Limitations to colour-based sexual preferences in three-spined sticklebacks (Gasterosteus aculeatus)

Female three-spined sticklebacks (Gasterosteus aculeatus) are suggested to select mates based on their red nuptial coloration, males with a redder display being more preferred. Although there are both laboratory and field data to support this view, there are also published accounts where females do not show a preference for the redder male. Here we report the results of a series of 19 trials where receptive gravid female three-spined sticklebacks were allowed to choose between two size-matched rival males. We used photographic and image analysis techniques to quantify male nuptial coloration to investigate how the magnitude of the colour difference between the two alternative males influenced female preferences. Using the amount of time a female spent oriented towards each male as a measure of his attractiveness to her, females were not always found to select the redder of the two presented males. We did, however, find that that the relative difference in coloration of the two males in each pair was important in determining the level of coloration-based preference, with females only selecting redder males consistently when the difference in coloration was sufficiently large. Received: 26 October 1999 / Received in revised form: 10 January 2000 / Accepted: 13 February 2000     

The effect of rearing environment on blue structural coloration of eastern bluebirds (<Emphasis Type="Italic">Sialia sialis</Emphasis>)

We used a brood-size manipulation to test the effect of rearing environment on structural coloration of feathers grown by eastern bluebird (Sialia sialis) nestlings. Ultraviolet (UV)-blue structural coloration has been shown to be sexually selected in this species. Our experimental design took advantage of the growth of UV-blue wing feathers in nestlings that are retained as part of the first nuptial plumage. We cross-fostered nestlings to create enlarged and reduced broods with the purpose of manipulating parental feeding rates and measured the effect on nestling growth and plumage coloration. Brood size influenced feeding rates to offspring, but the effect varied with season. In general, male nestlings reared in reduced broods were fed more often, weighed more, and displayed brighter structural plumage compared to nestlings reared in enlarged broods. Female nestlings appeared to experience less adverse affects of brood enlargement, and we did not detect an effect of brood-size manipulation on the plumage coloration of female nestlings. Measures of plumage coloration in both males and females, however, were correlated to hatching date and nestling mass during feather development. These data provide empirical evidence that environmental quality can influence the development of the blue structural coloration of feathers and that males may be more sensitive to environmental fluctuations than females.     

Egg coloration in ring-billed gulls (<Emphasis Type="Italic">Larus delawarensis</Emphasis>): a test of the sexual signaling hypothesis

Although many avian eggs appear to be cryptically colored, many species also lay vibrant blue green eggs. This seemingly conspicuous coloration has puzzled biologists since Wallace, as natural selection should favor reduced egg visibility to minimize predation pressure. The sexual signaling hypothesis posits that blue green egg coloration serves as a signal of female quality and that males exert post-mating sexual selection on this trait by investing more in the nests of females laying more intensely blue green eggs. This hypothesis has received mixed support to date, and most previous studies have been conducted in cavity-nesting species where male evaluation of his partner’s egg coloration, relative to that of other females, may be somewhat limited. In this study, we test the sexual signaling hypothesis in colonially nesting ring-billed gulls (Larus delawarensis) where males have ample opportunity to assess their mate’s egg coloration relative to that of other females. We used correlational data and an experimental manipulation to test four assumptions and predictions of the sexual signaling hypothesis: (1) blue green pigmentation should be limiting to females; (2) extent of blue green egg coloration should relate to female quality; (3) extent of blue green egg coloration should relate to offspring quality; and (4) males should provide more care to clutches with higher blue green chroma. Our data provide little support for these predictions of the sexual signaling hypothesis in ring-billed gulls. In light of this and other empirical data, we encourage future studies to consider additional hypotheses for the evolution of blue green egg coloration.     

Testosterone and the allocation of reproductive effort in male house finches (Carpodacus mexicanus)

Testosterone has been proposed to serve as the mediator that controls the relative effort that an individual male bird will devote to mating effort versus parental effort. Here, we demonstrate a testosterone-influenced trade-off between parental and mating efforts in male house finches. Male house finches with experimentally elevated testosterone fed nestlings at a significantly lower rate, but sang at a higher rate than males without manipulated testosterone levels. Females mated to testosterone-implanted males fed nestlings at a significantly higher rate than females mated to males without testosterone implants, resulting in similar feeding rates for both treated and untreated pairs. The effects of testosterone on male house finches, however, were not as dramatic as the effects of testosterone observed in some other socially monogamous species of birds. Because extra-pair copulations are uncommon in house finches and males provide substantial amounts of parental care, these more modest effects may be due to differences in how the allocation of reproductive effort affects the costs and benefits of different reproductive behaviors. Received: 6 June 2000 / Accepted: 17 July 2000     

The timing of chemical signaling with urine in dominance fights of male lobsters (Homarus americanus)

Chemicals carried in the urine are a rich source of information about the identity, sex, aggressive motivation, and other attributes of an animal. In agonistic interactions, animals should be careful about disclosing such information, since it can be used by receivers to the disadvantage of the sender. By adjusting the timing of urine release, the signaler may still influence the behavior of the receiver to its own benefit. Here we investigate the urine signaling of American lobsters (Homarus americanus), necessary for the maintenance of dominance, using a catheter technique with high temporal resolution. We hypothesize that urine release in lobster fights is not continuous but restricted to moments that may elicit a reaction in the receiver beneficial to the sender. We found that eventual winners signaled more readily in the initial phase of the fight. Eventual losers did not show such initial urination. This suggests that a confident lobster may use urine signals to influence the behavior of its opponent into giving up the fight at an early stage. For both winners and losers, urine release is linked to offensive behavior and increases as the intensity of agonistic behavior increases. The coupling of urine release to offensive behaviors appears to improve both the reliability and efficacy of the chemical signal. Releasing urine under the increasing risk of inflicting injury during the fight provides a handicap that may ensure reliability of the threat signal. Furthermore, the immediate coupling of urine components to offensive behavior may help to consolidate the receiver"s memory of the signaler"s individual scent and thus facilitate future recognition of the dominant animal. Received: 6 October 1999 / Accepted 28 August 2000     

Threat-sensitive feeding strategy of immature sticklebacks (Gasterosteus aculeatus) in response to recent experimental infection with the cestode Schistocephalus solidus

Threat-sensitive decision-making might be changed in response to a parasitic infection that impairs future reproduction. Infected animals should take more risk to gain energy to speed up their growth to achieve early reproduction and/or to strengthen their immune response. To avoid correlational evidence, we experimentally infected and sham-infected randomly selected immature three-spined sticklebacks with the cestode Schistocephalus solidus. For 7 weeks we determined the threat-sensitive foraging decisions and growth of individual sticklebacks in the presence of a live pike (Esox lucius). The experimenters were blind with respect to the infection status of the fish. In contrast to previous studies, our recently infected fish should have been almost unconstrained by the parasite and thus have been able to adopt an appropriate life history strategy. We found a strong predator effect for both infected and uninfected fish: the sticklebacks’ risk-sensitive foraging strategy resulted in significantly reduced growth under predation risk. Infected fish did not grow significantly faster under predation risk than uninfected fish. Since infected fish consumed much less prey in the presence of the predator than did infected fish in its absence, they obviously did not use the opportunity to maximize their growth rate to reach reproduction before the parasite impairs it. Received: 21 June 1999 / Revised: 27 November 1999 / Accepted: 5 September 2000     

Characteristics that influence male reproductive success on a lek of Lethrinops c.f. parvidens (Teleostei: Cichlidae)

Sexual selection has long been proposed as a mechanism leading to the diverse cichlid (Teleostei: Cichlidae) fauna of Lake Malawi, Africa. Many of the shallow-water, sand-dwelling, bower-building cichlid species are particularly well suited for studies of sexual selection because they participate in leks. Since females in lekking systems appear to acquire only genetic material from their mates, it has been suggested that leks are ideal systems to study female mate choice. The objectives of the investigation were to examine Lethrinops c.f. parvidens male bower characteristics (i.e., bower size and location) as well as other male characteristics (i.e., length, gular color, and duration on the lek) for their influence on male mating success as measured by the number of visits, circles, and eggs laid by females. These measures are nested in that a visit by a female may or may not lead to circling, and circling by a female may or may not lead to egg-laying. We found increased bower height and higher numbers of conspecific neighbors (analogous to shallow-water, near-shore bower positions) to be positively, significantly associated with the number of visits by females. The only significant correlate with the number of circles was visits, and similarly circles was the only significant correlate with the number of eggs laid. The R ² value for the egg-laying regression was quite low (19.8%) compared with visits (54.3%) and circling (78.9%), suggesting that females may be using additional cues, that we failed to measure, when in close proximity to males or simply that a small proportion of the females were ready to spawn. Both indirect selection and direct selection pressure due to egg predation may have influenced female choice on the lek. Received: 10 April 1999 / Received in revised form: 26 July 1999 / Accepted: 18 September 1999     

Female distribution affects mate searching and sexual selection in male northern water snakes (Nerodia sipedon)

Mating systems and sexual selection are assumed to be affected by the distribution of critical resources. We use observations of 312 mating aggregations to compare mate-searching success of male northern water snakes (Nerodia sipedon) in two marshes in which differences in mating substrate availability resulted in more than fourfold differences in female dispersion. Reproductive males had significantly larger home ranges where females were dispersed than where females were clumped. The number of females encountered by males increased significantly with male home range size where females were dispersed, and decreased significantly where females were clumped. Where females were clumped, males were more likely to encounter other males when they located females. We found no evidence in either population that mate searching was energetically expensive or that males with relatively more energy had larger home ranges. However, males with greater fat reserves at the start of the season participated in more mating aggregations when females were dispersed, suggesting that fat reserves could affect a male’s willingness to attempt mating or to persist in aggregations. When females were dispersed there was weak stabilizing selection acting to maintain male body size (β=–0.14), but strong directional selection favoring larger (β=0.50) and fatter (β=0.37) males. Over 7 years, the intensity of selection favoring larger males varied substantially (β=0.14–1.15), but that variation was not related to variation in the operational sex ratio. We found no evidence of directional selection on either body size (β=0.05) or fat reserves (β=0.10) of males when females were spatially clumped. Overall, the distribution of females had a pronounced effect on male behavior, on the factors that affected male success in locating females, and probably on the extent of sperm competition once females had been located. Received: 23 November 1998 / Received in revised form: 9 August 1999 / Accepted: 18 August 1999     

Seasonal variation in the relationship between cellular immune response and badge size in male house sparrows (Passer domesticus)

The immunocompetence handicap hypothesis postulates that secondary sexual traits are honest signals of male quality because steroid hormones (such as corticosteroids and sex steroids), which are supposed to favor the development of secondary sexual traits, may also have immunosuppressive effects. Certain secondary sexual traits are not only used as mate choice signals but also play a role as badges of status. In the house sparrow (Passer domesticus), males have a bib of black feathers which is used both as a signal of social status in male-male interactions and by females when choosing a mate. We investigated the relationships between bib size and cellular immune response in male house sparrows during and outside the reproductive season. Males with large badges were found to have lower levels of immunocompetence, as assessed using a T-cell-mediated immunity assay, during the reproductive season, as predicted by the immunocompetence handicap hypothesis. Conversely, in November, the correlation between badge size and cellular immune response was positive, possibly reflecting the better access to trophic resources of large-badged dominant males in winter flocks. Received: 24 September 1998 / Received in revised form: 2 February 1999 / Accepted: 14 February 1999     

Display rate by male fairy-wrens (Malurus cyaneus) during the fertile period of females has little influence on extra-pair mate choice

Empirical and theoretical studies have only recently begun to examine how females use complex multi-component displays when selecting mates. Superb fairy-wrens are well suited to the study of female choice because females have control over extra-group paternity and cuckold their mates at high rates, while males possess a variety of sexually selected traits. Available evidence suggests that females base their extra-group mate choice on the timing of male moult into breeding plumage or the onset of display. However, males continue to perform elaborate displays throughout the season, and direct most displays to females during their fertile period. We therefore conducted focal observations on fertile females to quantify the frequency of male display and used microsatellite genotyping to compare the role of display rate during the breeding season and the timing of male moult on female mate choice. We show that the addition of data on male display rate does not improve our ability to predict which males obtain extra-group paternity. The timing of male moult into breeding plumage remains the only predictor of male extra-group reproductive success. Nevertheless, we found that males displayed more to females that were unable to select extra-group mates on the basis of the timing of moult or the onset of display. This raises the possibility that there are circumstances when females use display rate to discriminate between potential extra-group sires. Overall this study supports the theoretical prediction that females are more likely to base their mate choice on reliable indicators of male quality such as fixed morphological traits and displays of endurance, in this case an early moult into breeding plumage and the performance of an elaborate display during the winter, than a flexible behavioural trait such as display rate during the breeding season. Received: 26 January 2000 / Revised: 1 August 2000 / Accepted: 26 August 2000     

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     

Assessing the aerodynamic effects of tail elongations in the house martin (Delichon urbica): implications for the initial selection pressures in hirundines

Of the three species of hirundine that breed sympatrically across the U.K., one, the barn swallow, has outer tail feathers elongated into streamers, whereas the other two species, the house martin and the sand martin, do not. The tail streamer of the barn swallow is regarded as a classic example of a sexually selected trait. Recent evidence, however, has suggested that streamers may have evolved largely through natural selection for enhanced flight performance and increased maneuverability. We tested the hypotheses that small streamers (1) increase performance in turning flight, but (2) decrease performance in flight variables related to velocity. We manipulated the lengths of house martin outer tail feathers and measured changes in their free-flight performance, using stereo-video to reconstruct the birds" three-dimensional flight paths. Five flight variables were found to best describe individual variation in flight performance. Of these five, the three variables determining maneuverability predicted that flight performance would be optimized by a 6- to 10-mm increase in the length of the outer tail feathers. In contrast, for mean velocity and mean acceleration, extension of the outer tail feathers appears to have a detrimental effect on flight performance. We suggest that the initial selection pressure for streamers in ancestral short-tailed "barn swallows" was via natural selection for increased maneuverability. In addition, we propose that the benefits of increased maneuverability have differed between hirundines in the past, such that the cost of increasing the length of the outer tail feather has, to date, outweighed the benefits of doing so in streamerless hirundines. Received: 14 February 2000 / Revised: 2 July 2000 / Accepted 18 July 2000     

Experimental evidence of a testosterone-induced shift from paternal to mating behaviour in a facultatively polygynous songbird

Previous studies have suggested that testosterone (T) profiles of male birds reflect a trade-off between mate attraction behaviours (requiring high T levels) and parental care activities (requiring low T levels). In this study, we experimentally elevated T levels of monogamous males in the facultatively polygynous European starling (Sturnus vulgaris), and compared mate attraction and paternal behaviour of T-treated males with those of controls (C-males). T-males significantly reduced their participation in incubation and fed nestlings significantly less often than C-males. Females paired to T-treated males did not compensate for their mate’s lower paternal effort. The observed reduction in a male’s investment in incubating the eggs was accompanied by an increased investment in typical female-attracting behaviours: T-males spent a significantly higher proportion of their time singing to attract additional females. They also occupied more additional nestboxes than C-males, although the differences just failed to be significant, and carried significantly more green nesting materials into an additional nestbox (a behaviour previously shown to serve a courtship function). T-males also behaved significantly more aggressively than C-males. During the nestling period, the frequency of mate-attracting behaviours by T-treated and control males no longer differed significantly. Despite the reduced paternal effort by T-males and the lack of compensation behaviour by females, hatching and breeding success did not differ significantly between T- and C-pairs. Received: 7 February 2000 / Revised: 10 August 2000 / Accepted: 3 September 2000     

Post-mating odor in females of the solitary bee, Andrena nigroaenea (Apoidea, Andrenidae), inhibits male mating behavior

We investigated odor changes and their behavioral significance in the solitary, ground-nesting bee Andrena nigroaenea. We used gas chromatography with electroantennographic detection and performed behavioral tests with males in the field using natural odor samples and synthetic compounds. We found that only cuticle extracts of young females elicited copulation attempts in the males. We demonstrated that among the 17 compounds which triggered electroantennographic responses, all-trans-farnesyl hexanoate and all-trans-farnesol were significantly more abundant in unattractive cuticle extracts of A. nigroaenea females. Dufour’s gland extracts of these females also contained greater amounts of both compounds. In bioassays using synthetic farnesyl hexanoate and farnesol we found that these compounds inhibit copulation behavior in the males. Farnesyl hexanoate is probably synthesized in Dufour’s gland and used by females for lining brood cells. We interpret the semiochemical function of farnesyl hexanoate and its precursor farnesol to have evolved secondarily. As an outcome of sexual selection, it facilitates the discrimination by males of receptive females from nesting and thus already mated individuals. The dual function of these compounds represents an elegant parsimony in the chemical communication system of this insect. Received: 19 January 2000 / Revised: 29 May 2000 / Accepted: 24 June 2000     

Function of the mating plug in Drosophila hibisci Bock

The mating plug in Drosophila hibisci Bock is a firm, gelatinous structure that forms within the female’s uterus during copulation. Two non-mutually exclusive hypotheses for the function of the plug were evaluated. The plug may serve as a nutritional gift that females digest, using the constituents for somatic maintenance or to provision eggs as they mature within the ovaries. Alternatively, the plug may act as a chastity enforcement device by preventing subsequent copulations, and thereby reducing sperm competition. Plug size did not decrease within females over a period of 2 days, and dietary treatment in females did not affect plug size. The extent of ovarian provisioning was also not related to plug size. These results weaken the nutritional gift hypothesis. In contrast, the probability of a second copulation increased sharply with an experimental decrease in plug size. Moreover, females with plugs experimentally reduced in size were courted significantly more and mated significantly faster than females with larger plugs. These results support the chastity enforcement hypothesis. The plug retains the ejaculate and concentrates sperm at the anterior end of the uterus near the apertures of the sperm storage organs. The presence of the plug thus probably facilitates the movement of sperm into storage by retaining sperm at the anterior end of the uterus near the apertures of the sperm storage organs, which may be especially important for D. hibisci, in which sperm length is nearly twofold greater than ventral receptacle length. Matings with newly eclosed virgin females were significantly shorter than with older virgins, and copulations with the younger virgins ended more often without any sperm having yet entered into storage. The effectiveness of the plug in safeguarding a male’s ejaculate may have favoured the evolution of shortened copula durations with young virgins. One fitness advantage of shortened copula duration could be time liberated for the pursuit of further mating opportunities. Received: 12 May 2000 / Revised: 22 September 2000 / Accepted: 15 October 2000     

Ectoparasites of house sparrows (Passer domesticus): an experimental test of the immunocompetence handicap hypothesis and a new model

Conspicuous secondary sexual traits may have evolved as handicap-revealing signals or as badges of status. We present results of an experiment using males of the sexually dimorphic house sparrow (Passer domesticus), that support the idea that the male-specific bib can be both a handicap-revealing signal and a reliable badge indicating the physical condition of the bird. In a test of the immunocompetence handicap hypothesis, wild-caught adult male house sparrows were studied in captivity. Birds implanted with elevated doses of testosterone were more dominant, had higher circulating levels of both testosterone and corticosterone and they also harboured relatively larger ectoparasite loads. Higher parasite loads were also associated with individuals showing lower immunocompetence and larger changes in bib size. A new model for immunocompetence effects in sexual selection is introduced, integrating actions that the hypothalamopituitary axis exerts on gonads, adrenals and the thyroid gland. The ”integrated immunocompetence model” synthesizes both the ”handicap” (i.e. survival-decreasing) and ”badge of status” (i.e. survival- enhancing) models for evolution of secondary sexual traits. Received: 15 May 1999 / Received in revised form: 2 November 1999 / Accepted: 6 November 1999     

Genetic evidence for mixed parentage in nests of the emu (Dromaius novaehollandiae)

Parentage in emus (Dromaius novaehollandiae) was examined by microsatellite analysis using four independent loci. Of 106 chicks sampled in one breeding season from 18 nests, 54 (51%) were not fathered by the nesting male, 12 (11%) were not from the observed mate of the sitting male, and 9 (8%) represented intra-specific brood parasitism, having no alleles in common with either nest parent. Some males (11%) fathered all chicks in their nests, but the majority showed high levels of cuckoldry. Those males commencing incubation earliest in the season tended to have the highest levels of paternity in their own nests. These results reveal a high frequency of extra-pair fertilisations and resultant cuckoldry in a predominantly socially monogamous bird and support recent reports which have described the emu mating system as a complexity of polyandrous, promiscuous and monogamous behaviour. Parentage assignment of chicks resulting from extra-pair fertilisations revealed an evenly scattered pattern of paternity that did not show any particular male dominance in reproductive success. These results lead to a reassessment of behavioural observations of emus, the consequences of parentage distribution, and theories about mating systems and sexual selection. The frequency of extra-pair copulations and intra-specific brood parasitism suggests patterns of descent that differ greatly from those implied by social monogamy. Received: 27 November 1999 / Received in revised form: 8 January 2000 / Accepted: 8 February 2000     

Female-biased immigration and male peace-keeping in groups of the shell-dwelling cichlid fish Neolamprologus multifasciatus

The small cichlid fish Neolamprologus multifasciatus is endemic to Lake Tanganyika and is unique among fish because it lives in complex social groups with several reproductive males and females, all of which participate in defending their territory against neighbors and intruders. Individuals use empty snail shells for breeding and shelter. Previous parentage analysis using microsatellites suggested occasional exchange of individuals between groups. In field experiments, we found that females showed a higher tendency than males to migrate into territories already occupied by a resident pair. The phenomenon and causes of female-biased immigration were further investigated in aquarium experiments: Nine of 15 females, but only 2 of 15 equally sized males, settled in territories of established pairs. Territorial males exhibited more aggression toward strange males (potential reproductive competitors) than toward strange females (potential additional mates); their females were more aggressive toward strange females (probably competitors for shells) than toward strange males. Apparently, a conflict exists between the sexes regarding the immigration of additional females. This conflict seems to be the selective basis for observed active male interference in aggressive disputes between females in the territory. Interfemale tolerance in a group was greater in the male’s presence than in his absence. Received: 1 February 2000 / Received in revised form: 2 May 2000 / Accepted: 5 May 2000     

Sexual selection in the northern pintail (Anas acuta): the importance of female choice versus male-male competition in the evolution of sexually-selected traits

We experimentally studied the relative importance of plumage, dominance status, and courtship behavior in determining male pairing success in the northern pintail Anas acuta and assessed whether these traits function in female choice, male-male competition or both. In an experiment (experiment IA) that eliminated the confounding effects of male-male competition and social courtship, females chose males with pure white breasts and colorful scapular feathers. When the same group of birds were free to interact (experiment 1B), male behavior was more important: females chose males that courted them intensely and were attentive to them, although preferred males again had whiter breasts and more colorful scapulars. In a second experiment (experiment 2), testing the effect of age on pairing success, females showed a significant preference for 2-year-old males over yearlings: 2-year-old males courted more and were more attentive to the female than yearlings; they were also more colorful than yearlings in a number of plumage measurements. Although males (in both experiments 1B and 2) were aggressive to one another while courting the female and dominant males were sometimes able to exclude subordinates from social courtship, contrary to expectation, we found no relationship between initial dominance rank and pairing success or dominance rank and age. In addition, dominance was not correlated with any of the morphological traits measured. Once chosen, however, subordinate males typically initiated fights with the higher-ranked male(s) and quickly achieved dominance. These results suggest that (1) females choose males based on a suite of morphological and behavioral characteristics, (2) male dominance relationships do not constrain active female choice, (3) a male's position in a dominance hierarchy is largely a result rather than a cause of female choice, and (4) female choice plays a more significant role than male-male competition in the evolution of several secondary sexual traits in male northern pintails.     

Measurement of parental investment and sex allocation in the European beewolf Philanthus triangulum F. (Hymenoptera: Sphecidae)

Fisher’s 1930 theory of sex allocation predicts a population-wide 1:1 ratio of parental investment. We tested this prediction in the European beewolf, a sphecid wasp that hunts for honeybees as larval food. Because the method to quantify parental investment is of crucial importance, we compared the suitability of several different investment measures. Female/male cost ratios were determined from a sample and the total investment in sons and daughters was calculated. In addition, the actual number of prey items for sons and daughters was directly determined by excavating nests and counting the cuticle remains of the prey. Though mortality was high (70%), it had only a weak effect on the estimate of the investment ratio. Based on commonly used measures like fresh and dry weight of emerged adults, the investment ratio did not deviate from Fisher’s prediction of equal investment. However, progeny weight considerably underestimates investment in males and investment in large progeny. Measures that reflect the allocation of resources more directly (amount of provisions, brood cell volume) revealed a significant male bias and thus contradicted Fisher’s theory. Three kinds of explanation are discussed. First, non-adaptive explanations are unlikely. Second, from the spectrum of alternative adaptive theories, only models that assume a non-linear relationship between amount of investment and progeny fitness seem to be relevant for the study species. Third, though the number of prey in a brood cell seems to be a rather good measure of parental investment in European beewolves, some problems in measuring parental investment remain. These problems are of broad significance. Received: 17 June 1999 / Received in revised form: 6 July 1999 / Accepted: 11 July 1999