Sexual selection and variation in reproductive strategy in male yellow warblers (Dendroica petechia)

Summary Male yellow warblers (Dendroica petechia) exhibit extensive variation in the amount and conspicuousness of the sexually distinctive brown streaking on the breast. We investigated this intraspecific variation in degree of sexual dichromatism to see if male plumage rank (essentially the amount of brown streaking) is correlated with the amount of reproductive effort allocated to parental investment, as sexual selection theory would predict. As predicted, the average rate of feeding visits to the nest by males was negatively correlated with the plumage rank of the male in 1982 (Fig. 1) and 1983 (Fig. 2), and varied little between years or among study areas. Within one study area, the higher nest visit rate of dull (low plumage rank) males was correlated with higher nestling growth rates. But in the other two study areas, where bright (high plumage rank) males predominated, nestlings of bright males tended to have the highest growth rates despite minimal nest visit rates. Thus, overall there was no correlation between male plumage rank and nestling growth rate, a potential index of relative reproductive success, in either year (Figs. 3 and 4). This overall equal nest success likely helps to maintain the behavioral and plumage polymorphism. Since we could find no evidence that the high nestling growth rates of bright males were due to an adjustment of female parental effort to compensate for low male parental effort, there must be some other benefit of being bright which contributes to nest success. To address this, we used the relationship between parental feeding rates and nestling growth rates for each nest as an index of relative territory quality. This analysis suggests that bright males generally occupy the best territories available, possibly because they are more aggressive in obtaining and defending them than dull males. Dull males appear to be relegated to poorer territories, where an increase in male parental contribution would be necessary to achieve high nestling growth rates. We propose that different males are using alternative but evolutionarily stable reproductive strategies, in a way that is consistent with the predictions of sexual selection theory. Allocation of limited reproductive effort to parental investment decreases as sexual dichromatism increases, which probably reflects an increased investment in intermale competition for the best territories.     

An experimental test of female choice relative to male structural coloration in eastern bluebirds

Several experimental studies have shown that female birds use ornamental melanin and carotenoid plumage coloration as criteria in mate choice. Whether females choose mates based on natural variation in structural coloration, however, has not been well established. Male eastern bluebirds (Sialia sialis) display brilliant ultraviolet (UV)-blue plumage coloration on their head, back, wings, and tail, which is positively correlated with condition, reproductive effort, and reproductive success. We experimentally tested the hypothesis that female eastern bluebirds prefer as mates males that display brighter structural coloration by presenting breeding-condition females with males of variable coloration. We conducted two types of mate-choice experiments. First, females chose between males whose coloration was manipulated within the natural range of variation in the population; feathers were either brightened with violet marker or dulled with black marker. Second, females chose between males with naturally dull or bright plumage coloration. In both manipulated and unmanipulated coloration trials, female choice did not differ significantly from random with respect to structural coloration. We found no support for the hypothesis that the UV–blue coloration of male eastern bluebirds functions as a criterion in female mate choice.     

Elaborately ornamented males avoid costly parental care in the house finch (<Emphasis Type="Italic">Carpodacus mexicanus</Emphasis>): a proximate perspective

Selection should favor flexibility in reproductive tactics when the combination of sexual traits and reproductive behaviors that achieve the highest fitness differs between males within a population. Understanding the functional significance of variation in male reproductive tactics can provide insight into their evolution. Male house finches (Carpodacus mexicanus) in a Montana population display continuous variation in parental tactics: males with more elaborated (redder) plumage color provide little or no parental care compared to less elaborated (dull) males. Here, we first determined whether elevation of prolactin (a pituitary hormone) was related to variation in male parental tactics and, second, we used the relationship between prolactin levels and parental behavior to investigate why redder males avoid a high investment in parental care. We found that prolactin elevation was closely associated with paternal care. In addition, males with redder plumage color had low prolactin levels, whereas dull males, which provision twice as frequently, had high levels of prolactin. We also found that male condition was unrelated to plumage color but negatively related to prolactin levels. These results suggest that the low provisioning of redder males was not due to physiological constraints, but instead reflected a tactic to avoid the costs associated with parental care. The condition benefits accrued by redder males may explain their higher post-breeding survival compared to dull males. Moreover, dull males were previously shown to have higher pairing success than redder males, suggesting that the relationship between male plumage color and parental care may reflect individually optimized parental tactics.Communicated by H. Kokko     

Delayed plumage maturation in Lazuli buntings: tests of the female mimicry and status signalling hypotheses

The evolutionary importance of delayed plumage maturation (DPM) in passerines, the condition when more than 1 year is required to achieve adult-like coloration, remains highly contentious. Adaptive hypotheses propose that aggression from after 2nd-year (ASY) males or predation favors DPM in 2nd-year (SY) males, thereby increasing SY male survivorship or reproductive success. However, each hypothesis suggests a distinct selective mechanism explaining “how” this is accomplished. Alternatively, DPM may be a consequence of a nonadaptive molt constraint. We tested the female mimicry and status signalling hypotheses in territorial ASY male lazuli buntings (Passerinaamoena) using three sets of model presentation experiments. The female mimicry hypothesis proposes that dull SY male plumage deceptively mimics female plumage, and predicts that ASY males can not distinguish SY male from female plumage. The status signalling hypothesis proposes that dull SY male plumage honestly signals low competitive threat, and predicts that ASY males respond less aggressively to dull versus bright, ASY-like plumage. Contrary to the female mimicry hypothesis, ASY males distinguished between SY male and female plumage, as they were aggressive to SY male models exclusively and attempted to copulate with female models. Supporting the status signalling hypothesis, ASY males were significantly less aggressive to SY versus ASY male plumage. While DPM may result from a physiological constraint on bright SY male plumage, our results support the idea that dull plumage in an SY male's first breeding season may be maintained by selection to reduce aggression from ASY males, serving as a signal of competitive status. Received: 21 February 1997 / Accepted after revision: 16 June 1997     

Correlates of male reproductive success in the browncheek blenny,Acanthemblemaria crockeri (Blennioidea: Chaenopsidae)

Summary Acanthemblemaria crockeri exhibits a resource defense polygyny mating system with male parental care. Females preferred to mate with the larger of two males in laboratory experiments, and male size was positively correlated with the number of eggs defended by males at two sites in the Gulf of California, Baja California, Mexico. Females appeared to avoid mating with males defending heavily-fouled shelters. The role of other factors including the intensity of male courtship coloration and displays in determining male reproductive success was studied in Bahia San Carlos, Sonora, Mexico by providing glass vials as shelters. This provided a non-destructive technique for counting of eggs defended by males and allowed repeated assessment of the reproductive success of individual males. Males varied greatly in their duration of residency of shelters and in their color score and intensity of courtship displays. The number of eggs received by males was positively correlated with their duration of residency in shelters, and females appeared to avoid mating with new residents. Mated males deserted shelters less frequently and were more likely to receive future matings than were unmated males. The mean color score of males was unrelated to their reproductive success, while the intensity of male courtship displays was negatively correlated with the number of eggs received. This may have resulted because female mate choice is based on multiple criteria, including some that more accurately reflect the quality of parental care afforded by males.     

Female pied flycatchers Ficedula hypoleuca choose male characteristics in homogeneous habitats

Summary The paper reports the results of a 2-year study of pairing success of male pied flycatchers in a homogeneous habitat. A handicapping experiment was carried out in which certain wing and tail feathers were removed from randomly selected males. Handicapped males had reduced pairing success, they lost weight, and they sang less frequently than control males. Male pairing success was positively correlated with the darkness of the plumage, body-size, and previous breeding experience. Earlier studies on the same species have failed to detect any relationships between pairing success and male characteristics, possibly because of habitat heterogeneity and variation in nest site quality. The evolutionary basis for female choice of male characteristics is discussed. There are reports that males with attractive traits (e.g. black plumage) provide a high quality of parental care. However, the fact that male pairing success was related to male conspicuousness makes it difficult to discriminate between active and passive female choice.     

Delayed maturation in plumage colour: Evidence for the female-mimicry hypothesis in the kestrel

Summary In many sexually dichromatic species, young males have female-like plumage during their first potential breeding year. The female-mimicry hypothesis (FMH) supposes that by possessing female-like plumage young males deceive older conspicuous males into believing that they are females, thus reducing competition from adult males. The status-signalling hypothesis (SSH) supposes that adult males can distinguish sex, but postulates that young males reduce competition from adult males by reliably signaling low status with their dull plumage. We tested these hypotheses in the European kestrel (Falco tinnunculus). Female-like young males settled to breed closer to adult males than did other adult males (Figs. 1a, b). By settling near adult males, young males seemed to increase their chance of mating with adult females. Adult female-young male pairs had better reproductive success than yearling-yearling pairs. These results suggest that there is an adaptive value in possessing a female-like plumage colour in the breeding season. To test the FMH, we measured sexual preference of adult males when adult females and young males were simultaneously shown in an aviary. Adult males were unable to recognize sex, because in half the cases they preferred young males (Fig. 3). However, when adult males and females were shown simultaneously, males preferred females (Fig. 2). Our results support the FMH rather than the SSH, because young males successfully deceived older males by their plumage.     

Age before beauty? Relationships between fertilization success and age-dependent ornaments in barn swallows

When males become more ornamented and reproduce more successfully as they grow older, phenotypic correlations between ornament exaggeration and reproductive success can be confounded with age effects in cross-sectional studies, and thus say relatively little about sexual selection on these traits. This is exemplified here in a correlative study of male fertilization success in a large colony of American barn swallows (Hirundo rustica erythrogaster). Previous studies of this species have indicated that two sexually dimorphic traits, tail length and ventral plumage coloration, are positively correlated with male fertilization success, and a mechanism of sexual selection by female choice has been invoked. However, these studies did not control for potential age-related variation in trait expression. Here, we show that male fertilization success was positively correlated with male tail length but not with plumage coloration. We also show that 1-year-old males had shorter tails and lower fertilization success than older males. This age effect accounted for much of the covariance between tail length and fertilization success. Still, there was a positive relationship between tail length and fertilization success among older males. But as this group consisted of males from different age classes, an age effect may be hidden in this relationship as well. Our data also revealed a longitudinal increase in both tail length and fertilization success for individual males. We argue that age-dependent ornament expression and reproductive performance in males complicate inferences about female preferences and sexual selection.     

Sexual selection in a socially monogamous bird: male color predicts paternity success in the mountain bluebird, <Emphasis Type="Italic">Sialia currucoides</Emphasis>

Ornamental traits are thought to evolve because they give individuals an advantage in securing multiple mates. Thus, the presence of ornamentation among males in many monogamous bird species presents something of a conundrum. Under certain conditions, extra-pair paternity can increase the variance in reproductive success among males, thus increasing the potential for sexual selection to act. We addressed this possibility in the mountain bluebird (Sialia currucoides), a socially monogamous songbird in which males possess brilliant ultraviolet (UV)-blue plumage. Specifically, we asked whether a male’s success at siring offspring within his own nest and within the nests of other males was related to his coloration. In pairwise comparisons, males that sired extra-pair offspring were not more colorful than the males that they cuckolded. However, males that sired at least one extra-pair offspring were, on average, brighter and more UV-blue than males that did not sire extra-pair offspring. Brighter, more UV-blue males sired more offspring both with their own mate and tended to sire more offspring with extra-pair mates and thus sired more offspring overall. Our results support the hypothesis that the brilliant UV-blue ornamental plumage of male mountain bluebirds evolved at least in part because it provides males with an advantage in fertilizing the eggs of multiple females.     

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     

Racketed tail of the male and female turquoise-browed motmot: male but not female tail length correlates with pairing success,performance, and reproductive success

Both males and females of many avian species maintain elaborate plumage traits, and elaborate monomorphic plumage may convey adaptive benefits to one or both sexes as inter- or intraspecific signals. Both sexes of the turquoise-browed motmot (Eumomota superciliosa) are elaborately plumed with long racket-tipped tail. I investigated whether the racketed tail functions as a sexually selected signal in one or both sexes by testing the predictions that males and/or females with the largest tails have: (1) greater pairing success, (2) greater reproductive performance (clutch-initiation date, clutch size, and hatching success), and (3) greater reproductive success. Yearling males with longer denuded rachises (wires) on the central tail feathers had greater pairing success. In addition, adult males with longer wires paired with females who laid larger clutches, had greater hatching success independent of clutch size, and fledged more young. There was no relationship between female tail plumage and pairing success, reproductive performance, or fledgling success. These results are consistent with the hypothesis that male tail plumage functions as a mate choice or status signal, but that the tail of the female does not function in a sexually selected context. I discuss alternative hypotheses for the evolutionary maintenance of the elaborate female tail plumage.     

Male traits expressed in females: direct or indirect sexual selection?

Summary Colored epaulets in female red-winged blackbirds (Agelaius phoeniceus) could be due either to direct sexual selection favoring the maintenance of this trait in females due to intrasexual competition for breeding opportunities, or to sexual selection favoring bright epaulets in males indirectly causing expression of the trait in females by genetic correlation. Older females tend to be brighter than younger females. Also, brighter females tend to breed earlier in the season than dull females. These patterns are consistent with both of the hypotheses. In a series of experiments in which males and females were presented with taxidermic mounts of dull and bright females, the plumage of these mounts had no influence on the response of either males or females. Also, the response of females was independent of their own plumage. The results of all of the experiments consistently supported the interpretation that male plumage characteristics expressed in female redwinged blackbirds have no functional value and are a consequence of genetic correlation with males. Since recent studies have also indicated that female aggression has no functional value, we speculate that this too could be due to genetic correlation with a trait favored in males.     

Female choice for parasite-free male satin bowerbirds and the evolution of bright male plumage

Summary Hamilton and Zuk proposed that bright male plumage may have evolved in males of polygynous species as a result of female preferences for males that are able to demonstrate their resistance to disease. They predicted an inverse correlation between female mating preferences and the level of parasitic infection of males. We found such a correlation between the level of infection by a common ectoparasite (Myrsidea ptilonorhynchi: Menoponidae) and mating success of male satin bowerbirds (Ptilonorhynchus violaceus). In addition, we tested and were able to confirm three other predictions derived from their model: that (1) older males had fewer parasites than their younger counterparts, (2) levels of individual parasitic infection are highly correlated between years, and (3) that individuals resighted in successive years are less parasitized than those that fail to return. These results support the bright male model, but they are also consistent with two other hypotheses that may explain plumage dimorphism based on the level of parasitic infection. The correlated infection model suggests that females choose males with few ectoparasites because of a correlation between the level of ectoparasitic infection and heritable resistance to internal infections. In the parasite avoidance model, females favor parasitefree males because it lowers their own prospects for parasitic infection. Our data did not show the predicted relationship between parasite numbers with plumage quality that is needed to support the bright male hypothesis, nor did it show the inverse correlation between male condition and parasite numbers that is predicted by both the bright male and correlated infection hypotheses. Our results are most consistent with the parasite avoidance hypothesis.     

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.     

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.     

Effects of nest site concealment on hatching success,reproductive success,and paternal behavior of the threespine stickleback,Gasterosteus aculeatus

Summary Male sticklebacks (Gasterosteus aculeatus) actively competed for a limited number of clay flowerpots as nest sites in circular wading pools. Males nesting in pots spawned earlier and more often, had higher mean and lower variance for hatching success (number of fry per gram of estimated clutch weight), and suffered fewer stolen fertilizations, nest raids, and territorial encounters than did males nesting outside pots, and in pools without pots (Table 2). Males nesting in pots had the more even temporal distribution of fanning bouts and interfanning intervals; the durations of fanning bouts and interfanning intervals were less variable for these males (Table 2).In replicate aquaria, one male nested on each side of a glass partition and both males spawned approximately simultaneously. Flowerpots were provided to one, both, or neither male. Males with pots had higher mean hatching success, fewer territorial encounters, and the more even temporal distribution of fanning bouts and interfanning intervals (Table 4).Thus it appears that nest site concealment, in the form of clay flowerpots, affected male hatching success, reproductive success, and parental behavior, even when physical contact between males was prevented. These data suggest that a causal relationship may exist between the paternal fanning regime and the subsequent hatching success. Furthermore, these data suggest that males which nest in high concealment increase their reproductive success, and that females which spawn with these males reduce the variance of their hatching success as well as increase their mean hatching success.     

Sexual selection and cuckoldry in a monogamous songbird: implications for sexual selection theory

Sexual selection is generally assumed to be weaker in monogamous than in polygynous animals. Recently, though, extra-pair fertilizations have been hailed as an important force in generating variance in reproductive success among males in socially monogamous species, thereby increasing the intensity of sexual selection. To see if extra-pair copulations contribute to variance in male reproductive success in the house finch (Carpodacus mexicanus), we used DNA fingerprinting to determine the paternity of chicks from 35 nests. This species is a socially monogamous passerine in which plumage brightness serves as a sexually selected indicator of male quality. Out of 119, nestlings 10 (8.3%) were fathered by a male other than the attending male, but cuckoldry occurred randomly with respect to the plumage colouration, size, or age of the attending male. Thus extra-pair fertilizations do not generate variance in male reproductive success with respect to plumage colour. On the other hand, a strongly male-biased sex ratio and asynchronous breeding by females may generate substantial variance in male reproductive success and could explain the evolution of ornamental colouration.     

Plumage characteristics, reproductive investment and assortative mating in tree swallows Tachycineta bicolor

Elaborate ornamental plumage has been associated with various measures of individual quality in many species of birds. Male plumage characteristics, which have been relatively well studied, have been shown to reflect past reproductive investment, as well as the potential for reproductive investment in the current breeding attempt. In contrast, the signalling functions of female traits remain largely unexplored. In this study, we investigated the relationship between plumage attributes of breeding adult tree swallows and past reproductive investment, current reproductive investment and social mate pairing strategy. Both males and older females possess metallic green to metallic blue iridescent plumage on their dorsal surface, making this a suitable species for this type of investigation. We did not find any effects of past reproductive investment and success on the plumage attributes of returning breeders. In contrast, female plumage hue covaried with fledging success, and female plumage brightness was positively associated with mean clutch egg mass. In addition, we found that social pairs mated assortatively with respect to plumage brightness. We argue that since plumage characteristics vary with age in both male and female tree swallows, plumage attributes in this species are indicative of breeding experience and may be honest signals of quality. Positive assortative pairing could be the result of mutual mate choice or intra-sexual competition for nest sites by both males and females.     

Sexual selection favours small and symmetric males in the polygynous greater sac-winged bat Saccopteryx bilineata (Emballonuridae, Chiroptera)

We investigated how morphological traits of territorial males in the polygynous bat Saccopteryx bilineata were related to their reproductive success. Because of the frequency of aerial courtship displays and defence manoeuvres, and the high energetic costs of flight, we expected small and symmetric males to be better able to court females on the wing and to monopolize copulations with females in their harems. We predicted that small and symmetric males would sire more offspring within the colony and a larger portion of the young born within their harem than large or asymmetric males. We measured size and fluctuating asymmetry of 21 territorial males and analysed their reproductive success in 6 offspring cohorts (n=209 juveniles) using 11 microsatellite loci. As predicted, small and symmetric males had, on average, a higher reproductive success in the colony than large and asymmetric males. The percentage of young sired by males within their harem increased as males decreased in size, but was not influenced by fluctuating asymmetry. As fluctuating asymmetry of males correlated with their reproductive success within the colony but not within their harems, we infer that fluctuating asymmetry is probably related to female choice, whereas male size is probably important for harem defence on the wing.Communicated by G. Wilkinson     

Alternative male mating tactics in a cichlid, Pelvicachromis pulcher: a comparison of reproductive effort and success

Pelvicachromis pulcher is a small African cichlid which breeds in holes. Males may either reproduce monogamously (pair males), polygynously (harem males), or be tolerated as helpers in a harem territory (satellite males). These helpers share in defence of the territory against conspecifics, heterospecific competitors and predators. There are two male colour morphs that are fixed for life and are apparently genetically determined. These differ in their potential mating strategy. Red morph males may become harem owners, while yellow morph males may become satellite males, and males of both morphs may alternatively pair up monogamously. We compared the reproductive effort and success of these three male reproductive strategies. Effort was measured as attack rates, time expenditure and the risk of being injured or killed when attacking competitors or predators of three sympatric fish species. Reproductive success was measured by observing how many eggs were fertilized by each male when this was possible, and by using genetic markers. The number of fry surviving to independence of parental care was used as a criterion of success. The reproductive success of harem males was 3.3 times higher than that of pair males and 7 times higher than that of the average satellite male. Dominant satellite males, however, were as successful as monogamous pair males, using the measure of fertilized eggs. To our knowledge, this has not been found previously in any fish species. Both harem and pair males had lower parental defence costs per sired offspring, however, than males using the alternative satellite tactic. Defence effort was significantly related to the risk of injury. Received: 17 January 1996 / Accepted after revision: 9 June 1997