Samuel Ellis

A simple threshold rule is sufficient to explain sophisticated collective decision-making.

Decision-making animals can use slow-but-accurate strategies, such as making multiple comparisons, or opt for simpler, faster strategies to find a ‘good enough’ option. Social animals make collective decisions about many group behaviours including foraging and migration. The key to the collective choice lies with individual behaviour. We present a case study of a collective decision-making process (house-hunting ants, Temnothorax albipennis), in which a previously proposed decision strategy involved both quality-dependent hesitancy and direct comparisons of nests by scouts. An alternative possible decision strategy is that scouting ants use a very simple quality-dependent threshold rule to decide whether to recruit nest-mates to a new site or search for alternatives. We use analytical and simulation modelling to demonstrate that this simple rule is sufficient to explain empirical patterns from three studies of collective decision-making in ants, and can account parsimoniously for apparent comparison by individuals and apparent hesitancy (recruitment latency) effects, when available nests differ strongly in quality. This highlights the need to carefully design experiments to detect individual comparison. We present empirical data strongly suggesting that best-of-n comparison is not used by individual ants, although individual sequential comparisons are not ruled out. However, by using a simple threshold rule, decision-making groups are able to effectively compare options, without relying on any form of direct comparison of alternatives by individuals. This parsimonious mechanism could promote collective rationality in group decision-making.

Polydomy in red wood ants

Polydomy, a single colony spread between multiple nests, is a widespread life history strategy in ants. The mechanisms by which a polydomous colony functions, and the fitness benefits this nesting strategy provides, are poorly understood. Here we review what is known about polydomy in the well-studied and ecologically important Formica rufa group. We focus particularly on the ecological fitness benefits polydomy may provide to members of the F. rufa group. We discuss the well-documented association in this group between polygyny (multiple queens in a colony) and polydomy, and how this relationship may favour colony reproduction by budding. We argue that although polygyny and reproduction by budding may drive a colony to spread between multiple nests, the maintenance of prolonged communication between these nests needs further explanation in terms of fitness benefits. The potential benefits of polydomy in the F. rufa group are discussed, specifically how polydomy may help a colony: exploit resources, dominate spaces, or lower the cost of stochastic nest destruction. The potential consequences of polydomy for the social organisation of a colony are explored. We also highlight gaps in current knowledge, and suggest future research directions.

Resource redistribution in polydomous ant nest networks: local or global?

Lay Summary Wood ants nests share resources with neighboring nests, not the whole colony. A single ant colony can either live all in one nest, or split into several separate, but communicating, nests. How and why ant colonies do this is unknown. By treating these separated colonies as networks we show that wood ants exchange food locally, with neighboring nests, without a colony-level plan.

The Role of Non-Foraging Nests in Polydomous Wood Ant Colonies

A colony of red wood ants can inhabit more than one spatially separated nest, in a strategy called polydomy. Some nests within these polydomous colonies have no foraging trails to aphid colonies in the canopy. In this study we identify and investigate the possible roles of non-foraging nests in polydomous colonies of the wood ant Formica lugubris. To investigate the role of non-foraging nests we: (i) monitored colonies for three years; (ii) observed the resources being transported between non-foraging nests and the rest of the colony; (iii) measured the amount of extra-nest activity around non-foraging and foraging nests. We used these datasets to investigate the extent to which non-foraging nests within polydomous colonies are acting as: part of the colony expansion process; hunting and scavenging specialists; brood-development specialists; seasonal foragers; or a selfish strategy exploiting the foraging effort of the rest of the colony. We found that, rather than having a specialised role, non-foraging nests are part of the process of colony expansion. Polydomous colonies expand by founding new nests in the area surrounding the existing nests. Nests founded near food begin foraging and become part of the colony; other nests are not founded near food sources and do not initially forage. Some of these non-foraging nests eventually begin foraging; others do not and are abandoned. This is a method of colony growth not available to colonies inhabiting a single nest, and may be an important advantage of the polydomous nesting strategy, allowing the colony to expand into profitable areas.

Mortality risk and social network position in resident killer whales: sex differences and the importance of resource abundance

An individuals ecological environment affects their mortality risk, which in turn has fundamental consequences for life-history evolution. In many species, social relationships are likely to be an important component of an individuals environment, and therefore their mortality risk. Here, we examine the relationship between social position and mortality risk in resident killer whales (Orcinus orca) using over three decades of social and demographic data. We find that the social position of male, but not female, killer whales in their social unit predicts their mortality risk. More socially integrated males have a significantly lower risk of mortality than socially peripheral males, particularly in years of low prey abundance, suggesting that social position mediates access to resources. Male killer whales are larger and require more resources than females, increasing their vulnerability to starvation in years of low salmon abundance. More socially integrated males are likely to have better access to social information and food-sharing opportunities which may enhance their survival in years of low salmon abundance. Our results show that observable variation in the social environment is linked to variation in mortality risk, and highlight how sex differences in social effects on survival may be linked to sex differences in life-history evolution.

Internest food sharing within wood ant colonies: resource redistribution behavior in a complex system

Lay Summary Wood ant workers forage to other nests in their spatially dispersed colonies. Ant colonies can be dispersed between several different nests. Sharing resources between these nests is an important challenge for these dispersed colonies. We show that in dispersed colonies of the wood ant Formica lugubris, resources are redistributed between nests by workers using the same behaviors as they use to forage. Resources are therefore shared between nests using simple, pre-existing behaviors. Twitter: @Samellisq

Postreproductive lifespans are rare in mammals

Abstract A species has a post‐reproductive stage if, like humans, a female entering the adult population can expect to live a substantial proportion of their life after their last reproductive event. However, it is conceptually and statistically challenging to distinguish these true post‐reproductive stages from the usual processes of senescence, which can result in females occasionally surviving past their last reproductive event. Hence, despite considerable interest, the taxonomic prevalence of post‐reproductive stages remains unclear and debated. In this study we use life tables constructed from published data on wild populations of mammals, and statistical measures of post‐reproductive lifespans, to distinguish true post‐reproductive stages from artefacts of senescence and demography in 52 species. We find post‐reproductive stages are rare in mammals and are limited to humans and a few species of toothed whales. By resolving this long‐standing debate, we hope to provide clarity for researchers in the field of evolutionary biology and a solid foundation for further studies investigating the evolution and adaptive significance of this unusual life history trait.

Weak effects of common genetic variation in oxytocin and vasopressin receptor genes on rhesus macaque social behavior

The neuropeptides oxytocin (OT) and arginine vasopressin (AVP) influence pair bonding, attachment, and sociality, as well as anxiety and stress responses in humans and other mammals. The effects of these peptides are mediated by genetic variability in their associated receptors, OXTR and the AVPR gene family. However, the role of these genes in regulating social behaviors in non‐human primates is not well understood. To address this question, we examined whether genetic variation in the OT receptor gene OXTR and the AVP receptor genes AVPR1A and AVPR1B influence naturally‐occurring social behavior in free‐ranging rhesus macaques—gregarious primates that share many features of their biology and social behavior with humans. We assessed rates of social behavior across 3,250 hr of observational behavioral data from 201 free‐ranging rhesus macaques on Cayo Santiago island in Puerto Rico, and used genetic sequence data to identify 25 OXTR, AVPR1A, and AVPR1B single‐nucleotide variants (SNVs) in the population. We used an animal model to estimate the effects of 12 SNVs (n = 3 OXTR; n = 5 AVPR1A; n = 4 AVPR1B) on rates of grooming, approaches, passive contact, contact aggression, and non‐contact aggression, given and received. Though we found evidence for modest heritability of these behaviors, estimates of effect sizes of the selected SNVs were close to zero, indicating that common OXTR and AVPR variation contributed little to social behavior in these animals. Our results are consistent with recent findings in human genetics that the effects of individual common genetic variants on complex phenotypes are generally small.

Inferring polydomy: a review of functional, spatial and genetic methods for identifying colony boundaries

Identifying the boundaries of a social insect colony is vital for properly understanding its ecological function and evolution. Many species of ants are polydomous: colonies inhabit multiple, spatially separated, nests. Ascertaining which nests are parts of the same colony is an important consideration when studying polydomous populations. In this paper, we review the methods that are used to identify which nests are parts of the same polydomous colony and to determine the boundaries of colonies. Specifically, we define and discuss three broad categories of approach: identifying nests sharing resources, identifying nests sharing space, and identifying nests sharing genes. For each of these approaches, we review the theoretical basis, the limitations of the approach and the methods that can be used to implement it. We argue that all three broad approaches have merits and weaknesses, and provide a methodological comparison to help researchers select the tool appropriate for the biological question they are investigating.

Wood ant reproductive biology and social systems

Transmitting genes from one generation to the next is the fundamental basis of natural selection and evolution. Understanding the reproductive biology of a species is, therefore, fundamental to understanding how the species evolved and how it is adapted to its environment. In eusocial insects such as the wood ants ( Formica rufa group), reproduction is invested in a specialised reproductive caste, which produces both the workers and the next generation of sexual individuals. This chapter introduces the reproductive biology of wood ants, and also gives a general view of the life cycle of a wood ant colony to put the reproductive biology in context. Wood ant life cycle The wood ant life cycle is strongly linked to seasonal changes in the environment. Wood ant colonies generally first become active, after winter quiescence, in early spring when the sun begins to heat the nest and, in some locations, melt the snow (Figure 2.1). The timing of the beginning of activity is strongly influenced by the local climate, altitude and nest location. In Switzerland, for example, significant colony activity usually begins in March or April (Cherix 1981; Chauternes 1988). On warm and sunny days, early in spring, worker activity begins and the internal temperature of the nest begins to rise to between 25°C and 30°C. Nest temperature remains at this level for the entire active season, even when, during early spring, the outside temperature is close to freezing (discussed in detail in Chapter 4) (Rosengren et al . 1987). Workers gather in the warm nest core and their nutrient-producing glands become more active, converting lipids and protein into food ready to be fed to queens and future larvae (Bausenwein 1960). The queen(s) remain(s) in the warm nest core for several days and lay ‘winter eggs’. The larvae hatching from these eggs are fed by the overwintering workers, usually developing into sexuals in about 6 weeks (Otto 2005). The queen(s) then retire(s) to the lower nest chambers to start to produce ‘summer eggs’, which usually develop into workers (Otto 2005). Reproduction generally takes place in early summer, with thousands of sexual individuals flying away from their nests and participating in nuptial flights (e.g. Cherix et al . 1991).