Role of Contests in the Scramble Competition Mating System of a Leaf Beetle

Male competition for mates can occur through contests or a scramble to locate females. We examined the significance of contests for mates in the leaf beetle Chrysomela aeneicollis, which experiences a short breeding season. During peak mating season, 18–52% of beetles are found in male-female pairs, and nearly half of these are copulating. Sex ratios do not differ from parity, females are larger than males, and positive size-assortative mating occurs. Males fight (2–4% of beetles) over access to females, and disruption of mating usually follows these contests. In the laboratory, we compared mating and fighting frequencies for males found in mating pairs (field-paired) and single males placed into an arena with a field-paired female. Mating pairs were switched in half of arenas (new male-female pairs) and maintained in the other half. For 2 days, each male was free to move about and fight; thereafter males were tethered to prevent contests. Mating frequencies were significantly greater for field-paired than single males in both situations. Male size was not related to mating frequency; however, large females received more matings than small ones. These data suggest that males fight for high quality females, but otherwise search for as many matings as possible.     

Pure Self‐Assessment of Size During Male–Male Contests in the Parasitoid Wasp Nasonia vitripennis

During contest competition, a competitor may persist in a given contest based on information regarding its own fighting ability (resource‐holding potential, RHP), or that of its opponent. Although a number of models formalize the ways in which competitors are hypothesized to use RHP‐related information to determine their persistence in contests, we focused on pure self‐assessment and mutual assessment models in this study. According to pure self‐assessment models, a competitor uses only information regarding its own RHP to determine its persistence in a contest. In contrast, according to mutual assessment models, persistence is based on information regarding a competitor's RHP relative to that of its opponent and therefore requires assessment between competitors. In this study, using size as a proxy for RHP, we tested whether the parasitoid wasp Nasonia vitripennis utilizes pure self‐assessment or mutual assessment during pairwise, male–male contests. When we examined competitors of varied sizes, we found that the losing male's size was positively related to contest duration, but the winning male's size was uncorrelated with contest duration. When we examined contests in which competitors were size‐matched, we found that the mean size of paired competitors was positively related to contest duration. These results suggest that male N. vitripennis engage in pure self‐assessment during contests.     

Competition Between the Sperm of a Single Male Can Increase the Evolutionary Rate of Haploid Expressed Genes

The population genetic behavior of mutations in sperm genes is theoretically investigated. We modeled the processes at two levels. One is the standard population genetic process, in which the population allele frequencies change generation by generation, depending on the difference in selective advantages. The other is the sperm competition during each genetic transmission from one generation to the next generation. For the sperm competition process, we formulate the situation where a huge number of sperm with alleles A and B, produced by a single heterozygous male, compete to fertilize a single egg. This “minimal model” demonstrates that a very slight difference in sperm performance amounts to quite a large difference between the alleles’ winning probabilities. By incorporating this effect of paternity-sharing sperm competition into the standard population genetic process, we show that fierce sperm competition can enhance the fixation probability of a mutation with a very small phenotypic effect at the single-sperm level, suggesting a contribution of sperm competition to rapid amino acid substitutions in haploid-expressed sperm genes. Considering recent genome-wide demonstrations that a substantial fraction of the mammalian sperm genes are haploid expressed, our model could provide a potential explanation of rapid evolution of sperm genes with a wide variety of functions (as long as they are expressed in the haploid phase). Another advantage of our model is that it is applicable to a wide range of species, irrespective of whether the species is externally fertilizing, polygamous, or monogamous. The theoretical result was applied to mammalian data to estimate the selection intensity on nonsynonymous mutations in sperm genes.     

Sperm: seminal fluid interactions and the adjustment of sperm quality in relation to female attractiveness

An important predictor of male fitness is the fertilizing efficiency of their ejaculates. Ejaculates are costly to produce and males are predicted to devote greater resources to copulations with reproductively superior females. It is well established that males allocate different numbers of sperm to ejaculates. However, less is known about how males adjust their sperm quality, which has important implications for our understanding of fertilization and the evolution of sexual strategies. Here we test in the fowl, Gallus gallus, whether males adjust their sperm velocity by differentially allocating seminal fluid to copulations with attractive and unattractive females. To disentangle the contributions of sperm and seminal fluid to sperm velocity, we separated and remixed sperm and seminal fluid from ejaculates allocated to females of different attractiveness. We show that dominant males increase the velocity of the sperm they invest in more attractive females by allocating larger ejaculates that contain seminal fluid that increases sperm velocity. Furthermore, we find weak evidence that males also allocate sperm with higher velocity, irrespective of seminal fluid, to more attractive females.     

Obstructive Role of the Dimorphic Sperm in a Non-copulatory Marine Sculpin, Hemilepidotus gilberti, to Prevent Other Males' Eusperm from Fertilization

In many species of animals, males normally produce parasperm (dimorphic sperm) along with eusperm (normal sperm) during spermatogenesis. In the present study, to clarify the role of parasperm of the non-copulatory sculpin Hemilepidotus gilberti, whose reproduction is characterized by polyandrous oviposition involving sneaking by neighboring territorial males, we observed the movements of parasperm. Parasperm could not move by themselves, but they were transported in solutions by passive movement due to collisions with actively swimming eusperm. In the viscous ovarian fluid (OF), which isolates eggs from seawater by covering them during spawning, parasperm did not exhibit any movement. However, they could be transported by eusperm movement in solutions with dissolved OF, partly because the viscosity of the fluid become lower. And then, in some solutions parasperm formed lumps. Lump formation of parasperm was also observed at the boundary surface of an egg mass where OF contacted seawater. Eusperm added experimentally to a solution in which parasperm were forming lumps were engulfed in the lumps and never escaped. Thus, lump formation of parasperm would be obstacles for the later arriving eusperm. Although lumps formed against both kin and non-kin eusperm, parasperm are thought to be available to overcome sperm competition which would occur during spawning that involves sneaking being almost concurrent with lump formation. The territorial male eusperm reach the eggs while his parasperm hinder other males' eusperm from reaching the eggs. Thus, we concluded that parasperm of H. gilberti play a role on protection of paternity by blocking rival eusperm physically from approaching eggs.     

Individual asymmetry as a predictor of fitness in the bat Carollia perspicillata

The measurement of fitness in wild populations is a challenging task, and a number of proxies have been proposed with different degrees of success. Developmental instability/stability (DI) is an organismal property associated with variance in bilateral asymmetry (fluctuating asymmetry—FA) and a correlated effect on fitness. This study provides evidence to corroborate the hypothesis that asymmetry partly reflects DI and is correlated with a reduction in fitness measured by survival and reproduction in bats. We studied two colonies of the bat Carollia perspicillata in southeastern Brazil over 5 years, marking and recapturing individuals. Gaussian mixture models for signed Forearm Asymmetry (ForA) distribution indicated that ~20% of asymmetry variation was due to DI heterogeneity among individuals. ForA, body condition (Scaled Mass Index—SMI) and Forearm Length (ForL) were used as predictors of survival probability in Cormack‐Jolly‐Seber models. Asymmetry was negatively associated with survival, whereas SMI and ForL were positively associated. The male C. perspicillata defend sites within the roost that are favoured by female harems, but there are mating opportunities for bachelor males, leading to both territorial disputes and sperm competition. As predicted by sexual selection, ForA was negatively associated with relative Testicle Length, a measure of reproductive potential. In females, ForA was negatively associated with the probability of two pregnancies (as opposed to one) in a given breeding season. The effect magnitudes and directions of associations suggest that asymmetry, even though not perfectly reflecting DI variation, is a useful predictor for fitness components in C. perspicillata.     

Nestmate recognition in burying beetles: the "breeder's badge" as a cue used by females to distinguish their mates from male intruders

Burying beetles use small vertebrate carcasses as food for theirlarvae and defend these carcasses against intra- and interspecificcompetitors. Breeding associations on carcasses can consistof single females, heterosexual pairs, or various combinationsof males and females. When a heterosexual pair collaborate ina breeding attempt, they do not typically exhibit aggressivebehavior toward each other, but do attack newly arrived conspecificsthat attempt to usurp the carcass. We investigated the cuesinvolved in discrimination between breeding partners and intrudersby female burying beetles. We found that resident females toleratemales that have cared for a brood, as well as males that havenot cared for a brood but have been on a carcass for a day ortwo. Males that have had no prior contact with a carcass areattacked. Females appear to use a chemical cue, the "breeder'sbadge," an apolar substance on the male's cuticle that can beremoved by washing with pentane. This cue is reliably correlatedwith recent male experience with a carcass that is suitablefor reproduction. The breeder's badge develops as a result ofprolonged contact with such a carcass, and disappears on removalfrom the carcass; its presence does not require contact witha female or with larvae. Female recognition of their male partnersin burying beetles thus does not involve individual recognition,but rather recognition of reproductive condition.