Factors affecting the allocation of paternal care in waterbugs (Belostoma flumineum Say)

Summary Male giant waterbugs (Belostoma flumineum Say) brood eggs oviposited on their dorsa by conspecific females. Laboratory observations indicate that viable egg pads are occasionally discarded before hatching. Theory predicts that such behavior should occur only if the costs incurred by brooding exceed the benefits of hatching the egg pad. We studied the effects of egg pad size, time invested in brooding, and egg viability upon the continuation of paternal care in the giant waterbug. We found that smaller egg pads are less likely to hatch than larger ones, and males appear to be less likely to discard egg pads as temporal investment increases. However, the inviability of eggs did not appear to affect the probability of an egg pad being discarded. Males of this species appear to have evolved a decision-making process involving the continuation of paternal care. Offprint requests to: K.C. Kruse     

Mate availability accelerates male filial cannibalism in a nest brooding fish: effects of number and fecundity of females

Theoretical models predict that increased mate availability accelerates filial cannibalism by the parental male, but we do not yet fully understand how the various aspects of mate availability contribute to this effect. We examined the effects of two elements of mate availability—female fecundity and sex ratio—on filial cannibalism by the lizard goby, Rhinogobius flumineus, which is a paternal nest brooding fish. We used three types of females (stimulus-females): a single female with slim belly (not ready to spawn), a single gravid female (ready to spawn), and two gravid females. Stimulus-females were housed in a transparent cage and shown to subject males just before and after spawning with a separate female, after which males cared for the brood. A single gravid stimulus-female accelerated filial cannibalism, compared with a control-stimulus consisting of an empty cage, but only during the early care period. In contrast, a single slim-bellied stimulus-female did not accelerate cannibalism. A stimulus of two gravid females accelerated cannibalism to the same degree as a single gravid female stimulus. Our results suggest that in lizard gobies, filial cannibalism by parental males is accelerated by female quality (fecundity) in the early care period, but not by a higher number of available females (sex ratio).     

On the timing of moulting processes in reproductively active Northern krill Meganyctiphanes norvegica

The interactions between moult phasing, growth and environmental cues in Northern krill (Meganyctiphanes norvegica) were examined through analysing populations at seasonal, weekly, and daily timescales. The analyses were carried out on resident populations of krill found in three different neritic locations that experience similar environmental signals (the Clyde Sea, Scotland; the Kattegat, Denmark; Gullmarsfjord, Sweden). Seasonal analyses were carried out on the Clyde Sea population and showed that moulting frequency increased significantly moving from winter to summer. The proportion of moulting females in summer samples was often more than double the proportion of moulting males, suggesting that females had a comparatively shorter intermoult period (IMP). Weekly samples taken from the Kattegat showed a similar pattern. However, although the difference between the proportion of female and male moulters was significant in one week, it was not another, mainly because of the variability in the proportion of female moulters. Such variability in females was equally evident in the daily samples taken at Gullmarsfjord. It suggests that females have a shorter IMP (12.5 days) than males (18.4 days) and are more likely to moult in synchrony. Nevertheless, the daily samples revealed that males are also capable of moult synchronisation, although less frequently than females. Shortened IMPs in females were not a result of the abbreviation of specific moult stages. Accordingly, reproductive activity did not alter the course of the normal moult cycle. There was no significant difference between the total body lengths of males and females indicating that females achieve the same levels of growth despite moulting more frequently and having to provision the energy-rich ovaries. This is in contrast to most other crustaceans where the energy costs of reproduction reduce female growth. The fact that females were less abundant than males, probably by suffering a greater level of mortality, suggests that different behavioural strategies, particularly vertical migration regimes, were adopted by each sex to maximise growth and reproduction.     

Operational sex ratios and behavioural sex differences in a pipefish population

In the pipefish Syngnathus typhle, only males brood embryos in specially developed brood pouches, supplying oxygen and nutrients. Laboratory studies have shown that this elaborate paternal care has led to sex-role reversal in this species: males limit female reproductive rate, females are the primary competitors for mates and males exercise greater selectivity in accepting mates. In the first field study of this pipefish, we describe mating behaviour in the wild and test the hypothesis that temporal variations in the operational sex ratio (OSR) determine sex differences in mating behaviour. Our study comprised two reproductive seasons of two sequential mating periods each, the latter separated by a lengthy interval of male brooding. During mating periods, females displayed to all males without wandering and males moved about searching for females, without reacting to all females. The OSR was least female-biased (or even male-biased) at the onset of the breeding season, when most pipefish were simultaneously available to mate, but became strikingly female-biased as males' pouches were filled. The OSR remained substantially female-biased during the second mating period, because few males became available to remate at any one time. As hypothesised, female-biased OSRs resulted in more female-female meetings. As well, females were above the eelgrass more often than brooding males, thus exposing themselves to conspecifics and/ or predators. In the second year, males arrived earlier than females on the breeding site and male pregnancies were shorter, because of higher water temperatures, so rematings occurred earlier. Males met more often during that year than the previous one, but male competitive interactions were still not observed. The field results support laboratory studies and demonstrate that behaviours associated with female-female competition are more prominent when the OSR is more female-biased. Correspondence to: A. Vincent     

Mate attraction by females in a sexually cannibalistic praying mantis

We studied mate attraction by females of the praying mantid, Tenodera aridifolia sinensis, testing honest signaling of mate availability versus deceptive signaling to attract males for sexual cannibalism. We experimentally varied female diet and mating history and measured the rate of attraction of a wild population of males to caged females. Honest signaling theory predicts that virgin females will attract males at the greatest rate whereas deceptive signaling predicts that hungry females (which are more likely to cannibalize males) will attract more males, particularly among non-virgin females. Our results show that hungry females did not attract more males than well-fed females. Indeed, the opposite was true: hungry females attracted significantly fewer males. Moreover, hungry females were no more likely than well-fed females to attract males subsequent to mating, and mated females attracted males at a lower rate than did virgin females. We also observed female T. aridifolia sinensis and male Mantis religiosa arriving at the caged females and we discuss the significance of these observations. The results refute the hypothesis of deceptive signaling and show that mate attraction signals of female T. aridifolia sinensis are honest indicators of female mate availability and a lower risk of sexual cannibalism.     

Female egg investment in relation to male sexual traits and the potential for transgenerational effects in sexual selection

Life-history theory predicts that individuals should increase their reproductive effort when the fitness return from reproduction is high. Females mated with high-quality males are therefore expected to have higher investment than females mated with low-quality males, which could bias estimates of paternal effects. Investigating the traits females use in their allocation decisions and the aspects of reproduction that are altered is essential for understanding how sexual selection is affected. We studied the potential for differential female allocation in a captive population of a precocial bird, the Chinese quail, Coturnix chinensis. Females paired with males with large sexual ornaments laid larger, but not more, eggs than females paired with males with small sexual ornaments. Furthermore, female egg mass was also significantly positively affected by male testis size, probably via some unknown effect of testis size on male phenotype. Testis size and ornament size were not correlated. Thus, both primary and secondary male sexual traits could be important components of female allocation decisions. Experimental manipulation of hormone levels during embryonic development showed that both male and female traits influencing female egg size were sensitive to early hormone exposure. Differences in prenatal hormone exposure as a result of maternal steroid allocation to eggs may explain some of the variation in reproductive success among individuals, with important implications for non-genetic transgenerational effects in sexual selection.Communicated by C. Brown     

Evidence for seasonal mate limitation in populations of an intertidal amphipod, Corophium volutator (Pallas)

Potential rates of reproduction (PRR) differ between the sexes of many animal species. Adult sex ratios together with PRR are expected to determine the operational sex ratio (OSR) defined as the ratio of fertilizable females to sexually active males at any given time. OSR is expected to determine the degree to which one sex competes for another—the limiting sex. We explored the potential for mate limitation in an intertidal amphipod, Corophium volutator (Pallas). Males have higher PRR than females, but males may be limiting because of extreme female-biased sex ratios observed in this species. Consistent with this idea, late season females were less likely to be ovigerous and had smaller size-specific clutches, both of which were associated with seasonal declines in availability of males of reproductive size. Seasonal changes in ovigery could not be explained by seasonal changes across sites in other factors (e.g., female body size or phenology of breeding). Smaller females were less likely to become ovigerous later in the season at three of four sites. Seasonal reductions in clutch size also occurred among small females expected to be reproducing for their first time. In complimentary laboratory experiments, reduced likelihood of ovigery and reduced fecundity occurred when the number of receptive females was increased relative to availability of a reproductively active male. Our results suggest male mate limitation can occur seasonally in this species and that male limitation is regionally widespread and may affect recruitment.     

Independent effects of male and female density on sexual harassment, female fitness, and male competition for mates in the western mosquitofish Gambusia affinis

Operational sex ratio (the ratio of sexually active males to fertilizable females) has a major influence on male competition for mates and male–female interactions. The contributions of male and female density per se to mating system dynamics, however, are rarely examined, and the fitness consequences are often inferred rather than quantified. Male mosquitofish (Gambusia affinis) compete aggressively and frequently harass females for copulations, a behavior thought to reduce female fitness. Female fitness can also be reduced by increases in female density, which may affect food availability, cannibalism rates, and chemical interactions between females. I manipulated male and female densities of G. affinis to measure their effects on male–male aggression, male harassment toward females, and female fitness. I found that males chased rivals more often and attempted fewer copulations when female density decreased, but surprisingly male density had no significant effect on the frequency of these male behaviors. In contrast, males’ agonistic displays toward other males increased with male density, but display behavior was unaffected by female density. These results suggest that male and female density do not always contribute equally or at all to the patterns of behavior we observe. Female fitness declined as female density increased, the opposite pattern expected if male harassment is costly to females. This suggests that a strong, negative effect of female density overwhelmed any potential costs of male harassment. Sources of female density dependence and the consequences of changes in male and female density to patterns of male behavior are discussed.     

Spatial organization and mating system of Microtus townsendii

Summary Space use by individual Townsend's voles, Microtus townsendii, was investigated in spring and summer by means of radiotelemetry and intensive live trapping in undisturbed grasslands near Vancouver, British Columbia. Home ranges of males were larger than those of females; females had significantly larger ranges in spring than in summer. Most males and females maintained territories free of individuals of the same sex in spring. Male-female pairs had their exclusive territories closely overlapping each other. The 1:1 operational sex ratio and the spatial association of pairs of males and females suggest that the voles were monogamous in the spring of 1988 and that 50% of the males were monogamous in the spring of 1989. In summer, there was more intrasexual overlap between home ranges of males and females and female ranges were considerably smaller than those of males. Females were more philopatric than males and females thought to be members of the same family group lived adjacent to each other or had overlapping home ranges. Males overlapped with more than one female in summer, but most females still overlapped with only one male, which suggests that the mating system is polygynous in summer. Thirty-five percent of the philopatric females became pregnant for the first time when the male spatially associated with their mother in the spring was still alive and thus could potentially have mated with their fathers. Male and female territoriality in spring is the proximate mechanism for the limitation of breeding density by spacing behaviour.[/p] Offprint requests to: C.J. Krebs     

The costs of male parental care and its evolution in a neotropical frog

Summary Parental care is practiced exclusively by males of the Puerto Rican frog, Eleutherodactylus coqui. Males brood clutches of direct-developing eggs in non-aquatic nest sites and defend eggs against cannibalistic nest intruders. Here, I report on energetic and mating costs incurred by males that provide parental care, and suggest how these proximate costs affect male fitness and the evolution of male parental care in this species. Energetic costs are small for brooding males in comparison to non-brooding, calling males. Brooding males had a higher frequency of empty stomachs and lost small, but significant, fractions of their initial body mass during parental care. Abdominal fat bodies of brooding males during the middle third of parental care were significantly smaller than those of calling males; those of males brooding eggs in earlier or later stages were not different. The mating cost of parental care is greater. Most brooding males cease calling during parental care. However, gravid females are available (i.e., known to mate) on most nights during the principal breeding season; hence non-calling males miss potential opportunities to mate. A mating cost was estimated by calculating nightly mating probabilities for calling males in a plot where nightly calling male densities and daily oviposition schedules were known. On average, a male exhibiting normal calling behavior would be expected to obtain a new mate once every 35.7 days. Hence a brooding male that ceased calling for a 20-day parental care period would miss, on average, 0.56 additional mates. Males that were more successful than average in attracting mates could miss up to 1.63 matings. A marginal value model (Fig. 1) is used to analyze the net effect on male fitness of parental care benefits and costs in E. coqui (Fig. 3). The model indicates that males garner the highest reproductive success by providing care from oviposition through hatching. There is no stage during the pre-hatching period at which a desertion strategy would yield higher reproductive success. In fact, the model suggests that males should provide full parental care even in the face of much higher mating costs than currently obtain in the system.     

Reproduction of the invasive slipper limpet, Crepidula fornicata, in the Bay of Brest, France

The reproduction of Crepidula fornicata was studied in the Bay of Brest in order to characterise the first step of the reproductive cycle of this invasive species. The survey was carried out from 2000 to 2003 and different parameters were measured, namely, the percentage of the different sexual stages, the straight length of the shell and the percentage of brooding females using a survey of the embryonic development and the fecundity. The juvenile frequency increases generally from mid-June or mid-August, depending on the year. In 2001 and 2003, a first peak was observed as early as May, but it was followed by a rapid disappearance of the individuals. The sex-ratio female/male increased from 0.22 to 0.46 between 2001 and 2003. The sex change between intermediates and females took place mainly in summer and was well marked in 2001 and 2003. The survey of the embryonic development in the egg capsules brooded by the females provided an annual phenology of the laying and hatching processes. The laying period extends from February to September with three to four major periods of egg-laying per year and corresponding hatching periods about 1 month later. Each female lays two to four times per year on average. The first egg-laying concerned fewer females than subsequent ones, except in 2003, and exhibited a higher fecundity. The annual mean of the number of eggs for each stage was not significantly different, thus indicating no significant mortality rate during embryonic development. For the C. fornicata population in the Bay of Brest, several reproductive characteristics tend to highlight its invasive capacity: (1) a long reproductive period, (2) reproduction in a ‘multi-trials’ process equivalent to a spreading out of the risks and (3) a relatively high fecundity.     

Sexual conflict in the snake den

Red-sided garter snakes (Thamnophis sirtalis parietalis) court and mate in spring, soon after they emerge from large communal overwintering dens in south-central Manitoba. Because of a massive bias in the operational sex ratio, every female attracts intense courtship from dozens to hundreds of males. We suggest that this courtship constitutes significant ”harassment,” because it delays the females’ dispersal from the den and hence increases their vulnerability to predation. Small females may face the greatest costs, because they are less able to escape from amorous males (who court all females, even juvenile animals). Our measurements show that males are stronger and faster than females. Experimental trials confirm that the locomotor ability of females (especially small females) is greatly reduced by the weight of a courting male. Arena trials show that intense courtship stimulates females to attempt to escape. Remarkably, some females that are too small to produce offspring may nonetheless copulate. This precocious sexual receptivity may benefit juvenile females because copulation renders them unattractive to males, and thus allows them to escape more easily from the den. Female ”tactics” to escape male harassment may explain other puzzling aspects of garter snake biology including size-assortative mating, temporal patterns in dispersal from the den, avoidance of communal dens by young-of-the-year snakes, and female mimicry. Hence, sexual conflict may have influenced important features of the mating system and behavioral ecology of these animals. Received: 8 May 2000 / Revised: 28 July 2000 / Accepted: 30 July 2000     

The effect of maternal body size on embryo survivorship in the broods of pregnant male pipefish

The occurrence of male pregnancy in the family Syngnathidae (seahorses, pipefishes, and sea dragons) provides an exceptionally fertile system in which to investigate issues related to the evolution of parental care. Here, we take advantage of this unique reproductive system to study the influence of maternal body size on embryo survivorship in the brood pouches of pregnant males of the broad-nosed pipefish, Syngnathus typhle. Males were mated with either two large females, two small females, a large then a small female, or a small then a large female. Our results show that offspring survivorship depends on an interaction between female body size and the number of eggs transferred by the female. Eggs of larger females deposited in large numbers are more likely to result in viable offspring than eggs of smaller females laid in large numbers. However, when females deposited smaller numbers of eggs, the eggs from smaller females were more likely to produce viable offspring compared to those from larger females. We found no evidence that this result was based on mating order, the relative sizes of competing females, or egg characteristics such as dry weight of eggs. Additionally, male body size did not significantly influence the survivorship of offspring during brooding. Our results suggest that the factors underlying offspring survivorship in pipefish may be more complex than previously believed, with multiple factors interacting to determine the fitness of individual offspring within the broods of pregnant males.     

Male limitation of female reproductive success in a pipefish: effects of body-size differences

Summary In the pipefish Syngnathus typhle, a species with exclusive male parental care, males limit female reproductive success because of their limited brood pouch space and long pregnancy. Sexual size dimorphism is absent in these 1-year-old animals but increases with age so that older females are larger than similarly aged males. Because fecundity is related to size in both sexes and increases more rapidly with body size in females than in males, the difference in growth increases female fecundity more, relative to male fecundity, as the fish get older. We therefore predicted that male limitation of female reproductive success is even more severe when all age classes are considered. To measure a female's maximum reproductive rate, she was provided with three males. Small 1-year-old females produced as many eggs, or produced eggs at the same rate, as a male of similar size could care for. Small females filled on average 1.06 males within the time span of one male pregnancy and actually produced on average 10 eggs fewer than needed to fill a similarly sized male. Large 2-year-old females, in contrast, produced on average a surplus of 149 eggs and filled 2.7 similarly sized males within the course of one pregnancy. The difference between females of the two size classes was highly significant. Males prefer to mate with larger females if given a choice. In nature sex ratios are equal, and males limit female reproductive success in the whole population. Therefore, small females are more severely constrained by mate availability than are larger females because males choose to mate with larger females. Offprint requests to: A Berglund     

Replacement males in the western bluebird: opportunity for paternity, chick-feeding rules, and fitness consequences of male parental care

In western bluebirds (Sialia mexicana), most pairs remain together for life and share equally in post-hatching parental care. We removed resident males of socially monogamous pairs during laying and after clutch completion to examine chick-feeding rules used by replacement males and current and future fitness consequences of paternal care. Replacement males were not infanticidal and 7 (47%) fed nestlings. Feeding replacement males and the females they joined fed at rates similar to controls. Females without a feeding replacement male compensated by feeding more themselves so that overall feeding rates were not compromised, but they reduced their brooding time. Unlike assisted females, unassisted females exhibited reduced nesting success and their 14-day-old chicks weighed less than controls. Field metabolic rates of unassisted females were 17% higher than those of control females, but the difference was not statistically significant. Older females were better able to raise young without the male's help than were yearling females. Female condition was not affected by male provisioning and unassisted females were as likely to survive to the next breeding season as assisted females. We found no future benefits of provisioning by replacement males; those that fed were no more likely to breed with the female on her subsequent attempt than were males that did not feed, and subsequent clutch sizes were not reduced for females rearing young without the male's help. These experiments suggest that male parental care increases nesting success in western bluebirds and that replacement males use an all-or-none rule to determine whether or not to feed chicks: if they are present during the fertile period they feed at typical rates; if they are not, they usually do not feed at all. Because chick-feeding by males is tied to opportunity for paternity, influences success in the current nest, and does not affect the male's future breeding success, it appears to be parental rather than mating effort. Received: 8 May 1998 / Accepted after revision: 23 July 1998     

Apparent resource competition among embryos in the brood pouch of a male pipefish

Embryo success was studied in the paternally brooding pipefish Syngnathus typhle. During brooding, which lasts about a month, males provide embryos in their brood pouch with nutrients and oxygen via a placenta-like structure. Egg size depends on female size. In aquaria, males were mated with differently sized females to give the following treatments: M, mixed-egg-size broods of approximately half large and half small eggs; L, single-egg-size broods of large eggs; S, single-egg-size broods of small eggs; and F, field mated males. All males were kept in aquaria for a full brooding period. For each egg-size category, the number of newborn was compared with the number of eggs the male initially fertilized in his brood pouch. Within mixed-egg-size broods, a higher proportion of large eggs survived and large eggs resulted in heavier newborn than small eggs. Indeed, small eggs from a mixed-egg-size brood had significantly lower relative success (proportion of embryos surviving to birth) than those from a brood entirely composed of small eggs. The implication is that embryos compete for resources within the brood pouch, and that competitive success depends on egg size. Given that females produce eggs corresponding in size to their body size, and that females are known to compete indirectly for access to mates (i.e., the sex-roles are reversed), this intrabrood competition could be seen as an extension of female-female competition, but alternative explanations are discussed. Received: 28 April 1995/Accepted after revision: 28 October 1995     

The amphipod Corophium multisetosum (Corophiidae) in Ria de Aveiro (NW Portugal). I. Life history and aspects of reproductive biology

The population of Corophium multisetosum Stock, 1952 in Areão displayed a semiannual, iteroparous life history. Mean longevity was ～6?mo, with the estimated life span longer for overwintering individuals born in autumn than for individuals born in spring. Length-frequency data indicated that the length increment per moult is probably higher in males than females; however females moulted more frequently and achieved a larger body size. Preliminary growth rates were 100?μm?d^?1 for juveniles and 19 to 29?μm?d^?1 for mature females, with the lower values occurring during the winter. It was estimated that under favourable conditions females may attain reproductive size and mature within 1?mo. Although incubating females were present all year round, recruitment occurred in spring, almost ceased during the summer, peaked in autumn, and decreased again during the winter. Extreme temperatures and very low salinities during winter and summer may have deterred breeding, while moderate temperatures (15 to 20?°C) and salinities > 1?psu in spring and autumn were apparently favourable for reproduction. The unfavourable summer conditions constrained breeding and synchronised the timing of reproduction. In late-autumn and during the winter, as temperature decreased and brooding time increased, synchrony was progressively lost. Brood size varied as a function of embryonic developmental stage, size of incubating females, and season. The life-history pattern and reproductive features of C. multisetosum in Areão are closely related to temperature and salinity; other environmental conditions such as oxygen content of the water and food availability may also be relevant.     

Female control of reproductive skew in cooperatively breeding brown jays (<Emphasis Type="Italic">Cyanocorax morio</Emphasis>)

Brown jays (Cyanocorax morio) are long-lived, social corvids that live in large, stable, territorial groups (mean = 10 individuals). In this study, I determined the distribution of reproductive success within groups using multi-locus DNA fingerprinting. Breeding females produced virtually all (99%) of the young within their own nests. Reproduction within groups was highly skewed towards a single primary female, although long term data indicate that secondary females (female breeders that were usually younger and subordinate to the primary female) were sometimes successful. The high reproductive skew observed for females was associated with primary female aggression. Successful reproduction by secondary females may have been due to parental facilitation or the inability of primary females to completely suppress secondary females. Multiple paternity occurred in 31–43% of broods and extra-group paternity occurred in a minimum of 22% of broods. Patterns of paternity also varied between years, since females often switched or included new genetic mates. Although male consorts of nesting females fathered relatively few offspring (20%), they still had a higher chance of fathering offspring than any other single group male. Reproduction was less skewed for males than females as a result of female mating patterns. Female reproductive patterns are consistent with some of the predictions and assumptions from optimal skew models, while male reproductive patterns are not. The factors affecting skew in species with complex social systems such as incomplete control by breeders over subordinate reproduction, female control of paternity, and resource inheritance have not been well incorporated into reproductive skew models.Communicated by: J. Dickinson     

The cause of size-assortative mating in the leaf beetle Trirhabda canadensis (Coleoptera : Chrysomelidae)

Summary Examples of positive assortative mating by body size are abundant but its causes remain controversial. I show that size-assortative mating occurs in the chrysomelid beetle Trirhabda canadensis and I test a series of alternative hypotheses to explain how this mating pattern comes about. Results suggest that assortative mating in this beetle is due to the greater ease with which size-matched pairs can achieve intromission, and not due to size-biased skews in the availability of mates or mate choice favoring large individuals. There was no correlation between male and female elytron length (a measure of body size) at the initiation of courtship, but pairs assorted positively by size at the onset of intromission. Moreover, in the laboratory, there was a negative correlation between male and female size for pairs engaged in courtship that terminated without mating. Assortative mating was not associated with a large-male mating advantage and there was no evidence of female choice of large males. Nor was there unequivocal evidence for male choice of large females; although mating females were slightly larger and considerably heavier than solitary females, males did not differ in the frequency with which they rejected large and small females. Assortative mating in T. canadensis appeared to be caused by the lower ability of mismatched pairs to achieve intromission after an encounter, both when males were larger and when they were smaller than the female.     

Reversed parasite-mediated selection in sticklebacks from eutrophied habitats

Sexual selection is often assumed to increase the viability of populations by increasing the quality of offspring produced. Presently, human-induced environmental changes are altering the process of sexual selection by influencing male–male interactions and female mate choice. Here, we show that increased density of filamentous algae due to eutrophication reverses parasite-mediated selection during reproduction in threespine sticklebacks (Gasterosteus aculeatus). When we manipulated the density of artificial algae in a breeding area in the Baltic Sea, more males nested in dense than in sparse vegetation, but the males in dense vegetation were more parasitized. Interestingly, heavily parasitized males acquired more eggs than less parasitized males in dense vegetation but not in sparse vegetation. The higher probability of reproduction for parasitized males in dense algae growth could be due to impaired visibility relaxing male–male competition or reducing female choosiness. This could favour males in poor condition that often invest more in attracting females than males in good condition. In sparse vegetation, parasitized males may have a lower reproductive success due to intense male–male competition, careful female choice and high predation rate selecting against parasitized males. The results suggest that eutrophication could alter the fraction of the population that reproduces, which could have long-term evolutionary consequences.