Female and male Texas cichlids (Herichthys cyanoguttatum) do not fight by the same rules

Selection usually acts differently on males and females duringintrasexual competition for resources and/or mates. Nevertheless,agonistic behavior has been examined both theoretically andempirically mostly in males. Our research questions whethermales and females follow the same rules of engagement in intrasexualcontests as predicted by the sequential assessment model (SAM).The SAM predicts negative correlations between contest intensityand duration and the magnitude of asymmetry in resource holdingpower (RHP) between the contestants, such that the most escalatedcontests are those between similarly endowed individuals. Westaged male and female intrasexual contests with varying degreesof body size asymmetry under a round robin design using themonogamous Texas cichlid fish (Herichthys cyanoguttatum) asa study case. We used Mantel's matrix analysis to compare howthe behavioral content, duration, structure, and outcome ofmale and female contests were affected by the relative bodysize of the contestants. In the case of males, relative sizein each contest predicted outcome, duration, and frequency ofconventional and escalated behaviors according to prevailingtheory. Female contest structure and outcome, however, werenot predicted by the relative size of contestants. We discussour results in terms of other asymmetries that might be importantin structuring female contests, and we propose potential approachesto study female–female aggression.     

Social stimulation and the resumption of copulation in Rhesus (Macaca mulatto) and stumptail (Macaca arctoides) Macaques

Copulatory data derived from observations of social groups of rhesus and stumptail macaques were analyzed to test the hypothesis that pairs of animals would resume copulation significantly sooner if a second male copulated with the female shortly after the first male’s ejaculation. Data from both groups supported the hypothesis. These results, extending previous studies in Macaca nemestrina,suggest that the shortening of copulatory intervals by social stimuli occurs in several species, both in social groups and in experimentally created triads. These findings also are consistent with the hypothesis that socially mediated resumption of mating is related to intrasexual competition among males.     

Darwin's vexing contrivance: a new hypothesis for why some flowers have two kinds of anther

Heteranthery, the presence of two or more anther types in the same flower, is taxonomically widespread among bee-pollinated angiosperms, yet has puzzled botanists since Darwin. We test two competing hypotheses for its evolution: the long-standing ‘division of labour'' hypothesis, which posits that some anthers are specialized as food rewards for bees whereas others are specialized for surreptitious pollination, and our new hypothesis that heteranthery is a way to gradually release pollen that maximizes pollen delivery. We examine the evolution of heteranthery and associated traits across the genus Clarkia (Onagraceae) and study plant–pollinator interactions in two heterantherous Clarkia species. Across species, heteranthery is associated with bee pollination, delayed dehiscence and colour crypsis of one anther whorl, and movement of that anther whorl upon dehiscence. Our mechanistic studies in heterantherous species show that bees notice, forage on and export pollen from each anther whorl when it is dehiscing, and that heteranthery promotes pollen export. We find no support for division of labour, but multifarious evidence that heteranthery is a mechanism for gradual pollen presentation that probably evolved through indirect male–male competition for siring success.     

Divergent sexual selection via male competition: ecology is key

Sexual selection and ecological differences are important drivers of speciation. Much research has focused on female choice, yet the role of male competition in ecological speciation has been understudied. Here, we test how mating habitats impact sexual selection and speciation through male competition. Using limnetic and benthic species of threespine stickleback fish, we find that different mating habitats select differently on male traits through male competition. In mixed habitat with both vegetated and open areas, selection favours two trait combinations of male body size and nuptial colour: large with little colour and small with lots of colour. This matches what we see in reproductively isolated stickleback species, suggesting male competition could promote trait divergence and reproductive isolation. In contrast, when only open habitat exists, selection favours one trait combination, large with lots of colour, which would hinder trait divergence and reproductive isolation. Other behavioural mechanisms in male competition that might promote divergence, such as avoiding aggression with heterospecifics, are insufficient to maintain separate species. This work highlights the importance of mating habitats in male competition for both sexual selection and speciation.     

Sexual selection and its evolutionary consequences in female animals

For sexual selection to act on a given sex, there must exist variation in the reproductive success of that sex as a result of differential access to mates or fertilisations. The mechanisms and consequences of sexual selection acting on male animals are well documented, but research on sexual selection acting on females has only recently received attention. Controversy still exists over whether sexual selection acts on females in the traditional sense, and over whether to modify the existing definition of sexual selection (to include resource competition) or to invoke alternative mechanisms (usually social selection) to explain selection acting on females in connection with reproduction. However, substantial evidence exists of females bearing characters or exhibiting behaviours that result in differential reproductive success that are analogous to those attributed to sexual selection in males. Here we summarise the literature and provide substantial evidence of female intrasexual competition for access to mates, female intersexual signalling to potential mates, and postcopulatory mechanisms such as competition between eggs for access to sperm and cryptic male allocation. Our review makes clear that sexual selection acts on females and males in similar ways but sometimes to differing extents: the ceiling for the elaboration of costly traits may be lower in females than in males. We predict that current and future research on female sexual selection will provide increasing support for the parsimony and utility of the existing definition of sexual selection.     

The relationship between plumage coloration and aggression in female tree swallows

Intrasexual competition is an important selective force that can favor the evolution of honest signals of fighting ability. Research has focused predominantly on male birds, but many female birds also possess plumage ornaments that could mediate the outcome of competitive interactions. We examined the relationship between blue and white structural coloration and aggression in female tree swallows Tachycineta bicolor. Tree swallows are secondary cavity nesters and females show delayed plumage maturation which may be related to intense competition for nest sites. We compared plumage reflectance of second‐year (SY) and after‐second year (ASY) females, and within‐individual changes in plumage reflectance of ASY females in two successive years. We assessed aggression by placing a caged SY female 2 m from the nest box of an ASY female and quantified the ASY female's response in relation to her own plumage coloration. We found substantial differences in plumage reflectance of SY and ASY females, but found that ASY females became greener with duller white coloration in the second year. This may be due to poor weather that made reproduction particularly costly in the first year of the study and suggests coloration is influenced by condition. We found no relationship between dorsal coloration and aggressiveness. Rather, brighter white females spent more time on their nest box and within 2 m of the intruder than females with dull breasts. Our findings suggest that brighter white ASY females may perceive a SY female as less of a threat or that there may be a trade‐off associated with aggression and parental care that is related to white brightness.     

Evolutionarily stable stealing: game theory applied to kleptoparasitism

We present an individual-based model of a group of foraginganimals. Individuals can obtain food either by discovering itthemselves or by stealing it from others (kleptoparasitism).Given that challenging another individual for a discovered fooditem costs time (which could otherwise be spent searching foran undiscovered item), attempting to steal from another maynot always be efficient We show that there is generally a uniquestrategy that maximizes uptake rate—always or never challengingothers. For any combination of parameter values, we can identifywhich strategy is appropraite. As a corollary to this, we predictthat small changes in ecolgical conditions can, under some circumstances,cause a dramatic change in the aggressive behavior of individuals.Further, we investigate situations where searching for undiscoveredfood and searching for potential opportunities for stealingare mutually exclusive activities (i.e., success at one canonly be improved at the expense of the other). Using game theory,we are able to find the evolutionarily stable strategy for investmentin these two activities in terms of the ecological parametersof the model.     

Lethal combat and sex ratio evolution in a parasitoid wasp

Sex allocation theory provides excellent opportunities for testinghow behavior and life histories are adjusted in response toenvironmental variation. One of the most successful areas fromthis respect is Hamilton's local mate competition theory. Aspredicted by theory, a large number of animal species have beenshown to adjust their offspring sex ratios (proportion male)conditionally, laying less female-biased sex ratios as the numberof females that lay eggs on a patch increases. However, recentstudies have shown that this predicted pattern is not followedby 2 parasitoid species in the genus Melittobia, which alwaysproduce extremely female-biased sex ratios. A possible explanationfor this is that males fight fatally and that males producedby the first female to lay eggs on a patch have a competitiveadvantage over later emerging males. This scenario would negatethe advantage of later females producing a less female-biasedsex ratio. Here we examine fatal fighting and sex ratio evolutionin another species, Melittobia acasta. We show that femalesof this species also fail to adjust their offspring sex ratioin response to the number of females laying eggs on a patch.We then show that although earlier emerging males do have anadvantage in winning fights, this advantage 1) can be reducedby an interaction with body size, with larger males more likelyto win fights and 2) only holds for a brief period around thetime at which the younger males emerge from their pupae. Thissuggests that lethal male combat cannot fully explain the lackof sex ratio shift observed in Melittobia species. We discussalternative explanations.