Comparing ant behaviour indices for fine-scale analyses

Foraging valor linked with aggression: selection against completely abandoning aggression in the high-elevation ant Tetramorium alpestre?

Aggression has multiple benefits and is often coupled with other behaviors ("behavioral syndromes"). The level of aggressiveness is influenced by an adaptive benefit-cost ratio suggesting that benefits should outweigh the costs of aggression. Here, we assess if several behaviors are coupled in two behaviorally different populations (aggressive, peaceful) of the high-elevation ant Tetramorium alpestre. For three weeks, we collected colony fragments and analyzed boldness, exploring, foraging, and risk-taking behaviors. We hypothesized that the aggressive population is bolder, more explorative and risk-prone, and forages more food than the peaceful population. To test whether (a) the combination of experiments and parameters used yields a good setup, (b) populations differ behaviorally, and (c) populations display behavioral syndromes, we assessed (a) the frequency of repeatable behaviors of each experiment, (b) the behavioral means among populations, and (c) the behavioral repeatability, respectively. We found that (a) boldness and exploring were most repeatable and represent a good experimental setup, (b) the aggressive population was bolder and more explorative and risk-prone than the peaceful population, (c) boldness and exploring behaviors were highly repeatable in both populations, thus corroborating our hypothesis. The results suggest that boldness, exploring, and risk-taking but not foraging are presumably coupled with aggression and indicate the presence of behavioral syndromes in this ant. Under specific ecological conditions, aggression may be coupled with other behaviors and important for finding food. Aggression is probably adaptive in T. alpestre, possibly indicating that selection favors aggression at least partially, which may counteract the complete loss of intraspecific aggression.

Short-time development of among-colony behaviour in a high-elevation ant

Standardised assays are often used to characterise aggression in animals. In ants,such assays can be applied at several organisational levels (e.g., colony, population) and at specific times during the season. However, whether the behaviour differs at these levels and changes over a few weeks remains largely unexplored. Here, six colonies from the high-elevation ant Tetramorium alpestre were collected weekly for five weeks from two behaviourally-different populations (aggressive and peaceful in intraspecific encounters). We conducted one-on-one worker encounters at the colony and population levels. When analysing the colony combinations separately, the behaviour was peaceful and remained so within the peaceful population; initial aggression became partially peaceful within the aggressive population; and initial aggression decreased occasionally and increased in one combination but remained constant for most across-population combinations. When analysing all colony combinations together, within-population behaviour remained similar, but acrosspopulation behaviour became peaceful. The observed behavioural differences among organisational levels emphasise the relevance of assessing both. Moreover, the effect of decreasing aggression is discernible already over a few weeks. Compression of the vegetation period at high elevations may compress such behavioural changes.Addressing both organisational levels and seasonality is important, particularly in studies of behavioural complexity such as in this ant.

Global change may make hostile - Higher ambient temperature and nitrogen availability increase ant aggression

Behaviour is a response of organisms to internal and external stimuli and comprises various activities such as searching for food. Aggression is important in such activities, for example, improving the chances of winning competition for food, but animals differ in their level of aggression. This behavioural plasticity allows individuals to respond to environmental changes and is important for the survival of animals. It may be an important asset in facing global changes, which affect all organisms, for example, via rising temperature and eutrophication. The latter have steadily increased since 1900, especially in high elevations. Their effects may first become visible in stationary organisms such as ants because their nests are strictly associated with the conditions on site. Here, we analysed eight populations of the high-elevation ant Tetramorium alpestre along several elevations spanning the European Alps. We conducted a correlative approach and analysed several genetic and environmental proxies, namely within- and across-colony genetic relatedness, cuticular hydrocarbons, body size, across-colony geographic distance, air temperature, and worker nitrogen values additionally to within-population aggressive behaviour. We hypothesised that a) these proxies and aggressive behaviour differ among populations and that b) one or more of these proxies influence aggression. We found that a) some environmental proxies and aggression differed among populations but not the genetic proxies and that b) air temperature and worker nitrogen-isotope values correlated positively with worker aggression. The results indicate an environmental but not social-structural influence on this ant's aggressive behaviour, even though social structure varied among populations (single- and multiple-queened colonies). We infer that global change affects aggression in our study system and propose five mutually non-exclusive scenarios to explain the behavioural change mechanistically. Using the space-for-time principle, we speculate that aggression may increase due to future increases in temperature and nitrogen availability in this ant and other species living in high elevations.

Genomic Signature of Shifts in Selection in a Subalpine Ant and Its Physiological Adaptations

Understanding how organisms adapt to extreme environments is fundamental and can provide insightful case studies for both evolutionary biology and climate-change biology. Here, we take advantage of the vast diversity of lifestyles in ants to identify genomic signatures of adaptation to extreme habitats such as high altitude. We hypothesized two parallel patterns would occur in a genome adapting to an extreme habitat: 1) strong positive selection on genes related to adaptation and 2) a relaxation of previous purifying selection. We tested this hypothesis by sequencing the high-elevation specialist Tetramorium alpestre and four other phylogenetically related species. In support of our hypothesis, we recorded a strong shift of selective forces in T. alpestre, in particular a stronger magnitude of diversifying and relaxed selection when compared with all other ants. We further disentangled candidate molecular adaptations in both gene expression and protein-coding sequence that were identified by our genome-wide analyses. In particular, we demonstrate that T. alpestre has 1) a higher level of expression for stv and other heat-shock proteins in chill-shock tests and 2) enzymatic enhancement of Hex-T1, a rate-limiting regulatory enzyme that controls the entry of glucose into the glycolytic pathway. Together, our analyses highlight the adaptive molecular changes that support colonization of high-altitude environments.