Charlotte Hemelrijk - Academia.edu (original) (raw)

Papers by Charlotte Hemelrijk

Theories of Small Groups: Interdisciplinary Perspectives

This chapter discusses evolutionary approaches to small group research. It defines basic assumpti... more This chapter discusses evolutionary approaches to small group research. It defines basic assumptions of the evolutionary perspective and then demarcates major debates within evolutionary theory relevant to the social scientific study of groups. Four distinct evolutionary perspectives on groups are defined and compared. Results of evolutionary research regarding group composition, typical group sizes, group interaction, outcomes, and development and change in groups are reviewed. We conclude with a discussion of strengths and weaknesses of the perspective and directions that future research may take. n research on evolution, the small group is something of an anomaly, located somewhere between genes and cultures. Our species has lived in groups throughout its existence and is descended from a long line of primate species that also lived in groups. Until very recently (in evolutionary terms), we lived in small groups that are somewhat approximated by hunter-gatherer societies (Boehm, 2000; Kelly, 1995). The advent of agriculture resulted in a transformation of human society over a period of only a few thousand years (Diamond, 1997). Because genetic evolution is a relatively slow process, in some respects, we are adapted to a physical and social environment with origins in the past. However, individual learning and cultural change can themselves be described as evolutionary processes, resulting in rapid adaptation to modern environments. Studying human groups from an evolutionary perspective need not be confined to genetic evolution (Boyd & Richerson, 1985; Dunbar, Knight, & Power, 1999; Durham, 1991). Although there are some common themes, the study of groups from an evolutionary viewpoint is best classified as an emerging perspective. Some evolutionists question whether groups are real, whereas others see groups as central to understanding humanity and have attempted to derive theories of how groups evolved and the impact of groups on evolution. Although to one outside the field evolution may seem a cut and dried theory, the study of evolution is in fact full of debate and has many different positions. Evolutionary thought relevant to groups is so broad and diverse that it cannot be summarized in a short article. This chapter will attempt to give readers an introduction to key positions and debates and to some conclusions drawn in this emerging research area. In writing this chapter, we have chosen to elide many differences among the authors and encourage readers to seek out the references we cite to understand the particularities and differences among us. In the next section, we will discuss some basic assumptions of evolutionary thought and debates relevant to the evolutionary view of groups. Following this, we will distinguish four alternative evolutionary viewpoints that differ in terms of their definition of group, the degree of importance they accord to groups in

EPL, Mar 17, 2022

Models of collective motion show a rich variety of patterns. One of these is milling, in which th... more Models of collective motion show a rich variety of patterns. One of these is milling, in which the individuals of a group are circling around a common center. Milling has been generated in a Vicsek-like model of collective motion, i.e. a minimal model where individuals coordinate their headings only via alignment with close neighbors, without being attracted to each other and without avoiding collisions. However, in this model information propagates instantaneously among neighbors, whereas in nature transfer and processing of information need time. How this delay aﬀects patterns of collective motion, particularly milling, is unknown. Here we investigate the eﬀect of time-delayed interactions on the emergence of milling in a Vicsek-like model. We show that delays may either destroy milling or induce it, depending on the parameters of the system. The range of speeds and ﬁelds of view of individuals at which milling occurs is shifted to smaller values if there are time-delays in the model. The presented ﬁndings may help to understand what causes milling in nature.

Physical Review E, 2009

Empirical measurements of hydrodynamics of swimming fish are very difficult. Therefore, modeling ... more Empirical measurements of hydrodynamics of swimming fish are very difficult. Therefore, modeling studies may be of great benefit. Here, we investigate the suitability for such a study of a recently developed mesoscale method, namely, multiparticle collision dynamics. As a first step, we confine ourselves to investigations at intermediate Reynolds numbers of objects that are stiff. Due to the lack of empirical data on the hydrodynamics of stiff fishlike shapes we use a previously published numerical simulation of the shapes of a fish and a tadpole for comparison. Because the shape of a tadpole resembles that of a circle with an attached splitter plate, we exploit the knowledge on hydrodynamic consequences of such an attachment to test the model further and study the effects of splitter plates for objects of several shapes at several Reynolds numbers. Further, we measure the angles of separation of flow around a circular cylinder and make small adjustments to the boundary condition and the method to drive the flow. Our results correspond with empirical data and with results from other models.

Physical Review E, 2012

The fluid dynamics of animal locomotion, such as that of an undulating fish, are of great interes... more The fluid dynamics of animal locomotion, such as that of an undulating fish, are of great interest to both biologists and engineers. However, experimentally studying these fluid dynamics is difficult and time consuming. Model studies can be of great help because of their simpler and more detailed analysis. Their insights may guide empirical work. Particularly the recently introduced multiparticle collision dynamics method may be suitable for the study of moving organisms because it is computationally fast, simple to implement, and has a continuous representation of space. As regards the study of hydrodynamics of moving organisms, the method has only been applied at low Reynolds numbers (below 120) for soft, permeable bodies, and static fishlike shapes. In the present paper we use it to study the hydrodynamics of an undulating fish at Reynolds numbers 1100-1500, after confirming its performance for a moving insect wing at Reynolds number 75. We measure (1) drag, thrust, and lift forces, (2) swimming efficiency and spatial structure of the wake, and (3) distribution of forces along the fish body. We confirm the resemblance between the simulated undulating fish and empirical data. In contrast to theoretical predictions, our model shows that for steadily undulating fish, thrust is produced by the rear 2/3 of the body and that the slip ratio U/V (with U the forward swimming speed and V the rearward speed of the body wave) correlates negatively (instead of positively) with the actual Froude efficiency of swimming. Besides, we show that the common practice of modeling individuals while constraining their sideways acceleration causes them to resemble unconstrained fish with a higher tailbeat frequency.

Fish and Fisheries, 2000

ABSTRACT Leadership is not an inherent quality of animal groups that show directional locomotion.... more ABSTRACT Leadership is not an inherent quality of animal groups that show directional locomotion. However, there are other factors that may be responsible for the occurrence of leadership in fish shoals, such as individual differences in nutritional state between group members. It appears that front fish have a strong influence on directional shoal movements and that individuals that occupy such positions are often characterised by larger body lengths and lower nutritional state. Potential interactions between the two factors and their importance for positioning within shoals need further attention. Initiation of directional movement in stationary shoals and position preferences in mobile shoals need to be addressed separately because they are potentially subject to different constraints. Individuals that initiate a swimming direction may not necessarily be capable of the sustained high swimming performance required to keep the front position or have the motivation to do so, for that matter. More empirical and theoretical work is necessary to look at the factors controlling positioning behaviour within shoals, as well as overall shoal shape and structure. Tracking of marked individuals whose positioning behaviour is monitored over extended time periods of hours or days would be useful. There is an indication that shoal positions are rotated by individuals according to their nutritional needs, with hungry fish occupying front positions only for as long as necessary to regain their nutritional balance. This suggests that shoal members effectively take turns at being leaders. There is a need for three-dimensional recordings of shoaling behaviour using high-speed video systems that allow a detailed analysis of information transfer in shoals of different size. The relationship between leadership and shoal size might provide an interesting field for future research. Most studies to date have been restricted to shoals of small and medium size and more information on larger shoals would be useful.

Behaviour, 2003

Lemur social systems have the striking social feature, that adult females consistently evoke subm... more Lemur social systems have the striking social feature, that adult females consistently evoke submissive behaviour of adult males. In the Alaotran gentle lemur, Hapalemur griseus alaotrensis, however, female dominance has not been studied yet. Here we confirm female dominance over males on the basis of a 5-month field study of the social behaviour of four groups, in the Lake Alaotra marshland of eastern Madagascar. Further, we found that dominant individuals initiated aggressive interactions significantly more often than lowerranking ones, they initiated group movements more often and higher-ranking individuals were groomed more often. The spatial configuration was remarkable, since individuals were closer in space to those more distant in rank.

Fish and Fisheries, 2014

There is increasing evidence that fish gain energetic benefits from the hydrodynamic interactions... more There is increasing evidence that fish gain energetic benefits from the hydrodynamic interactions when they swim in a school. The most recent indications of such benefits are a lower tail (or fin) beat at the back of a school and reduced oxygen consumption in schooling fish versus solitary ones. How such advantages may arise is poorly understood. Current hydrodynamic theories concern either fish swimming side by side or in a diamond configuration and they largely ignore effects of viscosity and interactions among wakes and individuals. In reality, however, hydrodynamic effects are complex and fish swim in many configurations. Since these hydrodynamic effects are difficult to study empirically, we investigate them in a computer model by incorporating viscosity and interactions among wakes and with individuals. We compare swimming efficiency of model fish (based on shapes of mullets of 126 mm) travelling solitarily and in schools at several interindividual distances in four different configurations (diamond, rectangular, phalanx and line). We show that these fish always swim more efficiently in a school than alone (except in a dense phalanx). We indicate how this efficiency may emerge from several kinds of interactions between wakes and individuals. As individuals in our simulations are not even intending to exploit the wake, gains in efficiency are obtained more easily than previously thought.

Aerial displays of starlings (Sturnus vulgaris) at their communal roosts are complex: thousands o... more Aerial displays of starlings (Sturnus vulgaris) at their communal roosts are complex: thousands of individuals form multiple flocks which are continually changing shape and density, while splitting and merging. To understand these complex displays both empirical data and models are needed. Whereas detailed empirical data were recently collected through video recordings and position measurements by stereo photography of flocks of thousands of starlings, there are as yet no models that generate these complex patterns. Numerous computer models in biology, however, suggest that patterns of single groups of moving animals may emerge by self-organisation from movement and local coordination (through attraction, alignment and avoidance of collision). In this paper, we investigated whether this approach can be extended to generate patterns resembling these aerial displays of starlings. We show in a model that to generate many of the patterns measured empirically in real starlings we have to...

In this paper, we show that differences in dominance and spatial centrality of individuals in a g... more In this paper, we show that differences in dominance and spatial centrality of individuals in a group may arise through selforganisation. Our instrument is a model, called DomWorld, that represents two traits that are often found in animals, namely grouping and competing. In this model individual differences grow under the following conditions: (1) when the intensity of aggression increases and grouping becomes denser, (2) when the degree of sexual dimorphism in fighting power increases. In this case the differences among females compared to males grow too, (3) when, upon encountering another individual, the tendency to attack is 'obligate' and not conditional, namely 'sensitive to risks'. Results resemble phenomena described for societies of primates, mice, birds and pigs. q

Lower visibility of female scientists, compared to male scientists, is a potential reason for the... more Lower visibility of female scientists, compared to male scientists, is a potential reason for the under-representation of women among senior academic ranks. Visibility in the scientific community stems partly from presenting research as an invited speaker at organized meetings. We analysed the sex ratio of presenters at the European Society for Evolutionary Biology (ESEB) Congress 2011, where all abstract submissions were accepted for presentation. Women were under-represented among invited speakers at symposia (15% women) compared to all presenters (46%), regular oral presenters (41%) and plenary speakers (25%). At the ESEB congresses in 2001-2011, 9-23% of invited speakers were women. This under-representation of women is partly attributable to a larger proportion of women, than men, declining invitations: in 2011, 50% of women declined an invitation to speak compared to 26% of men. We expect invited speakers to be scientists from top ranked institutions or authors of recent paper...

Experiments in Fluids

Obtaining accurate experimental data from Lagrangian tracking and tomographic velocimetry require... more Obtaining accurate experimental data from Lagrangian tracking and tomographic velocimetry requires an accurate camera calibration consistent over multiple views. Established calibration procedures are often challenging to implement when the length scale of the measurement volume exceeds that of a typical laboratory experiment. Here, we combine tools developed in computer vision and non-linear camera mappings used in experimental fluid mechanics, to successfully calibrate a four-camera setup that is imaging inside a large tank of dimensions \sim 10 \times 25 \times 6 \; \mathrm {m}^3$$∼10×25×6m3. The calibration procedure uses a planar checkerboard that is arbitrarily positioned at unknown locations and orientations. The method can be applied to any number of cameras. The parameters of the calibration yields direct estimates of the positions and orientations of the four cameras as well as the focal lengths of the lenses. These parameters are used to assess the quality of the calibr...

Behavioral Ecology and Sociobiology

When a predator attacks a flock of starlings (Sturnus vulgaris), involving thousands of individua... more When a predator attacks a flock of starlings (Sturnus vulgaris), involving thousands of individuals, a typical collective escape response is the so-called agitation wave, consisting of one or more dark bands (pulses) propagating through the flock and moving away from the predator (usually a Peregrine falcon, Falco peregrinus). The mechanism underlying this collective behavior remains debated. A theoretical study has suggested that the individual motion underlying a pulse could be a skitter (in the form of a zigzag), that is copied by nearby neighbors, and causes us to temporarily observe a larger surface of the wing because the bird is banking during turning while zigzagging. It is not known, however, whether pulses during a wave event weaken over time. This is of interest, because whereas during the usual turning by an undisturbed flock the motion is copied completely without weakening, we may expect that pulses dampen during a wave event because individuals that are further away from a predator react less because of reduced fear. In the present paper, we show in empirical data that pulses during a wave event weaken over time. Our computational model, StarDisplay, reveals that this is most likely a consequence of a reduction of the maximum banking angle during the zigzag escape maneuver rather than by a reduced tendency to copy this maneuver with time. The response seems adaptive because of lowered danger at a larger distance to the location of attack. Significance statement Huge flocks of starlings display amazing patterns of collective escape when attacked by an avian predator, such as a Peregrine falcon. One of them is the "agitation wave" in which dark bands move away from the predator. Dark bands arise probably from the temporarily larger wing area, which is observed when birds perform a skitter escape motion (zigzag) while temporarily banking sideward. Whereas during regular flock turns birds copy each other's motion completely, it is unknown whether this happens during agitation waves, because individuals further away from the attack may be less frightened. Studying this both empirically at the group level only and in a computational model at both the level of the individual and the group, we show that pulses of waves fade out with time and that this is probably due to a reduced maximum banking angle during the zigzag maneuver rather than a lower tendency of copying. This seems an adaptive response.

Journal of Physics D: Applied Physics

Take-down policy If you believe that this document breaches copyright please contact us providing... more Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Behavioral Ecology and Sociobiology

Collective behaviour of animals has been a main focus of recent research, yet few empirical studi... more Collective behaviour of animals has been a main focus of recent research, yet few empirical studies deal with this issue in the context of predation, a major driver of social complexity in many animal species. When starling (Sturnus vulgaris) flocks are under attack by a raptor, such as a peregrine falcon (Falco peregrinus), they show a great diversity of patterns of collective escape. The corresponding structural complexity concerns rapid variation in density and shape of the flock over time. Here, we present a first step towards unravelling this complexity. We apply a time series analysis to video footage of 182 sequences of hunting by falcons on flocks of thousands of starlings close to two urban roosts during winter. We distinguish several types of collective escape by determining the position and movement of individuals relative to each other (which determines darkness and shape of the flock over time) as well as relative to the predator, namely 'flash expansion', 'blackening', 'wave event', 'vacuole', 'cordon' and 'split'. We show that the specific type of collective escape depends on the collective pattern that precedes it and on the level of threat posed by the raptor. A wave event was most likely to occur when the predator attacked at medium speed. Flash expansion occurred more frequently when the predator approached the flock at faster rather than slower speed and attacked from above rather than from the side or below. Flash expansion was often followed by split, but in many cases, the flock showed resilience by remaining intact. During a hunting sequence, the frequencies of different patterns of collective escape increased when the frequency of attack by the raptor was higher. Despite their complexity, we show that patterns of collective escape depend on the predatory threat, which resembles findings in fish. Significance statement Patterns of collective escape in flocks of starlings have always intrigued laymen and scientists. A detailed analysis of their complex dynamics has been lacking so far, and is the focus of our present study: we analysed video footage of hunting by falcons on flocks of thousands of starlings and show how patterns of collective escape (namely flash expansion, blackening, wave event, vacuole, cordon and split) depend on the preceding pattern and on details of attack. A higher frequency of attack during a hunting sequence resulted in a higher frequency of collective escape events. Flash expansion happened most often when the predator attacks at greater speed. A wave event was most likely when the raptor attacks at medium (rather than high or low) speed. These results provide a first quantitative approach to social complexity in collective avoidance of a predator.

International Encyclopedia of the Social & Behavioral Sciences, 2001

Patterns of emergent social behavior refer to social patterns that arise automatically without re... more Patterns of emergent social behavior refer to social patterns that arise automatically without relevant specifications being in-built in the individual. Under what conditions such patterns may emerge, and of what kinds they are, is unpredictable, but can be learned from ‘bottom-up’ models. Models for this purpose use a so-called ‘process-oriented’ approach in which an artificial system is constructed in which agents are equipped with behavioral rules and mechanisms. While these agents interact with their environment and other agents, their behavior is studied in the same way as in ethological studies. Results show that in this way simple agents generate complex social structures, and that the same set of rules may lead to different patterns, depending on the past experiences of the individuals, the demography of their population, or the distribution of their food. Such studies were, of course, made possible by the development of computers, which have led to new modeling techniques. In this article, the major issues and characteristics of these models are exemplified. Their relation to different views on evolution and their general usage is discussed.

Journal of Theoretical Biology, 1991

The Biological Bulletin, 2002

Differences between related species are usually explained as separate adaptations produced by ind... more Differences between related species are usually explained as separate adaptations produced by individual selection. I discuss in this paper how related species, which differ in many respects, may evolve by a combination of individual selection, self-organization, and group-selection, requiring an evolutionary adaptation of only a single trait. In line with the supposed evolution of despotic species of macaques, we take as a starting point an ancestral species that is egalitarian and mildly aggressive. We suppose it to live in an environment with abundant food and we put the case that, if food becomes scarce and more clumped, natural selection at the level of the individual will favor individuals with a more intense aggression (implying, for instance, biting and fierce fighting). Using an individual-centered model, called DomWorld, I show what happens when the intensity of aggression increases. In DomWorld, group life is represented by artificial individuals that live in a homogeneous world. Individuals are extremely simple: all they do is flock together and, upon meeting one another, they may perform dominance interactions in which the effects of winning and losing are self-reinforcing. When the intensity of aggression in the model is increased, a complex feedback between the hierarchy and spatial structure results; via self-organization, this feedback causes the egalitarian society to change into a despotic one. The many differences between the two types of artificial society closely correspond to those between despotic and egalitarian macaques in the real world. Given that, in the model, the organization changes as a side effect of the change of one single trait proper to an egalitarian society, in the real world a despotic society may also have arisen as a side effect of the mutation of a single trait of an egalitarian species. If groups with different intensities of aggression evolve in this way, they will also have different gradients of hierarchy. When food is scarce, groups with the steepest hierarchy may have the best chance to survive, because at least a small number of individuals in such a group may succeed in producing offspring, whereas in egalitarian societies every individual is at risk of being insufficiently fed to reproduce. Therefore, intrademic group selection (selection within an interbreeding group) may have contributed to the evolution of despotic societies.

Lemur social systems have the striking social feature, that adult females consistently evoke subm... more Lemur social systems have the striking social feature, that adult females consistently evoke submissive behaviour of adult males. In the Alaotran gentle lemur, Hapalemur griseus alaotrensis, however, female dominance has not been studied yet. Here we con rm female dominance over males on the basis of a 5-month eld study of the social behaviour of four groups, in the Lake Alaotra marshland of eastern Madagascar. Further, we found that dominant individuals initiated aggressive interactions signi cantly more often than lowerranking ones, they initiated group movements more often and higher-rankingindividualswere groomed more often. The spatial con guration was remarkable, since individuals were closer in space to those more distant in rank.

Theories of Small Groups: Interdisciplinary Perspectives

EPL, Mar 17, 2022

Physical Review E, 2009

Physical Review E, 2012

Fish and Fisheries, 2000

Behaviour, 2003

Fish and Fisheries, 2014

Experiments in Fluids

Behavioral Ecology and Sociobiology

Journal of Physics D: Applied Physics

Behavioral Ecology and Sociobiology

International Encyclopedia of the Social & Behavioral Sciences, 2001

Journal of Theoretical Biology, 1991

The Biological Bulletin, 2002

Lemur social systems have the striking social feature, that adult females consistently evoke subm... more Lemur social systems have the striking social feature, that adult females consistently evoke submissive behaviour of adult males. In the Alaotran gentle lemur, Hapalemur griseus alaotrensis, however, female dominance has not been studied yet. Here we con rm female dominance over males on the basis of a 5-month eld study of the social behaviour of four groups, in the Lake Alaotra marshland of eastern Madagascar. Further, we found that dominant individuals initiated aggressive interactions signi cantly more often than lowerranking ones, they initiated group movements more often and higher-rankingindividualswere groomed more often. The spatial con guration was remarkable, since individuals were closer in space to those more distant in rank.