Flower chafers are well-known beetles with apparent sexual dimorphism. Yet, in contrast to other groups of scarab beetles, only little attention has been paid to various aspects of this phenomenon. It concerns also sexual dimorphism in size, which is very common in animals. Although ultimate causes of sexual size dimorphism have been extensively studied, the developmental mechanisms are still only poorly understood. We investigated proximate causes of sexual size dimorphism in several differentially dimorphic flower chafer species. We found that in highly dimorphic species the dimorphism started to develop already in the first instar and tended to accumulate through successive larval development. In contrast, the sexes in species with a relatively low dimorphism diverged in size during only one instar. Moreover, we found variability not only in when the dimorphism arises during development, but also how: sex-related differences in both the instar duration and average growth rate were the proximate causes of sexual size dimorphism. In addition, we showed that the sexes may differ also in growth trajectory - males had relatively longer period of rapid growth than females, even in cases where the absolute development times were similar in both sexes. Further, we focused on systematic survey of sexual...

The epaulets of male Red-winged Blackbirds (Agelaius phoeniceus) are frequently cited as a sexually selected plumage ornament, but a number of laboratory and field studies provide little evidence that they are currently experiencing sexual selection. We used hair dye to dull epaulets of free-living territorial males prior to pair formation to determine if manipulated males experienced disadvantages in comparison with control males. We found no differences between control males and males with dulled epaulets in territorial behavior (territory size, song rate, trespass rate, and loss of territory), paternal care (time spent on territory and in antipredator sentinel behavior, and response to a model crow to simulate the threat of predation), pairing success (number of social mates), apparent reproductive success (numbers of nesting attempts, eggs/nest, nestlings/egg, and fledglings/nestling), or realized reproductive success (numbers of within-pair, extra-pair, and total fledglings as determined by DNA fingerprinting). We then used a meta-analysis of 11 published studies of Red-winged Blackbirds to determine if there is an overall effect of epaulet color or size on male-male competition, female choice, or reproductive success. Our results show that epaulet size has a small positive effect on male reproductive success, but epaulet color has no effect on male-male competition, female choice, and male reproductive success. One explanation for the seeming contradiction between studies that show that epaulets are necessary for territory defense and those that conclude that epaulets are not currently under selection is that epaulets serve as one of several cues of species recognition, especially among males at close range. An alternative explanation proposes counter-balancing intersexual advantages and intrasexual disadvantages of epaulet expression. Additional studies are needed to test these alternatives.

Individuals in social species commonly form dominance relationships among each other, and are often observed to differ in behaviour depending on their social status. However, whether such behavioural differences are a consequence of dominance position, or also a cause to it, remains unclear. In this thesis I therefore investigated two perspectives of the relationship between social dominance and personality in the domestic fowl (Gallus gallus domesticus), a social species that forms relatively stable dominance hierarchies. In paper I I investigated the influence of social status on the expression and consistency of behaviours by experimentally changing status between repeated personality assays. The level of vigilance, activity and exploration changed with social status, while boldness and territorial crows appeared as stable individual properties, independent of status. These results showed that social status contribute to both variation and consistency in behavioural responses. Social status should therefore be taken into account when investigating and interpreting variation in personality. In paper II I showed that behaviour in a novel arena test and during encounter with an opponent can predict social status, more specifically that fast exploration and aggressiveness predicted a dominant social position. Together, these results highlight the dynamics of the two-way relationship between social position and individual behaviour and indicate that individual behaviour can both be a cause and a consequence of social status.

Les nombreuses différences qui existent entre les organismes illustrent que le processus de variation est un phénomène universel en biologie. Ces variations sont particulièrement observables chez les espèces sexuées, entre mâles et femelles. Comprendre les différents facteurs biologiques, environnementaux et génétiques, à l'origine de ce dimorphisme sexuel est le cœur de mon sujet de thèse. Pour cela, j’ai établi un nouveau modèle d'étude, l’insecte semi-aquatique Microvelia longipes. Ces insectes ont évolué un dimorphisme sexuel spectaculaire où les mâles présentent une croissance extrême et hypervariable spécifiquement au niveau de la troisième paire de pattes. Pour étudier ce phénomène, nous avons, en premier lieu, émis l’hypothèse que cette croissance exagérée était associée à des pressions de sélection sexuelle. Nous avons mis en évidence la présence de compétition intense entre males, qui utilisent leurs pattes arrière comme arme, pour s’accoupler avec les femelles. Les males à pattes plus longues gagnent souvent dans ces combats, expliquant l’importance adaptative de ces pattes exagérées chez les mâles. De plus, nous montrons que l’intensité que mettent les mâles à se battre est associée aux variations de taille de pattes chez les mâles, de la même espèce ou d’espèces différentes. Nous avons également développé un génome et une approche transcriptomique comparant les sexes et les pattes afin d’identifier les gènes responsables de cette croissance exagérée. Ceci a permis de dresser une liste de gènes dont l’expression corrèle avec l’exagération de la croissance des pattes chez les mâles et d’identifier des régions génomiques associées à la sélection sexuelle. / From the DNA molecule to the more complex phenotypes, variation is a universal process in life and living organisms. The innumerable differences that exist between species are probably one of the most manifest examples. Yet, all this diversity would never have occurred in nature without some pre-existing divergence within species. One of the most striking examples of intraspecies variation appears in sexual organisms, between males and females. Understanding the environmental and genetic factors influencing sexual divergence is a longstanding question in evolutionary biology. To this end, I focus here on a new insect model system, Microvelia longipes, which has the particularity to have evolved an extreme case of sexual dimorphism in the rear legs. Males display exaggerated long rear legs compared to females but also an extreme variability in these leg lengths from one male to another. We identified that M. longipes males use their exaggerated legs as weapons during male-male competition. Males with longer legs have more chance to access females on egg-laying sites and therefore increase their reproductive success. Moreover, fitness assays and comparative studies between Microvelia species revealed that the intensity of male competition was associated with the exaggeration and hypervariability of the rear legs in M. longipes males. In a second approach, we studied the developmental and genomic basis of this sexual dimorphism through a comparative transcriptomic analysis and identified genes and genomic regions associated with male exaggerated legs and ultimately with sexual selection. Overall, the integrative approach used in this work allows to establish Microvelia longipes as a promising new model system to study the influence of sexual selection in adaptive evolution.

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An optimal wing shape is necessary for the survival of winged insects since it enables different survival strategies and is expected to vary between environments due to different selection pressures. However, few studies have explored wing shape variation across the whole range of a species.  In this study, geographic variation in wing shape in male Lestes sponsa was examined from 14 localities along a latitudinal gradient in Europe. The wings were analyzed using a comparative geometric morphometric approach, where the different shapes were digitized, statistically analyzed, and visualized on thin spline deformation grids. The results showed significant differences in wing shape in both the fore- and hindwings between locations. Wings at some localities showed a slender appearance with a narrow tip, and wings from other localities showed a broader appearance with a convex tip. A significant trend in wing shape was found from the central populations to the northern one. However, no continuous trend in wing shape was found across the latitudinal range suggesting that local abiotic or biotic factors might drive the difference observed.

Vertebrate males frequently carry higher numbers of parasites than females. This bias in parasite loads could be a consequence of sexual selection. Grouping species are also assumed to be afflicted with larger numbers of parasites than solitary animals and associated costs of this parasitism could vary with group size or structure. I examined sex-biased parasitism and the influence of group size on parasite loads in Cape ground squirrels (Xerus inauris), a highly social species that occurs in the arid regions of southern Africa. Males carried three times as many ectoparasites as females, but females harbored nearly three times more endoparasites than males. Amount of time spent (per hour) autogrooming was similar between males and females, but amount time spent allogrooming by adult female was over eleven times that of adult males. Longer allogrooming of group members could be decreasing the numbers of ectoparasites of group members and ultimately the group. Males infrequently give or receive allogrooming and travel in very large home ranges, potentially increasing their exposure to ectoparasites. However, movement throughout a large home range may result in males foraging in areas with lower densities of fecal pellets, which could explain the lower endoparasite loads observed in males. When I considered the age class of group members, female age classes were similarly parasitized but male age classes were not. Sub-adult males carried similar ectoparasite loads to adult males and similar endoparasite loads to adult females. This result is of particular interest because sub-adult males are becoming scrotal but typically remain in the group until adulthood. Sexual selection does appear to influence parasite loads in this species, and parasite removal or avoidance potentially mitigates individual parasite loads and their associated costs. Parasites can be detrimental to the health, longevity, and reproduction of their hosts, but these costs are rarely quantified. I removed ectoparasites and endoparasites from Cape ground squirrels for three months and evaluated changes in female body mass, reproduction, burrow use, and grooming in response to parasite removal. Female body mass did not increase with parasite removal, but reproductive success (per capita offspring raised to emergence) increased nearly four-fold, while allogrooming by treated females decreased. Since breeding is highest in the late winter dry season when fewer resources are available, the impact of parasites may be highest during this season. Lactation and gestation are the most physiological stressful processes that females undergo, and the dramatic increase in reproductive success in treated females suggests that these females are able to allocate more resources to reproduction than females afflicted with parasites. These results suggest that studies investigating reproduction and fecundity must consider the vulnerability of the host to parasite infection and the potential impact on reproductive success.

Aspects of the behavioral ecology of bird song learning are examined in three parts. First, an approach from image analysis is extended to allow rapid, quantitative description of animal sounds. In this approach, sounds are summarized as sets of time-frequency-amplitude landmarks. Second, the role of dawn song in tree swallow (Tachycineta bicolor) breeding biology is examined. Song syllable sharing among tree swallows was found to be high among birds nesting at the same site, but sharing was lower between birds nesting at different sites. When birds nested at different sites, the distance between those sites was not related to the amount of difference between the birds' syllable repertoire compositions. All tree swallow song repertoires did not remain constant during the breeding season; some individuals added new syllable types, others modified existing types. Singing performance was correlated with reproductive success in tree swallows: males that sang more precise repetitions of their syllable types attracted more extra-pair mates. Furthermore, pairwise comparisons between the social and genetic fathers of extra-pair young found that the genetic fathers averaged higher syllable consistency than the cuckolded males. Third, a comparative study of the phylogenetic distribution of vocal mimicry examined the evolutionary history of song learning in oscine passerines. Vocal mimicry, defined as the habitual incorporation of heterospecific sounds into song displays, was found in twenty-eight separate clades of oscines. These clades were found in every major oscine superfamily, but made up a higher proportion of daughter groups within the most ancient superfamilies of oscines. The most plesiomorphic lineages of oscines were found to contain many highly-skilled mimics. These observations support the hypothesis that the course of song learning in oscines has run repeatedly from permissive learning rules that permit mimicry to restrictive learning rules that limit mimicry.

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