Ainda que existam diversos registros de formas extremas de dimorfismo sexual, a função do alongado quarto par de pernas nos machos de muitas espécies de opiliões permanece desconhecida. Neste estudo, testamos quatro previsões sobre a hipótese de que a seleção sexual está direcionando o alongamento do quarto par de pernas nos machos de Longiperna zonata e Promitobates ornatus: (1) o coeficiente alométrico do comprimento do fêmur IV em machos será maior do que o das fêmeas; (2) machos com pernas IV mais compridas vão residir próximos a grupos de fêmeas; (3) machos envolvidos em interações agressivas terão fêmures mais compridos do que o tamanho médio da população; (4) a diferença de tamanho de perna entre os oponentes será menor do que a diferença esperada entre pares de machos escolhidos ao acaso na população. Como o previsto, o coeficiente alométrico do fêmur IV foi significativamente maior em machos do que em fêmeas nas duas espécies. O comprimento do fêmur IV está positivamente relacionado à chance de um macho ser encontrado próximo a um grupo de fêmeas. Finalmente, a média do comprimento do fêmur IV dos machos envolvidos em brigas foi significativamente maior que a de machos selecionados ao acaso da população. Adicionalmente, a média da diferença de tamanho entre pares de machos que brigaram foi significativamente menor que a média da diferença de tamanho de perna entre pares de machos selecionados ao acaso na população. Assim, a seleção sexual na forma de competição entre machos parece estar direcionando o dimorfismo sexual extremo no comprimento do fêmur nessas duas espécies de opiliões. / Although there are many reports of extreme forms of sexual dimorphism in harvestmen, the function of the elongated fourth pair of legs in males of many species remains unknown. We tested four predictions to address the hypothesis that sexual selection is driving the enlargement of the fourth pair of legs in males of Longiperna zonata and Promitobates ornatus: (1) the allometric coefficient of femur IV length in males will be higher than in females; (2) males with longer legs IV will reside closer to groups of females; (3) males involved in aggressive interactions will have longer femur IV than the population average size; (4) the size difference between contenders will be smaller than the difference expected between randomly chosen pairs of males in the population. As predicted, the allometric coefficient of femur IV length was significantly higher in males than in females of both species. Femur IV length was positively related to the chance of a male being found close to a group of females. Finally, the mean femur IV length of the males involved in fights was significantly larger than the mean of random samples of males from the population, and the average difference between male contenders was smaller than the average random expected difference of the population. Therefore, sexual selection in the form of male-male competition seems to be driving the extreme sexual dimorphism in femur length in these species.

A critical mechanism underpinning current biological diversity is the extent to which one species mates with, or avoids mating with, another. However, little is known about the factors that mediate hybridisation, especially during the initial and rarely observed stages of secondary contact when interspecific interactions have not responded to selection. In particular, whilst hybridisation is ultimately a behavioural phenomenon, the role of behaviour in mediating hybridisation and how it is influenced by environmental and circumstantial factors is rarely investigated. Recently introduced species provide us with unequalled opportunities to study these factors. In this thesis I examine the role of behavioural mechanisms, in particular male-male competition and mate choice, in mediating mating patterns between two genetically and phenotypically distinct lineages of European wall lizard (Podarcis muralis) that have come into recent secondary contact through human introductions. In Chapter Two, I investigated how sexual selection during allopatry is responsible for creating stark differences in phenotypic traits such as body size and weapon performance evident in the two lineages today, ultimately explaining the strong biases in dominance during territorial disputes between males. However, I also show that even given this asymmetry in male competitive ability, the extent to which it extrapolates into greater access to females in naturalistic, outdoor enclosures depends strongly on the spatial clustering of basking sites, a critically important resource for many ectotherms. In contrast to initial predictions suggested by asymmetries in male competition outlined in the previous chapter, in Chapter Three I show that both paternity and courtship behaviour was strongly assortative in the outdoor enclosures. Further investigation through staged experiments on olfactory mate choice, mating trials and analyses on specific behavioural data obtained in an enclosure experiment, I show that lineage based dominance actually contributes to assortative mating patterns in conjunction with weak conspecific male choice. In contrast, female choice seems to play no role in mediating the mating patterns observed between the two lineages. In Chapter Four I had the rare opportunity to examine the morphological and behavioural factors that predict why animals should hybridise in the first place, using the data obtained in the enclosure experiment above. I found that hybridisation was particularly common between small individuals of the larger lineage and large individuals of the smaller lineage; a result that corroborates the mechanisms determining the assortative patterns uncovered in Chapter Three. Additionally, hybridisation rates were particularly high in less dominant individuals, which I suggest is due to subordinate males having reduced opportunities for courting conspecific females due to male-male competition, requiring them to become less ‘choosy’ and therefore more likely to mate with heterospecifics. Finally, secondary contact cannot occur without at least one lineage coming into a new environment, and yet relatively little attention is paid to how this environmental change can affect the signals involved in intraspecific communication and mate choice. In Chapter Five I show that a change in the amount of time male lizards spend thermoregulating (a likely consequence of arriving in a new environment) significantly changes the chemical composition of their scent marks. However, whilst female lizards were able to detect these effects, they did not seem to base their mating decisions on them. Nevertheless, this result raises interesting questions about the potential function and consequences of this plasticity, and highlights the importance of considering plasticity in chemical communication in heterogeneous environments. Overall, this thesis shows the critically important role of behaviour in mediating intra- and interspecific mating patterns during recent secondary contact. In particular, it highlights how the direction and extent of hybridisation and competition are influenced by the degree to which differing morphological and behavioural phenotypes interact over a heterogeneous environment, particularly during the initial stage of secondary contact when mate choice has not had the chance to respond to the selective pressures of hybridisation.

The availability of reproductive sites is a major factor shaping the behavior of males and females in species with resource-based mating systems. Using a meta-analytic approach, we tested five predictions directly or indirectly derived from the mating system theory. We expected that reproductive site limitation would lead to: (1) intense male-male competition for resource possession; (2) high variance in male reproductive success, generating high values of opportunity for sexual selection; (3) high intensity of selection on male traits related to resource possession; (4) high sperm competition risk; and (5) low quality of paternal care. We compiled information from observational and experimental studies that compared the reproductive behavior of individuals of the same species under low and high reproductive site limitation. We found that, when reproductive sites are limited, there is a slight increase in male-male competition, with higher rates of nest takeover and agonistic interactions, and a slight increase in the selection gradient on male traits, with successful males tending to be larger than unsuccessful males. Reproductive site limitation has no consistent effect on the opportunity for sexual selection and on the sperm competition risk. However, territorial males invest more in gonads and lose less paternity when reproductive sites are limited. There is also no clear effect of reproductive site limitation on the quality of paternal care, but few studies have addressed this subject. Taken together, our findings indicate that predictions on how reproductive site limitation affects several aspects of resource-based mating systems have weak empirical support. These predictions do not consider the plasticity in the mating tactics of males and females, which make them too simplistic. Moreover, since the original proposition of the mating system theory, our understanding of sperm competition increased a lot. We now know that accurate predictions on the intensity and direction of sexual selection should take into account both pre- and post-copulatory processes. Finally, the interplay between sexual selection and parental care is complex, and the original framework of mating system theory does not provide sufficient elements to derive clear and taxonomically broad predictions / A disponibilidade de sítios reprodutivos influencia tanto o comportamento de machos quanto de fêmeas em espécies cujo sistema de acasalamento depende da defesa de recursos. Usando uma abordagem meta-analítica, testamos cinco previsões direta ou indiretamente relacionadas à teoria de sistemas de acasalamento. Esperávamos que a limitação de sítios reprodutivos promoveria: (1) aumento na competição masculina pela posse de recursos; (2) aumento na variância do sucesso reprodutivo dos machos, gerando valores alto de oportunidade para a seleção sexual; (3) aumento da intensidade da seleção sobre características masculinas relacionadas à posse de recursos; (4) aumento no risco de competição espermática e (5) diminuição da qualidade do cuidado paternal. Compilamos informações de estudos observacionais e experimentais que compararam o comportamento reprodutivo de indivíduos da mesma espécie em situação de alta e baixa disponibilidade de sítios reprodutivos. Encontramos que, quando os sítios reprodutivos são escassos, há um ligeiro aumento na competição masculina, com maiores taxas de roubo de ninhos e interações agonísticas, e um ligeiro aumento no gradiente de seleção sobre características masculinas, com machos bem sucedidos tendendo a ser maiores do que machos mal sucedidos em monopolizar recursos. A disponibilidade de sítios reprodutivos não teve nenhum efeito consistente sobre a oportunidade para a seleção sexual e o risco de competição espermática. Entretanto, machos territoriais investiram mais em gônadas e perderam menos paternidade quando os sítios reprodutivos eram escassos. Não encontramos nenhum efeito da disponibilidade de sítios reprodutivos sobre a qualidade do cuidado paternal. Em conjunto, nossos resultados indicam que as previsões sobre como a disponibilidade de sítios reprodutivos influencia diversos aspectos de sistemas de acasalamento baseados na defesa de recursos têm fraco respaldo empírico. Tais previsões não consideram a plasticidade nas táticas de acasalamento de machos e fêmeas, tornando-as demasiadamente simplistas. Adicionalmente, desde a formulação original da teoria de sistemas de acasalamento, nossa compreensão sobre competição espermática aumentou. Sabemos atualmente que previsões acuradas sobre a intensidade e direção da seleção sexual devem levar em consideração processos pré- e pós-copulatórios. Finalmente, a interconexão entre seleção sexual e cuidado parental é complexa e a teoria original de sistemas de acasalamento não provê elementos suficientes para a construção de previsões claras e de amplo escopo taxonômico

Dans un contexte de sélection sexuelle, les systèmes de communication permettant l’attraction et la stimulation du partenaire sexuel et la compétition entre individus du même sexe sont indispensables. Ceci est particulièrement vrai chez les espèces à système d’appariement polygyne de type lek, où les mâles rassemblés dans l’espace sont en compétition pour l’accès aux femelles. Chez notre sujet d’étude, l’outarde houbara nord-africaine les males réalisent des parades incluant des démonstrations visuelles et des vocalisations appelées booms, sur des sites espacés les uns des autres par des distances importantes dans un système qualifié de lek explosé. Notre objectif était d’étudier les systèmes de codage-décodage des informations exprimées durant la parade des mâles chez cette espèce. Nous avons démontré l’existence d’un codage de l’identité individuelle porté par des paramètres visuels et des paramètres acoustiques des vocalisations. Des associations significatives entre des paramètres des booms et des caractéristiques physiques et comportementales des mâles susceptibles de refléter leur qualité, ont également été mises en évidence. Ainsi, les mâles qui produisent les booms les plus graves avaient les masses les plus importantes et paradaient le plus intensément. Lorsque des interactions agonistiques étaient simulées expérimentalement en diffusant des leurres acoustiques, les mâles avaient des réponses comportementales différentes en fonction de la fréquence des booms diffusés. Ainsi le niveau fréquentiel des booms des autres mâles est effectivement perçu et décodé par les individus en compétition. Par ailleurs, plusieurs paramètres des booms sont génétiquement déterminés et héritables, et pourraient porter une information sur l’apparentement entre individus.Certains paramètres sont également liés à la consanguinité des individus. Nous avons montré également que les booms sont particulièrement bien adaptés à la communication à grande distance. En effet, les booms sont de très basse fréquence, se propagent à des distances supérieures à 640 m, et les paramètres supportant le codage-décodage de l’information sont résistants à la propagation et produits de manière redondante. Enfin, les signaux acoustiques et des signaux visuels et semblent agir en synergie (multimodalité) ce qui pourraient améliorer l’efficacité de la communication à grande distance. Ainsi notre étude a démontré que des informations complexes sont transmises durant la parade des mâles au travers de signaux acoustiques et visuels au sein d’un réseau de communication à grande distance. / Résumé en anglais ;In a context of sexual selection, the communication between individuals is essential to optimize both intra-sexual (competition between male) and inter-sexual (mate choice) processes. This is particularly true in species with lek mating systems, where both processes reach their full expression. The North African Houbara bustard, Chlamydotis undulata undulata, is a lekking bird where courting males aggregate in a so called ‘exploded-lek’, i.e. where aggregation is detectable only when mapped over a large area. During the breeding season, males perform a conspicuous and spectacular courtship including visual and acoustic (booms) signals. Our aim was to decipher the coding-decoding systems of the courtship in this species. We found that males can be individually discriminated on the basis of visual and acoustic parameters. The booms produced during the courtship are also related to males’ characteristics susceptible to reflect their quality. Individuals who produced the lowest frequency booms were those with the greatest weights and performed the highest intensity courtships. Playback experiment shows that the frequency content of the booms was perceived and decoded by males during agonistic interaction, leading to significantly different behavioural responses according to the frequency of the signals played back. Furthermore, several acoustic parameters of the booms are significantly heritable through genetic transmission and these parameters are also related to the level of inbreeding. Finally, we showed that the booms and the acoustic parameters supporting information are particularly well adapted to the transmission of information at long range as required in a long distance communication network constituted by an exploded lek. First, booms are of very low-frequency and propagate up to 640 m. Second, coding-decoding of information relies on sequentially redundant and propagation-resistant features. Third, acoustic signals are combined with visual signalling (multimodality) which may improve the long distance transmission of the information. This study brought evidence that complex information is conveyed by visual and acoustic signals during the courtship of males within a long distance network.

The availability of reproductive sites is a major factor shaping the behavior of males and females in species with resource-based mating systems. Using a meta-analytic approach, we tested five predictions directly or indirectly derived from the mating system theory. We expected that reproductive site limitation would lead to: (1) intense male-male competition for resource possession; (2) high variance in male reproductive success, generating high values of opportunity for sexual selection; (3) high intensity of selection on male traits related to resource possession; (4) high sperm competition risk; and (5) low quality of paternal care. We compiled information from observational and experimental studies that compared the reproductive behavior of individuals of the same species under low and high reproductive site limitation. We found that, when reproductive sites are limited, there is a slight increase in male-male competition, with higher rates of nest takeover and agonistic interactions, and a slight increase in the selection gradient on male traits, with successful males tending to be larger than unsuccessful males. Reproductive site limitation has no consistent effect on the opportunity for sexual selection and on the sperm competition risk. However, territorial males invest more in gonads and lose less paternity when reproductive sites are limited. There is also no clear effect of reproductive site limitation on the quality of paternal care, but few studies have addressed this subject. Taken together, our findings indicate that predictions on how reproductive site limitation affects several aspects of resource-based mating systems have weak empirical support. These predictions do not consider the plasticity in the mating tactics of males and females, which make them too simplistic. Moreover, since the original proposition of the mating system theory, our understanding of sperm competition increased a lot. We now know that accurate predictions on the intensity and direction of sexual selection should take into account both pre- and post-copulatory processes. Finally, the interplay between sexual selection and parental care is complex, and the original framework of mating system theory does not provide sufficient elements to derive clear and taxonomically broad predictions / A disponibilidade de sítios reprodutivos influencia tanto o comportamento de machos quanto de fêmeas em espécies cujo sistema de acasalamento depende da defesa de recursos. Usando uma abordagem meta-analítica, testamos cinco previsões direta ou indiretamente relacionadas à teoria de sistemas de acasalamento. Esperávamos que a limitação de sítios reprodutivos promoveria: (1) aumento na competição masculina pela posse de recursos; (2) aumento na variância do sucesso reprodutivo dos machos, gerando valores alto de oportunidade para a seleção sexual; (3) aumento da intensidade da seleção sobre características masculinas relacionadas à posse de recursos; (4) aumento no risco de competição espermática e (5) diminuição da qualidade do cuidado paternal. Compilamos informações de estudos observacionais e experimentais que compararam o comportamento reprodutivo de indivíduos da mesma espécie em situação de alta e baixa disponibilidade de sítios reprodutivos. Encontramos que, quando os sítios reprodutivos são escassos, há um ligeiro aumento na competição masculina, com maiores taxas de roubo de ninhos e interações agonísticas, e um ligeiro aumento no gradiente de seleção sobre características masculinas, com machos bem sucedidos tendendo a ser maiores do que machos mal sucedidos em monopolizar recursos. A disponibilidade de sítios reprodutivos não teve nenhum efeito consistente sobre a oportunidade para a seleção sexual e o risco de competição espermática. Entretanto, machos territoriais investiram mais em gônadas e perderam menos paternidade quando os sítios reprodutivos eram escassos. Não encontramos nenhum efeito da disponibilidade de sítios reprodutivos sobre a qualidade do cuidado paternal. Em conjunto, nossos resultados indicam que as previsões sobre como a disponibilidade de sítios reprodutivos influencia diversos aspectos de sistemas de acasalamento baseados na defesa de recursos têm fraco respaldo empírico. Tais previsões não consideram a plasticidade nas táticas de acasalamento de machos e fêmeas, tornando-as demasiadamente simplistas. Adicionalmente, desde a formulação original da teoria de sistemas de acasalamento, nossa compreensão sobre competição espermática aumentou. Sabemos atualmente que previsões acuradas sobre a intensidade e direção da seleção sexual devem levar em consideração processos pré- e pós-copulatórios. Finalmente, a interconexão entre seleção sexual e cuidado parental é complexa e a teoria original de sistemas de acasalamento não provê elementos suficientes para a construção de previsões claras e de amplo escopo taxonômico

Darwin was troubled by elaborate male traits observed in many species that are seemingly maladaptive for survival, the peacock’s tail being the most iconic of all. He wrote "The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick" because it challenged his theory of evolution by natural selection for adaptive traits. The extreme length of the tail may render a peacock more vulnerable to predation and therefore maladaptive for survival. To account for the evolution of apparently maladaptive traits he proposed the theory of sexual selection, wherein, traits that directly enhance mating success may be selected for, either as weapons in male-male competition for mates or as ornaments preferred by females. Male and female elephants in the proboscidean evolutionary radiation have had tusks and show extreme exaggeration in size and form. However, tusk in the Asian elephant (Elephas maximus) is sexually dimorphic as it is expressed only in the males, hinting at a possibility that opposing selection (sexual selection advantage to males and natural selection disadvantage to females) may have been the processes behind this pattern of tusk expression. Intriguingly, tuskless males (male dimorphism with respect to tusk) also occur at fairly high frequencies in some Asian elephant populations (∼50% in norteastern India and ∼95% in Sri Lanka). Theory states that dimorphic males can also occur in a population in stable frequencies as a consequence of sexual selection. I explored sexual selection on elephant tusks as possible mechanism leading to the observed patterns of tusk dimorphism in the elephants. All elephant populations on earth have been harvested for ivory, therefore, artificial selection (selective poaching of tusked elephants for ivory) is another possible cause of tusk dimorphism. I developed mathematical models of population genetics, population dynamics and conducted field observations of mating behavior of Asian elephant in Kaziranga National Park, Assam to understand the evolution of tusk dimorphism in elephants. Darwin’s sexual selection theory was controversial when proposed in 1871 and continues to remain so in 2014. In the introduction of my thesis I have discussed Darwin’s two classical mechanisms of sexual selection, namely, male-male combats for mates and female mate choice based on male traits. The latter was viewed with considerable skepticism by his con-temporary Alfred Russell Wallace and more recently deemed "fundamentally flawed" by Joan Roughgarden. Therefore, I have also discussed the arguments against female mate choice for male traits found in literature. I have reviewed current knowledge about sexual selection for sexually dimorphic male traits of body size and musth, in the African and Asian elephant and state why I have hypothesized that tusks may also be under sexual selection. Sexually selected traits are expected to be genetically determined, therefore, I explored mathematically (Chapter 1) the genetic basis of evolution of sexual dimorphism. Fisher proposed that sexually selected male display traits originate in both the sexes but are suppressed in the females by modifier genes, when the trait becomes deleterious to females. Thus, sexually antagonistic selection on a trait and sex-specific gene expression can lead to the evolution of sexual dimorphism. Tusk is sexually monomorphic in the probocideans that are ancestral to both the African (Loxodonta africana) and Asian elephant (Elephas maximus). Tusk continues to remain monomorphic in the African elephant but has become sexually dimorphic in the Asian elephant. Tusk, therefore could be a sexually selected male trait that evolved according to the Fisherian model. Intriguingly, tuskless males occur at very high frequencies in some Asian elephant populations. The tusked and tuskless male morphs could be alternate male mating strategies, occurring at evolutionarily stable frequencies. Alternatively, the observed male tusk dimorphism, could be a consequence of artificial selection against tusked individuals, due to selective harvest of tusked males. Furthermore, male African elephants are more intensely poached for ivory than female elephants. Yet the frequency of tuskless individuals has increased more rapidly among females than in males. In essence, sexual dimorphism could be evolving among such poached populations. Is such rapid, contemporary evolution of sexual dimorphism, possible through the Fisherian modifier gene mechanism? A 2-loci genetic model (with X-linked trait gene and an autosomal modifier gene) (Rice 1984), a slight variant of the model (with X-linked modifier gene, and an autosomal trait gene) and an entirely autosomal model, were analyzed for the rate of evolution of sexual dimorphism, under different selection pressures for tusk possession. Negative frequency dependent selection was introduced into the model of tusk evolution in accordance with Gadgil’s model for the evolution of male dimorphism as consequence of sexual selection (Gadgil 1972). In two of the 2-loci models (in which tusk gene in autosomal), tusklessness evolved much more rapidly in females than in males, under equal negative selection pressures. The models predict several combinations of time-lines and negative selection pressures for effecting a particular change in the frequency of tusklessness. Model predictions were com-pared with observed changes in the frequency of female tusklessness, in one South Ugandan, African elephant population (∼2% to 10% in 5 to 9 generations) and male tusklessness (∼5% to 50% in 25 to 40 generations) in one north eastern Indian, Asian elephant population. The models predict strong selection pressures of 30% to 50% reduction in fitness, that can effect an 8% increase in tusklessness, in the African elephant population, within time-lines of 9 to 5 generations (∼225 to 125 years) respectively. For the male Asian elephants, natural selection against tusked males on an already sexually dimorphic population, must have been in operation and shifted the population to 5% male tusklessness. The models predict that artificial selection with 20 to 30% fitness cost to tusked males, operating for 40 to 25 generations (∼1000 to 600 years) respectively, can further shift the population from ∼5% to ∼50% tusklessness. Asian elephant populations may already have been in a transient phase of evolution, tending towards tusklessness, with recent artificial selection hastening the process. The two major pre-dictions from this modeling exercise are (1) artificial selection could have played a significant role in the evolution of male tusk dimorphism in the Asian elephant (2) a lack of or very mild current sexual selection on tusks in the male Asian elephant. Both these predictions may be empirically verified. Chapters 2 and 3 are attempts at empirical verification of prediction (1) and Chapters 4 and 5 of prediction (2). From historical references to elephant harvest in Assam, we do know that artificial selection has been in operation, but whether it has played a major role in causing male tusk dimorphism needs to be established. It may be possible to detect signatures of significant past harvest from current demographic structure of an elephant population. Sustained biased harvesting of a particular sex and or age class from an animal population alters the sex ratio and age structure (relative proportion of individuals in each age and sex class) of a population considerably (Sukumar 1989). It may be possible to back infer the harvest scenario by studying the deviation of current age and sex ratios from natural age and sex ratios. In Chapter 2, I explored models of population dynamics under different harvest regimes and its effect on age and sex ratios. I described a method to infer unknown harvest rates and numbers from age and sex ratios, namely, adult female to male ratio, male old-adult to young-adult ratio, and proportion of adult males in the population using Jensen’s(2000) 2-sex, density-dependent Leslie matrix model. The specific combination of male and female harvest rates and numbers can be deter-mined from the history of harvest and an estimate of population size. I validated this model with published data on age and sex ratios of one Asian and African elephant population with fairly reliable data on elephant harvest as well. In Chapter 3, I applied this model to the demographic data that I collected from a wild Asian elephant population in Kaziranga National Park, Assam, India (where more than 50% of the adult males are tuskless). Male polymorphism of sexually-selected male traits occur at stable frequencies in populations of several species. The different male morphs of the trait are hypothesized to be alternate male mating strategies with equal life time reproductive fitness. Male Asian elephants of Kaziranga National Park, Assam are dimorphic with respect to tusk possession: ∼50% of the males are tuskless (and are locally called makhnas). Makhnas could be trading tusk for either longevity, larger body size, testicular volume and or duration of musth as alternate mating strategies. On the other hand makhnas may have increased to a very high frequency primarily due to selective removal (captures for domestication and hunting for ivory) of tusked males from the population for centuries. The aim of Chapter 3 was to examine the role of artificial selection in the evolution of makhnas. Prolonged male-biased harvest(removal from the population) is bound to alter the demographic structure of the population and leave a signature of the intensity and type of harvest on the residual population structure. The Kaziranga elephant population was considered as representative of elephant populations of north east India; A harvest modeling approach (described and validated in Chapter 2) was used to infer unknown harvest of elephants from demographic parameters estimated by sampling this elephant population during 458 field days in the dry season months of 2008–2011. The Kaziranga elephant population appears to have been harvested approximately for the past 700 to 1000 years with adult tusked males being harvested at approximately twice the rate of adult tuskless males, adult females and their immature offspring of both the sexes. The currently observed high frequency of tuskless males in Kaziranga therefore, may be a consequence of sustained artificial selection against tusked males for several centuries. The previous two Chapters have only examined some mechanisms for the loss of tusks in elephants. I proceeded to examine the possibility of evolution of tusks through Darwin’s mechanisms of male-male competition for mates and female mate choice. Elephant tusks are cited as an example of a male trait that has evolved as a weapon in male-male combats. In Chapter 4 I examined the role of tusks in establishing dominance along with two other known male–male signals, namely, body size and musth (a temporary physiologically heightened sexual state) in an Asian elephant population in northeastern India with equal proportions of tusked and tuskless males. I observed 116 agonistic interactions with clear dominance outcomes between adult (>15 years) males during 458 field days in the dry season months of 2008–2011. A generalized linear mixed-effects model was used to predict the probability of winning as a function of body size, tusk possession and musth status relative to the opponent. A hierarchy of the three male–male signals emerged from this analysis, with musth overriding body size and body size overriding tusk possession. In this elephant population tusk possession thus played a relatively minor role in male–male competition. An important implication of musth and body size being stronger determinants of dominance than tusk possession is that it could facilitate rapid evolution of tuskless males in the population under artificial selection against tusked individuals, which are poached for ivory. If not a weapon, tusks could be a male ornament that female elephants find attractive. I explored the interplay of the three male traits (body size, musth and tusk), male mating strategies and female mate choice in Chapter 5. In some species males obtain mating opportunities by harassment of females. Given the striking size difference between an adult male and female elephant, with males weighing at least 30% more than females, male coercion of females to mate is a possibility. A detailed study of the courtship behavior revealed that overt male harassment of females is rare and the ability of a male to mount and stay mounted on a female for copulation is under female control. Therefore female Asian elephants can exercise choice to mate but this is subtly different from exercising mate choice itself. Age-related male mating strategy (reported for the first time in the Asian elephant) exists in the Kaziranga elephant population and this strategy limits the ability of females to exercise choice. Young males (<25 years) predominantly show a sneak mating strategy. Middle-aged males (25–40 years), when in musth, mate–guarded oestrous females from sneakers and attempted mating but sometimes resorted to sneak mating when out of musth. Old males (> 40 years) attempted mating only during their musth phase and were seldom sneakers. Large/musth males received positive responses from estrous females towards courtship attempts significantly more often than did small/non–musth males. Tusked non–musth males attempted courtship significantly more often than did their tuskless peers, and had a higher probability of receiving positive responses than did tuskless males. A positive response, however, may not translate into mating because of mate–guarding by the dominant male. Females permitted large/musth males to stay mounted significantly longer than small/non-musth males. Musth and large body size may be signals of male fertility. Female mate choice in elephants thus seems primarily for traits that signal direct benefits of assurance of conception. Tusked males may attain sexual maturity faster than tuskless males. Therefore it is worth exploring if tusks function as signals of male fertility when males are young (15 to 25 years); this may be possible through hormonal and behavioral profiling of young tusked and tuskless males from 10 to 20 years of age. Overall all musth and body size appear to play a larger role in enhancing male mating success than tusks. Tusked males appear to have a weak sexual selection advantage (male-male domi-nance and female preference) over their tuskless peers, only in the young age class (15 to 25 years) in this population. Males in this age age class, seldom come into musth that would over-ride tusk as a signal of male dominance. Current sexual selection on tusks in this population, appeared to be insignificant and this may be verified through genetic analysis of paternity success. An important implication of musth and body size being stronger determinants of mating success than tusk possession is that, it could facilitate rapid evolution of tuskless males in the population under artificial selection against tusked individuals, even in a slow breeder such as the elephant. Musth may have evolved much later than tusks in elephants, therefore it is possible that tusks evolved under sexual selection before musth evolved. However, body size, in mammals in gen-eral appear to be under both natural and sexual selection. Gould has shown that the absurdly large and palmate antlers of the extinct Irish elk, scales allometrically with body size (Gould & Lewontin 1979). Phylogenetic studies of elephant evolutionary radiation indicate a general trends towards increase in body-size with size reduction and tendency towards dwarfism occurring only in island habitats (Palombo 2001). Tusk development, which is essentially tooth development may be closely linked to cranium development. Cranium development in turn may be linked to body size through allometric scaling laws. If so, any selection on body size is bound to act on tusk size. I propose that the evolution of elaborate tusks seen in elephants is primarily due to natural and or sexual selection acting on body size, and tusk just hitched a ride with body size. Tusks may be maintained in spite of tuskless males occurring in the population only because of a rather weak sexual selection advantage to tusk possession in contests in which males are symmetrical with respect to body size and musth status.