat (i.e. the composition of the terrain that the ants are moving through) impacts seed harvesting preference. We addressed these questions by conducting seed harvesting experiments in three different Ephedra viridis populations that contain harvester ants (Pogonomyrmex occidentalis) by using only E. viridis seeds and conducting trials over multiple distances and varying degrees of microhabitat cover. We found that increased microhabitat cover and increased seed mass decrease the likelihood of E. viridis seeds being harvested much more dramatically than distance. However, we found no effect of distance or microhabitat cover on which E. viridis seeds were harvested. We conclude that harvester ant E. viridis seed preference is distance and microhabitat independent. However, increases in microhabitat cover negatively impacts the likelihood of P. occidentalis harvesting E. viridis seeds of any size. Our findings suggest that harvester ant foraging behavior is influenced by structure of the microhabitat more than by distance. This provides a new context on how harvester ant foraging behavior and effectiveness should be considered.

Transition of Vegetation States Positively Affects Harvester Ants in the Great Basin, United States

Invasions by non-native plants can alter ecosystems such that new ecological states are reached, but less is known about how these transitions influence animal populations. Sagebrush (Artemisia tridentata) ecosystems are experiencing state changes because of fire and invasion by exotic annual grasses. Our goal was to study the effects of these state changes on the Owyhee and western harvester ants (Pogonomyrmex salinus Olsen and P. occidentalis Cresson, respectively). We sampled 358 1-ha plots across the northern Great Basin, which captured unburned and burned conditions across 1−≥31 years postfire. Our results indicated an immediate and consistent change in vegetation states from shrubland to grassland between 1 and 31 years postfire. Harvester ant occupancy was unrelated to time since fire, whereas we observed a positive effect of fire on nest density. Similarly, we discovered that fire and invasion by exotic annuals were weak predictors of harvester ant occupancy but strong predictors of nest density. Occupancy of harvester ants was more likely in areas with finer-textured soils, low precipitation, abundant native forbs, and low shrub cover. Nest density was higher in arid locations that recently burned and exhibited abundant exotic annual and perennial (exotic and native) grasses. Finally, we discovered that burned areas that received postfire restoration had minimal influence on harvester ant occupancy or nest density compared with burned and untreated areas. These results suggest that fire-induced state changes from native shrublands to grasslands dominated by non-native grasses have a positive effect on density of harvester ants (but not occupancy), and that postfire restoration does not appear to positively or negatively affect harvester ants. Although wildfire and invasion by exotic annual grasses may negatively affect other species, harvester ants may indeed be one of the few winners among a myriad of losers linked to vegetation state changes within sagebrush ecosystems.

A test of the self-medication hypothesis for anting behavior in blue jays

Abstract. Anting is a curious behavior that has been recorded in over 200 species of songbirds. While anting, a bird will wipe several ants throughout its plumage. It has been proposed that birds select ants for their ability to spray formic acid, a chemical that is known to have antibacterial properties at high enough concentrations that may help to limit the growth of feather bacteria. To test this hypothesis, two blue jays (Cyanocitta cristata) were provided with daily rations of either ants capable of ejecting formic acid (black carpenter ants, Camponotus pennsylvanicus) or ants incapable of ejecting formic acid (Western harvester ants, Pogonomyrmex occidentalis). After 26 daily sessions, it was found that a significantly higher percentage of black carpenter ants were used for anting when compared to the percentage of Western harvester ants. In the second part of the study, feathers inoculated with the common feather bacterium Bacillus licheniformis were treated with black carpenter ants to determine ...

An ecological engineer maintains consistent spatial patterning, with implications for community‐wide effects

In many ecosystems, foundational species create spatial patterns that structure a broader community. It is unclear, however, how robust these patterns are across large areas and strong environmental gradients, and how the landscape‐level consequences of these patterns may vary. We investigated the robustness of non‐random patterning in the dispersion of the western harvester ant (Pogonomyrmex occidentalis), a widely recognized ecosystem engineer of western North America. We used remote imagery to characterize the spatial structure and densities of western harvester ant mounds at sites spanning their range within the sagebrush steppe and short‐grass prairie areas of Wyoming (581 × 450 km area). We found that ant mound densities varied substantially across the study region, but that mounds were strongly and consistently overdispersed (regularly patterned) across both climatic gradients and mound densities. Precipitation was the only abiotic factor that significantly affected either density or pattern, with stronger patterning among mounds at drier sites. This robustness in ecological patterning is likely to have strong effects on community function; mound dispersion increased the fraction of the landscape within typical ant foraging distances up to 30% over what density alone would predict. We estimated how patterning can modify one key ant effect at a landscape level by combining mound dispersion data with information from a seed removal experiment. Randomization tests based on these results showed that in a representative area, overdispersion could increase the mean landscape‐wide seed removal rate by 16%, and decrease its spatial variance by 50%. Western harvester ants are known to affect multiple aspects of community function and structure at a relatively fine scale, and our results show that their spatial dispersion may therefore influence many features of interspecific interactions and community dynamics.

Defending the nest: variation in the alarm aggression response and nest mound damage in the harvester ant Pogonomyrmex occidentalis

A number of studies have found that ant colonies vary in many colony-level phenotypes, including the level of aggression towards non-nestmates. The extent of a colony’s aggression and defense of the nest in response to attacks by predators is likely to affect its survival and reproduction, but the degree to which colonies vary in their defensive response is poorly known. We documented considerable variation in damage to the external nest mound of Pogonomyrmex occidentalis (Cresson) at our long-term study site in 2012. Heavily damaged colonies formed several spatial clusters, consistent with predation within a home range. We tested two competing hypotheses for the variation in nest damage: (1) colonies vary in their level of aggression, those with stronger defensive responses are better protected, and suffer less damage, versus (2) colonies have similar levels of aggression, but those that suffer predator-induced damage subsequently display a stronger defense response. We measured the alarm/aggression response in colonies exposed to a standardized stimulus and determined whether the level of aggression was correlated to the level of damage. Colonies with strong defensive responses were significantly less likely to exhibit damage than those with weak responses, suggesting that aggression level is a colony phenotype.

Upper Thermal Limits of Insects Are Not the Result of Insufficient Oxygen Delivery

Most natural environments experience fluctuating temperatures that acutely affect an organism's physiology and ultimately a species' biogeographic distribution. Here we examine whether oxygen delivery to tissues becomes limiting as body temperature increases and eventually causes death at upper lethal temperatures. Because of the limited direct, experimental evidence supporting this possibility in terrestrial arthropods, we explored the effect of ambient oxygen availability on the thermotolerance of insects representing six species (Acheta domesticus, Hippodamia convergens, Gromphadorhina portentosa, Pogonomyrmex occidentalis, Tenebrio molitor, and Zophobus morio), four taxonomic orders (Blattodea, Coleoptera, Hymenoptera, and Orthoptera), and multiple life stages (e.g., adults vs. larvae or nymphs). The survival curves of insects exposed to temperatures (45° or 50°C) under normoxic conditions (21% O(2)) were compared with those measured under altered oxygen levels (0%, 10%, 35%, and 95% O(2)). Kaplan-Meier log rank analyses followed by Holm-Sidak pairwise comparisons revealed that (1) anoxia sharply diminished survival times in all groups studied, (2) thermotolerance under moderate hyperoxia (35% O(2)) or moderate hypoxia (10% O(2)) was the same as or lower than that under normoxia, (3) half of the experimental treatments involving extreme hyperoxia (95% O(2)) caused reduced thermotolerance, and (4) thermotolerance differed with developmental stage. Adult G. portentosa exhibited much higher thermotolerance than their first-instar nymphs, but responses from larval and adult Z. morio were equivocal. We conclude that some factor(s) separate from oxygen delivery is responsible for death of insects at upper lethal temperatures.

Competitive Advantages of Earlier Onset of Foraging in Pogonomyrmex occidentalis (Hymenoptera: Formicidae)

ABSTBACT Colonies of the western harvester ant (Pogonomyrmex occidentalis Cresson) with greater genetic diversity begin foraging earlier, forage for longer durations, and collect more food than those with less genetic diversity. In this study we tested whether earlier onset of foraging improved the foraging success of colonies at baits placed midway between neighbors. Colonies that began foraging earlier in the morning were more likely to be the first to discover the food and to monopolize the food source. Colony size was also important with larger nests monopolizing significantly more baits than smaller nests. These results show that early foraging by colonies can provide an intraspecific foraging advantage at a clumped food resource. This advantage adds more evidence to a growing body of work showing how polyandry, genetic diversity, and colony-level performance are interrelated in P. occidentalis colonies.

Seed harvesting by a generalist consumer is context‐dependent: Interactive effects across multiple spatial scales

Granivore foraging decisions affect consumer success and determine the quantity and spatial pattern of seed survival. These decisions are influenced by environmental variation at spatial scales ranging from landscapes to local foraging patches. In a field experiment, the effects of seed patch variation across three spatial scales on seed removal by western harvester ants Pogonomyrmex occidentalis were evaluated. At the largest scale we assessed harvesting in different plant communities, at the intermediate scale we assessed harvesting at different distances from ant mounds, and at the smallest scale we assessed the effects of interactions among seed species in local seed neighborhoods on seed harvesting (i.e. resource–consumer interface). Selected seed species were presented alone (monospecific treatment) and in mixture with Bromus tectorum (cheatgrass; mixture treatment) at four distances from P. occidentalis mounds in adjacent intact sagebrush and non‐native cheatgrass‐dominated communities in the Great Basin, Utah, USA. Seed species differed in harvest, with B. tectorum being least preferred. Large and intermediate scale variation influenced harvest. More seeds were harvested in sagebrush than in cheatgrass‐dominated communities (largest scale), and the quantity of seed harvested varied with distance from mounds (intermediate‐scale), although the form of the distance effect differed between plant communities. At the smallest scale, seed neighborhood affected harvest, but the patterns differed among seed species considered. Ants harvested fewer seeds from mixed‐seed neighborhoods than from monospecific neighborhoods, suggesting context dependence and potential associational resistance. Further, the effects of plant community and distance from mound on seed harvest in mixtures differed from their effects in monospecific treatments. Beyond the local seed neighborhood, selection of seed resources is better understood by simultaneously evaluating removal at multiple scales. Associational effects provide a useful theoretical basis for better understanding harvester ant foraging decisions. These results demonstrate the importance of ecological context for seed removal, which has implications for seed pools, plant populations and communities.

NEOICHNOLOGICAL STUDY OF THE TRACES AND BURROWING BEHAVIORS OF THE WESTERN HARVESTER ANT POGONOMYRMEX OCCIDENTALIS (INSECTA: HYMENOPTERA: FORMICIDAE): PALEOPEDOGENIC AND PALEOECOLOGICAL IMPLICATIONS

Abstract New observations of burrowing behaviors, nest construction, and sediment-mixing patterns of the western harvester ant Pogonomyrmex occidentalis are described from a neoichnological laboratory study and compared with ichnofossil evidence. Fifty ants burrowed for six weeks in a glass-sided enclosure filled with silty clay simulating a soil with A, C, Ab, and Cb horizons. The galleries, chambers, and mound were mapped and digitally recorded every 12 hours to monitor nest development and changes in architecture. After six weeks the nest was cast with dental plaster to study above- and belowground architectural and surficial nest morphologies. Numerous, intricately interconnected galleries and chambers were constructed, with galleries averaging ~0.9 cm in diameter and chambers varying from ~3 to 10 cm long and up to 1.5 cm high. Burrowing behaviors included pulling, raking, pushing, forcing, cutting, and carrying. Sediment was removed from all horizons and deposited at the surface, resulting in a larg...

Patriline shifting leads to apparent genetic caste determination in harvester ants

The harvester ant, Pogonomyrmex occidentalis, is characterized by high levels of intracolonial genetic diversity resulting from multiple mating by the queen. Within reproductively mature colonies, the relative frequency of different male genotypes (patrilines) is not stable. The difference between samples increases with time, nearing an asymptote after a year. Patriline distributions in gynes and workers display similar patterns of change. A consequence of changing patriline distributions is that workers and gynes appear to have different fathers. However, apparent genetic differences between castes are caused by changing paternity among all females. Temporal variation in the relative frequency of patrilines may be a consequence of processes that reflect sexual conflict, such as sperm clumping. Recent work documenting genotype differences between physical castes (workers and gynes; major and minor workers) in several species of ants has been interpreted as evidence of genetic caste determination. Reanalysis of these studies found little support for this hypothesis. Apparent caste determination may result from temporal variation in sperm use, rather than from fertilization bias among male ejaculates.

The structure of foraging activity in colonies of the harvester ant, Pogonomyrmex occidentalis

The timing of activity by desert dwelling poikilotherms can be critical to survival. In the western harvester ant, colonies that have higher levels of genetic diversity forage for longer time periods in the morning than colonies with less diversity. We determined whether the advantage of early foraging colonies was consistent by examining foraging behavior at other times of day and year. We used a combination of activity monitoring and temperature measurement at the nest entrance to quantify foraging activity during the morning and evening summer foraging periods in both June and August. The duration of morning and evening foraging was significantly positively correlated both within and across seasons—some colonies have a consistent advantage in foraging. The temperature range over which colonies foraged was also consistent across time, suggesting that intercolony differences are a consequence of variation in the thermal ranges/preferences of the colony's workers. The duration of foraging during this study was correlated with the duration of foraging measured 6 years earlier, suggesting that it is an aspect of colony phenotype.

Expression patterns of a circadian clock gene are associated with age-related polyethism in harvester ants, Pogonomyrmex occidentalis.

BackgroundRecent advances in sociogenomics allow for comparative analyses of molecular mechanisms regulating the development of social behavior. In eusocial insects, one key aspect of their sociality, the division of labor, has received the most attention. Age-related polyethism, a derived form of division of labor in ants and bees where colony tasks are allocated among distinct behavioral phenotypes, has traditionally been assumed to be a product of convergent evolution. Previous work has shown that the circadian clock is associated with the development of behavior and division of labor in honeybee societies. We cloned the ortholog of the clock gene, period, from a harvester ant (Pogonomyrmex occidentalis) and examined circadian rhythms and daily activity patterns in a species that represents an evolutionary origin of eusociality independent of the honeybee.ResultsUsing real time qPCR analyses, we determined that harvester ants have a daily cyclic expression of period and this rhythm is endogenous (free-running under dark-dark conditions). Cyclic expression of period is task-specific; foragers have strong daily fluctuations but nest workers inside the nest do not. These patterns correspond to differences in behavior as activity levels of foragers show a diurnal pattern while nest workers tend to exhibit continuous locomotor activity at lower levels. In addition, we found that foragers collected in the early fall (relative warm, long days) exhibit a delay in the nightly peak of period expression relative to foragers collected in the early spring (relative cold, short days).ConclusionThe association of period mRNA expression levels with harvester ant task behaviors suggests that the development of circadian rhythms is associated with the behavioral development of ants. Thus, the circadian clock pathway may represent a conserved 'genetic toolkit' that has facilitated the parallel evolution of age-related polyethism and task allocation in social insects.