Competition for mates and predominant juvenile male dispersal in mammals

Competition among males for mates may partially explain the predominant dispersal of juvenile male California ground squirrels (Spermophilus beecheyi). I tested the generality of this hypothesis by comparing dispersal patterns in many species of mammals. In most polygynous and promiscuous species of mammals, juvenile males are the predominant dispersers. In most monogamous species of mammals, juvenile males and juvenile females are predominant dispersers with little distinction as to sex. While many factors may influence dispersal, achieving outcrossing and advantages in competition for mates are most likely to explain predominant dispersal by juvenile males in many mammal species.     

Oxytocin increases generosity in humans

Human beings routinely help strangers at costs to themselves. Sometimes the help offered is generous-offering more than the other expects. The proximate mechanisms supporting generosity are not well-understood, but several lines of research suggest a role for empathy. In this study, participants were infused with 40 IU oxytocin (OT) or placebo and engaged in a blinded, one-shot decision on how to split a sum of money with a stranger that could be rejected. Those on OT were 80% more generous than those given a placebo. OT had no effect on a unilateral monetary transfer task dissociating generosity from altruism. OT and altruism together predicted almost half the interpersonal variation in generosity. Notably, OT had twofold larger impact on generosity compared to altruism. This indicates that generosity is associated with both altruism as well as an emotional identification with another person.     

A polypeptide from male accessory glands which triggers maturation of tick spermatozoa

The maturation of tick (Acari: Ixodoidea) spermatozoa, after their transfer to the female, is induced by secretions of the male accessory gland which are added to the prospermia during ejaculation. Analysis of the secretions from male Ornithodoros moubata (Argasidae) and Dermacentor variabilis (Ixodidae) indicates that the factor responsible for inducing maturation is a polypeptide with a molecular weight of about 12,500. Although the polypeptides from the two species are similar in most respects, each will not activate the prospermia of the other species. The mode of action of the factor is hormone-like, inasmuch as a few minutes of exposure of the sperm to the factor are sufficient to trigger the entire maturation sequence, which takes 12–24 h in vitro.     

Mating and hormonal triggers regulate accessory gland gene expression in male Drosophila

Drosophila melanogaster males transfer accessory gland proteins, as part of their seminal fluid, to females during each mating. Since accessory gland proteins are important for male reproductive success, it is important that the male replenish the proteins he transferred during mating. Previous studies had shown that mating induces the resynthesis of accessory gland proteins, but since mating includes a set of stereotyped behavior patterns as well as the act of copulation, it was not known which aspect of the mating process induces accessory gland protein synthesis. By exposing males to females whose ovipositors had been sealed shut, we have shown that resynthesis of accessory gland proteins occurs only when seminal fluid is transferred to females. By applying juvenile hormone or 20-hydroxyecdysone topically to the cuticle of male flies, we showed that these hormones can act in vivo to stimulate the synthesis of accessory gland proteins to levels similar to those observed after mating.     

Skull trophies of the Pacific War: transgressive objects of remembrance

This article discusses the use of enemy body parts as war trophies, focusing on the collection of Japanese skulls as trophies by Allied servicemen in the Second World War, and on the treatment of these objects after the war. I argue that such human trophy-taking tends to occur in societies, including modern states, in which two conditions hold: the hunting of animals is an important component of male identity; and the human status of enemies is denied.     

Competition for male reproductive investment elevates testosterone levels in female dunnocks,Prunella modularis

In many songbirds, females occasionally sing in contexts of high female-female competition. Testosterone may be involved in the activation of song, because testosterone implants elicit female song in many species with rare female song. A possible mechanism for the hormonal control of female song is provided by the challenge hypothesis, which predicts a rise in testosterone in response to aggressive interactions during socially unstable situations. We tested this by comparing faecal testosterone levels in polygynandrous and monogamous female dunnocks. In groups with two to three females (polygynandry and polygyny) males provide less help at each nest than in groups with a single female (monogamy and polyandry). Polygynandrous and polygynous females are aggressive towards one another and attempt to expel rivals. Polygynandrous females had significantly higher testosterone levels than monogamous females. Competition between females that was induced by removal of males caused testosterone levels to rise. Further, female testosterone levels were correlated with the rate of 'tseep' calls, which are produced during aggressive encounters between females. Finally, polygynandrous and polygynous females sang significantly more than monogamous females. To the best of our knowledge, these results provide the first experimental support for the challenge hypothesis in female birds, and suggest that testosterone can regulate facultative female song in songbirds.     

Competition drives specialization in pollination systems through costs to male fitness

Specialization in pollination systems played a central role in angiosperm diversification, yet the evolution of specialization remains poorly understood. Competition through interspecific pollen transfer may select for specialization through costs to male fitness (pollen lost to heterospecific flowers) or female fitness (heterospecific pollen deposited on stigmas). Previous theoretical treatments of pollination focused solely on seed set, thus overlooking male fitness. Here we use individual-based models that explicitly track pollen fates to explore how competition affects the evolution of specialization. Results show that plants specialize on different pollinators when visit rates are high enough to remove most pollen from anthers; this increases male fitness by minimizing pollen loss to foreign flowers. At low visitation, plants generalize, which minimizes pollen left undispersed in anthers. A model variant in which plants can also evolve differences in sex allocation (pollen/ovule production) produces similar patterns of specialization. At low visitation, plants generalize and allocate more to female function. At high visitation, plants specialize and allocate equally to both sexes (in line with sex-allocation theory). This study demonstrates that floral specialization can be driven by selection through male function alone and more generally highlights the importance of community context in the ecology and evolution of pollination systems.     

Attentiveness to eyes predicts generosity in a reputation-relevant context

Eyes play a vital role in human social interactions. In fact, some prior work indicates that simply the presence of eyes or eye-like stimuli increases people's prosocial behavior, arguably because the eyes serve as cues of being watched and thus elicit reputational concerns. The current study was designed to address two questions in this regard. First, we examined how salient the eyes are among the human features. Second, we asked whether individual differences in attentiveness to eyes (but not other human features such as ears or hands) are predictive of reputation-enhancing behavior. Using an eye-tracking paradigm, we found that participants looked longest to eyes compared to other human features. Critically, greater attentiveness to eyes correlated with greater generosity on a donation task, but only in a reputation-relevant context (i.e., when donations were public but not when they were anonymous). Attentiveness to other human features did not predict donation behavior. Eyes are thus an especially salient human feature, and attentiveness to eyes may signal individuals' concerns about their reputations.     

Artificial surveillance cues do not increase generosity: two meta-analyses

Many studies have seemingly demonstrated that anonymous individuals who are shown artificial cues of being watched behave as if they are being watched by real people. However, several studies have failed to replicate this surveillance cue effect. In light of these mixed results, we conducted two meta-analyses investigating the effect of artificial observation cues on generosity. Overall, our meta-analyses found no evidence to support the claim that artificial surveillance cues increase generosity, either by increasing how generous individuals are, or by increasing the probability that individuals will show any generosity at all. Therefore, surveillance cue effects should be interpreted cautiously.     

The long-term benefits of human generosity in indirect reciprocity

Among the theories that have been proposed to explain the evolution of altruism are direct reciprocity and indirect reciprocity. The idea of the latter is that helping someone or refusing to do so has an impact on one's reputation within a group. This reputation is constantly assessed and reassessed by others and is taken into account by them in future social interactions. Generosity in indirect reciprocity can evolve if and only if it eventually leads to a net benefit in the long term. Here, we show that this key assumption is met. We let 114 students play for money in an indirect and a subsequent direct reciprocity game. We found that although being generous, i.e., giving something of value to others, had the obvious short-term costs, it paid in the long run because it builds up a reputation that is rewarded by third parties (who thereby themselves increase their reputation). A reputation of being generous also provided an advantage in the subsequent direct reciprocity game, probably because it builds up trust that can lead to more stable cooperation.     

Competition for females on leks when male competitive abilities differ: empirical test of a model

Leks are mating arenas visited by females seeking copulationsand can be thought of as patches differing in female encounterrate. Recently, the ideal free distribution model of unequalcompetitors with interference has been applied to explain maledistributions between leks. This model predicts that the malesof highest competitive ability should be present on the lekswith the highest female encounter rates and should be most successful.I tested the predictions from the model with empirical datafrom the ruff, Philomachus pugnax. Contrary to the predictionsfrom the model, low-ranking males preferentially visited theleks with highest female encounter rates, where the degree ofmale aggression was greatest Furthermore, there was no generalrelationship between female encounter rate and male success,and the empirical data again refute the predictions from themodel. The results illustrate the problem of using male per-capitasuccess when predicting individual behavior. Several more generalproblems with applying the ideal free distribution of unequalcompetitors model to competition for mates are also discussed.     

Competition for land

A key challenge for humanity is how a future global population of 9 billion can all be fed healthily and sustainably. Here, we review how competition for land is influenced by other drivers and pressures, examine land-use change over the past 20 years and consider future changes over the next 40 years.Competition for land, in itself, is not a driver affecting food and farming in the future, but is an emergent property of other drivers and pressures. Modelling studies suggest that future policy decisions in the agriculture, forestry, energy and conservation sectors could have profound effects, with different demands for land to supply multiple ecosystem services usually intensifying competition for land in the future.In addition to policies addressing agriculture and food production, further policies addressing the primary drivers of competition for land (population growth, dietary preference, protected areas, forest policy) could have significant impacts in reducing competition for land. Technologies for increasing per-area productivity of agricultural land will also be necessary. Key uncertainties in our projections of competition for land in the future relate predominantly to uncertainties in the drivers and pressures within the scenarios, in the models and data used in the projections and in the policy interventions assumed to affect the drivers and pressures in the future.     

The economic basis of cooperation: tradeoffs between selfishness and generosity

The current study examined the economics of cooperation in controlled-payoffgames by using captive blue jays, Cyanocitta cristata. Thisinvestigation used a special feeding apparatus to test for thestability of cooperative choice in a series of iterated games.The jays experienced experimentally determined game theoreticalpayoff matrices, which determined the distribution of food tothemselves and their opponent, depending on their decision tocooperate or defect. The experiment tested four game matrices,called the cooperate only, defect only, prisoner's dilemma,and opponent control treatments. This study found little cooperationin the defect only and prisoner's dilemma treatments. Cooperationoccurred significantly more often in the opponent control treatment.These findings suggest that the jays attend to short-term consequences;they do not cooperate in the absence of an immediate benefit(defect only), even if a long-term benefit may exist (prisoner'sdilemma). The opponent control treatment suggests that cooperationcan occur when an individual's benefits depend completely onthe actions of others; therefore, generosity is cheap. Thisstudy, therefore, agrees with recent studies in proposing alternativemodels of cooperation.     

The selfish nature of generosity: harassment and food sharing in primates

Animals may share food to gain immediate or delayed fitness benefits. Previous studies of sharing have concentrated on delayed benefits such as reciprocity, trade and punishment. This study tests an alternative model (the harassment or sharing-under-pressure hypothesis) in which a food owner immediately benefits because sharing avoids costly harassment from a beggar. I present an experiment that varies the potential ability of the beggar to harass, and of the owner to defend the food, to examine the effects of harassment on food sharing in two primate species: chimpanzees (Pan troglodytes) and squirrel monkeys (Saimiri boliviensis). For both species, high levels of harassment potential significantly increased both beggar harassment and sharing by the owner. Food defensibility did not affect harassment or sharing. Interestingly, squirrel monkeys and chimpanzees shared equally frequently with conspecifics despite a much higher natural sharing rate in chimpanzees. These results suggest that harassment can play a significant role in primate food sharing, providing a simple alternative to reciprocity. The selfish nature of harassment has implications for economic, psychological and evolutionary studies of cooperative systems.     

Reputation management: in autism, generosity is its own reward

A recent study has found that autistic people donate the same to charity regardless of whether they are observed. This is not because they are oblivious to others, but because they are free of hypocrisy.     

Competition for two resources

Summary Tadpoles of the tree frog, Hyla chrysoscelis, were raised at low and high levels of two independent, growth-limiting resources: solid food substrates and ingestible particles. The effect of resource level on intraspecific competition was measured as the relative importance of interference versus exploitative competitive mechanisms. The levels of both solid food substrates and particles affected the outcome of competition among the tadpoles, despite their independent effects on growth. Current models of competition generally allow only one resource to limit population growth. This result implies that more than one resource may influence the outcome of intraspecific competition.