Consistent Individual Differences Research Articles

Sex- and context-specific associations between personality and a measure of fitness but no link with life history traits

The pace-of-life syndrome hypothesis posits that personality traits (i.e. consistent individual differences in behaviour) are linked to life history and fitness. Specifically, fast-paced individuals are predicted to be proactive (i.e. active and aggressive) with an earlier age at first reproduction, a shorter life span and higher fecundity than slow-paced individuals. Environmental conditions and sex differences may be important in maintaining behavioural and life history variation in populations and may influence the covariance of personality with life history or lifetime fitness. However, these effects are rarely tested together. We investigated whether the occurrence of a resource pulse (called a mast year) during adulthood altered the associations between personality and life history traits or lifetime offspring production in adult North American red squirrels, Tamiasciurus hudsonicus. Despite accounting for environmental context during adulthood, we found no evidence of an overall pace-of-life syndrome in this population as personality was not associated with age at first reproduction or longevity in either sex. Males and females had similar activity levels, but females were more aggressive, potentially due to the fitness benefits of protecting their offspring from predation. In all females, regardless of mast experience, there was no association between activity and lifetime pup production, but there was a positive association between aggression and lifetime pup production. In males that experienced a mast, there was a positive association between lifetime pup production and both activity and aggression. In males that did not experience a mast, there was no association between activity and lifetime pup production, but there was a negative association between aggression and lifetime pup production. Lifetime recruit production (number of adult offspring recruited into the breeding population) in either sex was not influenced by activity or aggression regardless of mast experience. Overall, our results suggest that the infrequent occurrence of mast years may contribute to maintaining variation in personality traits in red squirrels.

Reactive stress-coping styles show more variable reproductive expenditure and fitness outcomes

Stress-coping styles dictate how individuals react to stimuli and can be measured by the integrative physiological parameter of resting heart-rate variability (HRV); low resting HRV indicating proactive coping styles, while high resting HRV typifies reactive individuals. Over 5 successive breeding seasons we measured resting HRV of 57 lactating grey seals. Mothers showed consistent individual differences in resting HRV across years. We asked whether proactive and reactive mothers differed in their patterns of maternal expenditure and short-term fitness outcomes within seasons, using maternal daily mass loss rate to indicate expenditure, and pup daily mass gain to indicate within season fitness outcomes. We found no difference in average rates of maternal daily mass loss or pup daily mass gain between proactive and reactive mothers. However, reactive mothers deviated more from the sample mean for maternal daily mass and pup daily mass gain than proactive mothers. Thus, while proactive mothers exhibit average expenditure strategies with average outcomes, expenditure varies much more among reactive mothers with more variable outcomes. Overall, however, mean fitness was equal across coping styles, providing a mechanism for maintaining coping style diversity within populations. Variability in reactive mothers’ expenditures and success is likely a product of their attempts to match phenotype to prevailing environmental conditions, achieved with varying degrees of success.

Does consistent individual variability in pelagic fish larval behaviour affect recruitment in nursery habitats?

Individual animals across all taxa differ consistently in behaviour, i.e. they show personality traits. This inter-individual variability has significant ecological and evolutionary consequences, since it affects a range of population-level processes. Here, we focus on the selection and recruitment of nursery habitats in temperate fish larvae. The “Sense Acuity and Behavioural Hypothesis” has proposed that fish larvae could detect and follow environmental cues to actively choose suitable nursery habitats. We empirically tested this hypothesis questioning if this non-random active process occurs and if it could be linked to consistency in individual behaviours. Individual larvae of the white seabream Diplodus sargus (Linnaeus, 1758) were tested repeatedly at different ages in a two-channel choice-chamber apparatus exposing them to a flow with different stimuli, as nursery habitats (lagoon, coastal), different temperatures or salinities and recording exploratory activity and preference in the different conditions. Most larvae changed behaviour during ontogeny, but they were also significantly consistent in their behaviour, revealing strong individuality; yet, no significant preference for the presented stimuli emerged, nor it was related to individuality. Exploratory activity was higher when larvae showed unresponsive or inconclusive behaviours, meaning that the larvae tried to find a different stimulus from the one that we were offering or had random habitat selection. Individual behavioural consistency could influence the process of searching for suitable nursery habitats and, consequently, dispersion and connectivity of white seabream population. Characterizing the behaviour of temperate pelagic marine fish larvae may shed light on fish recruitment variability, help refining larval dispersion models and possibly help understanding effects of climate change on population distribution and connectivity. A Chinese idiom says that “It is easier to change mountains and rivers than to alter one’s character.” What about fish? Well, fish can exhibit individuality traits that control autoecological and demecological processes. For example, shy fish have lower fitness while the rate of invasion progress is faster in populations with bolder individuals. Individuality studies rarely focused on fish larvae, except for coral fish. So, we tested if temperate fish larvae display consistent behaviour throughout ontogeny. This goal delves into the Sense Acuity And Behavioural Hypothesis which incorporated behaviour into the hypotheses deeming to explain fish recruitment variability. We found that temperate fish larvae display consistent individual behavioural differences in exploratory activity since early in ontogeny. This confirms the deterministic role of pelagic fish larvae behaviour on population connectivity processes, namely to control their dispersion and choose a nursery habitat.

Personality traits change after an opportunity to mate.

There is growing evidence that personality traits can change throughout the life course in humans and nonhuman animals. However, the proximate and ultimate causes of personality trait change are largely unknown, especially in adults. In a controlled, longitudinal experiment, we tested whether a key life event for adults--mating--can cause personality traits to change in female threespine sticklebacks. We confirmed that there are consistent individual differences in activity, sociability and risk-taking, and then compared these personality traits among three groups of females: (i) control females; (ii) females that had physically mated, and (iii) females that had socially experienced courtship but did not mate. Both the physical experience of mating and the social experience of courtship caused females to become less willing to take risks and less social. To understand the proximate mechanisms underlying these changes, we measured levels of excreted steroids. Both the physical experience of mating and the social experience of courtship caused levels of dihydroxyprogesterone (17α,20β-P) to increase, and females with higher 17α,20β-P were less willing to take risks and less social. These results provide experimental evidence that personality traits and their underlying neuroendocrine correlates are influenced by formative social and life-history experiences well into adulthood.

Contact calling in context: intra- and intergroup variation in vocalization rates depend on a call’s function

To maintain group cohesion while coordinating group movements, individuals might use signals to advertise the location of a route, their intention to initiate movements, or their position at a given time. In highly mobile animals, the latter is often accomplished through contact calls that are emitted at different rates by group members. Here, we describe and quantify intra- and intergroup variation in contact calling rates in Spix’s disc-winged bats (Thyroptera tricolor), a species that employs distinct inquiry and response calls to coordinate group movements during flight and while announcing roost locations. We evaluate the extent to which groups are composed of individuals with similar calling rates and estimate variation among and within groups. Our results show large variation in response calling rates among and within groups, both in terms of calling rates and the probability of being vocal or not; for example, a large portion (35%) of bats sampled did not produce these signals. For inquiry calls, we found that variation in calling rates was greater within than among groups, and in contrast to response calls, only a few individuals (3%) did not produce inquiry calls. Overall, we found support for the existence of intra- and intergroup differences in the context of contact calling in disc-winged bats, and our results suggest that different mechanisms may promote the evolution and maintenance of varying calling rates for the two types of calls studied. Animal communication networks are comprised of individuals with different vocal behaviors. However, we still do not fully understand the mechanisms that facilitate individual and group variation in vocal behavior and how these differences affect group cohesion in highly mobile organisms. Here, we describe and quantify contact calling behavior in Spix’s disc-winged bats (Thyroptera tricolor) and evaluate whether vocal behavior is associated with group membership and call type. We found consistent individual differences in contact calling rates, as well as among-group variation in calls associated to roost finding. Our study provides a baseline for generating hypotheses regarding the costs and benefits of contact call production during group movements.

Individuality in Egg Colouration of Black-Headed Gulls Chroicocephalus ridibundus across the Years Confirmed through DNA Analyses

The eggshell colouration among and within certain bird species is highly variable. Although many studies have addressed this variation, the reasons for it remain largely unclear. Consistent individual differences in egg colouration may improve the ability of colonially breeding birds to recognise their clutch among other neighbouring nests. Moreover, in species with a high incidence of intraspecific brood parasitism, such as the Black-head Gull Chroicocephalus ridibundus, egg colouration patterns may allow parents to expel foreign eggs from their own clutch. Here, we used standardised photography and image processing, including UV spectral information, to investigate egg colour patterns in female Black-headed Gulls (476 eggs in 170 clutches) during four breeding seasons, from 2015 to 2018 in NE Germany. We confirmed maternity by comparing DNA extracted from eggshells with feathers from the breeding female. Our results showed a greater similarity of eggs within a clutch than among clutches (ANOSIM: R = 0.43, P = 0.001). Black-headed Gulls showed consistent individual-specific eggshell colour pattern across consecutive breeding seasons. Genetic analyses further revealed one foreign egg was present in three out of 25 clutches (12%), but these eggs were not expelled by the gulls, possibly because these eggs did not differ significantly in colouration from their own. Additionally, we conducted a clutch exchange experiment to assess the reaction of the breeding pair to a foreign clutch. After 21 neighbouring clutches had been exchanged, adults came back immediately and incubated the interchanged clutch without any negative reaction. It seems that they do not use the specific colour pattern for recognition of their clutches, but possibly use other cues that remain to be investigated. While the function of individual egg colour patterns remains unclear, our results strongly suggest colour variation is driven by internal rather than external factors in this colonially breeding species.

Aggregation and social interaction in garter snakes (Thamnophis sirtalis sirtalis)

Previous research has shown that competition, familiarity, diet, and relatedness can all influence aggregation patterns in garter snakes. We controlled for these factors and examined social aggregation patterns in juvenile Eastern garter snakes (Thamnophis sirtalis sirtalis). We assayed snakes individually for consistent individual differences in boldness and sociability. We then placed snakes in groups of 10 in an enclosure with four shelters and observed their social interactions over a period of 8 days. We demonstrate that the snakes actively seek social interaction, prefer to remain with larger aggregates, and associate nonrandomly with specific individuals or groups. We show that their social interaction patterns are influenced by individual boldness, sociability, and age. The snakes’ social networks were perturbed twice a day by “shuffling” their locations. Despite these disturbances, the snakes eventually re-formed their preferred social environment. Aggregation and exploration patterns also varied across time, with most activity occurring later in the day. These results highlight the complexity of snake sociality and may have important implications for conservation efforts. Snakes are often considered nonsocial animals, but this is inaccurate. We used social network analyses to compare juvenile Eastern garter snakes’ group behaviors to their individual personalities. Our research demonstrates that these snakes actively seek out social interaction and prefer to join and remain with larger groups and that their social interaction patterns are influenced by consistent individual differences in boldness and sociability. Our work contributes to a sparse but growing body of literature on sociability in reptiles. This work is important for changing perceptions among the scientific community and the public as a whole. As a consequence, it may aid efforts in developing appropriate conservation techniques that consider sociability patterns in relocated reptiles.

Consistent individual differences in ecto‐parasitism of a long‐lived lizard host

Individual hosts vary substantially in their parasite loads. However, whether individual hosts have consistently different loads remains uncertain. If so, hosts that have consistently high parasite loads may serve as key reservoirs or super‐spreaders. Thus, identifying whether individuals persistently differ in their parasitism and the factors that explain these patterns constitute important issues for disease ecology and management. To investigate these topics, we examined nine years of tick counts in a wild population of sleepy lizardsTiliqua rugosa. Lizards were individually marked, and throughout their activity season, often across several years, we repeatedly assessed lizards’ ticks (to stage – larva, nymph, adult male and adult female – and species, eitherBothriocroton hydrosauriorAmblyomma limbatum). Using these repeated individual measures, we determined whether tick counts were repeatable. Then, we tested predictors of average tick counts, particularly lizard mass, sex, behavioural type (aggression and boldness), and the distance between lizards’ home range centre and a road transecting the study site (an area of greater food and lizard activity). We found that lizards exhibited consistent individual differences in tick loads both within and across years. Within‐lizard yearly average counts of larvae and nymphs were positively correlated. Lizards closer to the road tended to have more larvae and nymphs of both species and more adultB. hydrosauri. Sex did not affect tick counts. Mass differentially affected adult femaleA. limbatumand adult maleB. hydrosauritick counts. Intriguingly, lizards with above average aggression but below average boldness, or vice versa, tended to have higher average adult femaleB. hydrosauritick counts. Ultimately, our results demonstrate that lizards differed consistently in their tick counts, indicating that lizard parasitism may constitute a phenotypic trait of the individual, with implications for both host–parasite dynamics and broader host ecology.

When personality matters: personality and social structure in wild bottlenose dolphins, Tursiops truncatus

There is increasing evidence that animal personality can affect many aspects of an individual's behaviour, life history and fitness. However, there have been few studies about the link between personality and social organization in the context of wild mammals in their own natural environments. This article reports on ecologically relevant data, linking experimental data from the wild to long-term social association data in a socially and cognitively complex mammal species (bottlenose dolphin, Tursiops truncatus). Here, I used behavioural data to describe personality differences between bottlenose dolphins and social network analysis to assess the relationship between personality and social structure. First, I measured the reaction of photo-identified individuals over time and across contexts as a trade-off between a novelty-seeking behaviour (boldness) and a novelty-averse behaviour (shyness). Second, I applied social network analysis to understand the link between the observed shy–bold continuum and social organization, while controlling for other factors that could contribute to affiliation. This study presents for the first time consistent individual differences in behavioural response to novelty, as a proxy for the shy–bold continuum, in wild bottlenose dolphins. Bold individuals had a central role in the social network with stronger associations than shy individuals, suggesting that bold individuals may play an important role in group cohesion, group stability and the spread of information through the network. Together, these findings provide insights into how a social network is structured by personality in wild bottlenose dolphins, with potential fitness consequences. Furthermore, this study provides additional evidence of the existence of social personalities in nonhuman animals and contributes to the understanding of the role of personality in determining the extent to which mammals associate with others.

Does the tie fit the female? Melanin-based colouration, aggressive personality and reproductive investment in female great tits (Parus major)

The evolution and function of female trait exaggeration in species with traditional sex roles are poorly understood. Competition for social and ecological resources, rather than mating opportunities, may be a key selection pressure in females of socially monogamous species. Moreover, such pre-existing resource competition-selected traits (armaments) may become the object of mating preferences (ornaments); however, the benefits that males may gain in choosing more ornamented females remain unclear. Here, using observational data on free-living female great tits (Parus major), we explore whether tie size, a well-known melanin-pigmented plumage trait, covaries with intrasexual aggressiveness during the breeding season and the investment in current reproduction (fledgling number and mass). We found consistent individual differences (i.e. personality variation) in most of the aggressiveness parameters studied. Moreover, we found support for a signalling role of tie size with respects to how close females approached intruders. Interestingly, this relationship was only evident in older females, not first-year breeding females, suggesting age-related changes in the information content of tie size in the context of same-sex aggressiveness. Additionally, although first-year breeding females, on average, produced fledglings with lower body mass compared to older females, tie size was positively associated with fledgling mass, suggesting that males could potentially gain benefits by choosing females with larger tie size. Experimental studies are now necessary to confirm whether females with larger tie size have a selective advantage in female competition (i.e. armament), as well as whether males base their mate choice on female tie size (i.e. ornament). Why females of many species with traditional sex roles often have exaggerated traits very similar to those found in males remains poorly understood. We explored the functional significance of a well-known melanin-based plumage trait (tie size) in females of a socially monogamous passerine bird, the great tit. Female tie size positively covaried with consistent individual differences in aggressiveness during female-female interactions, but only so in older females and not first-year breeding females. In addition, females with larger tie size produced fledglings with higher body mass, suggesting that it could potentially be used by males in mate choice. Our study emphasizes that greater attention should be paid to female trait exaggeration in socially monogamous species in the context of female-female competition and male mate choice, as well as to potential age-related changes in the information they convey.

Individual differences in processing orientation and proximity as emergent features

Numerous examples of meaningful inter-individual differences in visual processing have been documented in low- and high-level vision. For mid-level vision or perceptual organization, vision scientists have only recently started to study the inter-individual differences structure. In this study, we focus on orientation and proximity as emergent features and combine a quantitative information processing approach with an individual differences approach. We first replicated the results reported in Hawkins, Houpt, Eidels, and Townsend (2016) in a set of 52 observers. That is, observers showed higher processing capacity for detecting a change in a stimulus configuration when the emergent features orientation or proximity were changed. Next, we asked whether individual differences processing capacities were similar across emergent features. The capacity to detect any type of change correlated moderately across individuals, whereas the capacity to detect changes in either emergent feature alone was not strongly correlated. This indicates that there is no general sensitivity to emergent features and that observers can be good at detecting orientation changes whilst being poor at detecting proximity changes (and vice versa). An additional exploratory multivariate analysis of the data revealed that response times and accuracies correlated strongly within each emergent feature. Moreover, specific factors related to change detection and inward displacements were observed, revealing consistent individual differences in our data. We discuss the results in the context of the literature on individual differences in vision where both specific, fragmented factors as well as broad, general factors have been reported.

A longitudinal assessment of behavioral development in wild capuchins: Personality is not established in the first 3 years.

Animal personality is defined as consistent individual differences across time and situations, but little is known about how or when those differences are established during development. Likewise, several studies described the personality structure of adult capuchin monkeys, without assessing the ontogeny of these personality traits. We analyzed the behavioral repertoire of 12 wild infants (9 males, 3 females) yellow-breasted capuchin monkeys (Sapajus xanthosternos), in Una Biological Reserve (Bahia, Brazil). Each infant was observed and filmed weekly from birth until 36 months, through daily focal sampling. We analyzed the behavior of each individual in 10 developmental points. By means of component reduction (principal component analysis), we obtained four behavioral traits: Sociability, Anxiety, Openness, and Activity. We investigated whether there were developmental effects on those traits by fitting regression models for the effect of time on behavioral traits, controlling for monkey identity, sex, and cohort. Sociability (decreasing) and Anxiety (increasing) changed significantly alongdevelopment. By means of repeatability analysis, we did not find intra-individual consistency across time in those traits, so we cannot discriminate stable personality traits in early ontogeny. Our results show that the personality structure of capuchin monkeys is not established during early development, in agreement with the literature on human personality.

Comparing behavioural syndromes across time and ecological conditions in a free-ranging predator

Personality traits (i.e. consistent individual differences in behaviour) often covary, forming behavioural syndromes. Such associations, if driven by an underlying proximate mechanism, could limit the independent evolution of each behaviour. In contrast, a behavioural syndrome may be the result of selection favouring the behavioural correlation under certain ecological circumstances. In this context, investigating the stability and the potential drivers of behavioural syndromes is a fundamental step towards understanding how they might shape the evolution of behaviours. We assessed syndrome stability across time in two subpopulations of free-ranging juvenile lemon sharks, Negaprion brevirostris. We measured two behaviours, exploration in an open field and sociability in a group, over 5 years. We tested whether these behaviours were associated in a behavioural syndrome, how stable this syndrome was and whether its strength was predicted by ecological conditions in this system: predator abundance, juvenile survival, newborn recruitment and total subpopulation size. We found that a behavioural syndrome was only observed in the subpopulation where predatory shark abundance was higher. Across years, newborn recruitment was the main driver for the strength of the syndrome. During years of low recruitment, the subpopulations probably experienced lower competition due to fewer newborns joining the population. In these years, sharks that were more explorative were also less social. These results indicate that the behaviours we tested were plastic and responsive to ecological conditions. Since this could signify that they are only favoured under certain ecological conditions, we argue in this case that the correlation of two behavioural traits is adaptive.

Do different food amounts gradually promote personality variation throughout the life stage in a clonal gecko species?

Various hypotheses have been proposed to explain mechanisms of personality formation. Several theoretical models predict that interactions between intrinsic state and behaviour gradually increase, or sometimes decrease, individual variation across ontogeny. However, these models should be empirically tested with a simple study system. Here, I investigated whether state differences in energy intake promote consistent behavioural differences in clonal geckos. I controlled food supply of two groups (high-energy or low-energy intake) and evaluated their life-history traits and behavioural traits throughout ontogeny. The high-energy group grew faster and laid eggs earlier than the low-energy group. Geckos at several age classes exhibited consistent individual differences in risk boldness, foraging boldness and activity. However, significant behavioural change was detected only in risk boldness; individuals in the high-energy group maintained boldness over their lifetimes, whereas individuals in the low-energy group changed to being shy at early stages and returned to being bold at later stages. These results imply that behaviours do not change gradually, but drastically, in response to energy intake, suggesting that personality formation does not represent simple divergence with ontogeny. Because predation risk and food availability change across an individual's lifetime, the state-dependent safety model could explain divergence and convergence of risk boldness in geckos at different life stages. The present study highlights the importance of a ‘life-stage specific’ effect of energy state on behaviour for personality formation.

Error-monitoring across social and affective processing contexts

The error-related negativity (ERN) is one of the most researched event-related potentials in the study of cognitive control, and it is thought to capture preconscious error-monitoring. ERN amplitude is known to be modulated by trait and state differences in affect, yet most ERN studies use ‘cold’ cognitive tasks that do not directly target affective processes involved in cognitive control. For example, speeded response-time tasks that elicit the ERN typically use neutral stimuli (e.g., letters, arrows), yet these paradigms are also flexible enough such that affective or social stimuli can readily be incorporated to target the role of affect in error-monitoring. In this project, the commonly-used arrow flanker task was modified to examine whether the expected behavioral and psychophysiological indices of error-monitoring would be observed using affective and social stimuli. Specifically, four different flanker tasks were administered using a within-subjects design with the following stimuli: arrows, neutral faces, unpleasant images, and pleasant images. Analyses indicated that the flanker tasks using arrows and faces elicited expected behavioral patterns (e.g., lower accuracy and slower reaction time on incongruent versus congruent trials) and ERN modulation by error versus correct trials. Although flanker tasks using unpleasant and pleasant stimuli also modulated the ERN, flanker effects on behavioral performance were not as consistent as the other tasks. Further, within incongruent trials, the ERN was larger when affective stimuli needed to be suppressed for a correct response. The correlations of the ERN and behavioral measures across tasks indicated some consistent individual differences in the ERN across tasks as well as substantial task-specific variances. This project lays the foundation for modifying classic error-monitoring tasks in a manner that may better target social and affective constructs that are of interest to clinical researchers.

Effects of trap confinement on personality measurements in two terrestrial rodents.

In recent years, consistent individual differences in behavior, or personalities, have been a topic of increasing interest as researchers strive to understand and predict the responses of individuals and populations to anthropogenic changes. Behavioral studies in wild populations often require that animals are live trapped before behavioral observation can occur, and this is especially true in studies investigating animal personalities. However, it is unknown whether the amount of time confined to a live trap may regulate the behavior of trapped individuals. Specifically, if the duration of trap confinement directly influences behavior, then by obtaining wild animals through live trapping we may be confounding the very measurements of greatest interest. To investigate whether the duration of trap confinement influences the behavior of trapped individuals, we performed a study on two small mammal species, focusing specifically on personality traits. We positioned high-definition trail cameras facing Longworth small mammal traps in the field to observe capture events and record the time of capture. We then measured personality in captured deer mice (Peromyscus maniculatus) and southern red-backed voles (Myodes gapperi) using three standardized tests, and through linear and generalized linear models we found that the time an animal had spent confined to a trap before testing did not affect 86% of behaviors exhibited. Our results showed two weak behavioral effects of confinement duration on boldness and docility resulting from an interaction between the duration of confinement and whether or not an individual was naïve to trapping. Our results suggest that behavioral measurements of wild, trapped small mammals are not determined by the time spent confined to a trap. However, researchers should use caution and consider whether an animal is naïve to trapping during analysis since habituation to the live trap may play a role in the effects of confinement duration on behavior.

Animals exhibit consistent individual differences in their movement: A case study on location trajectories of Japanese macaques

Researching individual recognition (IR) is essential to understand the life history and adaptive behavior of social animals. Investigation of possible IR clues may also help us build new hypotheses about how social animals distinguish between different individuals. This study investigates behavioral individuality in Japanese macaques (Macaca fuscata), focusing on one specific trait that is movement. Using a recently developed tracking system based on Bluetooth® Low Energy beacons, we collected three-dimensional (3D) location data from five Japanese macaques living in a group cage. A non-parametric, neural network-based analysis of the data revealed the existence of consistent individual differences in extremely limited aspects of the movement data (2-min trajectory of 3D location). Our results support the validity of multimodal approaches in studying IR, beyond the typical single-frame face recognition method, both for researchers and animal agents.

Parasitic Personalities: Consistent Individual Differences in Behavior in a Facultatively Parasitic Mite

Host attachment is a necessary behavior in the evolution of parasitism. It is hypothesized that populations pass through a stage of facultative parasitism before the evolution of obligate parasitism. Previous research has revealed the impact of extrinsic factors on the expression of parasitic tendencies, but to our knowledge, the role of individual behavioral variation driving parasitic tendencies remains to be tested. Here, we used repeated behavioral assays to quantify attachment propensity and activity level of individual Macrocheles muscaedomesticae, facultatively parasitic mites of flies, from two different populations. Mites from both populations exhibited repeatability in attachment propensity and mites from one population exhibited repeatability in activity level. We did not find a relationship between an individual’s activity level and attachment propensity. Our data suggest that facultative parasitism may not simply describe a phenotypically plastic strategy that responds to environmental cues, but perhaps that individual differences in parasitic tendencies may appear like facultative parasitism at the population or species level.

Light-based monitoring devices to assess range use by laying hens

Access to an outdoor range has many potential benefits for laying hens but range use can be poor due to factors only partly understood. Techniques to monitor individual range use within commercial flocks are crucial to increase our understanding of these factors. Direct observation of individual range use is difficult and time-consuming, and automatic monitoring currently relies on equipment that is difficult to use in an on-farm setting without itself influencing range use. We evaluated the performance of a novel small, light and readily portable light-based monitoring system by validating its output against direct observations. Six commercial houses (2000 hens/house) and their adjacent ranges were used, three of which were equipped with more structures on the range than the others (to determine whether cover would influence monitoring accuracy). In each house, 14 hens were equipped with light monitoring devices for 5 discrete monitoring cycles of 7 to 8 consecutive days (at 20, 26, 32, 36 and 41 weeks of age). Light levels were determined each minute: if the reading on the hen-mounted device exceeded indoor light levels, the hen was classified as outside. Focal hens were observed directly for 5 min/hen per week. Accuracy (% of samples where monitoring and direct observations were in agreement) was high both for ranges with more and with fewer structures, although slightly better for the latter (92% v. 96% ± 1 SEM, F1,19 = 5.2, P = 0.034). Furthermore, accuracy increased over time (89%, 94%, 95%, 98% ± 1 SEM for observations at 26, 32, 36 and 41 weeks, respectively, F3,19 = 3.2, P = 0.047), probably due to progressively reduced indoor light levels resulting from partial closing of ventilation openings to sustain indoor temperature. Light-based monitoring was sufficiently accurate to indicate a tendency for a greater percentage of monitored time spent outside when more range structures were provided (more: 67%, fewer: 56%, SEM: 4, χ12 = 2.9, P = 0.089). Furthermore, clear and relatively consistent individual differences were detected. Individuals that were caught outside at the start of the experiment ranged more throughout its duration (caught outside: 72%, caught inside 51%, SEM: 4, χ12 = 10.0, P = 0.002), and individual range use was correlated between monitoring cycles (for adjacent monitoring cycles: rs2 = 0.5−0.7, P < 0.0001). This emphasizes the importance of studying range use on an individual level. In conclusion, our light-based monitoring system can assess individual range use accurately (although accuracy was affected by house characteristics to some extent) and was used to show that both cover availability and individual characteristics affected range use.

Social influence and interaction bias can drive emergent behavioural specialization and modular social networks across systems.

In social systems ranging from ant colonies to human society, behavioural specialization—consistent individual differences in behaviour—is commonplace: individuals can specialize in the tasks they perform (division of labour (DOL)), the political behaviour they exhibit (political polarization) or the non-task behaviours they exhibit (personalities). Across these contexts, behavioural specialization often co-occurs with modular and assortative social networks, such that individuals tend to associate with others that have the same behavioural specialization. This raises the question of whether a common mechanism could drive co-emergent behavioural specialization and social network structure across contexts. To investigate this question, here we extend a model of self-organized DOL to account for social influence and interaction bias among individuals—social dynamics that have been shown to drive political polarization. We find that these same social dynamics can also drive emergent DOL by forming a feedback loop that reinforces behavioural differences between individuals, a feedback loop that is impacted by group size. Moreover, this feedback loop also results in modular and assortative social network structure, whereby individuals associate strongly with those performing the same task. Our findings suggest that DOL and political polarization—two social phenomena not typically considered together—may actually share a common social mechanism. This mechanism may result in social organization in many contexts beyond task performance and political behaviour.