"Leading according to need" in self-organizing groups.

Self-organizing-system approaches have shed significant light on the mechanisms underlying synchronized movements by large groups of animals, such as shoals of fish, flocks of birds, or herds of ungulates. However, these approaches rarely consider conflicts of interest between group members, although there is reason to suppose that such conflicts are commonplace. Here, we demonstrate that, where conflicts exist, individual members of self-organizing groups can, in principle, increase their influence on group movement destination by strategically changing simple behavioral parameters (namely, movement speed, assertiveness, and social attraction range). However, they do so at the expense of an increased risk of group fragmentation and a decrease in movement efficiency. We argue that the resulting trade-offs faced by each group member render it likely that group movements are led by those members for which reaching a particular destination is most crucial or group cohesion is least important. We term this phenomenon leading according to "need" or "social indifference," respectively. Both kinds of leading can occur in the absence of knowledge of or communication about the needs of other group members and without the assumption of altruistic cooperation. We discuss our findings in the light of observations on fish and other vertebrates.

Democracy in animals: the evolution of shared group decisions.

A 'consensus decision' is when the members of a group choose, collectively, between mutually exclusive actions. In humans, consensus decisions are often made democratically or in an 'equally shared' manner, i.e. all group members contribute to the decision. Biologists are only now realizing that shared consensus decisions also occur in social animals (other than eusocial insects). Sharing of decisions is, in principle, more profitable for groups than accepting the 'unshared' decision of a single dominant member. However, this is not true for all individual group members, posing a question as to how shared decision making could evolve. Here, we use a game theory model to show that sharing of decisions can evolve under a wide range of circumstances but especially in the following ones: when groups are heterogeneous in composition; when alternative decision outcomes differ in potential costs and these costs are large; when grouping benefits are marginal; or when groups are close to, or above, optimal size. Since these conditions are common in nature, it is easy to see how mechanisms for shared decision making could have arisen in a wide range of species, including early human ancestors.

Nonrandom movement behavior at habitat boundaries in two butterfly species: implications for dispersal.

We observed meadow brown (Maniola jurtina) and gatekeeper (Pyronia tithonus) butterflies at habitat boundaries and observed spontaneous movements out of suitable habitat in order to investigate such movements in relation to dispersal. We found that butterflies of both species were aware of the position of a highly permeable habitat boundary without needing to cross it. Nevertheless, a considerable proportion of butterflies close to the boundary left their habitat (25-43%). Butterflies that crossed the boundary, and moved substantial distances into unsuitable habitat (up to 350 m in M. jurtina and 70 m in P. tithonus), usually returned to their original habitat patch (98-100%). Movement trajectories, at least in M. jurtina, were significantly different from, and more directed and systematic than, a correlated random walk. Approximately 70-80% of spontaneous movements into unsuitable habitat in both species were "foray" loops comparable to those described in mammals and birds. We conclude that, since migrants seemed to have considerable control over leaving their patch and over their subsequent movement trajectories, chance encounter rates with habitat boundaries, and indeed habitat leaving rates, might be less crucial in determining dispersal rates than is usually assumed. In addition, random dispersal trajectories should not be taken for granted in population or evolution models.

Foray search: an effective systematic dispersal strategy in fragmented landscapes.

In the absence of evidence to the contrary, population models generally assume that the dispersal trajectories of animals are random, but systematic dispersal could be more efficient at detecting new habitat and may therefore constitute a more realistic assumption. Here, we investigate, by means of simulations, the properties of a potentially widespread systematic dispersal strategy termed "foray search." Foray search was more efficient in detecting suitable habitat than was random dispersal in most landscapes and was less subject to energetic constraints. However, it also resulted in considerably shorter net dispersed distances and higher mortality per net dispersed distance than did random dispersal, and it would therefore be likely to lead to lower dispersal rates toward the margins of population networks. Consequently, the use of foray search by dispersers could crucially affect the extinction-colonization balance of metapopulations and the evolution of dispersal rates. We conclude that population models need to take the dispersal trajectories of individuals into account in order to make reliable predictions.

Group decision-making in animals.

Groups of animals often need to make communal decisions, for example about which activities to perform, when to perform them and which direction to travel in; however, little is known about how they do so. Here, we model the fitness consequences of two possible decision-making mechanisms: 'despotism' and 'democracy'. We show that under most conditions, the costs to subordinate group members, and to the group as a whole, are considerably higher for despotic than for democratic decisions. Even when the despot is the most experienced group member, it only pays other members to accept its decision when group size is small and the difference in information is large. Democratic decisions are more beneficial primarily because they tend to produce less extreme decisions, rather than because each individual has an influence on the decision per se. Our model suggests that democracy should be widespread and makes quantitative, testable predictions about group decision-making in non-humans.

Ecological and evolutionary processes at expanding range margins.

Many animals are regarded as relatively sedentary and specialized in marginal parts of their geographical distributions. They are expected to be slow at colonizing new habitats. Despite this, the cool margins of many species' distributions have expanded rapidly in association with recent climate warming. We examined four insect species that have expanded their geographical ranges in Britain over the past 20 years. Here we report that two butterfly species have increased the variety of habitat types that they can colonize, and that two bush cricket species show increased fractions of longer-winged (dispersive) individuals in recently founded populations. Both ecological and evolutionary processes are probably responsible for these changes. Increased habitat breadth and dispersal tendencies have resulted in about 3- to 15-fold increases in expansion rates, allowing these insects to cross habitat disjunctions that would have represented major or complete barriers to dispersal before the expansions started. The emergence of dispersive phenotypes will increase the speed at which species invade new environments, and probably underlies the responses of many species to both past and future climate change.

Non-random dispersal in the butterfly Maniola jurtina: implications for metapopulation models.

The dispersal patterns of animals are important in metapopulation ecology because they affect the dynamics and survival of populations. Theoretical models assume random dispersal but little is known in practice about the dispersal behaviour of individual animals or the strategy by which dispersers locate distant habitat patches. In the present study, we released individual meadow brown butterflies (Maniola jurtina) in a non-habitat and investigated their ability to return to a suitable habitat. The results provided three reasons for supposing that meadow brown butterflies do not seek habitat by means of random flight. First, when released within the range of their normal dispersal distances, the butterflies orientated towards suitable habitat at a higher rate than expected at random. Second, when released at larger distances from their habitat, they used a non-random, systematic, search strategy in which they flew in loops around the release point and returned periodically to it. Third, butterflies returned to a familiar habitat patch rather than a non-familiar one when given a choice. If dispersers actively orientate towards or search systematically for distant habitat, this may be problematic for existing metapopulation models, including models of the evolution of dispersal rates in metapopulations.

Sex differences in weather sensitivity can cause habitat segregation: red deer as an example.

Sex differences in habitat use (habitat segregation) are widespread in sexually dimorphic ungulates. A possible cause is that males are more sensitive to weather than females, leading to sex differences in sheltering behaviour (the 'weather sensitivity hypothesis'). However, this hypothesis has never been tested. We considered the allometric rates of net energy gain during times of cold weather and food shortage in a model. We argue that the higher absolute heat losses relative to intake rates of larger ungulates should indeed lead to higher weather sensitivity in males than in females. Furthermore, we tested the weather sensitivity hypothesis empirically in red deer, Cervus elaphus, on the Isle of Rum, U.K. We predicted that (1) use of relatively exposed, high-quality forage habitat should be negatively influenced by bad weather; and (2) this influence should be stronger in males. We found that bad weather (strong wind, low temperature, heavy rain) in winter and spring influenced use of high-quality forage habitat negatively in all deer; that adult males responded more strongly to low temperature and strong wind than did females; and that adult males foraged on windy days at better sheltered sites than did females. Thus, the weather sensitivity hypothesis is supported both theoretically and empirically. We suggest that the weather sensitivity hypothesis can potentially explain winter habitat segregation in a large number of ungulate species. Copyright 2000 The Association for the Study of Animal Behaviour.

Activity synchrony and social cohesion: a fission-fusion model.

A social group can only be spatially coherent if its members synchronize activities such as foraging and resting. However, activity synchronization is costly to individuals if it requires them to postpone an activity that would be personally more profitable in order to do what the rest of the group is doing. Such costs will be particularly high in groups whose members belong to different age, size or sex classes since the optimal allocation of time to various activities is likely to differ between such classes. Thus, differences in the costs of activity synchronization between and within classes could cause non-homogenous groups to be less stable than homogenous groups, with the result that homogenous groups predominate in the population: that is, they could cause 'social segregation' of animals of different sex, size or age. We develop a model that predicts the degree of social segregation attributable to differences in activity synchronization between homogenous and non-homogenous groups and use this model in determining whether activity synchronization can explain intersexual social segregation in red deer (Cervus elaphus). Differences in activity synchronization between mixed-sex and unisex groups of red deer explained 35% of the observed degree of intersexual social segregation, showing that activity synchronization is an important cause of social segregation in this species.

Social segregation is not a consequence of habitat segregation in red deer and feral soay sheep.

In many sexually dimorphic mammals, adults tend to form single-sex groups ('social segregation'). It has been assumed that social segregation is simply a by-product of sex differences in habitat use ('habitat segregation'). I tested this assumption on red deer, Cervus elaphus, and feral soay sheep, Ovis aries, using data on group composition, habitat use and space use collected on the Scottish islands of Rum (1974-1993) and Hirta (1985-1994), respectively. If social segregation had been a by-product of habitat segregation, then (1) social segregation should have been influenced by the same environmental parameters that influence habitat segregation and (2) degree of social segregation should have equalled (and in no case been larger than) degree of habitat segregation. However, I found that weather parameters that influence habitat segregation did not influence social segregation in red deer and that degree of social segregation was significantly larger than degree of habitat segregation in both species. I conclude that social segregation is not a by-product of habitat segregation in either species, and discuss the implications of this finding. Copyright 1999 The Association for the Study of Animal Behaviour.

Habitat segregation in ungulates: are males forced into suboptimal foraging habitats through indirect competition by females?

Sex differences in habitat use (termed `habitat segregation') are widespread in sexually dimorphic ungulate species. They are a puzzling phenomenon, particularly when females use better foraging habitats than males. It has been suggested that males, owing to their larger body size and higher forage requirements, are inferior in indirect competition to females and are forced by female grazing pressure into marginal habitats (`indirect-competition hypothesis'). This hypothesis has been widely cited and has important theoretical and practical implications. However, evidence for it is inconclusive. The present paper presents the results of the first experimental test of the indirect-competition hypothesis. We manipulated female and male numbers of red deer (Cervus elaphus L.) on a large scale on the Isle of Rum, Scotland, and tested the influence of this manipulation on deer habitat use. We predicted that where female numbers were reduced, male use of preferred habitat should increase and sex differences in habitat use should decrease, while a reduction in male numbers should not have such effects. In contrast, we found that the manipulation of female and male numbers did not affect habitat use, and conclude that the indirect-competition hypothesis does not explain habitat segregation on Rum.

The relationship between habitat choice and lifetime reproductive success in female red deer.

In non-territorial species, individuals can move freely and should be distributed in an ideal free manner between habitats and areas with respect to resources that influence lifetime reproductive success (LRS). Consequently, no relationship between diet quality and LRS should be expected. However, there have been no attempts to test this prediction. The present paper investigates the relationship between forage habitat use and LRS in red deer (Cervus elaphus) hinds within three neighbouring areas on the Isle of Rum which differed in their amounts of high-quality-forage habitat. Within areas, hinds move widely and have access to the same resources. We found no correlation between LRS of individual hinds and their use of high-quality-forage habitat (i.e. short Agrostis/Festuca grassland). Our analysis suggests that high hind densities on short Agrostis/Festuca grassland offset any advantages of increased access to preferred forage. These results support the hypothesis that red deer hinds are distributed in an ideal free manner with respect to the use of high-quality-forage habitat. However, hinds rarely leave areas where they are born, and the analysis suggests that constraints in changing areas hindered an ideal free distribution on a larger spatial scale. Consequently, mean LRS was not the same within the three investigated areas: one area, with a low amount of short Agrostis/Festuca grassland and a low hind density, contributed more male offspring (and more total offspring) per hind to the population than the other two areas.

Could asynchrony in activity between the sexes cause intersexual social segregation in ruminants?

In many sexually dimorphic mammal species, the sexes live outside the mating season in separate social groups ('social segregation'). Social segregation occurs in a wide range of environmental conditions, but its cause in unknown. I suggest that social segregation is caused by a lower level of activity synchrony between individuals in mixed-sex groups than in single-sex groups, owing to sex differences in activity rhythm. As a consequence, mixed-sex groups are more likely to break up than single-sex groups, resulting in a predominance of single-sex groups at equilibrium. To test this hypothesis in red deer (Cervus elaphus L.), I developed an index of activity synchronization and showed that deer in mixed-sex groups were significantly less synchronized in their activity than deer in single-sex groups. Thus, low intersexual synchrony in activity can lead to social segregation. However, a lower level of intrasexual (female-female and male-male) activity synchrony within mixed-sex than within single-sex groups implies that additional factors (other than sex differences in foraging rhythm) contribute to the higher degree of instability of mixed-sex groups.