Vincent Fourcassié

Are ants sensitive to the geometry of tunnel bifurcation

The ability to orient and navigate in space is essential for all animals whose home range is organized around a central point. Because of their small home range compared to vertebrates, central place foraging insects such as ants have for a long time provided a choice model for the study of orientation mechanisms. In many ant species, the movement of individuals on their colony home range is achieved essentially collectively, on the chemical trails laid down by their nest mates. In the initial stage of food recruitment, these trails can cross each other and thus form a network of interconnected paths in which ants have to orient. Previous simulation studies have shown that ants can find the shortest path between their nest and a food source in such a network only if there is a bias in the branch they choose when they reach an asymmetrical bifurcation. In this paper, we studied the choice of ants when facing either a symmetrical or an asymmetrical bifurcation between two tunnels. Ants were tested either on their way to a food source or when coming back to their nest, and either in the presence or in the absence of a chemical trail. Overall, our results show that the choice of an ant at a tunnel bifurcation depends more on the presence/absence of a trail pheromone than on the geometry of the bifurcation itself.

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

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

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.

Nesting patterns, ecological correlates of polygyny and social organization in the neotropical arboreal ant Odontomachus hastatus (Formicidae, Ponerinae)

Queen number varies in the population of O. hastatus in SE Brazil. Here, we evaluate how nesting ecology and colony structure are associated in this species, and investigate how reproduction is shared among nestmate queens. Queen number per colony is positively correlated with nesting space (root cluster of epiphytic bromeliads), and larger nest sites host larger ant colonies. Plant samplings revealed that suitable nest sites are limited and that nesting space at ant-occupied bromeliads differs in size and height from the general bromeliad community. Dissections revealed that queens in polygynous colonies are inseminated, have developed ovaries, and produce eggs. Behavioral observations showed that reproduction in polygynous colonies is mediated by queen–queen agonistic interactions that include egg cannibalism. Dominant queens usually produced more eggs. Field observations indicate that colonies can be initiated through haplometrosis. Polygyny in O. hastatus may result either from groups of cofounding queens (pleometrosis) or from adoption of newly mated queens by established colonies (secondary polygyny). Clumping of bromeliads increases nest space and probably adds stability through a strong root system, which may promote microhabitat selection by queens and favor pleometrosis. Rainstorms that frequently knock down bromeliads can be a source of colony break-up and may promote polygyny. Bromeliads are limited nest sites and may represent a risk for young queens leaving a suitable nest, thus favoring secondary polygyny. Although proximate mechanisms mediating queen number are poorly understood, this study suggests that heterogeneous microhabitat conditions probably contribute to the coexistence of variable forms of social structure in O. hastatus.

Effect of substrate roughness on load selection in the seed-harvester ant Messor barbarus L. (Hymenoptera, Formicidae)

We investigated the effect of substrate roughness on load selection in the seed-harvester ant Messor barbarus. Ants were forced to travel either on sand or on gravel to reach a seed patch containing seed fragments of different weights. We hypothesized that foragers travelling on a rough substrate could either increase their load as a result of the increased distance travelled (due to a more sinuous path and an increase in the vertical component of the path) or decrease their load because of the anticipated difficulty of moving with a heavy load on a rough surface. Our results were consistent with neither of these hypotheses: Load selection by ants did not depend on the roughness of the substrate encountered during their outbound trip. The main effect of substrate roughness was to slow down the progression of the ants and increase their probability of dropping or transferring heavy seeds on their way back to the nest, thus resulting in an overall reduction of the rate of seed return to the nest.

Comparative study of resistance to heat in two species of leaf-cutting ants

Atta laevigata and Atta capiguara are two species of leaf-cutting ants that are found in the pastures of Central Brazil and build huge underground nests linked to the outdoor environment by underground tunnels, which can reach several tens of meters and further extend through foraging trails to distant foraging grounds. The tunnels built by mature colonies of A. capiguara are usually longer and deeper than those built by mature colonies of A. laevigata. The physical trails are also shorter on average. We hypothesized that these differences could be related to differences in thermotolerance between the two species. To test this we collected ants on foraging trails and placed them individually in waterproof test tubes plunged in a thermostatic bath at 25xa0°C (control), 37 and 39xa0°C (test temperatures). The results showed that at both 37 and 39xa0°C, the survival time of A. laevigata was much more extended than that of A. capiguara. A possible explanation for the longer and deeper underground foraging tunnels, as well as the shorter foraging trails, built by A. capiguara may thus be their lower resistance to heat stress. The longer tunnels built by A. capiguara colonies may reduce the exposure to heat of the foraging workers that commute between their nest and the foraging grounds or act as a thermal refuge in which the workers can find temporary protection against high outdoor temperatures.

Dynamics of foraging trails in the Neotropical termite Velocitermes heteropterus (Isoptera: Termitidae)

Foraging behavior in termites varies with the feeding habits of each species but often occurs through the formation of well-defined trails that connect the nest to food sources in species that build structured nests. We studied the formation of foraging trails and the change in caste ratio during foraging in the termite Velocitermes heteropterus. This species is widespread in Cerrado vegetation where it builds epigeal nests and forages in open-air at night. Our aim was to understand the processes involved in the formation of foraging trails, from the exploration of new unmarked areas to the recruitment of individuals to food and the stabilization of traffic on the trails, as well as the participation of the different castes during these processes. Foraging trails were videotaped in the laboratory and the videos were then analyzed both manually and automatically to assess the flow of individuals and the caste ratio on the trails as well as to examine the spatial organization of traffic over time. Foraging trails were composed of minor workers, major workers, and soldiers. The flow of individuals on the trails gradually increased from the beginning of the exploration of new areas up to the discovery of the food. The caste ratio remained constant throughout the foraging excursion: major workers, minor workers and soldiers forage in a ratio of 8:1:1, respectively. The speed of individuals was significantly different among castes, with major workers and soldiers being significantly faster than minor workers. Overall, our results show that foraging excursions in V. heteropterus may be divided in three different phases, characterized by individual speeds, differential flows and lane segregation.

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

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

Foraging activity pattern and herbivory rates of the grass-cutting ant Atta capiguara

The objective of this study was to investigate the seasonal change in the pattern of foraging activity in the leaf-cutting ant Atta capiguara and to provide an estimate of the annual herbivory rate of its colonies. We surveyed at least once a month during 12xa0months, the foraging activity of two colonies located in a pasture of central Brazil. Each survey consisted in counting the number of ants exiting and returning (laden or unladen) to their nests during 10xa0min every hour during 24xa0h. The air temperature and relative humidity were noted at the beginning of each survey. Independent of the season, the colonies presented two peaks of foraging activity, one small peak at the beginning of the day and another peak at the end of the day. During the dry season, the diurnal activity ceased almost completely between 10:00 and 16:00. The proportion of ants returning to the nest laden with a leaf fragment was also lower than during the humid season. Activity rhythm depended significantly on both air temperature and humidity: foraging activity peaked when the temperature was around 25xa0°C and relative humidity around 66xa0%. Based on the statistical model used to describe the activity rhythm, we estimated that over a period of 1 year, the total dry biomass of vegetation collected by the two colonies studied was, respectively, 25.16xa0kg for the bigger nest, and 11.69xa0kg for the smaller one, or an equivalent of 104.83-kg and 48.71-kg fresh biomass, respectively.

Ant community organization along elevational gradients in a temperate ecosystem

The aim of our study was to characterize the factors that shape the pattern of change in ant species richness and community structure along altitudinal gradients in two valleys located on the northern and southern side of the Pyrenees. During three summers, we sampled 20 sites distributed across two Pyrenean valleys ranging in elevation from 1,009 to 2,339xa0m using pitfall traps and hand collection. We employed diversity index, degree of nestedness of ant assemblages, ordination method, and multiple regression analysis to examine the effects of various environmental factors on ant species communities. In total, 41 ant species were found in the two valleys. The number of species was 26xa0% lower in the valley located on the northern side than in that located on the southern side. At the valley scale, the number of ant species, as well as the evenness, decreased with elevation. A significant nested pattern was observed, indicating that the species found in the poorest site represented a subset of those found at the richest one. Ants collected at mid- and high-elevation sites had a wider altitudinal range than those collected at low-elevation sites, thus complying with Rapoport’s rule. Our results suggest that, although elevation strongly influences the organization of ant communities, ecological factors such as temperature and local habitat features (sun exposure, vegetation density) are the main factors explaining the pattern of ant diversity along altitudinal gradients.