Sperm allocation in an uncertain world

Models of optimal sperm allocation are constructed using stochastic dynamic programming techniques, yielding predictions about sperm allocation when males mate sequentially, breeding time is finite, sperm stores are limited and females vary in quality. The models suggest that uncertainty of future reproductive opportunities should favour withholding of sperm, so that males tend to allocate less (for a given level of stored sperm) earlier in the breeding season. This effect is more pronounced the greater the variance in female quality. We also show that while allocation will be influenced by mate value, it is not necessarily optimal to allocate preferentially to high-quality females, since the benefits of a higher-quality mate may be offset by increased risk of rejection of sperm or higher sperm competition. The relationship between mate quality and level of allocation will depend strongly on the amount of remaining stored sperm, with males whose supplies are depleted being more likely to favour lower-quality partners. Received: 12 September 1997 / Accepted after revision: 28 June 1998     

Sexual conflict over fertilizations: female bluethroats escape male paternity guards

Extra-pair copulations create a potential for sexual conflict in pair-bonding birds. Here we report an experimental study of the bluethroat, Luscinia s. svecica, in which the throat ornament of males was blackened with Nyanzol D in order to reduce their sexual attractiveness and thus increase the sexual conflict over fertilizations. In an earlier study, we showed that males blackened before pairing had a lower success in attracting social mates than controls, whereas males blackened after pairing guarded their mates more intensely and sang less than controls. Here we add behavioural data from one more year on males blackened after pairing and corroborate our previous finding that the manipulation caused males to guard their mates more intensely and advertise less for additional mates. Blackened males did not suffer more intrusions from neighbouring males than did controls. Paternity analyses of the combined data set, using multilocus DNA fingerprinting and microsatellite typing, revealed that blackened males lost significantly more paternity than controls. There was also a tendency for blackened males to show a lower success in achieving extra-pair fertilizations. These results indicate that females have the upper hand in the sexual conflict over fertilizations, as females paired with unattractive males can achieve more extra-pair paternity despite the greater constraint posed by the intensified mate guarding. Still, within the blackened group, there were some indications that males guarding more intensely and singing less had higher paternity than males guarding less and singing more, suggesting a marginal positive effect of guarding for unattractive males. Male mate guarding must nevertheless be considered a best-of-a-bad-job strategy in this species. Received: 4 December 1997 / Accepted after revision: 14 June 1998     

Measuring mate choice using correlation: the effect of female sampling behaviour

For intersexual selection to occur, it is necessary that females choose between males. It is now well appreciated that constraints exist, which preclude females sampling all the available males in a population. These constraints are likely to have caused the evolution of sampling rules (such as the “best-of-n” rule) by which females sample males. Here we investigate the impact of female subsampling of the male population, not on the evolution of sampling behaviour, but on the population-level correlation between a male trait and currencies such as reproductive success. This study is important as it illustrates when population-level correlations can be safely used to infer the presence and strength of sexual selection in the field. We find that the correlation between a male trait and a mate choice variable rises steeply as the number of males sampled by each female increases, flattening above seven to ten males sampled. This shape is found to be remarkably robust, and little affected by, for example, the mate choice variable used, by noise in assessment, by sampling behaviour depending on female quality, or by population size. The only variable found to have a large impact is male clumping according to their “quality”. If females are sampling about four males, the maximum correlation that can be found at the population level is in the range 0.4–0.6, perhaps as little as 0.1 if males are strongly clumped. A recent review of the literature suggests that four is the average number of males that females sample. Thus, the absence of a strong correlation cannot by itself be used to infer that sexual selection is weak, as it may be due to females sampling few males. Received: 18 May 1998 / Accepted after revision: 18 July 1998     

Changes in male guppy courting distance in response to a fluctuating light environment

In the guppy (Poecilia reticulata), effective courting by a male requires visual contact with the female. Therefore, environmental light intensity may affect male display behavior, particularly initial courtship distance. We found that male guppies courted at exact and predictable distances from the female given a particular light level, both in field and laboratory studies. In lower light levels (<0.1 μmol m⁻² s⁻¹), for example at dawn, dusk, or under heavy canopy, males court females at closer and less variable distances (<3 cm). At higher light levels, which occur during most of the day and with less canopy cover, males often court from twice or three times further out. Light levels over guppy streams change over relatively short time periods and ranges, correlating with variation in courtship distances. Laboratory manipulations of irradiance confirmed that courtship distance depends on illumination. Hence, courtship distances may be set by the effect of lighting on signal efficiency, minimization of energy or time expenditures, or predation risk. Received: 16 December 1997 / Accepted after revision: 8 August 1998     

Sex ratio, sexual dimorphism and mating structure in bethylid wasps

Sexual dimorphism has been linked to parasitoid mating structure by several authors. In turn mating structure has an important influence on predicted sex ratio optima. Here we test the relationship between sexual dimorphism and sex ratio using data from 19 species of bethylid wasps. Using phylogenetically based comparative methods we confirm the findings of a previous cross-species analysis that sex ratio (proportion of males) is strongly and negatively correlated with clutch size. Using cross-species comparisons we show an additional positive correlation of sex ratio and relative male size, as predicted. The relationship however is not significant when using phylogenetically based methods. The cross-species result is largely due to differences between two bethylid sub-families: the Epyrinae have relatively large males and relatively high sex ratios, whereas the Bethylinae have relatively small males and lower sex ratios. Our study illustrates the benefits and drawbacks of using cross-species versus phylogenetically based comparisons. Received: 13 May 1997 / Accepted after revision: 12 January 1998     

Testosterone is involved in acquisition and maintenance of sexually selected male plumage in superb fairy-wrens, Malurus cyaneus

Testosterone has been proposed as a physiological link between the level of sexual signalling and male condition. Bright plumage is one of the most noticeable sexual signals and is often used by females as a basis for mate choice. Yet bright male plumage is not necessarily testosterone dependent. We investigated the role of testosterone in the moult into seasonal nuptial plumage in male superb fairy-wrens. Early pre-nuptial moult is under intense intersexual selection and males can acquire the bright plumage any time between autumn and the next spring. Testosterone was always undetectable or very low in males in dull eclipse plumage. During the pre-nuptial moult, both the number of males with detectable testosterone and average testosterone levels increased sharply. High testosterone was more correlated with nuptial plumage than with presence of the cloacal protuberance (indicative of sperm storage). Subcutaneous testosterone implants always induced the pre-nuptial moult within 2–3 weeks after implantation, even well outside the natural time range of moulting. Moreover, removal of the implants before the nuptial plumage was completed, arrested the moult process. The evidence suggests that development of the nuptial plumage is testosterone dependent, although we cannot exclude that testosterone exerts its action after conversion to a metabolite such as oestrogen. Once the nuptial plumage was completed, all males maintained substantially elevated testosterone, sometimes months before the onset of breeding. These high levels could be necessary to maintain the plumage, and/or are involved in courtship displays. The results are discussed with respect to potential costs involved in acquiring and maintaining the nuptial plumage. Received: 17 January 2000 / Received in revised form: 24 February 2000 / Accepted: 25 February 2000     

Behavioral and demographic changes following the loss of the breeding female in cooperatively breeding marmosets

Recent models of the evolution and dynamics of family structure in cooperatively breeding vertebrates predict that the opening of breeding vacancies in cooperatively breeding groups will result in (1) dispersal movements to fill the reproductive position, and (2) within-group conflict over access to reproduction. We describe the behavioral and demographic changes that followed the creation of breeding vacancies in three wild groups of cooperatively breeding common marmosets (Callithrix jacchus). Replacement of the breeding female was rapid when no adult females remained in the group, but did not occur for several months when other adult females were present. Aggression of adult animals towards same-sex potential immigrants was associated with a period of reduced affiliation, increased intragroup agonism, no intragroup sexual behavior, and frequent extragroup copulations. This ended with the fissioning of groups along sexual lines. After replacement, multiple males copulated with multiple females and vice versa, with no increases in sexually related aggression. Female-female conflict was resolved through infanticide. The lack of direct conflict between males is consistent with cooperative polyandry. After a breeding vacancy appeared, marmoset groups showed conflict of interests among group members similar to those shown by cooperatively breeding birds, but they used different behavioral mechanisms to resolve those conflicts. Our data provide important evidence from a cooperatively breeding mammal to support Emlen’s model for the evolution of vertebrate families, but they suggest that species-specific inter- and intrasexual competitive strategies should be considered before the model can be applied to other cooperatively breeding vertebrates. Received: 23 November 1999 / Received in revised form: 28 March 2000 / Accepted: 1 April 2000     

Does promiscuous mating preclude female choice? Female sexual strategies in chimpanzees (Pan troglodytes verus) of the Taï National Park, Côte d’Ivoire

According to sexual selection theory, females should selectively mate with high-quality males to enhance offspring survival and maximize reproductive success. Yet, chimpanzee (Pan troglodytes) females are known to mate promiscuously. Although there is substantial rationale for a promiscuous mating strategy, there is also a strong expectation that females should be selective, and the question arises as to whether promiscuity precludes female choice. The aims of this study are to: (1) compare wild female chimpanzee sexual strategies throughout estrus, and (2) determine whether females exhibit mate preferences for particular males. Over 2,600 h of data were collected on two habituated chimpanzee (Pan troglodytes verus) communities in the Taï National Park, Côte dIvoire. Female mate preferences were measured by quantifying proceptivity and receptivity toward males. Results indicate that all females exhibited proceptivity and resistance to male solicitations, but that there was substantial variation in their magnitudes within and among females. Female proceptivity rates were lower and resistance rates were higher in the periovulatory period (POP) when conception is most likely. Females were more selective during POP, and more promiscuous outside of POP, suggesting that females may follow a mixed reproductive strategy, being selective when conception is likely and more promiscuous when conception is unlikely. Results from this study emphasize the importance of considering the fertility window when determining female mate preferences, and of examining female behavior in POP and non-POP phases separately when evaluating hypotheses for multi-male mating.Communicated by C. Nunn     

Patterns of sexual cohabitation and female ejaculate storage in the American lobster (<Emphasis Type="Italic">Homarus americanus</Emphasis>)

Little is known of the time and ejaculate allocation strategies during mating of American lobster, Homarus americanus. This study investigated sexual cohabitation and female ejaculate accumulation patterns in a laboratory mating experiment, as well as female seminal receptacle load in exploited populations in the waters of the Magdalen and Anticosti Islands, in eastern Canada. In the laboratory experiment, the length of sexual cohabitation was proportionate to female size for large but not for small males. Also, large males cohabited with pre- and postmolt females longer than small males. These different time investment strategies can be explained by different mutual benefits. In the field and laboratory, larger females accumulated more ejaculate than smaller ones. This suggests that male lobsters tailor ejaculate to female size, a reliable index of her reproductive potential. Moreover, similarly-sized females accumulated more ejaculate when mated with large compared to small males. Comparison of receptacle loads between wild-mated and laboratory-mated females suggests that the former were mating mainly with smaller males, although some evidence of positive size-assortative mating existed, especially at the less exploited Anticosti site. The results are discussed in the context of evolutionary theory and of proposed management measures to increase egg production in exploited populations.Communicated by T. Czeschlik     

Testing the ability of males and females to respond to altered songs in the dueting green lacewing, Chrysoperla plorabunda (Neuroptera: Chrysopidae)

Green lacewings in the carnea group of Chrysoperla engage in species-specific heterosexual duets using low-frequency substrate-borne signals. Within each species, both sexes sing nearly identical songs. Songs are the principal barriers to hybridization between sympatric species in the complex. Here, we investigated the responsiveness of males and females of Chrysoperla plorabunda to synthesized, prerecorded songs that differed from the species mean in the period between repeated volleys of abdominal vibration. We tested 15–16 males and 15–16 females using playbacks of two signals that gradually increased or decreased in volley period, starting at the species mean. We found that (1) duets during courtship are accurate, interactive, and adjustable by each participant; (2) in staged duets, both sexes respond best to song tempos near the mean volley period of their population, but can nonetheless maintain duets with signals of nearly twice, or half, the normal volley period; (3) individuals fine-tune their adjustments to signals of different volley periods by changing their own volley duration and latent period, or less often by inserting extra volleys or skipping every other volley; (4) males are significantly better at matching signals of changing tempo than females; and (5) the range of song responsiveness of C. plorabunda does not overlap the natural range of volley periods found in Chrysoperla adamsi, an acoustically similar sibling species, thus reaffirming strong behavioral isolation. In sum, the precise, almost unbreakable heterosexual duets characteristic of song species of the carnea group result from tight mutual feedback between partners. Effective reproductive isolation between species can be based on song differences alone.