Diacamma ants adjust liquid foraging strategies in response to biophysical constraints.

Ant foragers provide food to the rest of the colony, often requiring transport over long distances. Foraging for liquid is challenging because it is difficult to transport and share. Many social insects store liquids inside the crop to transport them to the nest, and then regurgitate to distribute to nest-mates through a behaviour called trophallaxis. Some ants instead transport fluids with a riskier behaviour called pseudotrophallaxis-holding a drop of liquid between the mandibles through surface tension. Ants share this droplet with nest-mates without ingestion or regurgitation. We hypothesised that ants optimize their liquid-collection approach depending on viscosity. Using an ant that employs both trophallaxis and pseudotrophallaxis, we investigated the conditions where each liquid-collection behaviour is favoured by measuring biophysical properties, collection time and reaction to food quality for typical and viscosity-altered sucrose solutions. We found that ants collected more liquid per unit time by mandibular grabbing than by drinking. At high viscosities ants switched liquid collection method to mandibular grabbing in response to viscosity and not to sweetness. Our results demonstrate that ants change transport and sharing methods according to viscosity-a natural proxy for sugar concentration-thus increasing the mass of sugar returned to the nest per trip.

Socially transferred materials: why and how to study them.

When biological material is transferred from one individual's body to another, as in ejaculate, eggs, and milk, secondary donor-produced molecules are often transferred along with the main cargo, and influence the physiology and fitness of the receiver. Both social and solitary animals exhibit such social transfers at certain life stages. The secondary, bioactive, and transfer-supporting components in socially transferred materials have evolved convergently to the point where they are used in applications across taxa and type of transfer. The composition of these materials is typically highly dynamic and context dependent, and their components drive the physiological and behavioral evolution of many taxa. Our establishment of the concept of socially transferred materials unifies this multidisciplinary topic and will benefit both theory and applications.

Individual Ants Do Not Show Activity-Rest Rhythms in Nest Conditions.

Circadian rhythms, which respond to the day-night cycle on the earth, arise from the endogenous timekeeping system within organisms, called the "biological clock." For accurate circadian rhythms, daily fluctuations in light and temperature are considered one of the important time cues. In social insects, both abiotic and biotic factors (i.e., social interactions) play a significant role in activity-rest rhythm regulation. However, it is challenging to monitor individual activity-rest rhythms in a colony because of the large group size and small body size. Therefore, it is unclear whether individuals in a colony exhibit activity-rest rhythms and how social interactions regulate their activity-rest rhythms in the colony. This study developed an image-based tracking system using 2D barcodes for Diacamma cf. indicum from Japan (a monomorphic ant) and measured the locomotor activities of all colony members under laboratory colony conditions. We also investigated the effect of broods on activity-rest rhythms by removing all broods under colony conditions. Activity-rest rhythms appeared only in isolated ants, not under colony conditions. In addition, workers showed arrhythmic activities after brood removal. These results suggested that a mixture of social interactions, and not light and temperature, induces the loss of activity-rest rhythms. These results contribute to the knowledge of a diverse pattern of circadian activity rhythms in social insects.

Bipartite network analysis of ant-task associations reveals task groups and absence of colonial daily activity.

Social insects are one of the best examples of complex self-organized systems exhibiting task allocation. How task allocation is achieved is the most fascinating question in behavioural ecology and complex systems science. However, it is difficult to comprehensively characterize task allocation patterns due to behavioural complexity, such as the individual variation, context dependency and chronological variation. Thus, it is imperative to quantify individual behaviours and integrate them into colony levels. Here, we applied bipartite network analyses to characterize individual-behaviour relationships. We recorded the behaviours of all individuals with verified age in ant colonies and analysed the individual-behaviour relationship at the individual, module and network levels. Bipartite network analysis successfully detected the module structures, illustrating that certain individuals performed a subset of behaviours (i.e. task groups). We confirmed age polyethism by comparing age between modules. Additionally, to test the daily rhythm of the executed tasks, the data were partitioned between daytime and nighttime, and a bipartite network was re-constructed. This analysis supported that there was no daily rhythm in the tasks performed. These findings suggested that bipartite network analyses could untangle complex task allocation patterns and provide insights into understanding the division of labour.

A specific type of insulin-like peptide regulates the conditional growth of a beetle weapon.

Evolutionarily conserved insulin/insulin-like growth factor (IGF) signaling (IIS) has been identified as a major physiological mechanism underlying the nutrient-dependent regulation of sexually selected weapon growth in animals. However, the molecular mechanisms that couple nutritional state with weapon growth remain largely unknown. Here, we show that one specific subtype of insulin-like peptide (ILP) responds to nutrient status and thereby regulates weapon size in the broad-horned flour beetle Gnatocerus cornutus. By using transcriptome information, we identified five G. cornutus ILP (GcorILP1-5) and two G. cornutus insulin-like receptor (GcorInR1, -2) genes in the G. cornutus genome. RNA interference (RNAi)-mediated gene silencing revealed that a certain subtype of ILP, GcorILP2, specifically regulated weapon size. Importantly, GcorILP2 was highly and specifically expressed in the fat body in a condition-dependent manner. We further found that GcorInR1 and GcorInR2 are functionally redundant but that the latter is partially specialized for regulating weapon growth. These results strongly suggest that GcorILP2 is an important component of the developmental mechanism that couples nutritional state to weapon growth in G. cornutus. We propose that the duplication and subsequent diversification of IIS genes played a pivotal role in the evolution of the complex growth regulation of secondary sexual traits.

Ant circadian activity associated with brood care type.

In group-living animals, social interactions influence various traits including circadian activity. Maternal care, in particular, can have a strong effect on the circadian activity of parents or nurses across taxa. In social insects, nest-mates are known to have diverse activity rhythms; however, what kind of social environment is crucial in shaping an individual's rhythm is largely unknown. Here, we show that the focal brood types being taken care of (i.e. egg, larva and pupa) have significant effects on individual activity/rest rhythm, using the monomorphic ant Diacamma (putative species indicum). When isolated from a colony, nurses exhibited a clear circadian rhythm. However, when paired with eggs or larvae, they exhibited around-the-clock activity with no apparent rhythm. In contrast, a clear activity rhythm emerged when nurses were paired with a pupa, requiring little care. Such brood-type-specific changes in circadian activity are considered to arise from the difference in caretaking demands. Our finding may contribute to the understanding of the organization of a colony in the context of behavioural variability under different microenvironments.