Merging social hierarchies: Effects on dominance rank in male green swordtail fish (Xiphophorus helleri).

When the same set of individuals are placed in different social contexts, some groups members often experience a change in dominance status. We examined the context-dependence of social status using a group fusion protocol in male green swordtail fish (Xiphophorus helleri). Six individuals were matched for size and separated into two groups of three fish. Each triad established a stable hierarchy after which time the two subgroups were merged into one larger assemblage. The maintenance of within- and between-group rank relationships was examined. Relative rank was preserved within each subgroup across social contexts but we found no evidence that familiarity with dominant animals assists individuals of one subgroup in achieving higher rank (coat-tail effects). Dominant individuals from the pre-fusion groups were significantly likely to obtain high status in the merged group and vice versa for subordinate pre-fusion animals. These results demonstrate that social rank in swordtails is relatively impervious to changes in social context, but we address some deviations from this trend. Small differences in standard length were a significant predictor of the most dominant rank in the post-fusion hierarchy, with the largest animals tending to occupy the alpha position. We discuss our results in terms of the potential factors involved in within- and between-group rank maintenance, including individual recognition, winner and loser effects, or asymmetries in dominance-related characteristics.

Coalition formation in animals and the nature of winner and loser effects.

Coalition formation has been documented in a diverse array of taxa, yet there has been little formal analysis of polyadic interactions such as coalitions. Here, we develop an optimality model which examines the role of winner and loser effects in shaping coalition formation. We demonstrate that the predicted patterns of alliances are strongly dependent on the way in which winner and loser effects change with contestant strength. When winner and loser effects decrease with the resource-holding power (RHP) of the combatants, coalitions will be favoured between the strongest members of a group, but not between the weakest. If, in contrast, winner and loser effects increase with RHP, exactly the opposite predictions emerge. All other things being equal, intervention is more likely to prove worthwhile when the beneficiary of the aid is weaker (and its opponent is stronger), because the beneficiary is then less likely to win without help. Consequently, intervention is more probable when the impact of victory on the subsequent performance of a combatant increases with that individual's strength because this selects for intervention in favour of weaker combatants. The published literature on hierarchy formation does not reveal how winner and loser effects actually change with contestant strength and we therefore hope that our model will spur others to collect such data; in this light we suggest an experiment which will help to elucidate the nature of winner and loser effects and their impact on coalition formation in animals.

Female mating preference for bold males in the guppy, Poecilia reticulata.

Although females prefer to mate with brightly colored males in numerous species, the benefits accruing to such females are virtually unknown. According to one hypothesis of sexual selection theory, if the expression of costly preferred traits in males (such as conspicuous colors) is proportional to the male's overall quality or reveals his quality, a well-developed trait should indicate good condition and/or viability for example. A female choosing such a male would therefore stand to gain direct or indirect fitness benefits, or both. Among potential phenotypic indicators of an individual's quality are the amount and brightness of its carotenoid-based colors and its boldness, as measured by its willingness to risk approaching predators without being killed. Here, we show experimentally that in the Trinidadian guppy (Poecilia reticulata) the visual conspicuousness of the color pattern of males correlates positively with boldness toward, and with escape distance from, a cichlid fish predator. Bold individuals are thus more informed about nearby predators and more likely to survive encounters with them. Mate-choice experiments showed that females prefer colorful males as mates, but prefer bolder males irrespective of their coloration when given the opportunity to observe their behavior toward a potential fish predator. By preferentially mating with colorful males, female guppies are thus choosing on average, relatively bold, and perhaps more viable, individuals. In doing so, and to the extent that viability is heritable, they potentially gain indirect fitness benefits by producing more viable offspring than otherwise.

Interface between culturally based preferences and genetic preferences: female mate choice in Poecilia reticulata.

The relative contribution of genetic and socio-cultural factors in the shaping of behavior is of fundamental importance to biologists and social scientists, yet it has proven to be extremely difficult to study in a controlled, experimental fashion. Here I describe experiments that examined the strength of genetic and cultural (imitative) factors in determining female mate choice in the guppy, Poecilia reticulata. Female guppies from the Paria River in Trinidad have a genetic, heritable preference for the amount of orange body color possessed by males. Female guppies will, however, also copy (imitate) the mate choice of other females in that when two males are matched for orange color, an "observer" female will copy the mate choice of another ("model") female. Three treatments were undertaken in which males differed by an average of 12%, 24%, or 40% of the total orange body color. In all cases, observer females viewed a model female prefer the less colorful male. When males differed by 12% or 24%, observer females preferred the less colorful male and thus copied the mate choice of others, despite a strong heritable preference for orange body color in males. When males differed by 40% orange body color, however, observer females preferred the more colorful male and did not copy the mate choice of the other female. In this system, then, imitation can "override" genetic preferences when the difference between orange body color in males is small or moderate, but genetic factors block out imitation effects when the difference in orange body color in males is large. This experiment provides the first attempt to experimentally examine the relative strength of cultural and genetic preferences for a particular trait and suggests that these two factors moderate one another in shaping social behavior.

Cooperation among unrelated individuals: reciprocal altruism, by-product mutualism and group selection in fishes.

Cooperation among unrelated individuals can evolve not only via reciprocal altruism but also via trait-group selection or by-product mutualism (or some combination of all three categories). Therefore the (iterated) prisoner's dilemma is an insufficient paradigm for studying the evolution of cooperation. We replace this game by the cooperator's dilemma, which is more versatile because it enables all three categories of cooperative behavior to be examined within the framework of a single theory. Controlled studies of cooperation among fish provide examples of each category of cooperation. Specifically, we describe reciprocal altruism among simultaneous hermaphrodites that swap egg parcels, group-selected cooperation among fish that inspect dangerous predators and by-product mutualism in the cooperative foraging of coral-reef fish.

Evolving cooperation: the role of individual recognition.

To evaluate the role of individual recognition in the evolution of cooperation, we formulated and analyzed genetic algorithm model (EvCo) for playing the Iterated Prisoner's Dilemma (IPD) game. Strategies compete against each other during each generation, and successful strategies contribute more of their attributes to the next generation. Each strategy is encoded on a 'chromosome' that plays the IPD, responding to the sequences of most recent responses by the interacting individuals (chromosomes). The analysis reported in this paper considered different memory capabilities (one to five previous interactions), pairing continuities (pairs of individuals remain together for about one, two, five, or 1000 consecutive interactions), and types of individual recognition (recognition capability was maximal, nil, or allowed to evolve between these limits). Analysis of the results focused on the frequency of mutual cooperation in pairwise interactions (a good indicator of overall success in the IPD) and on the extent to which previous responses by the focal individual and its partner were associated with the partner's identity (individual recognition). Results indicated that a fixed, substantial amount of individual recognition could maintain high levels of mutual cooperation even at low pairing continuities, and a significant but limited capability for individual recognition evolved under selection. Recognition generally increased mutual cooperation more when the recent responses of individuals other than the current partner were ignored. Titrating recognition memory under selection using a fitness cost suggested that memory of the partner's previous responses was more valuable than memory of the focal's previous responses. The dynamics produced to date by EvCo are a step toward understanding the evolution of social networks, for which additional benefits associated with group interactions must be incorporated.

Monozygotic twinning: an evolutionary hypothesis.

Monozygotic twinning is rare within populations yet taxonomically widespread. We explore the evolution of monozygotic twinning by modeling an allele in a newly formed offspring that causes it to undergo mitosis and separation to form one or more clones (twins), potentially in conflict with the parents' best interest. The success of this twinning allele in our haploid models depends on the balance of the benefit of increased frequency in the clutch and the cost of reduced survival resulting from limited parental resources. The trait reaches high frequency in a broad range of plausible conditions but also fails to spread or is kept at low frequency in others when the survival cost is high (e.g., in small clutch sizes). Interestingly, there are two reasonable conditions that predict high frequency of the trait but low visibility: random parental abortion and selection for low penetrance. Thus our models suggest reasons why monozygotic twinning might be rare, or alternatively, be common yet appear rare. In addition, we discuss the implications for sex-linked twinning, dizygotic twinning, and twinning by gametes.

Reversal of female mate choice by copying in the guppy (Poecilia reticulata).

Ever since Fisher (1958) formalized models of sexual selection, female mate choice has been assumed to be a genetically determined trait. Females, however, may also use social cues to select mates. One such cue might be the mate choice of conspecifics. Here we report the first direct evidence that a female's preference for a particular male can in fact be reversed by social cues. In our experiments using the Trinidadian guppy (Poecilia reticulata), this reversal was mediated by mate-copying opportunities, such that a female (the 'focal' female) is given the opportunity to choose between two males, followed by a period in which she observes a second female (the 'model' female) displaying a preference for the male she herself did not prefer initially. When allowed to choose between the same males a second time, compared with control tests, a significant proportion of focal females reversed their mate choice and copied the preference of the model female. These results provide strong evidence for the role of non-genetic factors in sexual selection and underlie the need for new models of sexual selection that explicitly incorporate both genetic and cultural aspects of mate choice.

Beyond the prisoner's dilemma: Toward models to discriminate among mechanisms of cooperation in nature.

The iterated prisoner's dilemma game, or IPD, has now established itself as the orthodox paradigm for theoretical investigations of the evolution of cooperation; but its scope is restricted to reciprocity, which is only one of three categories of cooperation among unrelated individuals. Even within that category, a cooperative encounter has in general three phases, and the IPD has nothing to say about two of them. To distinguish among mechanisms of cooperation in nature, future theoretical work on the evolution of cooperation must distance itself from economics and develop games as a refinement of ethology's comparative approach.

N-person games and the evolution of co-operation: a model based on predator inspection in fish.

Two N-person game theoretical models examining the evolution of co-operation during predator inspection in fish are presented. Predator inspection occurs in small shoals of fish, in which one to a few individuals, the "inspectors" (co-operators) break away from the shoal and cautiously approach a predator to obtain information on this potential danger. In the models presented here, remaining with the shoal and not inspecting is considered an act of defection. Both model I and II produce a stable internal polymorphism of inspectors and noninspectors. While the equilibrial frequency of inspectors can be low (i.e. less than 10%) at large shoal size, the proportion of shoals containing any inspectors--and therefore exhibiting the inspection behavior--is much greater. Both models presented here, and N-person games in general are equivalent to intrademic group selection models of evolution in structured populations, in which shoals are trait groups and co-operation evolves by between-shoal selection. While the results are cast in terms of predator inspection, the model itself is general and applies to any multi-group scenario where co-operators benefit entire groups at their own expense. The results presented here add to the mounting theoretical and empirical evidence that co-operation is frequently not a pure evolutionarily stable strategy, and that many metapopulations should be polymorphic for both co-operators and defectors.