Mate choice in the brown thornbill (Acanthiza pusilla): are settlement decisions, divorce and extrapair mating complementary strategies?

In socially monogamous birds, females may express mate preferences when they first select a breeding partner, through divorce and subsequent breeding dispersal to a new partner and through extrapair mating. We examined settlement patterns, divorce and breeding dispersal in a sedentary Australian passerine, the brown thornbill (Acanthiza pusilla), in relation to two traits known to influence extrapair paternity (male age and male size). Settlement decisions, divorce and territory switching behaviour were all female strategies that reduced their likelihood of breeding with 1-year-old males. Females preferred to settle in territories with 2+ -year-old males, were more likely to divorce 1-year-old males, and only switched territories if they had an opportunity to form a new pair bond with an old male. In contrast, female settlement and divorce decisions were not influenced by male size. Female thornbills obtain a direct benefit from preferring older males as social mates because breeding success improves with male age in brown thornbills. Nevertheless, divorce rates in this species were low (14% of pair bonds were terminated by divorce), and individuals rarely switched territories following the death of a mate. Both of these mating strategies appeared to be primarily constrained by the distance adults moved to initiate a new pair bond (1-2 territories) and by the limited availability of unpaired older males in the immediate neighbourhood.

Sexual selection for male dominance reduces opportunities for female mate choice in the European bitterling (Rhodeus sericeus)

Sexual selection involves two main mechanisms: intrasexual competition for mates and intersexual mate choice. We experimentally separated intrasexual (male-male interference competition) and intersexual (female choice) components of sexual selection in a freshwater fish, the European bitterling (Rhodeus sericeus). We compared the roles of multiple morphological and behavioural traits in male success in both components of sexual competition, and their relation to male reproductive success, measured as paternity of offspring. Body size was important for both female choice and male-male competition, though females also preferred males that courted more vigorously. However, dominant males often monopolized females regardless of female preference. Subordinate males were not excluded from reproduction and sired some offspring, possibly through sneaked ejaculations. Male dominance and a greater intensity of carotenoid-based red colouration in their iris were the best predictors of male reproductive success. The extent of red iris colouration and parasite load did not have significant effects on female choice, male dominance or male reproductive success. No effect of parasite load on the expression of red eye colouration was detected, though this may have been due to low parasite prevalence in males overall. In conclusion, we showed that even though larger body size was favoured in both intersexual and intrasexual selection, male-male interference competition reduced opportunities for female choice. Females, despite being choosy, had limited control over the paternity of their offspring. Our study highlights the need for reliable measures of male reproductive success in studies of sexual selection.

Dissecting the complex genetic basis of mate choice

The genetic analysis of mate choice is fraught with difficulties. Males produce complex signals and displays that can consist of a combination of acoustic, visual, chemical and behavioural phenotypes. Furthermore, female preferences for these male traits are notoriously difficult to quantify. During mate choice, genes not only affect the phenotypes of the individual they are in, but can influence the expression of traits in other individuals. How can genetic analyses be conducted to encompass this complexity? Tighter integration of classical quantitative genetic approaches with modern genomic technologies promises to advance our understanding of the complex genetic basis of mate choice.

Male reproductive tactics and female choice in the solitary, promiscuous bridled nailtail wallaby (Onychogalea fraenata)

Several behavioral studies of large, gregarious, and sexually dimorphic macropods have shown that males form dominance hierarchies and large males have the highest reproductive success. The bridled nailtail wallaby (Onychogalea fraenata) is a smaller and strongly sexually dimorphic macropod, but is also highly solitary and males do not form dominance hierarchies that are maintained temporally or spatially. Genetic studies of paternity have shown that large males are the most reproductively successful and only one-quarter of males sire offspring at any one time. The aim of this study was to investigate the tactics that males adopt to secure access to females at the time of estrus and to investigate whether females can influence which males have access to them. This study was conducted using 2 wild, free-ranging populations of bridled nailtail wallabies. Females in estrus were located and observed. and the total number of males present, the relative weight rank of each mate, and interactions between individuals were recorded. Females showed a preference for large males and incited male-male competition when the group of males present was large. Unlike other dimorphic macropods, fights among males were rare and were restricted to males of similar size. Large males gained access to females by guarding and following them closely and threatening other males who attempted to gain access. Smaller males spent less time with females, suggesting that small males may leave multimale groups in an attempt to locate unguarded females. Given the solitary nature of this species and the lack of a stable dominance hierarchy to influence male reproductive success. mate searching and mate guarding may be important male reproductive tactics in this species.

Communication in coral reef fish: the role of ultraviolet colour patterns in damselfish territorial behaviour

Many coral reef fish possess ultraviolet (UV) colour patterns. The behavioural significance of these patterns is poorly understood and experiments on this issue have not been reported for free-living reef fish in their natural environment. The damselfish Pomacentrus amboinensis has UV facial patterns, and spectroradiometric ocular media measurements show that it has the potential for UV vision. To test the potential behavioural significance of the UV patterns, I studied the response of males, in natural territories on the reef and in aquaria, to two conspecific intruders, one presented in a UV-transmitting (UV+) container and the other in a UV-absorbing (UV-) one. Territory owners attacked intruders viewed through UV+ filters significantly more often and for longer than intruders viewed through the UV- filter. In general, the results of the field experiment confirmed those of the laboratory experiment. The results support the hypothesis that P. amboinensis males are sensitive to UV light and that reflectance patterns, which appear in high contrast only in UV, modulate the level of aggressive behaviour. A recent survey showed that many predatory fish may not have UV vision and the use of UV colours in select species of reef fish may therefore serve as a 'private communication channel'. (C) 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Contrasting mutual sexual selection on homologous signal traits in Drosophila serrata

The nature of male mating preferences, and how they differ from female mating preferences in species with conventional sex roles, has received little attention in sexual selection studies. We estimated the form and strength of sexual selection as a consequence of male and female mating preferences in a laboratory-based population of Drosophila serrata. The differences between sexual selection on male and female signal traits (cuticular hydrocarbons [CHCs]) were evaluated within a formal framework of linear and nonlinear selection gradients. Females tended to exert linear sexual selection on male CHCs, whereas males preferred intermediate female CHC phenotypes leading to convex (stabilizing) selection gradients. Possible mechanisms determining the nonlinear nature of sexual selection on female CHCs are proposed.

Parallel evolution of sexual isolation in sticklebacks

Mechanisms of speciation are not well understood, despite decades of study. Recent work has focused on how natural and sexual selection cause sexual isolation. Here, we investigate the roles of divergent natural and sexual selection in the evolution of sexual isolation between sympatric species of threespine sticklebacks. We test the importance of morphological and behavioral traits in conferring sexual isolation and examine to what extent these traits have diverged in parallel between multiple, independently evolved species pairs. We use the patterns of evolution in ecological and mating traits to infer the likely nature of selection on sexual isolation. Strong parallel evolution implicates ecologically based divergent natural and/or sexual selection, whereas arbitrary directionality implicates nonecological sexual selection or drift. In multiple pairs we find that sexual isolation arises in the same way: assortative mating on body size and asymmetric isolation due to male nuptial color. Body size and color have diverged in a strongly parallel manner, similar to ecological traits. The data implicate ecologically based divergent natural and sexual selection as engines of speciation in this group.

Reinforcement drives rapid allopatric speciation

Allopatric speciation results from geographic isolation between populations. In the absence of gene flow, reproductive isolation arises gradually and incidentally as a result of mutation, genetic drift and the indirect effects of natural selection driving local adaptation(1-3). In contrast, speciation by reinforcement is driven directly by natural selection against maladaptive hybridization(1,4). This gives individuals that choose the traits of their own lineage greater fitness, potentially leading to rapid speciation between the lineages(1,4). Reinforcing natural selection on a population of one of the lineages in a mosaic contact zone could also result in divergence of the population from the allopatric range of its own lineage outside the zone(4-6). Here we test this with molecular data, experimental crosses, field measurements and mate choice experiments in a mosaic contact zone between two lineages of a rainforest frog. We show that reinforcing natural selection has resulted in significant premating isolation of a population in the contact zone not only from the other lineage but also, incidentally, from the closely related main range of its own lineage. Thus we show the potential for reinforcement to drive rapid allopatric speciation.

Can non-directional male mating preferences facilitate honest female ornamentation?

Recent studies have demonstrated male mate choice for female ornaments in species without sex-role reversal. Despite these empirical findings, little is known about the adaptive dynamics of female signalling, in particular the evolution of male mating preferences. The evolution of traits that signal mate quality is more complex in females than in males because females usually provide the bulk of resources for the developing offspring. Here, we investigate the evolution of male mating preferences using a mathematical model which: (i) specifically accounts for the fact that females must trade-off resources invested in ornaments with reproduction; and (ii) allows male mating preferences to evolve a non-directional shape. The optimal adaptive strategy for males is to develop stabilizing mating preferences for female display traits to avoid females that either invests too many or too few resources in ornamentation. However, the evolutionary stability of this prediction is dependent upon the level of error made by females when allocating resources to either signal or fecundity.

Effect of thermal acclimation on female resistance to forced matings in the eastern mosquitofish

All copulations in the eastern mosquitofish, Gambusia holbrooki, are coercive-and-achieved by force. Female G. holbrooki never appear to cooperate with males, but vigorously resist matings at all times. We examined the role of females within a sexually coercive mating system by investigating the ability of female G. holbrooki to resist forced copulations after acclimation to 16 degrees C and 32 degrees C for 4-5 weeks. We also examined burst swimming performance of female G. holbrooki after acclimation, as this trait is likely to underlie a female's ability to resist forced matings. We predicted that if female G. holbrooki indiscriminately resist matings from all males, acclimation would enhance female resistance at their acclimation temperature. However, we found that it did not. We also predicted that if females are able to influence the outcome of mating interactions, acclimation to an optimal thermal environment may induce females to reduce resistance. In support of this prediction, females acclimated at 32 degrees C were able to modify their resistance behaviour between exposure to 16 degrees C and 32 degrees C. The rate of copulations experienced by 32 inverted perpendicular C-acclimated females was 2.5 times greater at 32 degrees C than at 16 degrees C. In addition, acclimation at 32 degrees C significantly enhanced burst swimming performance at 32 degrees C but no effect of acclimation was observed at 16 degrees C. Our results suggest that female G. holbrooki are able to play a greater role in determining the outcome of sexual coercive mating interactions than previously thought. (c) 2006 The Association for the Shidy of Animal Behavioor. Published by Elsevier Ltd. All rights reserved.

Restricted mating dispersal and strong breeding group structure in a mid-sized marsupial mammal (Petrogale penicillata)

Ecological genetic studies have demonstrated that spatial patterns of mating dispersal, the dispersal of gametes through mating behaviour, can facilitate inbreeding avoidance and strongly influence the structure of populations, particularly in highly philopatric species. Elements of breeding group dynamics, such as strong structuring and sex-biased dispersal among groups, can also minimize inbreeding and positively influence levels of genetic diversity within populations. Rock-wallabies are highly philopatric mid-sized mammals whose strong dependence on rocky terrain has resulted in series of discreet, small colonies in the landscape. Populations show no signs of inbreeding and maintain high levels of genetic diversity despite strong patterns of limited gene flow within and among colonies. We used this species to investigate the importance of mating dispersal and breeding group structure to inbreeding avoidance within a 'small' population. We examined the spatial patterns of mating dispersal, the extent of kinship within breeding groups, and the degree of relatedness among brush-tailed rock-wallaby breeding pairs within a colony in southeast Queensland. Parentage data revealed remarkably restricted mating dispersal and strong breeding group structuring for a mid-sized mammal. Breeding groups showed significant levels of female kinship with evidence of male dispersal among groups. We found no evidence for inbreeding avoidance through mate choice; however, anecdotal data suggest the importance of life history traits to inbreeding avoidance between first-degree relatives. We suggest that the restricted pattern of mating dispersal and strong breeding group structuring facilitates inbreeding avoidance within colonies. These results provide insight into the population structure and maintenance of genetic diversity within colonies of the threatened brush-tailed rock-wallaby.

The roles of natural and sexual selection during adaptation to a novel environment

The net effect of sexual selection on nonsexual fitness is controversial. On one side, elaborate display traits and preferences for them can be costly, reducing the nonsexual fitness of individuals possessing them, as well as their offspring, In contrast, sexual selection may reinforce nonsexual fitness if an individual's attractiveness and quality are genetically correlated. According to recent models, such good-genes mate choice should increase both the extent and rate of adaptation. We evolved 12 replicate populations of Drosophila serrata in a powerful two-way factorial experimental design to test the separate and combined contributions of natural and sexual selection to adaptation to a novel larval food resource. Populations evolving in the presence of natural selection had significantly higher mean nonsexual fitness when measured over three generations (13-15) during the course of experimental evolution (16-23% increase). The effect of natural selection was even more substantial when measured in a standardized, monogamous mating environment at the end of the experiment (generation 16; 52% increase). In contrast, and despite strong sexual selection on display traits, there was no evidence from any of the four replicate fitness measures that sexual selection promoted adaptation. In addition, a comparison of fitness measures conducted under different mating environments demonstrated a significant direct cost of sexual selection to females, likely arising from some form of male-induced harm. Indirect benefits of sexual selection in promoting adaptation to this novel resource environment therefore appear to be absent in this species, despite prior evidence suggesting the operation of good-genes mate choice in their ancestral environment. How novel environments affect the operation of good-genes mate choice is a fundamental question for future sexual selection research.