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In species where males do not provide immediate benefits to females in the form of resources or parental care to offspring, adaptive mate choice may be maintained by selection because preferred males sire offspring that are genetically superior. I test the hypothesis that female mating preferences result in enhanced performance of their larvae, owing to genetic variation among sires in two species of red-eyed treefrogs. This variation in males may be manifested as body size or in advertisement call parameters. I examined natural pairing patterns and found that both species display a size-based mating pattern with substantial seasonal effects. In lean years, females mate with males that are larger on average, yet females also seem to favor a constant size ratio of the male relative to her body size. I documented significant call trait variation both within and among males and have evidence to support size-related call traits as indicators of mating success for some properties. Finally, I provide evidence for a pronounced effect of sire size on offspring traits, most notably upon hatching that may have significant post-metamorphic fitness benefits. My study provides data on the basis of female choice and may provide a link between female preference for male traits and enhanced offspring performance by larvae sired by preferred males in the evolution of anuran mating systems.

Some male sexually selected traits are sensitive to stressors early in life and provide females with information to discriminate among males with different developmental experiences. Moreover, female early life experiences could also impact which males they choose. Females might either choose honest traits indicative of male quality, no matter their own experiences, or they might choose mates to match or compensate for their own experiences. To determine how developmental stressors alter male sexually-selected traits and female preference thereof, I exposed zebra finches (Taeniopygia guttata, ZEFI) to i) lipopolysaccharide (LPS), an immunogenic, Gram-negative bacterial component, ii) corticosterone (CORT), an avian stress steroid, iii) both challenges (CORT/LPS), or iv) none of the above (control vehicles). Finches were exposed during development (12-28 days post-hatch) and male traits (e.g., body size, bill and cheek coloration) and female behaviors (e.g., general activity, male sampling effort, and male preference) were then measured in adulthood. Control males were predicted to express the most elaborate traits followed by LPS, CORT, and then CORT/LPS males. If female preference was generally driven by male quality, control females were predicted to be most selective followed by LPS, CORT, and CORT/LPS females. Alternatively, if female choice was contingent on her own experience, females might choose males with similar (i.e, matching) or distinct (i.e, complementarity) developmental histories. Of the male characteristics measured, only cheek coloration was impacted by treatment early in life; CORT/LPS males had duller, less orange cheeks than controls. For females, overall activity was reduced in CORT/LPS females. More importantly in regards to mate choice, females exhibited a blend of matching and complimentary behavior; females not exposed to LPS or CORT preferred males also not exposed to LPS or CORT. In general, females avoided LPS males no matter their own experience. Altogether, this study suggests that female mate preference is quite sensitive to early-life experiences and driven by a mix of choice of outright male quality and relative complementarity.

Sexual selection drives the evolution of male morphology, life history, physiology, and behaviour across taxa. Here I examine the mechanisms of sexual selection that arise at various stages in mating interactions to identify congruence or conflict between the traits selected by choice and competition. I first examine plasticity of developing male Australian redback spiders (Latrodectus hasselti) and show that male metabolic rates vary adaptively to facilitate the scramble to reach virgins. Next, I show that females cease sex pheromone production after mating and re-advertise receptivity later in their reproductive season effectively creating two windows in which males may compete. I show that females discriminate against males that do not meet a threshold courtship duration suggesting that courtship is the trait selected through choice. However, male-male competition leads to reductions in courtship effort provided to females. During the first window paternity is split equally if rival males mate in quick succession with a virgin female. However, if the second mating is delayed, there is a strong bias in the paternity of the second male. A delay in the second mating is beneficial to females as it reduces longevity costs of polyandry. However, delays in the initial mating decrease female longevity, likely because of elevated metabolic rates of virgins. My research shows that the trait favoured by female choice is in opposition to selection via male-male competition. Females’ sex pheromone production yields windows during which mating will optimize female, but not male, fitness. Studies that isolate the mechanisms of sexual selection are valuable in that they can identify the traits under selection. However, my research shows that considering these processes in isolation can lead to incorrect inferences about the net effect of sexual selection.

Sexual selection drives the evolution of male morphology, life history, physiology, and behaviour across taxa. Here I examine the mechanisms of sexual selection that arise at various stages in mating interactions to identify congruence or conflict between the traits selected by choice and competition. I first examine plasticity of developing male Australian redback spiders (Latrodectus hasselti) and show that male metabolic rates vary adaptively to facilitate the scramble to reach virgins. Next, I show that females cease sex pheromone production after mating and re-advertise receptivity later in their reproductive season effectively creating two windows in which males may compete. I show that females discriminate against males that do not meet a threshold courtship duration suggesting that courtship is the trait selected through choice. However, male-male competition leads to reductions in courtship effort provided to females. During the first window paternity is split equally if rival males mate in quick succession with a virgin female. However, if the second mating is delayed, there is a strong bias in the paternity of the second male. A delay in the second mating is beneficial to females as it reduces longevity costs of polyandry. However, delays in the initial mating decrease female longevity, likely because of elevated metabolic rates of virgins. My research shows that the trait favoured by female choice is in opposition to selection via male-male competition. Females’ sex pheromone production yields windows during which mating will optimize female, but not male, fitness. Studies that isolate the mechanisms of sexual selection are valuable in that they can identify the traits under selection. However, my research shows that considering these processes in isolation can lead to incorrect inferences about the net effect of sexual selection.

Evolutionary psychology explains and predicts human behaviour based on its adaptive value. Some apparently non-adaptive behaviours such as humans’ devotion to music can be explained by sexual selection of costly signals, since it takes time and effort to learn to play an instrument well. Here, participants rated pictures of persons of the opposite sex that were said to play a piece of music that was heard while watching each picture. The music performances were either of low, medium, or high level of skill, and a better performance was predicted to lead to higher ratings of partner attractiveness because it is more costly. No effect of the music was found, except that women rated men as less desirable for a long-term relationships when the skill level was high than when it was medium. / Evolutionspsykologiska teorier förklarar och predicerar mänskligt beteende utifrån dess adaptiva värde. Vissa uppenbart icke-adaptiva beteenden, som människans hängivenhet till musik, kan förklaras som sexuell selektion av kostsamma signaler, eftersom det kräver tid och möda att lära sig bemästra ett instrument. Deltagare fick skatta bilder på personer av motsatt kön som påstods spela det musikstycke som hördes medan man tittade på varje bild. Musikutförandet var antingen av låg, medel, eller hög skicklighet, och ett bättre utförande förväntades ge högre skattningar av partnerattraktivitet eftersom det är mer kostsamt. Ingen effekt av musiken förelåg, förutom att kvinnorna skattade män som mindre attraktiva för ett längre förhållande när skicklighetsnivån var hög jämfört med när den var medelhög.

Marine and freshwater environments support numerous species of teleost fish with a wide and diverse range of reproductive strategies. Despite the considerable interest in fish reproduction, our knowledge regarding ejaculate traits and factors affecting them is limited. Using computer-assisted sperm analysis (CASA) I measured ejaculate traits (sperm swimming speed, motility, path trajectory, longevity and concentration) from sexually mature chinook salmon (Oncorhynchus tshawytscha) activated in freshwater and ovarian fluid. I also looked at these ejaculate traits in relation to measures of male quality (body condition) and investment into reproduction (relative testes mass). Furthermore, I determined the chemical composition of seminal and ovarian fluid and looked at the effect these fluids have on sperm behaviour. A considerable amount of intraspecific variation existed in all ejaculate traits measured, and investment into reproduction (relative testes mass) was dependent on male body condition, as males in better condition had relatively larger testes. However, these males did not have superior quality ejaculates or ejaculates with a higher density of spermatozoa; hence the potential reproductive advantage of having larger relative testes in this species remains unknown and requires further investigation. In addition, a positive relation between sperm longevity and sperm swimming speed was observed defying the expected trade-off between ejaculate traits according to theory. There was also a weak negative trend in our data between body condition and sperm swimming speed, linearity, and longevity. All sperm traits measured were greatly enhanced when activated in a solution containing ovarian fluid (a viscous fluid which is excreted with the egg batch during spawning) from female chinook salmon. Interestingly, sperm swimming speed activated in fresh water only accounted for < 12% of the observed variation in mean sperm swimming speed in ovarian fluid. This result suggests the sperm traits measured in fresh water are not relevant to those same traits measured in ovarian fluid, so caution should be applied when comparing the potential for individual males to fertilize ova when sperm traits are activated in water, especially in studies of sperm competition in an externally fertilising species. Sperm competition between males is known to strongly influence sperm and ejaculate traits, but less is known about female sperm choice after copulation via a process called cryptic female choice (CFC). In CFC, females may have the ability to favour the sperm of one male over another and bias fertilisation accordingly. To test whether ovarian fluid could act as a mechanism of CFC in an externally fertilising fish species, I measured sperm traits from each male activated in the ovarian fluids from different females. I found that mean sperm swimming speed, longevity, and path trajectory differed significantly among males, but most importantly, the pattern of within-male variation in these traits also varied significantly among males in response to different females’ ovarian fluids. This result suggests that ovarian fluid may be a potential mechanism of CFC whereby females differentially enhance the swimming speed of sperm from different males. In addition, I found that sperm longevity was negatively correlated with variation in [Ca²⁺] and [Mg²⁺] concentration in the ovarian fluid, while percent motility increased with increasing concentration of [Mg²⁺]. These observations provide a possible chemical basis for cryptic female choice whereby female ovarian fluid differentially influences the behaviour of sperm from different males and thus their fertilisation success. This finding is particularly exciting, as we may have uncovered a potential mechanism of CFC in an externally fertilising species, which is poorly understood. In addition, results from this study suggest new directions for genetic studies to provide direct evidence for CFC. For example, does sperm selection via ovarian fluid promote favoured genetic combinations that enhance male reproductive success?

Sexual reproduction entails complex co-evolution between the sexes, necessary for successful fertilization, ensuring individual and population-level fitness. Interfertility is the main criterion for species definition and understanding speciation requires detailed studies of reproductive barriers. However, many studies on reproductive barriers are constrained to infer evolutionary processes from patterns. In this thesis, I focus on a hybrid zone between collared and pied flycatchers (Ficedula albicollis and hypoleuca) on the island of Öland, and a trait that is essential for fertilization: sperm. Long-term monitoring of these species, combined with recent advances in molecular tools, allow me to study how complex on-going intersexual and interspecific interactions influence reproductive isolation in this young hybrid zone. I start by exploring the links between pre- and postmating sexual selection within collared flycatchers (paper I and II). I show that secondary sexual characters and indirect mate-choice benefits are tightly linked to physiology (paper I), and that a male’s attractiveness and dominance status dictate which sperm traits are optimal, as a male’s fertilization success depends on an interaction between sperm and display traits (paper II). I then report a source of strong postzygotic isolation between recently diverged collared and pied flycatchers: impaired spermatogenesis resulting in absence of mature sperm cells in hybrid males (paper III). I show however that pied flycatcher females, who are most exposed to hybridization, can mitigate these costs through mechanisms of cryptic female choice impairing heterospecific sperm performance, allowing them to bias paternity towards pure-species offspring (paper IV). Finally, by exploring the testes transcriptomes and sperm proteomes of both species, I highlight the importance of gene and protein regulation mechanisms in facilitating phenotypic divergence between these species (paper V). Thus, my thesis reveals complex interactions between primary and secondary sexual characters in a wild bird and suggests that mechanisms of sexual selection are tightly linked to essential physiological functions. I also show that genetic incompatibilities can evolve rapidly despite low genome-wide levels of divergence but that divergence in regulatory regions and proteins potentially allows fast evolution of molecular mechanisms impairing or preventing costly heterospecific fertilization.

In birds, song can evolve quickly through cultural transmission and due to errors in the learning process may result in regional dialects. A lack of dialectal recognition may be a critical component of reproductive isolation through female mate preference. I investigated the role of cultural divergence in reproductive isolation in a widespread Neotropical passerine the rufous-collared sparrow (Zonotrichia capensis). In Chapter II, I investigated, the role of female preference for and male territorial response towards, local and non-local dialects in two allopatric populations. Females in both populations preferred their local dialect to the dialect of an allopatric population only 25 km away. In contrast, males showed similar territorial response to all conspecific dialects. Premating reproductive isolation based on culture may exist among the study populations. In chapter III, I investigated if cultural divergence can drive population divergence by examining dialects and variable microsatellite loci among eight populations of Z. capensis. I investigated the presence of population divergence and then identified the mechanism that may be driving the pattern. Apart from culture, a geographic barrier (the Andean ridge), elevation, and geographic distance were potential mechanisms of population divergence that I considered. All sites exhibited localized dialects, except for two closest neighboring populations. Populations exhibited genetic differentiation with support for the presence of five genetic clusters. Culture does not appear to be driving population divergence because song dialects and genetic population structure were not correlated. Populations separated by an Andean ridge did not display higher genetic or song differences than distance would predict, suggesting that the ridge is not driving population divergence. Elevation was not correlated to song or genetic differentiation. Both song divergence and microsatellite allele frequency differentiation were correlated with geographic distance suggesting a pattern of isolation by distance. Overall, geographic distance is the best predictor of population divergence in this system. Cumulatively, I found that culture might promote assortative mating via female mate choice, perhaps generating partial reproductive isolation; however, song dialect differences among contiguous populations is not currently driving population divergence in this species. / Ph. D.

Due to an extreme asymmetry in parental investment, raising the potential for sexual conflict, orangutans provide unique insights into the role of sexual conflict in male and female reproductive strategies. Sexual selection acts on reproductive strategies, selecting for strategies that increase an individual's reproductive success. Sexual conflict results when the reproductive strategies of one sex impose costs on the other. Three forms of sexual selection have been documented in orangutans—male-male competition, female choice, and sexual coercion. This dissertation asks (1) how males employ reproductive strategies to increase their likelihood of reproductive success in the face of multi-male mating by females and (2) how females employ reproductive strategies to enact female choice and increase infant survival in the face of male-male competition and sexual coercion. I investigate these two questions using genetic paternity, long-term association data, and a year of behavioral data from orangutans (Pongo pygmaeus wurmbii) in Gunung Palung National Park, West Kalimantan, Indonesia. Brown-modified Hinde Indices, initiation and termination of associations, and following data reveal that female participation is necessary to sustain longer male-female associations and females use facultative associations to employ mate choice. Genetic paternity determination found low reproductive skew, with flanged males siring more offspring than unflanged males. Investigation of female infanticide avoidance strategies showed that females with dependent offspring under age six associate with males less often than other categories of females do, and mothers decrease the distance with their offspring during associations with males. These results highlight the importance of both female choice and male-male competition in reproductive outcomes, and illustrate how facultative associations play a key role in the reproductive strategies of both male and female orangutans. Both sexes’ strategies are studied, demonstrating the dynamic co-evolution of strategies and counterstrategies. I also consider the implications of these findings for the evolution of male bimaturism. Examining orangutan mating and reproductive behaviors deepens our understanding of how sexual conflict and sexual selection have shaped their unusual mating system and broadens our understanding of sexual conflict in reproductive strategies in a species with an extreme disparity between maternal and paternal investment. / 2025-08-31T00:00:00Z