Final Report Summary - SEXUAL SELECTION (Ecological and life-history bases of sexual selection and sex roles in birds)
The main aims of the project were to investigate fundamental but controversial aspects of sexual selection and sex role evolution using phylogenetic comparative methods and birds as model organisms. We made substantial progress toward achieving these objectives.
In a series of analyses we tested how mate availability, as estimated by adult sex ratio of the populations (ASR, proportion of males in all adults), influences various aspects of sex roles and mating behaviour. Recent theories suggest that skewed ASR may play an important role in sex role evolution. This is because members of the rarer sex have greater mating opportunities and thus should invest more in mating and less in parental activities then members of the more common sex. We tested this idea using a group with the maximal range of sex role variation, shorebirds, and showed that conventional sex roles (male mating competition, female care of offspring) occur in species with female-biased ASR whereas sex-role reversal (female mating competition, male care of offspring) is associated with male-biased ASR (Liker et al. 2013). This study provided the first evidence that skewed ASR facilitates sex-role reversal, one of the most enigmatic breeding systems in animals.
To investigate the relationship between sexual selection and ASR across a broader taxonomic coverage, we focused on 187 species from 59 avian families (Liker et al. submitted). This work showed that skewed ASR is related to both short-term and long-term pair-bonding in birds. Specifically, we found that divorce rate is higher in species with female-biased sex ratio, indicating that mate change by pair members and/or breaking of pair-bonds by unmated individuals is more frequent when females outnumber males. Furthermore, infidelity (or extra-pair paternity) increases with male-biased ASR in socially monogamous birds, suggesting that male coercion and/or female willingness to cheat the partner are facilitated by male-biased ASR.
Finally, we used comparative data to investigate which mechanisms can generate skewed ASRs in wild populations. These analyses revealed that interspecific variation in ASR is predicted by sex differences in adult mortality rates, whereas it is unrelated to initial sex ratio disparities, i.e. to hatching and fledging sex ratios (Szekely et al. in prep).
In the second line of comparative studies, we focused on the evolutionary causes and consequences of parental care are division between sexes. Using detailed data on male participation in different components of care, we showed that care components evolve in a correlated fashion: one group of correlated components includes nest building, incubation and brooding, whereas an other group includes offspring feeding and defence (Szekely et al. 2013).
More recently we investigated general hypotheses explaining the evolution of parental sex roles, including the controversial anisogamy hypothesis, which proposes that initial sex differences in the investment in gametes (eggs versus sperms) influence post-gametic investment. Following this rationale, one would expect predominant female parental care in animals given that the female invest more into production of the offspring than do males. Contrary to this theory, anisogamy was consistently unrelated to the share of the sexes in care (Liker et al. in prep). In addition, we also found that relative male contribution to care decreases with increasing variation in male reproductive success and increasing frequency of extra-pair paternity, and increases (at least in some components of care) with male-biased ASR. Finally, using theoretical modelling, we showed that the specialisation of the sexes to provide different components of care (i.e. sex role specialization) can stabilise parental cooperation, i.e. promote the evolution of biparental care even in the presence of strong sexual selection (Barta et al. submitted).
These results are significant because they provide the first demonstration that social environment as represented by ASR is strongly associated with several fundamental aspects of sex roles. We believe that due to lack of data and empirical support, the importance of ASR has been downplayed or neglected in earlier studies. We expect that our results will stimulate further ASR research, which seems essential for a better understanding of sexual selection and sex role evolution. Second, our project provides the most comprehensive analyses to date of the current general evolutionary hypotheses of parental care division between the sexes, and support the conclusions of recent theoretical models that sexual selection and social environment (ASR), as opposed to anisogamy, are major forces influencing parental sex role evolution.
In a series of analyses we tested how mate availability, as estimated by adult sex ratio of the populations (ASR, proportion of males in all adults), influences various aspects of sex roles and mating behaviour. Recent theories suggest that skewed ASR may play an important role in sex role evolution. This is because members of the rarer sex have greater mating opportunities and thus should invest more in mating and less in parental activities then members of the more common sex. We tested this idea using a group with the maximal range of sex role variation, shorebirds, and showed that conventional sex roles (male mating competition, female care of offspring) occur in species with female-biased ASR whereas sex-role reversal (female mating competition, male care of offspring) is associated with male-biased ASR (Liker et al. 2013). This study provided the first evidence that skewed ASR facilitates sex-role reversal, one of the most enigmatic breeding systems in animals.
To investigate the relationship between sexual selection and ASR across a broader taxonomic coverage, we focused on 187 species from 59 avian families (Liker et al. submitted). This work showed that skewed ASR is related to both short-term and long-term pair-bonding in birds. Specifically, we found that divorce rate is higher in species with female-biased sex ratio, indicating that mate change by pair members and/or breaking of pair-bonds by unmated individuals is more frequent when females outnumber males. Furthermore, infidelity (or extra-pair paternity) increases with male-biased ASR in socially monogamous birds, suggesting that male coercion and/or female willingness to cheat the partner are facilitated by male-biased ASR.
Finally, we used comparative data to investigate which mechanisms can generate skewed ASRs in wild populations. These analyses revealed that interspecific variation in ASR is predicted by sex differences in adult mortality rates, whereas it is unrelated to initial sex ratio disparities, i.e. to hatching and fledging sex ratios (Szekely et al. in prep).
In the second line of comparative studies, we focused on the evolutionary causes and consequences of parental care are division between sexes. Using detailed data on male participation in different components of care, we showed that care components evolve in a correlated fashion: one group of correlated components includes nest building, incubation and brooding, whereas an other group includes offspring feeding and defence (Szekely et al. 2013).
More recently we investigated general hypotheses explaining the evolution of parental sex roles, including the controversial anisogamy hypothesis, which proposes that initial sex differences in the investment in gametes (eggs versus sperms) influence post-gametic investment. Following this rationale, one would expect predominant female parental care in animals given that the female invest more into production of the offspring than do males. Contrary to this theory, anisogamy was consistently unrelated to the share of the sexes in care (Liker et al. in prep). In addition, we also found that relative male contribution to care decreases with increasing variation in male reproductive success and increasing frequency of extra-pair paternity, and increases (at least in some components of care) with male-biased ASR. Finally, using theoretical modelling, we showed that the specialisation of the sexes to provide different components of care (i.e. sex role specialization) can stabilise parental cooperation, i.e. promote the evolution of biparental care even in the presence of strong sexual selection (Barta et al. submitted).
These results are significant because they provide the first demonstration that social environment as represented by ASR is strongly associated with several fundamental aspects of sex roles. We believe that due to lack of data and empirical support, the importance of ASR has been downplayed or neglected in earlier studies. We expect that our results will stimulate further ASR research, which seems essential for a better understanding of sexual selection and sex role evolution. Second, our project provides the most comprehensive analyses to date of the current general evolutionary hypotheses of parental care division between the sexes, and support the conclusions of recent theoretical models that sexual selection and social environment (ASR), as opposed to anisogamy, are major forces influencing parental sex role evolution.