Olof Leimar

Life-history trade-offs favour the evolution of animal personalities

In recent years evidence has been accumulating that personalities are not only found in humans but also in a wide range of other animal species. Individuals differ consistently in their behavioural tendencies and the behaviour in one context is correlated with the behaviour in multiple other contexts. From an adaptive perspective, the evolution of animal personalities is still a mystery, because a more flexible structure of behaviour should provide a selective advantage. Accordingly, many researchers view personalities as resulting from constraints imposed by the architecture of behaviour (but see ref. 12). In contrast, we show here that animal personalities can be given an adaptive explanation. Our argument is based on the insight that the trade-off between current and future reproduction often results in polymorphic populations in which some individuals put more emphasis on future fitness returns than others. Life-history theory predicts that such differences in fitness expectations should result in systematic differences in risk-taking behaviour. Individuals with high future expectations (who have much to lose) should be more risk-averse than individuals with low expectations. This applies to all kinds of risky situations, so individuals should consistently differ in their behaviour. By means of an evolutionary model we demonstrate that this basic principle results in the evolution of animal personalities. It simultaneously explains the coexistence of behavioural types, the consistency of behaviour through time and the structure of behavioural correlations across contexts. Moreover, it explains the common finding that explorative behaviour and risk-related traits like boldness and aggressiveness are common characteristics of animal personalities.

Evolution of fighting behaviour: Decision rules and assessment of relative strength☆

Abstract A mathematical model of fighting behaviour is developed. The contestants belong to a population with varying fighting abilities and the fights consist of the repetition of one type of interaction. At each interaction in the sequence the opponents acquire some information about the true fighting abilities. The fights are seen as a motion of each opponent through a causal factor space; the current position of an animal in the space represents all information obtained by the animals so far. A decision rule (strategy) is a specification of what action to take at each point in the causal factor space. Evolutionarily stable strategies are calculated numerically and are found to be pure and unique. The distribution of fighting times and the probabilities of winning are calculated for pairs of contestants from a population using the ESS. Expected utilities are also computed and in the situations investigated they are fairly close to the maximum value that would obtain if the contested resource were divided equally between the contestants without any cost.

Evolution of cooperation through indirect reciprocity

How can cooperation through indirect reciprocity evolve and what would it be like? This problem has previously been studied by simulating evolution in a small group of interacting individuals, assuming no gene flow between groups. In these simulations, certain ‘image scoring’ strategies were found to be the most successful. However, analytical arguments show that it would not be in an individuals interest to use these strategies. Starting with this puzzle, we investigate indirect reciprocity in simulations based on an island model. This has an advantage in that the role of genetic drift can be examined. Our results show that the image scoring strategies depend on very strong drift or a very small cost of giving help. As soon as these factors are absent, selection eliminates image scoring. We also consider other possibilities for the evolution of indirect reciprocity. In particular, we find that the strategy of aiming for ‘good standing’ has superior properties. It can be an evolutionarily stable strategy and, even if not, it usually beats image scoring. Furthermore, by introducing quality variation among individuals into the model, we show that the standing strategy can be quality revealing, adding a new dimension to indirect reciprocity. Finally, we discuss general problems with currently popular modelling styles.

Evolution of fighting behaviour: The effect of variation in resource value*

A number of empirical studies have shown that animals adjust their fighting behaviour when resource value is changed. We apply evolutionary game theory to investigate how variation in resource value influences the evolution of fighting behaviour. Although no completely general predictions can be made concerning the cost of fighting and the probability of victory, for most situations of biological relevance the cost of fighting will increase when resource value increases and the probability of victory for an animal will increase when resource value is increased only to that animal. In order to study the effect of variation in resource value when differences in fighting ability exist and are assessed, sequential assessment games are developed for two situations. In the first situation, contestants do not know each others subjective resource value. In the second situation, there is an owner-intruder asymmetry where the owner is better informed about the value of the resource than the intruder. The models give predictions for fight duration, cost, and probability of victory. The predictions are compared with empirical data, and a good qualitative agreement is found.

The Effect of Flexible Growth Rates on Optimal Sizes and Development Times in a Seasonal Environment

The interrelationships among development time, growth rate, and adult size are investigated using simple optimization models of a seasonal life history in which larger adults have greater reproductive output. Unlike most previous studies, our models assume that growth rate is an adaptively flexible character that can be increased at the expense of a greater juvenile mortality rate. Three components of fitness are considered: the cost of developing at a suboptimal time of the year, the reproductive advantage of larger adult size, and the increased mortality from rapid juvenile growth. The study focuses on the optimal responses of size, development time, and growth rate to changes in the amount of time available for completion of the life cycle. The models show that the optimal growth rate and size at maturity may respond in several different ways. Perhaps the most likely effect is that growth becomes faster and size smaller with less time available. It is also possible, however, for either growth rate or size (but not both) to stay constant; in other cases, less time available leads to slower growth or larger size. The effects of increased mortality on the juvenile stage are also explored; here, the optimal size is likely to decrease, but growth rate and development time may increase or decrease.

A test of the sequential assessment game: fighting in the cichlid fish Nannacara anomala

Many species have a repertoire of behaviour patterns that are used in contests over resources. It is likely that the function of these behaviour patterns is the assessment of asymmetries between con- testants in physical variables (e.g. size or strength). Theoretical models of fighting behaviour such as the war of attrition and the hawk-dove game do not incorporate any behavioural mechanisms allowing assessment, and, therefore, yield no predictions about the use of behaviour patterns. In a sequential assessment game, on the other hand, the assessment of asymmetries is a major activity during a fight. Recently, a version of the sequential assessment game with several behavioural options has been devel- oped, and here predictions from this model are tested using data from 102 staged fights between males of the cichlid fish Nannacara anomala. The model predicts that the sequence of behaviour patterns in a fight should be maximally efficient in assessing relative fighting ability. Specific predictions are that (1) the sequence should be organized into phases consisting of one or several behaviour patterns with constant rates of behaviour within a phase, (2) the division into phases should be independent of relative fighting ability, and (3) contests with great asymmetry in relative fighting ability should end in an early phase, whereas matched individuals may proceed through a series of escalations reaching a final phase of more dangerous fighting. The results show that the fighting behaviour of N. anomala is rather well predicted by the model.

Effects of asymmetries in owner-intruder conflicts*

A common situation when animals compete for resources is that there is an owner-intruder asymmetry. Many studies show that owners win most of these conflicts. We investigate how various asymmetries between contestants that might be present in owner-intruder conflicts will affect the outcome and nature of such interactions. A mathematical model is used to represent a fight between an owner and an intruder. A fight in this model consists of a sequence of behaviours; at each step in the sequence the contestants assess their relative strength and each of them decides whether to give up or to continue to fight on the basis of these assessments. For such contests it is shown that the role asymmetry inherent in an owner-intruder conflict can give rise to an ESS where the individual in one role is less willing to continue fighting than the individual in the other role. We also consider that the resource might be more valuable for an owner than for an intruder and that owners might be stronger on the average. Asymmetry in average strength will appear when a resource typically is contested several times and strong individuals have an advantage in such contests. This process of accumulation of strong individuals as owners is studied in some detail. ESSs for contests with these different types of asymmetries are computed numerically. A common feature is that owners will be more persistent than intruders and from this some predictions follow, e.g., owners will win more also when opponents are of equal strength, contests won by the owner will tend to be shorter than those won by the intruder, and the longest contests will be those where the intruder is slightly stronger.

Evolutionary Stability of Aposematic Coloration and Prey Unprofitability: A Theoretical Analysis

Species that have evolved some defense against predators, here called unprofitable prey, often show aposematic (warning) coloration. The process whereby a predator learns not to attack unprofitable prey is an essential ingredient in the interaction between predators and such prey. Using learning theory and evolutionary game theory, we develop a model of predator-prey interactions, focusing on two characters: prey unprofitability and prey coloration. Both these characters are treated as one-dimensional variables, and it is assumed that variation in coloration causes variation in the rate at which prey are detected by predators. Regarding the two characters as strategies used by prey individuals, we determine evolutionarily stable strategies (ESSs). The coloration yielding the smallest possible rate of detection is called cryptic, and colorations yielding higher rates of detection are called aposematic. Two aspects of predator behavior are shown to be important for the stability of an aposematic strategy: a reluctance to attack prey more conspicuously colored than those so far encountered, and faster avoidance learning vis-a-vis prey with more conspicuous coloration. At least one of these factors must be present for an aposematic strategy to be an ESS. Similarly, two factors are important for the evolution of prey unprofitability: increased survival of attacks by a predator, and faster avoidance learning vis-a-vis more-unprofitable prey. If there is a nonpredatory cost associated with unprofitability, at least one of these factors must be present for an unprofitable strategy to be an ESS. The first factor mentioned, both for coloration and unprofitability, results in individual selection. The second factor, variation in the rate of avoidance learning caused by variation in the character, produces a selection pressure only if there is a correlation between prey phenotypes encountered by the predator. Such correlation may be the result of relatedness among prey individuals (resulting in kin selection) or repeated encounters with the same prey individual.

Sex-biased dispersal in sperm whales: contrasting mitochondrial and nuclear genetic structure of global populations.

The social organization of most mammals is characterized by female philopatry and male dispersal. Such sex–biased dispersal can cause the genetic structure of populations to differ between the maternally inherited mitochondrial DNA (mtDNA) and the bi–parental nuclear genome. Here we report on the global genetic structure of oceanic populations of the sperm whale, one of the most widely distributed mammalian species. Groups of females and juveniles are mainly found at low latitudes, while males reach polar waters, returning to tropical and subtropical waters to breed. In comparisons between oceans, we did not find significant heterogeneity in allele frequencies of microsatellite loci (exact test; p = 0.23). Estimates of GST = 0.001 and RST = 0.005 also indicated negligible if any nuclear DNA differentiation. We have previously reported significant differentiation between oceans in mtDNA sequences. These contrasting patterns suggest that interoceanic movements have been more prevalent among males than among females, consistent with observations of females being the philopatric sex and having a more limited latitudinal distribution than males. Consequently, the typical mammalian pattern may have operated on a global scale in sperm whales.

The Evolution of Phenotypic Polymorphism: Randomized Strategies versus Evolutionary Branching

A population is polymorphic when its members fall into two or more categories, referred to as alternative phenotypes. There are many kinds of phenotypic polymorphisms, with specialization in reproduction, feeding, dispersal, or protection from predators. An individual’s phenotype might be randomly assigned during development, genetically determined, or set by environmental cues. These three possibilities correspond to a mixed strategy of development, a genetic polymorphism, and a conditional strategy. Using the perspective of adaptive dynamics, I develop a unifying evolutionary theory of systems of determination of alternative phenotypes, focusing on the relative possibilities for random versus genetic determination. The approach is an extension of the analysis of evolutionary branching in adaptive dynamics. It compares the possibility that there will be evolutionary branching, leading to genetic polymorphism, with the possibility that a mixed strategy evolves. The comparison is based on the strength of selection for the different outcomes. An interpretation of the resulting criterion is that genetic polymorphism is favored over random determination of the phenotype if an individual’s heritable genotype is an adaptively advantageous cue for development. I argue that it can be helpful to regard genetic polymorphism as a special case of phenotypic plasticity.