Predictions Of Sexual Selection Theory Research Articles

The effect of individual state on the strength of mate choice in females and males.

Animals are thought to gain significant fitness benefits from choosing high-quality or compatible mates. However, there is large within-species variation in how choosy individuals are during mating. This may be because the costs and benefits of being choosy vary according to an individual's state. To test this, I systematically searched for published data relating the strength of animal mate choice in both sexes to individual age, attractiveness, body size, physical condition, mating status, and parasite load. I performed a meta-analysis of 108 studies and 78 animal species to quantify how the strength of mate choice varies according to individual state. In line with the predictions of sexual selection theory, I find that females are significantly choosier when they are large and have a low parasite load, thus supporting the premise that the expression of female mate choice is dependent on the costs and benefits of being choosy. However, female choice was not influenced by female age, attractiveness, physical condition, or mating status. Attractive males were significantly choosier than unattractive males, but male mate choice was not influenced by male age, body size, physical condition, mating status, or parasite load. However, this dataset was limited by a small sample size, and the overall correlation between individual state and the strength of mate choice was similar for both sexes. Nevertheless, in both males and females individual state explained only a small amount of variation in the strength of mate choice.

Divergence in pollen performance between Clarkia sister species with contrasting mating systems supports predictions of sexual selection.

Animal taxa that differ in the intensity of sperm competition often differ in sperm production or swimming speed, arguably due to sexual selection on postcopulatory male traits affecting siring success. In plants, closely related self- and cross-pollinated taxa similarly differ in the opportunity for sexual selection among male gametophytes after pollination, so traits such as the proportion of pollen on the stigma that rapidly enters the style and mean pollen tube growth rate (PTGR) are predicted to diverge between them. To date, no studies have tested this prediction in multiple plant populations under uniform conditions. We tested for differences in pollen performance in greenhouse-raised populations of two Clarkia sister species: the predominantly outcrossing C. unguiculata and the facultatively self-pollinating C. exilis. Within populations of each taxon, groups of individuals were reciprocally pollinated (n = 1153 pollinations) and their styles examined four hours later. We tested for the effects of species, population, pollen type (self vs. outcross), the number of competing pollen grains, and temperature on pollen performance. Clarkia unguiculata exhibited higher mean PTGR than C. exilis; pollen type had no effect on performance in either taxon. The difference between these species in PTGR is consistent with predictions of sexual selection theory.

Genetic Evidence for Polygynandry in the Black-Striped Pipefish Syngnathus abaster: A Microsatellite-Based Parentage Analysis

Sexual selection theory predicts that, in organisms with reversed sex roles, more polyandrous species exhibit higher levels of sexual dimorphism. In the family Syngnathidae (pipefish, seahorses, and seadragons), males provide all parental care by carrying developing embryos on their ventral surfaces, and females develop secondary sex characters. Syngnathids exhibit a variety of genetic mating patterns, making them an ideal group to test predictions of sexual selection theory. Here, we describe the mating system of the black-striped pipefish Syngnathus abaster, using 4 highly variable microsatellites to analyze parentage of 102 embryos. Results revealed that 1) both sexes mate multiple times over the course of a pregnancy (polygynandrous mating system), 2) eggs are spatially segregated by maternity within each brood pouch, and 3) larger females have higher mating success (Kolmogorov-Smirnov test; P < 0.05). Together with similar studies of other syngnathid species, our results support the hypothesis that the mating system is related to the intensity of sexual dimorphism.

Do Cloacal Pathogenic Microbes Behave as Sexually Transmitted Parasites in Birds?

In birds, microparasites found in both the reproductive and the digestive tracts may be transmitted through copulations via cloacal contact (male-to-female and vice versa) and/or through the seminal fluid (mainly male-to-female). Most importantly, such cloacal microparasites are affected by and may in turn affect sexual selection processes and the evolution of mating systems. Here I provide preliminary comparative evidence that at least some cloacal microparasites tend to be distributed in hosts according to the host's mating system and as broadly expected from predictions of sexual selection theory. The patterns, however, are more suggestive than conclusive. There is a non-significant trend for polyg- amy to be associated with higher richness of cloacal microparasite taxa; with body size, however, also having a positive association with both polygamy and parasite richness. Although increased sexual plumage dichromatism tends to be asso- ciated with decreased cloacal microparasite richness, indicating that secondary sexual traits may be used by sexual part- ners to discriminate between infected and uninfected individuals, qualitative trends also suggest that non-mating periods of the year tend to be associated with slightly higher levels of prevalence and richness of cloacal microparasites. Given this variability of results, it is suggested that future studies should focus on specialist sexually transmitted microbes, to be compared with more generalist one.

Male‐biased nectar production in a protandrous herb matches predictions of sexual selection theory in plants

Nectar production may disproportionately benefit male relative to female pollination success. In such cases, sexual selection is often suggested as the cause of asymmetric benefits, yet sexual selection in plants-particularly plants with hermaphroditic flowers-is infrequently tested empirically. Here, I used a protandrous herb with male-biased nectar production (Chrysothemis friedrichsthaliana, Gesneriaceae) to test predictions from sexual selection theory. During three flowering seasons, I measured nectar production, pollinator visits, and male and female fecundity following different numbers of cross-pollination events. In accordance with sexual selection predictions, (1) nectar production was greater during the male phase by at least 65%; (2) visits by the main pollinator (hummingbird Phaethornis striigularis) were limiting for part of the season, indicating that plants had to compete for pollinator visits; (3) pollinators spent 53% more time per visit and made 86% more visits to male- vs. female-phase flowers, suggesting that nectar increased male more than female pollination success; and (4) female fecundity was maximized by one visit, whereas male fecundity continued to increase with additional visits. Autonomous self-pollination further reduced visit requirements for maximum female seed set. These findings match specific sexual selection predictions: they link an observable male bias in a secondary sexual trait (nectar) to positive responses of mating participants (pollinators), resulting in more mating opportunities for mate-limited males, relative to apparently resource-limited females. This field-testing of theoretical predictions provides unique evidence that sexual selection helps maintain nectar production patterns in this and, quite likely, other hermaphroditic plant species.

Locomotor Performance and Sexual Selection: Individual Variation in Sprint Speed of Collared Lizards (Crotaphytus Collaris)

Whole-animal performance traits, such as locomotor performance, are central to current concepts of phenotypic adaptation, yet the possible evolution of such traits via sexual selection is an underexplored hypothesis. We studied a cursorial, polygynous, territorial lizard to test two predictions of sexual selection theory applied to wholeanimal performance: a secondary sexual performance characteristic may be sexually dimorphic, and variation in performance among males should be correlated to mating success. Maximal sprinting performance was measured in a series of wild-caught adult collared lizards (Crotaphytus collaris) from a single Oklahoma population. Sprint speed varied repeatably among individuals, but did not scale to body size among adults. Contrary to our first prediction, sprint speed did not differ between sexes. However, among 11 adult males sprint speed was strongly correlated to territory size and a spatial index of potential mating success (independent of body size), which indicates phenotypic intrasexual selection, whether direct or indirect, for whole-animal locomotor performance. The lack of sexual dimorphism in sprint speed may reflect trade-offs with other sexually selected traits (e.g., head size) and/or conditiondependence of running capability. Sexual selection of social behavior may underlie more generally the evolution of physiological performance, and therefore of suborganismal physiology and morphology.

MATING FREQUENCY IN SCHIZOCOSA OCREATA (HENTZ) WOLF SPIDERS: EVIDENCE FOR A MATING SYSTEM WITH FEMALE MONANDRY AND MALE POLYGYNY

Courtship behavior has been studied extensively in the wolf spider Schizocosa ocreata (Hentz) (Araneae, Lycosidae). While much research has tested predictions of sexual selection theory regarding male traits and female mate choice, some critical assumptions about female behavior remain untested. To determine if females mate more than once, and to what degree copulation influences subse- quent female mating, a multiple mating experiment was conducted. Virgin females were paired randomly with males in laboratory containers. If mating occurred, females were paired with a second male within 24 hr, after 3 days, or after 30 days (enough time for an egg sac to be produced). Of the 101 females tested, 83 (82%) mated with the first male they encountered. The probability of a female mating the first time was not influenced by female size, male size, or male age, but varied significantly with female age post-maturity. Of the 18 males that failed to mate, 3 were cannibalized. Of the 83 males that did mate, 12 were cannibalized after mating. There was no difference between re-mating treatments (1 d, 3 d and 30 d), and analysis of pooled data showed a highly significant difference in the proportion of virgin and mated females accepting males; most females mated only once (93%). In contrast, males appeared to court and attempt mating with every female encountered (virgin and mated), and a majority of males paired with more than one virgin female mated more than once (64.5%). Results suggest that female S. ocreata are essentially monandrous, while males are polygynous, and are discussed in the context of potential conflicts-of-interest between the sexes.

Sexual Dichromatism and Female Preference in Eulemur fulvus Subspecies

We experimentally tested the hypothesis that sexual dichromatism in the subspecies of Eulemur fulvus is the evolutionary result of female preference for brightly colored males. Ten female lemurs representing 6 different subspecies of Eulemur fulvus were subjects in the experiment; controls were 4 females of non-sexually dichromatic lemurid taxa. For each taxon we presented photographs of the face of a male of that taxon whose colors had been digitally altered to make him less and more colourful. Median viewing times of the pooled female Eulemur fulvus are significantly correlated with colorfulness. Viewing times in the control females are not correlated with color or brightness of the stimulus photographs. We concluded that the females of the Eulemur fulvus sspp. preferred to view photographs of more colorful males, which is consistent with the predictions of sexual selection theory.

THE DYNAMICS OF MALE BROODING, MATING PATTERNS, AND SEX ROLES IN PIPEFISHES AND SEAHORSES (FAMILY SYNGNATHIDAE)

Modern theory predicts that relative parental investment of the sexes in their young is a key factor responsible for sexual selection. Seahorses and pipefishes (family Syngnathidae) are extraordinary among fishes in their remarkable adaptations for paternal care and frequent occurrences of sex-role reversals (i.e., female-female competition for mates), offering exceptional opportunities to test predictions of sexual selection theory. During mating, the female transfers eggs into or onto specialized egg-brooding structures that are located on either the male's abdomen or its tail, where they are osmoregulated, aerated, and nourished by specially adapted structures. All syngnathid males exhibit this form of parental care but the brooding structures vary, ranging from the simple ventral gluing areas of some pipefishes to the completely enclosed pouches found in seahorses. We present a molecular phylogeny that indicates that the diversification of pouch types is positively correlated with the major evolutionary radiation of the group, suggesting that this extreme development and diversification of paternal care may have been an important evolutionary innovation of the Syngnathidae. Based on recent studies that show that the complexity of brooding structures reflects the degree of paternal investment in several syngnathid species, we predicted sex-role reversals to be more common among species with more complex brooding structures. In contrast to this prediction, however, both parsimony- and likelihood-based reconstructions of the evolution of sex-role reversal in pipefishes and seahorses suggest multiple shifts in sex roles in the group, independent from the degree of brood pouch development. At the same time, our data demonstrate that sex-role reversal is positively associated with polygamous mating patterns, whereas most nonreversed species mate monogamously, suggesting that selection for polygamy or monogamy in pipefishes and seahorses may strongly influence sex roles in the wild.

Female Dominance in Captive Red Ruffed Lemurs, Varecia variegata rubra (Primates , Lemuridae)

Malagasy primates (Lemuriformes) pose a puzzle to evolutionary biologists because polygynous lemur species are not consistent with fundamental predictions of sexual selection theory with respect to several behavioral and morphological traits. Such species lack sexual dimorphism [1], exhibit even sex ratios in adult social groupings [2] and exhibit female dominance [3]. Although female dominance is considered a rare trait among mammals [4], it is a widespread behavioral trait among lemurs [3], but comparative data are lacking on the red ruffed lemur, Varecia variegata rubra: does female dominance occur, and does it occur only during feeding?

Physiological Suppression of Sexual Function of Subordinate Males: A Subtle Form of Intrasexual Competition Among Male Sifakas ( Propithecus verreauxi)?

In contrast to most anthropoid primates, sifakas ( Propithecus verreauxi), like many group-living lemurs, exhibit a number of features that deviate from predictions of sexual selection theory. Despite a promiscuous mating system, they lack sexual dimorphism, suggesting that physical combat plays only a minor role in intrasexual competition for receptive females. In this study, we investigated the hypothesis that socioendocrinological mechanisms contribute to suppression of reproductive function of subordinate males. For that purpose, 10 male sifakas from five social groups were observed for 669 focal animal hours for 4 months, including the mating season, in Western Madagascar. Concomitantly, 315 fecal samples of these animals were collected and the concentration of immunoreactive testosterone was quantified by enzymeimmunoassay procedures. Clear dominance relationships existed among coresident males. Testosterone levels of dominant males were significantly higher than those of subordinates during, as well as outside, the mating season. Additionally, the increase in testosterone levels prior to the mating season was more pronounced for dominant than for subordinate males. These findings are in accordance with the hypothesis of suppression of sexual function of subordinate males, probably providing dominant males with an advantage in sperm competition. If reproductive success is mainly determined by this nonagonistic form of intrasexual competition, the results of this study contribute an important piece to the puzzle of lacking sexual dimorphism in P. verreauxi.

Fluctuating sexual dimorphism and differential hibernation by sex in a primate, the gray mouse lemur ( Microcebus murinus )

The aim of this study was to investigate reproductive strategies and their consequences in gray mouse lemurs (Microcebus murinus), small solitary nocturnal primates endemic to Madagascar. Previous reports of sexual dimorphism in favor of males and females, respectively, a high potential for sperm competition and pheromonal suppression of mating activity among captive males, led us to investigate mechanisms of intrasexual competition in a wild population. Based on 3 years of mark-recapture data, we demonstrate that sexual dimorphism in this species fluctuated annually as a result of independent changes in male and female body mass. Male body mass increased significantly prior to the short annual mating season. Because their testes increased by 100% in the same period and because their canines are not larger than those of females, we suggest that large male size may be advantageous in searching for estrous females and in enabling them to sustain periods of short-term torpor. In contrast to reports from captive colonies, we found no evidence for two morphologically distinct classes of males. Finally, we also show that most adult males are active throughout the cool dry season that precedes the mating season, whereas most adult females hibernate for several months. This is in contrast to other solitary hibernating mammals, where males typically emerge 1–2 weeks before females. Thus, this first extended field study of M.␣murinus clarified previous conflicting reports on sexual dimorphism and male reproductive strategies in this primitive primate by showing that their apparent deviation from predictions of sexual selection theory is brought about by specific environmental conditions which result in sex-specific life history tactics not previously described for mammals. A general conclusion is that sexual selection can operate more strongly on males without resulting in sexual dimorphism because of independent selection on the same traits in females.

Intrasexual selection in Mirza coquereli : evidence for scramble competition polygyny in a solitary primate

The primates of Madagascar (Lemuriformes) deviate from fundamental predictions of sexual selection theory in that polygynous species lack sexual dimorphism, have even adult sex ratios and often live in female-dominated societies. It has been hypothesized that intrasexual selection in these species is either reduced or primarily focused on traits related to scramble competition. The goal of this study was to examine these hypotheses by studying the mating system of a solitary nocturnal species, Mirzacoquereli. During a 4-year field study in western Madagascar, I captured and followed 88 individually marked animals. I found that adult males were significantly larger than females, providing the first evidence for sexual size dimorphism in lemurs. In addition, the adult sex ratio was biased in favour of females in 3 out of 4 years. There was no significant sex difference in canine size, however. Males showed pronounced seasonal variation in testis size with a 5-fold increase before and during the short annual mating season. During the mating season, males had more injuries than females and more than quadrupled their home ranges, overlapping with those of more than ten females, but also with about the same number of rivals. Only about one social interaction per 10 h of observation was recorded, but none of them were matings. Together, these results indicate that these solitary lemurs are clearly subject to intrasexual selection and that male-male competition is primarily, but not exclusively, of the scramble type. In addition, they suggest that the above-mentioned idiosyncracies may be limited to group-living lemurs, that social systems of solitary primates are more diverse than previously thought, and that the temporal distribution of receptive females is responsible for this particular male mating strategy.

Intrasexual selection and testis size in strepsirhine primates

The main objective of this study was to investigate two predictions of sexual selection theory concerning interspecific variation in testis size among strepsirhine primates (Lemuriformes and Lorisiformes). First, the unique evolutionary history of lemurs provides an opportunity for an independent test of the predictions of sperm competition theory regarding die relationship between mating system and relative testis size. Second, I examined the evolutionary relationship between the morphological correlates of preand postcopulatory competition (Le., between sexual dimorphism and testis size) because polygamous lemurs, in contrast to other polygamous primates, lack sexual dimorphism. Based on measurements from 174 captive strepsirhines from 24 species, I found that multi-male species had significantly larger testes than pair-Irving ones, but that they did not differ significantly from solitary species. This result deviates from theoretical expectations, but may be the result of yet-unknown heterogeneity in mechanisms of male-male competition in both multi-male and solitary species. There was no difference in relative testis size between nonmonogamous lemurs and lorises, indicating that presumably lower levels of precopulatory competition are not necessarily compensated by more intense sperm competition. Body size and phylogenetic effects were also found to considerably affect interspecific variability in testis size. Analyses of independent contrasts revealed that evolutionary changes in mating system, testis size, sexual size, and canine dimorphism were not, or only weakly, associated in this monophyletic group of primates. Additional comprehensive comparative studies of sexual dimorphism, testis size, mating system, and copulatory behavior in these and other taxa are indicated to illuminate general patterns and causes of covariation among these traits. Kty words: mating system, primates, sexual dimorphism, sexual selection, sperm competition, testis. [Behav Ecot 8:10-19 (1997)]

Intrasexual selection and phylogenetic constraints in the evolution of sexual canine dimorphism in strepsirhine primates

AbstractThe goals of this study were to analyze the origin and function of sex differences in the size of canine teeth among Malagasy lemurs and other strepsirhine primates. These analyses allowed me to illuminate interactions between different mechanisms of sexual selection and to elucidate constraints on this sexually‐selected trait. In contrast to central predictions of sexual selection theory, polygynous lemurs lack both sexual dimorphism in body size and male social dominance, but the degree of sexual dimorphism in the size of their canines is not known. A comparison of male and female canine size in 31 species of lemurs and lorises revealed significant male‐biased canine dimorphism in only 6 of 13 polygynous lemur species. This result is in contrast to predictions of a hypothesis that would explain the lack of size dimorphism in lemurs as a result of high viability costs because canine teeth presumably have low maintenance costs and because they are used as weapons in male‐male combat. Moreover, because females had significantly larger maxillary canines than males in only one lemur species, female dominance is not generally based on female physical superiority and selective forces favoring female dominance do not constrain sexual canine dimorphism in the sense of a pleiotropic effect. Contrary to predictions of sexual selection theory, species differences in canine dimorphism across strepsirhines were neither associated with differences in mating system, nor with the potential frequency of aggression. Variation in canine dimorphism was also unrelated to differences in body size, but there were significant differences among families, pointing to strong phylogenetic constraints. This study demonstrated that polygynous lemurs are at most subject to weak intrasexual selection on dental traits used in male combat and that traits thought to be under intense sexual selection are strongly influenced by phylogenetic factors.

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

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.