John Lazarus

Tendency-distance models of social cohesion in animal groups

Although it has been assumed that attraction and repulsion between social individuals constitute a basis for group cohesion, there has been no systematic study of the possible ways in which these tendencies might vary with inter-individual distance (IID), or of associated implications for group structure. In this paper, a family of attraction/repulsion--distance functions is described. Computer simulation was used to examine the effects of each function on group cohesion, as reflected by mean values and variability in IID and group shape. Our results showed that: (a) all models led to stability in group structure, but differed significantly in terms of stable IID and group shape characteristics; (b) cohesion was best served by an upwardly convex behaviour--distance function in which maximum attraction equaled maximum repulsion (and the biological plausibility of this function is discussed); (c) group elongation and variability in mean IID were significantly positively correlated; (d) although dyads maintained an equilibrial separation distance, at which attraction balanced repulsion, in larger groups stable nearest neighbour distances were often less than the equilibrium distance; and (e) individuals needed to monitor and respond to only relatively few of their companions in order to avoid group fragmentation.

The early warning function of flocking in birds: An experimental study with captive quelea

Abstract Two tests are described, using the weaverbird quelea, of the proposed early warning function of flocking in birds (i.e. larger flocks detect a predator sooner). In experiment 1 flocks detected a goshawk flying over their cage with a greater probability than single birds. In experiment 2 the probability of detection of a brief, artificial, alarm stimulus increased with flock size in the same way, but at a lower absolute level, as that predicted by a simple model of detection. Possible reasons for the discrepancy (non-independence of detection probabilities; flock size influences each individuals probability of detection or response threshold) are evaluated. As flock size increased the type of response elicited changed from taking wing to flight intention movements to orienting responses, a trend which can be understood functionally in terms of the reduced risk of capture in larger flocks once the flock has been detected by a predator.

The Problem of Vigilance in Animal Life

Vigilance is an important facet of animal life andparticularly so in relation to three areas: external danger, the exploitation of environmental resources, and intraspecific communication. The second of these involves, in addition to vigilance, an active search of the environment. The organization of the nervous system, however, imposes certain limitations on the effectiveness of vigilance and these are described, with particular reference to experimental studies of vigilance in man. Three types of strategy are then described which function to offset the neural limitations on vigilance. These are: neural mechanisms, individual behavioural strategies, and social strategies. These are considered in relation to those areas of life already defined as requiring a watchkeeping function.

An information primacy model of exploratory and foraging behaviour

We describe a stochastic model of an animal exploring and foraging within an uncertain environment. Behaviour is determined not by an optimizing algorithm but by fuzzy systems using linguistic rules derived from the information primacy hypothesis which stresses the importance of continual information gathering under conditions of uncertainty. In the model, the animals hunger increases steadily over time and is reduced by visiting locations that may contain varying amounts of food. Uncertainty arises from three sources: (1) location novelty or ambiguity, that is, the animal is uncertain whether it has visited the same location before; (2) variation in the amounts of food in a given location; and (3) the recency of information concerning these two aspects of a given location. In complex and changing environments fresh information is likely to be more accurate than old information and consequently our model gives most weight to recently gathered information. All sources of uncertainty are reduced by visiting locations and gathering fresh information. The model is successful in simulating results from experiments investigating such phenomena as: spontaneous alternation; patrolling; the effects of hunger on the variability of learnt responses; latent learning; contrafreeloading; and behaviour following changes in food availability.

The logic of mate desertion.

Abstract Since the fitness consequences of mate desertion are markedly affected by whether the partner also deserts, natural selection is expected to favour desertion decisions that take the mates decision into account. A model for the evolution of mate desertion is described in which individuals make desertion decisions repeatedly during the period of parental care, choosing the option (desert or stay) that gives them the greater fitness, taking into account whether their mate has already deserted. If the partner still remains, the optimal decision is made assuming that the partner responds optimally in turn. Four patterns of time-dependent payoffs define all possible desertion decisions, three of which produce different kinds of stable evolutionary outcome: both parents stay with the young; both desert; and one deserts, one stays. The fourth pattern (‘pre-emptive desertion’) also results in both parents deserting but paradoxically, in this case, they sacrifice fitness by so doing. Such uncooperative parents are caught in the well-known Prisoners Dilemma of game theory and provide a link with recent thinking on the evolution of cooperation. Under conditions of mate choice or repeated interaction between potential mates the payoff pattern that normally produces pre-emptive desertion may result instead in a ‘both stay’ outcome. When desertion decisions are conditional on the partners decision, in the way assumed here, desertion outcomes cannot be predicted simply by comparing the costs and benefits of desertion for the male and female at any one time. Rather, one needs to know the way in which these payoffs vary for each partner over the whole of the parental care period. Applications of the model are discussed, with particular reference to uniparental care in fish and biparental care in birds.

Flock Size and Behaviour in Captive Red-Billed Weaverbirds (Quelea Quelea): Implications for Social Facilitation and the Functions of Flocking

As a consequence of the anti-predator functions of flocking it is predicted that as flock size increases individual birds will (i) spend less time vigilant for predators and more time feeding, and (2) show fewer escape responses. These predictions are corroborated here in experiments with captive flocks of the red-billed weaverbird Quelea quelea varying in size from I to 32 individuals. In Experiment i the presence of companions reduced feeding latency and increased the rate of pecking. Feeding behaviour by the companion(s) was not necessary to produce these effects and did not produce additional facilitation of the rate of pecking, but feeding companions were more potent facilitators of the onset of pecking than non-feeding companions. The number of companions influenced different measures of feeding in different ways but facilitation reached a ceiling with three companions. These and other findings in the social facilitation literature are interpreted in the light of various propositions concerning the functions of flocking behaviour. In Experiment 2 undisturbed birds with companions made less frequent head-turns (possibly vigilant responses), wing-flicks (flight intention movements), hops and flights than birds on their own but the number of companions had no effect on the frequency of these responses. All responses correlated significantly and positively across individuals. Solitary individuals were more often perched, and less often on the ground, than those in flocks, a finding compatible with greater safety in flocks in the normal feeding habitat of the species. These (undisturbed) birds, in flocks of 1, 2 and 4, had lower rates of head-turning than those in another experiment (LAZARUS, 1979), which had been subjected to 10 alarm stimuli; in larger flocks there was no difference in the rate of head-turning. An alternative interpretation of the experimental results is evaluated and a possible reason for the weak effect of flock size considered.

Prey scan at random to evade observant predators.

Anti–predator scans by animals occur with very irregular timing, so that the initiation of scans resembles a random, Poisson–like, process. At first sight, this seems both dangerous (predators could exploit the long intervals) and wasteful (scans after very short intervals are relatively uninformative). We explored vigilance timing using a new model that allows both predators and prey to vary their behaviour. Given predators that attack at random with respect to prey behaviour, constant inter–scan intervals minimize predation risk. However, if prey scan regularly to minimize their risk from randomly attacking predators, they become more vulnerable to predators that initiate attacks when the inter–scan intervals begin. If, in order to defeat this tactic, prey choose extremely variable inter–scan intervals, they become more vulnerable to predators who wait for long intervals before launching attacks. Only if predators can monitor the variability of inter–scan intervals and either attack immediately (if variability is too low) or wait for long intervals to attack (if variability is too high) does the empirically observed pattern of Poisson–like scanning become the optimal prey strategy.

Environmental adversity and uncertainty favour cooperation

BackgroundA major cornerstone of evolutionary biology theory is the explanation of the emergence of cooperation in communities of selfish individuals. There is an unexplained tendency in the plant and animal world – with examples from alpine plants, worms, fish, mole-rats, monkeys and humans – for cooperation to flourish where the environment is more adverse (harsher) or more unpredictable.ResultsUsing mathematical arguments and computer simulations we show that in more adverse environments individuals perceive their resources to be more unpredictable, and that this unpredictability favours cooperation. First we show analytically that in a more adverse environment the individual experiences greater perceived uncertainty. Second we show through a simulation study that more perceived uncertainty implies higher level of cooperation in communities of selfish individuals.ConclusionThis study captures the essential features of the natural examples: the positive impact of resource adversity or uncertainty on cooperation. These newly discovered connections between environmental adversity, uncertainty and cooperation help to explain the emergence and evolution of cooperation in animal and human societies.

The effects of luteinizing hormone, oestrogen and ovariectomy on the agonistic behaviour of female Quelea quelea

Abstract Previous work with male Quelea showed that agonistic behaviour in relation to individual distance is controlled by luteinizing hormone (LH), rather than by testosterone, and that males are more aggressive than females. Experiments with female groups are reported which show that: (a) LH injections increase aggressive encounter frequency; (b) ovariectomy in the breeding season (but not outside it) also increases encounter frequency; and (c) oestrogen injections decrease encounter frequency. The effects of LH were shown to be specific to agonistic responses rather than mediated through changes in activity. Correlations between changes in natural hormone levels and encounter frequency support the injection findings. These results are consistent with the hypothesis that LH controls aggressive encounters over individual distance in the female as in the male and that oestrogenic inhibition of this LH-mediated aggressiveness is a cause of female subordination and the lower encounter frequency found in female groups. The annual cycle of encounter frequency is described and the significance of different systems of hormonal control is discussed.

Environmental risk, cooperation, and communication complexity

The evolution of cooperation and communication in communities of individuals is a puzzling problem for a wide range of scientific disciplines, ranging from evolutionary theory to the theory and application of multi-agent systems. A key issue is to understand the factors that affect collaboration and communication evolution. To address this problem, here we choose the environmental risk as a compact descriptor of the environment in a model world of simple agents. We analyse the evolution of cooperation and communication as a function of the environmental risk. Our findings show that collaboration is more likely to rise to high levels within the agent society in a world characterised by high risk than in one characterised by low risk. With respect to the evolution of communication, we found that communities of agents with high levels of collaboration are more likely to use less complex communication than those which show lower levels of collaboration. Our results have important implications for understanding the evolution of both cooperation and communication, and the interrelationships between them.