The evolution of male infanticide in relation to sexual selection in mammalian carnivores

The evolution of infanticide by males has often been explained by the sexual selection hypothesis, which posits that infanticide improves male reproductive success by shortening the interbirth intervals of the mothers of the killed offspring. In Carnivora, however, the fitness advantages assumed in this hypothesis have been shown in only a few species, and it has been argued that male infanticide may be nonadaptive in pinniped carnivores. According to the sexual selection hypothesis, male infanticide is expected to be more prevalent in species in which males are subjected to stronger sexual selection through intrasexual competition over mates. We examined a phylogenetically corrected relationship between male infanticide and sexual size dimorphism (SSD) as a measure of the intensity of sexual selection in carnivores. Our analyses failed to detect a significant association between the occurrence of male infanticide and SSD across carnivores, although they showed that, among fissipeds (typically terrestrial carnivores), males in species with stronger male-biased SSD are significantly more likely to commit infanticide. This suggests that the evolution of male infanticide is correlated with intense sexual selection in fissipeds. In pinnipeds (Odobenidae, Otariidae, and Phocidae), there was no significant association between male infanticide and SSD. Assuming that SSD represents the intensity of sexual selection on males, this result is consistent with the argument that infanticide by male pinnipeds is not a sexually selected behaviour.     

Male sexual attractiveness and aggressiveness in rodents having different mating systems

Male aggressiveness can affect male reproductive success both directly by increasing competitiveness and indirectly through female preference. Assuming that significance of male aggressiveness in species having different mating systems can be different, we studied how male aggressiveness relates to sexual attractiveness in polygynous rodents, the water vole (Arvicola terrestris) and the house mouse (Mus musculus), and in a monogamous species, the steppe lemming (Lagurus lagurus). Our analysis revealed that the relation between odor attractiveness and aggressiveness is nonlinear. In polygynous species, males are more aggressive, so females opt for aggressive, albeit not too aggressive, males. In the monogamous steppe lemming, males show low level of intermale aggressiveness, and the most attractive are slightly aggressive males who have greater reproductive potential.     

The intensity of sexual selection in relation to male sexual behaviour,female choice,and sperm precedence

In this paper we define sexual selection on males as the variance in numbers of mates per male and show how the intensity of this selection is affected by male sexual behaviour, female choice, sex ratio, and modes of sperm precedence. This definition coincides with Darwin's conception of sexual selection but differs from some post-Darwinian views. For systems of single-male paternity, we show that the intensity of total selection on male reproductive success equals the intensity of natural selection on female fertility, times the sex ratio, plus the intensity of sexual selection on males. The absolute intensity of sexual selection is unaffected by the system of sperm precedence. The application of the results to field studies is discussed.     

Stabilizing sexual selection for female ornaments in a dance fly

Ornamental traits function by improving attractiveness and are generally presumed to experience directional selection for mating success. However, given the greater investment of females in offspring than males, female-specific ornaments can in theory signal fecundity yet be constrained by fecundity costs. Theoretical work predicts that such constraints can lead to stabilizing selection via male choice for intermediately ornamented females. Female dance flies Rhamphomyia longicauda (Diptera: Empididae) display two female-specific ornaments in mating swarms - inflatable abdominal sacs and pinnate tibial scales. We investigated the intensity and form of sexual selection on female traits including ornaments and found no evidence for directional sexual selection. Instead, we found marginally nonsignificant quadratic selection for all three measures of ornament expression. Canonical analysis confirmed that the strongest vectors of nonlinear selection were associated with ornamental traits, although the significance of the quadratic coefficients associated with these vectors depended on the statistical approach. Direct Mitchell-Olds and Shaw tests for the location of the maximum fitted fitness value for both raw morphological traits and canonical axes revealed only one marginally nonsignificant result for the multivariate axis loading most heavily on pinnate leg scales. Together, these results provide the first tentative support for stabilizing selection on female-specific ornaments.     

Wing pigmentation in territorial male damselflies, Calopteryx haemorrhoidalis: a possible relation to sexual selection

One striking characteristic in adult males of some odonate species is the presence of wing pigmentation. In Calopteryx species, males show a series of pre- and postcopulatory behavioural displays during which they face females while showing their pigmented wings. One hypothesis to explain the precopulatory flying displays and the associated wing pigmentation is that they may serve a sexual selection function. I investigated this in the territorial damselfly Calopteryx haemorrhoidalis. Males of this species defend aquatic substrates that females use for oviposition. Observational evidence indicated that males with a higher proportion of wing pigmentation were more likely to defend a territory, obtained more matings, had fewer gut parasites, survived in the study site and stayed in territories for longer. Experimental evidence suggested that the relationship mating success and wing pigmentation still held when controlling for the size of the substrate defended by territorial males. Similar to other studies in the Calopterygidae, these results suggest that wing pigmentation may be favoured by sexual selection. I discuss, however, whether an alternative function for male copulatory courtship displays and wing pigmentation, as sexual and/or species recognition, may also explain the evolution of these traits. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Stabilizing selection of protein function and distribution of selection coefficient among sites

In this study, I take a new approach to modeling the evolutionary constraint of protein sequence, introducing the stabilizing selection of protein function into the nearly-neutral theory. In other words, protein function under stabilizing selection generates the evolutionary conservation at the sequence level. With the help of random mutational effects of nucleotides on protein function, I have derived the distribution of selection coefficient among sites, called the S-distribution whose parameters have clear biological interpretations. Moreover, I have studied the inverse relationship between the evolutionary rate and the effective population size, showing that the number of molecular phenotypes of protein function, i.e., independent components in the fitness of the organism, may play a key role for the molecular clock under the nearly-neutral theory. These results are helpful for having a better understanding of the underlying evolutionary mechanism of protein sequences, as well as human disease-related mutations.     

Effect of male-to-male aggressiveness and feed-restriction during rearing on sexual behaviour and aggressiveness towards females by male domestic fowl

This experiment was one part of a larger study investigating problems of aggression towards females by male broiler breeder fowl. To investigate causal mechanisms, we were interested in determining (1) if feed-restriction during rearing affects behaviour towards females at sexual maturity and (2) if aggressiveness towards females is correlated with general levels of aggressiveness. We compared broiler breeder males with commercial laying strain males, which were either fed ad libitum or were feed-restricted during the rearing phase, and with game strain males, bred for fighting. Differences in behaviour were determined by observing males during interactions with small groups of females.Laying strain males did not behave aggressively towards females, whether feed-restricted or fed ad libitum during rearing. Despite genetic selection for fighting ability, game strain males also were not aggressive towards females. Conversely, broiler breeder males displayed significantly higher levels of aggression towards females than did feed-restricted laying strain males (P<0.02). Broiler breeder males were rough with females during mating, whereas laying strain and game strain males were not. Females struggled more frequently during mating attempts by broiler breeder males (P<0.002) and interfered frequently when these males attempted to mate with other females.From our results, we conclude that (1) feed-restriction during rearing has little effect on the sexual and aggressive behaviour of laying strain males at maturity and (2) selection for aggressiveness has not resulted in males which are more aggressive to females. Aggression towards females appears to be a unique problem occurring in broiler breeder male strains and not a function of feed-restriction.     

Care of young, aggressiveness, and secretion of testosterone in male rodents: A correlation analysis

To test the current hypotheses on the relationship between the mating system, reproductive strategy, aggression, and secretion of testosterone, a comparative study of interactions in pair encounters, the level of parental care, and the gonadal testosterone level in males was performed in six rodent species (Clethrionomys rutilus, Meriones meridianus, Microtus arvalis, Lagurus lagurus, Lasiopodomys mandarinus, and Meriones unguiculatus) with different types of spatial-and-ethological population structures (SEPSs). It is shown that this dependence is absent in species with promiscuous mating and dominance hierarchy among males (C. rutilus and M. meridianus, SEPS type II). A trade-off, or negative correlation, was found in M. arvalis—a species with weak pair bonds and male competition for receptive females (SEPS type III). In species with persistent pair bonds and structured family groups (L. mandarinus and M. unguiculatus, SEPS type IV), no inverse relationship between the secretion of testosterone and paternal behavior was found either. Moreover, in male L. mandarinus androgens appear to stimulate paternal care.     

Quantitative genetic models of sexual selection by male choice

There are many examples of male mate choice for female traits that tend to be associated with high fertility. I develop quantitative genetic models of a female trait and a male preference to show when such a male preference can evolve. I find that a disagreement between the fertility maximum and the viability maximum of the female trait is necessary for directional male preference (preference for extreme female trait values) to evolve. Moreover, when there is a shortage of available male partners or variance in male nongenetic quality, strong male preference can evolve. Furthermore, I also show that males evolve to exhibit a stronger preference for females that are more feminine (less resemblance to males) than the average female when there is a sexual dimorphism caused by fertility selection which acts only on females.     

Hormone induced lordosis and its relation to masculine sexual activity in male rats

Intact and castrated male rats were injected with a single dose of estrogen (75 μg), followed by progesterone (1 mg) or by oil. Intact males showed higher lordosis quotients as compared to castrated males. Progesterone facilitated lordosis behavior in castrated as well as in intact males. An estrogen-progesterone treated group of sexually inactive male rats and of sexually exhausted males showed lower levels of lordosis as compared to the intact sexually active males.     

Host immune function and sexual selection in birds

Parasites have been hypothesized to affect sexual selection of their hosts, if secondary sexual characters reliably signal absence of infectious parasites, superior parenting ability caused by the absence of parasites, or heritable resistance to parasites, for which there is some intraspecific and interspecific evidence. Measures of immune defence of hosts provide reliable information on the current infection status of individuals of the chosen sex, usually males, and correlations between immune defence and development of secondary sexual characters thus provide a novel critical test of parasite-mediated sexual selection. In a comparative study of birds, sexually dichromatic species had higher immune defences, measured in terms of leukocyte concentration and the size of spleen and bursa of Fabricius, respectively, than closely related, monochromatic species. Male plumage brightness was consistently negatively related to the size of the spleen in males of sexually dichromatic species, but not in males of monochromatic species. Hence, the brightest males, which frequently are preferred as mates by choosy females, had low levels of immune defence, suggesting that such males were healthy. This provides evidence for a general role of parasites in sexual selection among their bird hosts.     

The intensity of sexual selection predicts weapon size in male bovids

As a classical example of a sexually selected trait, the horns of male bovids offer a prime opportunity to identify predictors of the intensity of sexual selection. Here I use the comparative method to quantify sexual and natural selection pressures behind interspecific variation in horn length. I show that male horn length depends on factors proposed to affect the mean mate number per mating male, correlating positively with group size and negatively with male territoriality. This suggests that whereas group size increases the opportunity for sexual selection, territoriality reduces it because territorial males are unable to follow and monopolize female groups as effectively as males in nonterritorial species. Sexual body size dimorphism also correlates positively with group size and negatively with territoriality, corroborating these factors as predictors of the intensity of sexual selection on males. Female horn length was unaffected by the factors related to mating system, suggesting that this trait is mainly under natural selection. Using female horn length as a proxy for forces of natural selection revealed a negative effect on male horn length. Thus where natural selection favors female horns, possibly as effective weapons against predators, a similar selection pressure on males might prevent them from evolving too elaborate horns through sexual selection. There was no correlation found between horn length and latitude, thus providing no support for the hypothesis that horns have a thermoregulatory function.     

Stabilizing natural selection on the early expression of a secondary sexual trait in a passerine bird

Natural selection is a central tenet of evolutionary theory, yet the estimation of the direction and intensity of selection remains problematic. Here, we assess the strength of selection on the early expression of a secondary sexual ornament, bill colour, in male European blackbirds (Turdus merula) using 5 years of capture-mark-recapture (CMR) data. The best-fitting model consisted of a quadratic relationship between survival rate and bill colour, indicating stabilizing natural selection on the early expression of a secondary sexual trait. There was no evidence for sexual selection acting on bill colour in the first year. We suggest that the consideration of early selection and the adoption of refined statistical methods may reveal patterns of selection in the wild that have, as yet, remained undetected.     

A change in competitive context reverses sexual selection on male size

In studies of sexual selection, larger size is often argued to increase male fitness, and relatively smaller males are explained by genetic and/or environmental variation. We demonstrate that a size–development life‐history trade‐off could underlie the maintenance of a broad, unimodal distribution of size in male redback spiders (Latrodectus hasselti). Larger males are superior in direct competition, but redback males mature rapidly at small size in the presence of females. In field enclosures, we simulated two competitive contexts favouring development of divergent male sizes. Relatively smaller males lost when competing directly, but had 10 times higher fitness than relatively larger males when given the temporal advantage of rapid development. Linear selection gradients confirmed the reversal of selection on size, showing that it is critical to consider life‐history decisions underlying the development of traits related to fitness.     

The social selection alternative to sexual selection

Social selection offers an alternative to sexual selection by reversing its logic. Social selection starts with offspring production and works back to mating, and starts with behavioural dynamics and works up to gene pool dynamics. In social selection, courtship can potentially be deduced as a negotiation, leading to an optimal allocation of tasks during offspring rearing. Ornaments facilitate this negotiation and also comprise 'admission tickets' to cliques. Mating pairs may form 'teams' based on the reciprocal sharing of pleasure. The parent-offspring relation can be managed by the parent considered as the owner of a 'family firm' whose product is offspring. The cooperation in reproductive social behaviour evolves as a mutual direct benefit through individual selection rather than as some form of altruism requiring kin or multi-level selection.     

Stabilizing survival selection on presenescent expression of a sexual ornament followed by a terminal decline

Senescence is a decrease in functional capacity, increasing mortality rate with age. Sexual signals indicate functional capacity, because costs of ornamentation ensure signal honesty, and are therefore expected to senesce, tracking physiological deterioration and mortality. For sexual traits, mixed associations with age and positive associations with life expectancy have been reported. However, whether these associations are caused by selective disappearance and/or within‐individual senescence of sexual signals, respectively, is not known. We previously reported that zebra finches with redder bills had greater life expectancy, based on a single bill colour measurement per individual. We here extend this analysis using longitudinal data and show that this finding is attributable to terminal declines in bill redness in the year before death, with no detectable change in presenescent redness. Additionally, there was a quadratic relationship between presenescent bill colouration and survival: individuals with intermediate bill redness have maximum survival prospects. This may reflect that redder individuals overinvest in colouration and/or associated physiological changes, while below‐average bill redness probably reflects poorer phenotypic quality. Together, this pattern suggests that bill colouration is defended against physiological deterioration, because of mate attraction benefits, or that physiological deterioration is not a gradual process, but accelerates sharply prior to death. We discuss these possibilities in the context of the reliability theory of ageing and sexual selection.