Andrea Pilastro

Directional postcopulatory sexual selection revealed by artificial insemination

Postcopulatory sexual selection comprises both sperm competition, where the sperm from different males compete for fertilization, and cryptic female choice, where females bias sperm use in favour of particular males. Despite intense current interest in both processes as potential agents of directional sexual selection, few studies have attributed the success of attractive males to events that occur exclusively after insemination. This is because the interactions between pre- and post-insemination episodes of sexual selection can be important sources of variation in paternity. The use of artificial insemination overcomes this difficulty because it controls for variation in male fertilization success attributable to the females perception of male quality, as well as effects due to mating order and the relative contribution of sperm from competing males. Here, we adopt this technique and show that in guppies, when equal numbers of sperm from two males compete for fertilization, relatively colourful individuals achieve greater parentage than their less ornamented counterparts. This finding indicates that precopulatory female mating preferences can be reinforced exclusively through postcopulatory processes occurring at a physiological level. Our analysis also revealed that relatively small individuals were advantaged in sperm competition, suggesting a possible trade-off between sperm competitive ability and body growth.

Colourful male guppies produce faster and more viable sperm

In guppies (Poecilia reticulata) precopulatory sexual selection (via female choice) and post‐copulatory selection (via sperm competition) both favour males with relatively high levels of carotenoid (orange) pigmentation, suggesting that colourful males produce more competitive ejaculates. Here we test whether there is a positive association between male orange pigmentation and sperm quality. Our analysis of sperm quality focused on sperm swimming speeds (using CASA: computer‐assisted sperm analysis to estimate three parameters of sperm velocity in vitro), sperm viability (proportion of live sperm per stripped ejaculate) and sperm lengths. We found that males with relatively large areas of orange pigmentation had significantly faster and more viable sperm than their less ornamented counterparts, suggesting a possible link between dietary carotenoid intake and sperm quality. By contrast, we found no relationship between sperm length (head length and total sperm length) and male phenotype. These findings, in conjunction with previous work showing that highly ornamented male guppies sire higher quality offspring, suggest that female preference for colourful males and sperm competition work in concert to favour intrinsically higher quality males.

CRYPTIC FEMALE PREFERENCE FOR COLORFUL MALES IN GUPPIES

Abstract Cryptic female choice (CFC) refers to female‐mediated processes occurring during or after copulation that result in biased sperm use in favor of preferred or compatible males. Despite recent empirical support for this hypothesis, evidence that CFC contributes towards the evolution of male body ornaments, in the same way that precopulatory female choice does, is currently lacking. Here, we tested the possibility that CFC selects for increased male attractiveness in the guppy Poecilia reticulata, a freshwater fish exhibiting internal fertilization. Specifically, we examined whether females are able to manipulate the number of sperm transferred or retained at copulation in favor of relatively attractive males. In support of this prediction, we found that following solicited copulations the number of sperm inseminated is influenced exclusively by the females perception of relative male coloration, independent of any direct manipulation of males themselves. Because females prefer brightly colored males during precopulatory mate choice, our finding that colorful males are also favored as a consequence of enhanced insemination success indicates that cryptic female choice can reinforce precopulatory preferences for extravagant male ornaments.

Female aggregation and male competition reduce costs of sexual harassment in the mosquitofish Gambusia holbrooki

Male mosquitofish allocate a large proportion of their time budget to attempting to inseminate unreceptive females. Because females invest considerable time and energy in avoiding unwanted copulations, sexual harassment is expected to conflict with other activities such as foraging. We found that sexual harassment more than halved the foraging efficiency of the female in a task requiring the retrieval of food items scattered on the water surface. The presence of shoalmates relieved the female from harassment and foraging efficiency increased with the number of females in the group. Small males attempted to mate significantly more than large males, causing a greater reduction in the female’s foraging efficiency. When several males competed for the same female, the larger, dominant male prevented all the other males from attempting to mate and so had fewer opportunities to attempt copulation. Harassment from solitary males appeared to be more costly, as females foraged more efficiently when chased by a group of males. Because male sexual activity has a large impact on a female’s feeding efficiency and possibly on her survival and reproduction, sexual conflict is expected to be important in shaping association patterns of female mosquitofish with conspecifics of both sexes.  2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour

Cryptic female preference for genetically unrelated males is mediated by ovarian fluid in the guppy

As inbreeding is costly, it has been suggested that polyandry may evolve as a means to reduce the negative fitness consequences of mating with genetically related males. While several studies provide support for this hypothesis, evidence of pure post-copulatory mechanisms capable of biasing paternity towards genetically unrelated males is still lacking; yet these are necessary to support inbreeding avoidance models of polyandry evolution. Here we showed, by artificially inseminating a group of female guppies with an equal number of sperm from related (full-sib) and unrelated males, that sperm competition success of the former was 10 per cent lower, on average, than that of the unrelated male. The paternity bias towards unrelated males was not due to differential embryo survival, as the size of the brood produced by control females, which were artificially inseminated with the sperm of a single male, was not influenced by their relatedness with the male. Finally, we collected ovarian fluid (OF) from virgin females. Using computer-assisted sperm analysis, we found that sperm velocity, a predictor of sperm competition success in the guppy, was significantly lower when measured in a solution containing the OF from a sister as compared with that from an unrelated female. Our results suggest that sperm–OF interaction mediates sperm competition bias towards unrelated mates and highlight the role of post-copulatory mechanisms in reducing the cost of mating with relatives in polyandrous females.

Males prefer ornamented females: a field experiment of male choice in the rock sparrow

Recent evidence suggests that female ornaments in birds not only are genetic correlates of selection on males but may also have evolved by selection acting directly on females. However, most evidence comes from studies in captivity and it is therefore still debated whether male choice is important under natural conditions. The rock sparrow, Petronia petronia, is a monomorphic passerine in which both sexes show a yellow breast patch, although females have a smaller patch than males. Breast patch in females is positively correlated with body mass and fecundity (number of broods per year). We tested whether males preferentially mate with more ornamented females in this species. We examined male mate choice by reducing the size of the yellow breast patch of free-ranging females during the pair formation period in a colony in southeast Spain. We found that females with an experimentally reduced breast patch were courted less intensely and chased less frequently, than control females were. Furthermore, females with an experimentally reduced ornament were less likely to mate and, when mated, did so at a significantly later date than controls. These results are supported by correlational observations from another rock sparrow population, for which a larger data set was available, showing that pairs mated assortatively with respect to breast patch size. Altogether, these results therefore suggest that female ornamentation may be sexually selected in this species.

Individual adjustment of sperm expenditure accords with sperm competition theory

Sperm competition theory predicts that males should strategically allocate their sperm reserves according to the level of sperm competition, defined as the probability that the sperm of two males compete for fertilizing a given set of ova. Substantial evidence from numerous animal taxa suggests that, at the individual level, sperm expenditure increases when the risk of sperm competition is greater. In contrast, according to the “intensity model” of sperm competition [Parker, G. A., Ball, M. A., Stockley, P. & Gage, M. J. G. (1996) Proc. R. Soc. London Ser. B 263, 1291–1297], when more than two ejaculates compete during a given mating event, sperm expenditure should decrease as the number of competing males increases. Empirical evidence supporting this prediction, however, is still lacking. Here we measured sperm expenditure in two gobiid fishes, the grass (Zosterisessor ophiocephalus) and black goby (Gobius niger), in which up to six sneakers can congregate around the nest of territorial males and release their sperm when females spawn. We show that, in accordance with theory, sneaker males of both species release fewer sperm as the number of competitors increases.

Sperm number and velocity affect sperm competition success in the guppy ( Poecilia reticulata )

While both sperm number and quality are now recognized to be important in determining the outcome of sperm competition, very few studies have experimentally assessed the influence of these two parameters simultaneously. We studied the effect of sperm quality and number on competitive fertilization success in an internal-fertilizing fish, the guppy (Poecilia reticulata), which is characterized by high levels of sperm competition. We artificially inseminated virgin females with varying proportion of sperm from two competing males, while holding constant the total number of sperm transferred to the female. Sperm morphology and sperm swimming velocity were also determined prior to insemination. The paternity outcome of sperm competition trials was assessed through molecular analyses of the resulting offspring using polymorphic microsatellite loci. We found that both sperm number and sperm velocity affected the outcome of sperm competition, with males that contributed more and faster sperm achieving a greater paternity share.

Male phenotype predicts insemination success in guppies.

Theory predicts that mate choice can lead to an increase in female fecundity if the secondary sexual traits used by females to assess male quality covary with the number of sperm transferred during copulation. Where females mate multiply, such a relationship between male attractiveness and ejaculate size may, additionally (or alternatively), serve to augment the effect of indirect selection by biasing paternity in favour of preferred males. In either case, a positive correlation between male attractiveness and the size of ejaculates delivered at copulation is predicted. To date, some of the most convincing (indirect) evidence for this prediction comes from the guppy, a species of fish exhibiting a resource–free mating system in which attractive males tend to have larger sperm reserves. We show that, during solicited copulations, male guppies with preferred phenotypes actually transfer more sperm to females than their less–ornamented counterparts, irrespective of the size of their initial sperm stores. Our results also reveal that, during coercive copulations, the relationship between ejaculate size and the males phenotype breaks down. This latter result, in conjunction with our finding that mating speed—a factor under the females control—is a significant predictor of ejaculate size, leads us to speculate that females may exert at least partial control over the number of sperm inseminated during cooperative matings.

LONG LIVING AND REPRODUCTION SKIPPING IN THE FAT DORMOUSE

We studied the reproductive strategy of a population of fat dormice (Myoxus glis) breeding in nest boxes in a beech forest on the southern Alps, Italy. In eight years of data collection (1991-1998), we observed births only in 1991, 1992, and 1995. We did not observe young during the autumn dispersal in the remaining five years. Reproduction was not correlated with high rainfall, low temperature in summer, or with population density. Reproduction was only observed in years of beech mast seeding. Body condition of adults when they emerged from hibernation did not differ between years with and without repro- duction, suggesting that the adults may avoid breeding when the young have a low prob- ability of surviving their first winter. A capture-recapture analysis on individually marked adult fat dormice identified a very high yearly survival rate (0.86-0.92), close to that of large herbivorous mammals and much higher than that typical of other rodent species with similar body size. No influence of sex on survival or recapture probability was detected. The long life span expectancy of an adult dormouse (averaging nine years) is the prerequisite for the infrequent breeding strategy adopted by this rodent species to cope with an unpre- dictable food source. A high survival rate despite the small body size may have been possible because of the long hibernation time ( .6 months). Long life span and intermittent breeding make the life history of fat dormice unique among rodents.