Biomedical subjects

Rufus A Johnstone

Publications and source records attributed to Rufus A Johnstone.

17 recordsLinked to original sources

PubMed2006

Statistical measures for defining an individual's degree of independence within state-dependent dynamic games.

BACKGROUND: For organisms living or interacting in groups, the decision-making processes of an individual may be based upon aspects of both its own state and the states of other organisms around it. Much research has sought to determine how group decisions are made, and whether some individuals are more likely to influence these decisions than others. State-dependent modelling techniques are a powerful tool for exploring group decision-making processes, but analyses conducted so far have lacked methods for identifying how dependent an individual's actions are on the rest of the group. RESULTS: Here, we introduce and evaluate two easy-to-calculate statistics that quantify how dependent an individual's actions are upon the state of a co-player in a two-player state-dependent dynamic game. We discuss the merits of these statistics, and situations in which they would be useful. CONCLUSION: Our statistical measures provide a means of quantifying how independent an individual's actions are. They also allow researchers to quantify the output of state-dependent dynamic games, and quantitatively assess the predictions of these models.

Animals↗

PubMed2006

Early experience and parent-of-origin-specific effects influence female reproductive success in mice.

Recent studies on mammals investigating parent-of-origin-specific effects such as genomic imprinting and maternal effects have demonstrated their impact on short-term measures of fitness, for example offspring growth. However, the long-term fitness consequences of parent-of-origin-specific effects and their role outside the immediate mother-offspring interaction remain largely unexplored. Here, we show that female mice mated to males that inherited the same set of paternal and maternal genes as themselves have a higher reproductive success than females mated to males of reciprocal genotype. Furthermore, we demonstrate that the early social environment experienced by an individual influences its reproductive success. Females raised with unrelated siblings in a mixed litter had a subsequent lower reproductive success than those that were fostered together with all their biological siblings in unmixed litters. Our results highlight the important influence of parent-of-origin-specific effects and conditions in early development on long-term reproductive success in mammals and suggest that parent-of-origin-specific effects may provide the underlying mechanism for beneficial coadaptation between genotypes, for example, in mate choice.

Animals↗

PubMed2006

The influence of phenotypic and genetic effects on maternal provisioning and offspring weight gain in mice.

Close interactions between mother and offspring are said to result in a coevolution of parental and offspring genotypes such that offspring are adapted in their solicitation behaviour to obtain maternal provisioning that maximizes their fitness. Few empirical studies have been conducted in this field and it remains unclear whether maternal provisioning and offspring weight gain are influenced by the same set of maternal and offspring phenotypic and genotypic factors. Using a cross-foster, split-litter design in mice, we found that overall maternal provisioning and offspring weight gain are significantly correlated but are affected by a different set of parameters, except for the effect of maternal bodyweight. While the level of maternal provisioning was influenced by both offspring and foster mother genotype, offspring weight gain was only affected by the number of males in the mixed litter. We suggest that this disparity may hint at the inefficiency of offspring solicitation behaviour or effects of sibling competition.

Animals↗

PubMed2006

Context-dependent discrimination and the evolution of mimicry.

Many mimetic organisms have evolved a close resemblance to their models, making it difficult to discriminate between them on the basis of appearance alone. However, if mimics and models differ slightly in their activity patterns, behavior, or use of microhabitats, the exact circumstances under which a signaler is encountered may provide additional clues to its identity. We employ an optimality model of mimetic discrimination in which signal receivers obtain information about the relative risk of encountering mimics and models by observing an external background cue and flexibly adjust their response thresholds. Although such flexibility on the part of signal receivers has been predicted by theory and is supported by empirical evidence in a range of biological settings, little is known about the effects it has on signalers. We show that the presence of external cues that partly reveal signaler identity may benefit models and harm mimics, harm both, or even benefit both, depending on ecological circumstances. Moreover, if mimetic traits are costly to express, or mimics are related to their neighbors, context-dependent discrimination can dramatically alter the outcome of mimetic evolution. We discuss context-dependent discrimination among signal receivers in relation to small-scale synchrony in model and mimic activity patterns.

Adaptation, Physiological↗

PubMed2004

Wide faces or large canines? The attractive versus the aggressive primate.

Hominids display marked body size dimorphism, suggestive of strong sexual selection, yet they lack significant sex differences in canine size that are commonly associated with intrasexual competition in primates. We resolve this paradox by examining sex differences in hominoid facial morphology. We show that chimpanzees, but not gorillas, exhibit clear sexual dimorphism in face width, over and above that expected based on sex differences in body size. We show that this facial dimorphism, expressed as an index, is negatively correlated with canine dimorphism among anthropoid primates. Our findings suggest that a lack of canine dimorphism in anthropoids is not owing to weak sexual selection, but rather is associated with strong sexual selection for broader face width. Enlarged cheek-bones are linked with attractiveness in humans, and we propose that the evolution of a broad face and loss of large canines in hominid males results from mate choice.

Animals↗

PubMed2004

The evolution of mimicry under constraints.

The resemblance between mimetic organisms and their models varies from near perfect to very crude. One possible explanation, which has received surprisingly little attention, is that evolution can improve mimicry only at some cost to the mimetic organism. In this article, an evolutionary game theory model of mimicry is presented that incorporates such constraints. The model generates novel and testable predictions. First, Batesian mimics that are very common and/or mimic very weakly defended models should evolve either inaccurate mimicry (by stabilizing selection) or mimetic polymorphism. Second, Batesian mimics that are very common and/or mimic very weakly defended models are more likely to evolve mimetic polymorphism if they encounter predators at high rates and/or are bad at evading predator attacks. The model also examines how cognitive constraints acting on signal receivers may help determine evolutionarily stable levels of mimicry. Surprisingly, improved discrimination abilities among signal receivers may sometimes select for less accurate mimicry.

Animals↗

PubMed2004

Evolution of spite through indirect reciprocity.

How can cooperation persist in the face of a temptation to 'cheat'? Several recent papers have suggested that the answer may lie in indirect reciprocity. Altruistic individuals may benefit by eliciting altruism from observers, rather than (as in direct reciprocity) from the recipient of the aid they provide. Here, we point out that indirect reciprocity need not always favour cooperation; by contrast, it may support spiteful behaviour, which is costly for the both actor and recipient. Existing theory suggests spite is unlikely to persist, but we demonstrate that it may do so when spiteful individuals are less likely to incur aggression from observers (a negative form of indirect reciprocity).

Aggression↗

PubMed2004

The evolution of paternal care with overlapping broods.

Most attempts to model the evolution of parental care assume that caring and mating are mutually exclusive activities (i.e., individuals acquire and guard broods "sequentially"). However, in most fish and certain insects, males can keep mating and collecting additional eggs while continuing to guard broods obtained earlier (i.e., males guard "overlapping" broods). We present a model of parental care with overlapping broods in which males can mate and guard simultaneously, even though there is a trade-off between these two activities. Within this framework, we show that male care is favored by short female processing times and high population densities, which minimize the mating cost of care. Relatively low mortality while guarding is also important for the stability of male care. Female care, on the other hand, is favored by long female processing times and low populations densities, which lead to longer intermating intervals. Biparental care is stable only when the cost to benefit ratio of care was not biased toward either sex. We derive quantitative estimates of fitness for different strategies for two species of assassin bugs with male and female uniparental care and show that the model predicts the correct form of care for both species. We believe our model might help explain the prevalence of male uniparental care in certain taxa, such as fish.

Animals↗

PubMed2003

Optimal assessment of multiple cues.

In a wide range of contexts from mate choice to foraging, animals are required to discriminate between alternative options on the basis of multiple cues. How should they best assess such complex multicomponent stimuli? Here, we construct a model to investigate this problem, focusing on a simple case where a 'chooser' faces a discrimination task involving two cues. These cues vary in their accuracy and in how costly they are to assess. As an example, we consider a mate-choice situation where females choose between males of differing quality. Our model predicts the following: (i) females should become less choosy as the cost of finding new males increases; (ii) females should prioritize cues differently depending on how choosy they are; (iii) females may sometimes prioritize less accurate cues; and (iv) which cues are most important depends on the abundance of desirable mates. These predictions are testable in mate-choice experiments where the costs of choice can be manipulated. Our findings are applicable to other discrimination tasks besides mate choice, for example a predator's choice between palatable and unpalatable prey, or an altruist's choice between kin and non-kin.

Animals↗

PubMed2003

Spontaneous emergence of leaders and followers in foraging pairs.

Animals that forage socially often stand to gain from coordination of their behaviour. Yet it is not known how group members reach a consensus on the timing of foraging bouts. Here we demonstrate a simple process by which this may occur. We develop a state-dependent, dynamic game model of foraging by a pair of animals, in which each individual chooses between resting or foraging during a series of consecutive periods, so as to maximize its own individual chances of survival. We find that, if there is an advantage to foraging together, the equilibrium behaviour of both individuals becomes highly synchronized. As a result of this synchronization, differences in the energetic reserves of the two players spontaneously develop, leading them to adopt different behavioural roles. The individual with lower reserves emerges as the 'pace-maker' who determines when the pair should forage, providing a straightforward resolution to the problem of group coordination. Moreover, the strategy that gives rise to this behaviour can be implemented by a simple 'rule of thumb' that requires no detailed knowledge of the state of other individuals.

Animals↗

PubMed2003

The limits to cost-free signalling of need between relatives.

Theoretical models have demonstrated the possibility of stable cost-free signalling of need between relatives. The stability of these cost-free equilibria depends on the indirect fitness cost of cheating and deceiving a donor into giving away resources. We show that this stability is highly sensitive to the distribution of need among signallers and receivers. In particular, cost-free signalling is likely to prove stable only if there is very large variation in need (such that the least-needy individuals stand to gain much less than the most-needy individuals from additional resources). We discuss whether these conditions are likely to be found in altricial avian breeding systems--the most intensively studied instance of signalling of need between relatives. We suggest that cost-free signalling is more likely to prove stable and will provide parents with more information during the earlier phases of chick growth, when parents can more easily meet the demands of a brood (and chicks are more likely to reach satiation). Later, informative yet cost-free signalling is unlikely to persist.

Animal Communication↗

PubMed2003

The genetic basis of family conflict resolution in mice.

Asymmetries in the costs and benefits of parental investment for mothers, fathers and offspring result in family conflict over the production and provisioning of young. In species where females provide most resources before and after birth, the resolution of this conflict may be influenced by genes expressed in mothers and by maternally and paternally inherited genes expressed in offspring. Here we disentangle these effects by means of reciprocal mating and cross-fostering of litters between two strains of mice that differ with respect to the typical resolution of family conflict. We find that differences in litter size between these two strains are determined by paternal genotype, whereas differences in provisioning are under maternal control, showing that there is antagonistic coadaptation of maternal and paternal effects on distinct life-history traits. Maternal provisioning is also influenced by the type of foster offspring. Contradictory to theoretical expectations, however, we find no evidence for a negative correlation across strains between maternal provisioning and offspring demand. Instead, we show that there is positive coadaptation such that offspring obtain more resources from foster mothers of the same strain as their natural mother, irrespective of their father's strain.

Animals↗

PubMed2002

Managing uncertainty: information and insurance under the risk of starvation.

In an uncertain world, animals face both unexpected opportunities and danger. Such outcomes can select for two potential strategies: collecting information to reduce uncertainty, or insuring against it. We investigate the relative value of information and insurance (energy reserves) under starvation risk by offering model foragers a choice between constant and varying food sources over finite foraging bouts. We show that sampling the variable option (choosing it when it is not expected to be good) should decline both with lower reserves and late in foraging bouts; in order to be able to reap the reduction in uncertainty associated with exploiting a variable resource effectively, foragers must be able to afford and compensate for an initial increase in the risk of an energetic shortfall associated with choosing the option when it is bad. Consequently, expected exploitation of the varying option increases as it becomes less variable, and when the overall risk of energetic shortfall is reduced. In addition, little activity on the variable alternative is expected until reserves are built up early in a foraging bout. This indicates that gathering information is a luxury while insurance is a necessity, at least when foraging on stochastic and variable food under the risk of starvation.

Animals↗

PubMed2002

Cost, competition and information in communication between relatives.

Recent signalling models have shown that honest, cost-free communication between relatives can be stable. Moreover, cost-free signalling equilibria are in some cases more efficient than costly equilibria. However, we show that they are also relatively uninformative, particularly when relatedness between signaller and receiver is low. We explore the trade-off between signal cost and information, and further demonstrate that incorporating competition among signallers into a model of communication between relatives can reduce the propensity of any one signaller to display. As a result, there is a general increase in the amount of broadcast information in a non-costly signal with increasing competitor number.

Animal Communication↗

PubMed2002

The evolution of inaccurate mimics.

Instances of strikingly accurate Batesian mimicry (in which a palatable prey organism closely resembles an aversive model) are often cited to illustrate the power of natural selection. Less attention has been paid to those mimics, such as many hoverfly (Syrphidae) mimics of wasps or bees, that resemble their models only poorly. Attempts to provide an adaptive explanation for imperfect mimicry have suggested that what seems a crude resemblance to human observers may appear a close match to predators, or that inaccurate mimics may bear a general resemblance to several different model species. I show here, however, that truly inaccurate mimicry of a single model organism may be favoured over perfect resemblance, by kin selection. Signal detection theory predicts that predators will modify their level of discrimination adaptively in response to the relative frequencies and similarity of models and mimics. If models are rare and/or weakly aversive, greater local similarity of mimics can thus lead to greater attack rates. Where individual mimics are related to others in their vicinity, kin selection will then oppose the evolution of accurate mimicry.

Adaptation, Physiological↗

PubMed2002

Why is mutual mate choice not the norm? Operational sex ratios, sex roles and the evolution of sexually dimorphic and monomorphic signalling.

Biases in the operational sex ratio (OSR) are seen as the fundamental reason behind differential competition for mates in the two sexes, and as a strong determinant behind differences in choosiness. This view has been challenged by Kokko and Monaghan, who argue that sex-specific parental investment, mortalities, mate-encounter rates and quality variation determine the mating system in a way that is not reducible to the OSR. We develop a game-theoretic model of choosiness, signalling and parental care, to examine (i) whether the results of Kokko and Monaghan remain robust when its simplifying assumptions are relaxed, (ii) how parental care coevolves with mating strategies and the OSR and (iii) why mutual mate choice is observed relatively rarely even when both sexes vary in quality. We find qualitative agreement with the simpler approach: parental investment is the primary determinant of sex roles instead of the OSR, and factors promoting choosiness are high species-specific mate-encounter rate, high sex-specific mate-encounter rate, high cost of breeding (parental investment), low cost of mate searching and highly variable quality of the opposite sex. The coevolution of parental care and mating strategies hinders mutual mate choice if one parent can compensate for reduced care by the other, but promotes it if offspring survival depends greatly on biparental care. We argue that the relative rarity of mutual mate choice is not due to biases in the OSR. Instead, we describe processes by which sexual strategies tend to diverge. This divergence is prevented, and mutual mate choice maintained, if synergistic benefits of biparental care render parental investment both high and not too different in the two sexes.

Animals↗

PubMed2000

Power Struggles, Dominance Testing, and Reproductive Skew.

Models of reproductive skew are concerned with the partitioning of reproduction between dominant and subordinate members of a group. In an interesting extension of these models, Reeve and Ratnieks briefly considered whether it might benefit subordinates to engage in aggressive behavior to test the fighting ability of a dominant. Their analysis suggested that such testing should be more probable in groups that feature high skew and, hence, perhaps among closer relatives (because high relatedness favors high skew). Here we explore in more detail the possibility of dominance testing. Three models that differ in the outcome of fights over dominance are presented: in the first model, the loser of the challenge is killed; in the second model, the loser is evicted from the nest; and, in the third model, the loser becomes (or remains) subordinate. In each case we consider the independent effects of the parameters that determine skew (namely, relatedness, group productivity, and ecological constraints) on the predicted level of dominance testing. We then construct an amalgamated model to examine situations where fights may lead to any one of the three outcomes. Our analysis reveals that, in the majority of cases, higher relatedness will in fact lead to lower levels of aggression. Moreover, dominance testing need not be associated with high skew. Rather, the relationship between skew and dominance testing will depend on which factor (relatedness, group productivity, or level of ecological constraints) is principally responsible for variation in the distribution of reproduction.

aggression↗