Clint A. Penick

Neurohormonal changes associated with ritualized combat and the formation of a reproductive hierarchy in the ant Harpegnathos saltator

Dominance rank in animal societies is correlated with changes in both reproductive physiology and behavior. In some social insects, dominance status is used to determine a reproductive division of labor, where a few colony members reproduce while most remain functionally sterile. Changes in reproduction and behavior in this context must be coordinated through crosstalk between the brain and the reproductive system. We investigated a role for biogenic amines in forming this connection in the ant Harpegnathos saltator. In this species, workers engage in an elaborate dominance tournament to establish a group of reproductive workers termed gamergates. We analyzed biogenic amine content in the brains of gamergates, inside-workers and foragers under stable colony conditions and found that gamergates had the highest levels of dopamine. Dopamine levels were also positively correlated with increased ovarian activity among gamergates. Next, we experimentally induced workers to compete in a reproductive tournament to determine how dopamine may be involved in the establishment of a new hierarchy. Dopamine levels rose in aggressive workers at the start of a tournament, while workers that were policed by their nestmates (a behavior that inhibits ovarian activity) showed a rapid decline in dopamine. In addition to dopamine, levels of serotonin and tyramine differed among castes, and these changes could contribute to differences in caste-specific behavioral patterns observed among non-reproductive workers. Overall, these results provide support that biogenic amines link changes in behavior and dominance with reproductive activity in H. saltator as well as drive differences in worker task performance.

An Engineered orco Mutation Produces Aberrant Social Behavior and Defective Neural Development in Ants

Ants exhibit cooperative behaviors and advanced forms of sociality that depend on pheromone-mediated communication. Odorant receptor neurons (ORNs) express specific odorant receptors (ORs) encoded by a dramatically expanded gene family in ants. In most eusocial insects, only the queen can transmit genetic information, restricting genetic studies. In contrast, workers in Harpegnathos saltator ants can be converted into gamergates (pseudoqueens) that can found entire colonies. This feature facilitated CRISPR-Cas9 generation of germline mutations in orco, the gene that encodes the obligate co-receptor of all ORs. orco mutations should significantly impact olfaction. We demonstrate striking functions of Orco in odorant perception, reproductive physiology, and social behavior plasticity. Surprisingly, unlike in other insects, loss of OR functionality also dramatically impairs development of the antennal lobe to which ORNs project. Therefore, the development of genetics in Harpegnathos establishes this ant species as a model organism to study the complexity of eusociality.

Reproduction, dominance, and caste: endocrine profiles of queens and workers of the ant Harpegnathos saltator

The regulation of reproduction within insect societies is a key component of the evolution of eusociality. Differential patterns of hormone levels often underlie the reproductive division of labor observed among colony members, and further task partitioning among workers is also often correlated with differences in juvenile hormone (JH) and ecdysteroid content. We measured JH and ecdysteroid content of workers and queens of the ant Harpegnathos saltator. In this species, new colonies are founded by a single queen, but after she dies workers compete in an elaborate dominance tournament to decide a new group of reproductives termed “gamergates.” Our comparisons revealed that queens, gamergates, and inside workers (non-reproductive) did not differ in levels of JH or ecdysteroids. However, increased JH and decreased ecdysteroid content was observed in outside workers exhibiting foraging behavior. Application of a JH analog to virgin queens of H. saltator, although effective at inducing dealation, failed to promote egg production. Together, these results support the hypothesis that JH has lost its reproductive function in H. saltator to regulate foraging among the worker caste.

Using Physiology to Predict the Responses of Ants to Climatic Warming

Physiological intolerance of high temperatures places limits on organismal responses to the temperature increases associated with global climatic change. Because ants are geographically widespread, ecologically diverse, and thermophilic, they are an ideal system for exploring the extent to which physiological tolerance can predict responses to environmental change. Here, we expand on simple models that use thermal tolerance to predict the responses of ants to climatic warming. We investigated the degree to which changes in the abundance of ants under warming reflect reductions in the thermal niche space for their foraging. In an eastern deciduous forest system in the United States with approximately 40 ant species, we found that for some species, the loss of thermal niche space for foraging was related to decreases in abundance with increasing experimental climatic warming. However, many ant species exhibited no loss of thermal niche space. For one well-studied species, Temnothorax curvispinosus, we examined both survival of workers and growth of colonies (a correlate of reproductive output) as functions of temperature in the laboratory, and found that the range of thermal tolerances for colony growth was much narrower than for survival of workers. We evaluated these functions in the context of experimental climatic warming and found that the difference in the responses of these two attributes to temperature generates differences in the means and especially the variances of expected fitness under warming. The expected mean growth of colonies was optimized at intermediate levels of warming (2-4°C above ambient); yet, the expected variance monotonically increased with warming. In contrast, the expected mean and variance of the survival of workers decreased when warming exceeded 4°C above ambient. Together, these results for T. curvispinosus emphasize the importance of measuring reproduction (colony growth) in the context of climatic change: indeed, our examination of the loss of thermal niche space with the larger species pool could be missing much of the warming impact due to these analyses being based on survival rather than reproduction. We suggest that while physiological tolerance of temperature can be a useful predictive tool for modeling responses to climatic change, future efforts should be devoted to understanding the causes and consequences of variability in models of tolerance calibrated with different metrics of performance and fitness.

Juvenile hormone induces queen development in late-stage larvae of the ant Harpegnathos saltator

A link between hormones and developmental plasticity has long been established, but understanding how evolution has shaped the physiological systems underlying plasticity remains a major question. Within the eusocial insects, developmental plasticity helps define a reproductive division of labor through the production of distinct queen and worker castes. Caste determination may be triggered via changes in juvenile hormone (JH) levels during specific JH-sensitive periods in development. The timing of these periods, however, can vary and may relate to phenotypic differences observed among species. In order to gain insight into the evolution of caste determining systems in eusocial insects, we investigated the presence of a JH-sensitive period for queen determination in the ant Harpegnathos saltator. This species displays a number of ancestral characteristics, including low queen-worker dimorphism, and should allow insight into the early evolution of caste determining systems in ants. We identified four larval instars in H. saltator, and we found that the application of a JH analog (JHA) to third and fourth instar larvae induced queen development while treatment of early instars did not. This indicated the presence of a JH-sensitive period for queen determination at the end of the larval stage. These results contrast with what has been found in other ant species, where queen determination occurs much earlier in development. Therefore, our results suggest that caste determination originally occurred late in the larval stage in the ancestral condition but has shifted earlier in development in species that began to acquire advanced characteristics. This shift may have facilitated the development of greater queen-worker dimorphism as well as multiple worker castes.

Stable isotopes reveal links between human food inputs and urban ant diets

The amount of energy consumed within an average city block is an order of magnitude higher than that consumed in any other ecosystem over a similar area. This is driven by human food inputs, but the consequence of these resources for urban animal populations is poorly understood. We investigated the role of human foods in ant diets across an urbanization gradient in Manhattan using carbon and nitrogen stable isotopes. We found that some—but not all—ant species living in Manhattans most urbanized habitats had δ13C signatures associated with processed human foods. In particular, pavement ants (Tetramorium sp. E) had increased levels of δ13C similar to δ13C levels in human fast foods. The magnitude of this effect was positively correlated with urbanization. By contrast, we detected no differences in δ15N, suggesting Tetramorium feeds at the same trophic level despite shifting to human foods. This pattern persisted across the broader ant community; species in traffic islands used human resources more than park species. Our results demonstrate that the degree urban ants exploit human resources changes across the city and among species, and this variation could play a key role in community structure and ecosystem processes where human and animal food webs intersect.

Beyond thermal limits: comprehensive metrics of performance identify key axes of thermal adaptation in ants

Summary How species respond to temperature change depends in large part on their physiology. Physiological traits, such as critical thermal limits (CTmax and CTmin), provide estimates of thermal performance but may not capture the full impacts of temperature on fitness. Rather, thermal performance likely depends on a combination of traits—including thermal limits—that vary among species. Here we examine how thermal limits correlate with the main components that influence fitness in ants. First, we compare how temperature affected colony survival and growth in two ant species that differ in their responses to warming in the field—Aphaenogaster rudis (heat-intolerant) and Temnothorax curvispinosus (heat-tolerant). We then extended our study to compare CTmax, thermal requirements of brood, and yearly activity season among a broader set of ant species. While thermal limits were higher for workers of T. curvispinosus than A. rudis, T. curvispinosus colonies also required higher temperatures for survival and colony growth. This pattern generalized across 17 ant species, such that species whose foragers had a high CTmax also required higher temperatures for brood development. Finally, species whose foragers had a high CTmax had relatively short activity seasons compared with less heat-tolerant species. The relationships between CTmax, thermal requirements of brood, and seasonal activity suggest two main strategies for growth and development in changing thermal environments: one where ants forage at higher temperatures over a short activity season, and another where ants forage at lower temperatures for an extended activity season. Where species fall on this spectrum may influence a broad range of life-history characteristics and aid in explaining the current distributions of ants as well as their responses to future climate change. This article is protected by copyright. All rights reserved.

Climatic warming destabilizes forest ant communities

A field-based climate warming experiment reveals a loss of dynamical community stability due to altered species interactions. How will ecological communities change in response to climate warming? Direct effects of temperature and indirect cascading effects of species interactions are already altering the structure of local communities, but the dynamics of community change are still poorly understood. We explore the cumulative effects of warming on the dynamics and turnover of forest ant communities that were warmed as part of a 5-year climate manipulation experiment at two sites in eastern North America. At the community level, warming consistently increased occupancy of nests and decreased extinction and nest abandonment. This consistency was largely driven by strong responses of a subset of thermophilic species at each site. As colonies of thermophilic species persisted in nests for longer periods of time under warmer temperatures, turnover was diminished, and species interactions were likely altered. We found that dynamical (Lyapunov) community stability decreased with warming both within and between sites. These results refute null expectations of simple temperature-driven increases in the activity and movement of thermophilic ectotherms. The reduction in stability under warming contrasts with the findings of previous studies that suggest resilience of species interactions to experimental and natural warming. In the face of warmer, no-analog climates, communities of the future may become increasingly fragile and unstable.

Modulation of the heat shock response is associated with acclimation to novel temperatures but not adaptation to climatic variation in the ants Aphaenogaster picea and A. rudis

Ecological diversification into thermally divergent habitats can push species toward their physiological limits, requiring them to accommodate temperature extremes through plastic or evolutionary changes that increase persistence under the local thermal regime. One way to withstand thermal stress is to increase production of heat shock proteins, either by maintaining higher baseline abundance within cells or by increasing the magnitude of induction in response to heat stress. We evaluated whether environmental variation was associated with expression of three heat shock protein genes in two closely-related species of woodland ant, Aphaenogaster picea and A. rudis. We compared adult workers from colonies collected from 25 sites across their geographic ranges. Colonies were maintained at two different laboratory temperatures, and tested for the independent effects of environment, phylogeny, and acclimation temperature on baseline and heat-induced gene expression. The annual maximum temperature at each collection site (Tmax) was not a significant predictor of either baseline expression or magnitude of induction of any of the heat shock protein genes tested. A phylogenetic effect was detected only for basal expression of Hsp40, which was lower in the most southern populations of A. rudis and higher in a mid-range population of possible hybrid ancestry. In contrast, a higher acclimation temperature significantly increased baseline expression of Hsc70-4, and increased induction of Hsp40 and Hsp83. Thus, physiological acclimation to temperature variation appears to involve modulation of the heat shock response, whereas other mechanisms are likely to be responsible for evolutionary shifts in thermal performance associated with large-scale climate gradients.

The Role of Anchor-Tipped Larval Hairs in the Organization of Ant Colonies

The spatial organization within a social insect colony is a key component of colony life. It influences individual interaction rates, resource distribution, and division of labor within the nest. Yet studies of social insect behavior are most often carried out in artificial constructions, which may change worker behavior and colony organization. We observed how workers of the ant Pheidole rhea organized brood in nests with deep chambers and textured walls that were designed to mimic their natural constructions more closely. Instead of clumping larvae into piles on the chamber floor, workers suspended fourth-instar larvae from the vertical walls and ceiling of each chamber while young larvae and pupae were clumped at the base. Fourth-instar larvae possess five rows of anchor-tipped hairs on their dorsal side, and we predicted that these hairs functioned to attach larvae to the nest walls. We gave larvae “haircuts,” where only the anchor-tipped hairs were removed, and then tested their ability to adhere to a textured surface raised to an angle of 90° and then 120° with respect to the horizontal plane. Larvae whose hairs had been clipped came unattached in almost all trials, while larvae whose hairs remained intact stayed attached. This confirmed that anchor-tipped hairs functioned to attach larvae to the walls of the nest. The presence of anchor-tipped hairs is widespread and has been documented in at least 22 genera from the ant subfamily Myrmicinae, including species that occur in a variety of environments and represent a broad range of nesting habits. Based on our results, it is likely that many species exhibit this larval hanging behavior, and this could impact colony characteristics such as spatial organization and the care of developing larvae by nurse workers.