Sexual Selection Pressures Research Articles

Sex differences in cognitive performance and style across domains in mosquitofish (Gambusia affinis).

Given that the sexes often differ in their ecological and sexual selection pressures, sex differences in cognitive properties are likely. While research on sexually dimorphic cognition often focuses on performance, it commonly overlooks how sexes diverge across cognitive domains and in behaviors exhibited during a cognitive task (cognitive style). We tested male and female western mosquitofish (Gambusia affinis) in three cognitive tasks: associative learning (numerical discrimination), cognitive flexibility (detour task), and spatio-temporal learning (shuttlebox). We characterized statistical relationships between cognitive performances and cognitive style during the associative learning task with measures of anxiety, boldness, exploration, reaction time, and activity. We found sex differences in performance, cognitive style, and the relationships between cognitive domains. Females outperformed males in the spatio-temporal learning task, while the sexes performed equally in associate learning and cognitive flexibility assays. Females (but not males) exhibited a 'fast-exploratory' cognitive style during associative learning trials. Meanwhile, only males showed a significant positive relationship between domains (associative learning and cognitive flexibility). We propose that these sexually dimorphic cognitive traits result from strong sexual conflict in this taxon; and emphasize the need to explore suites of sex-specific cognitive traits and broader comparative work examining sexual selection and cognition.

Legs morphometric characters of the <i>Dolichopus</i> Latreille species, 1796 (Diptera, Dolichopodidae)

A comparative analysis of 30 species of the Dolichopodidae family in a phylogenetic context was conducted to examine interspecific variation in the legs morphometry. Five relative traits of legs from 12 and seven absolute traits from nine showed significant phylogenetic signal. A set of traits, such as relatively short hind tibia and relatively long fore and middle tibia and the first segments of the hind legs allowed to allocate Dolichopus species from the other ones. The projection of the phylogenetic tree of Dolichopus species into the morphospace allowed us to divide it into four individual areas: not closely related species, but species having similar modifications of males legs tended to cluster. This suggests that the legs morphometric traits should be mainly under pressure of sexual selection. It has also been revealed that the elongation of the first segment of hind tarsi in Dolichopus species is associated with the distal displacement of the insertion point of dm-m with M4 and the decrease of the length of R4+5 . The functional significance of these characters set is discussed.

Contrasting patterns of sexually selected traits in Mediterranean and continental populations of European mouflon.

The expression of sexually selected traits in highly dimorphic ungulates may be influenced by environmental quality. Variations in habitat conditions can impose different constraints on the allocation of energy resources to male life‐history traits, and possibly alter the female preferences for specific features. Here, we compared the horn growth patterns in male European mouflon Ovis aries musimon living in different habitats (Mediterranean vs. continental) but sharing a common genetic origin. We hypothesized that the expression of sexually selected traits such as horn development should be promoted in more favorable habitat conditions (i.e., Mediterranean). Using linear mixed models on data retrieved from individuals harvested under the same hunting regime, we found longer horns and greater individual variance in horn segment length in the Mediterranean population than in the continental one. Furthermore, Mediterranean rams showed no evidence of compensatory horn growth, as opposed to the continental rams. Unexpectedly, horn base circumference was greater in the continental habitat than in the Mediterranean one. The overall results suggest different patterns of investment in horns in the two populations, with seemingly stronger pressure and consequences of sexual selection on mouflon rams living in more favorable environments. Although the role of hunters' selectivity cannot be excluded a priori, our data suggest that the differences in the expression of sexually selected traits in our study populations may be influenced by environmental conditions. Because sexual selection can impose substantial fitness costs on individuals, further investigations on the trade‐offs between reproduction and survival would improve our understanding of the dynamics of mouflon populations living in different environmental conditions.

Male morphological traits are heritable but do not predict reproductive success in a sexually-dimorphic primate

Sexual selection favours traits that increase reproductive success via increased competitive ability, attractiveness, or both. Male rhesus macaque (Macaca mulatta) morphological traits are likely to reflect the effects of multiple sexual selection pressures. Here, we use a quantitative genetic approach to investigate the production and maintenance of variation in male rhesus macaque morphometric traits which may be subject to sexual selection. We collected measurements of body size, canine length, and fat, from 125 male and 21 female free-ranging rhesus macaques on Cayo Santiago. We also collected testis volumes from males. We used a genetic pedigree to calculate trait heritability, to investigate potential trait trade-offs, and to estimate selection gradients. We found that variation in most male morphometric traits was heritable, but found no evidence of trait trade-offs nor that traits predicted reproductive success. Our results suggest that male rhesus macaque morphometric traits are either not under selection, or are under mechanisms of sexual selection that we could not test (e.g. balancing selection). In species subject to complex interacting mechanisms of selection, measures of body size, weaponry, and testis volume may not increase reproductive success via easily-testable mechanisms such as linear directional selection.

Reproductive transitions in plants and animals: selfing syndrome, sexual selection and speciation.

The evolution of predominant self-fertilisation frequently coincides with the evolution of a collection of phenotypes that comprise the 'selfing syndrome', in both plants and animals. Genomic features also display a selfing syndrome. Selfing syndrome traits often involve changes to male and female reproductive characters that were subject to sexual selection and sexual conflict in the obligatorily outcrossing ancestor, including the gametic phase for both plants and animals. Rapid evolution of reproductive traits, due to both relaxed selection and directional selection under the new status of predominant selfing, lays the genetic groundwork for reproductive isolation. Consequently, shifts in sexual selection pressures coupled to transitions to selfing provide a powerful paradigm for investigating the speciation process. Plant and animal studies, however, emphasise distinct selective forces influencing reproductive-mode transitions: genetic transmission advantage to selfing or reproductive assurance outweighing the costs of inbreeding depression vs the costs of males and meiosis. Here, I synthesise links between sexual selection, evolution of selfing and speciation, with particular focus on identifying commonalities and differences between plant and animal systems and pointing to areas warranting further synergy.

Sexual selection in the Kinda baboon

Due to wide variation in the presence and degree of expression of a diverse suite of sexually-selected traits, the tribe Papionini represents an outstanding model for understanding how variation in sexual selection pressures and mechanisms leads to trait evolution. Here, we discuss the particular value of Papio as a model genus for studies of sexual selection, emphasizing the presence of multiple mating systems, and differences in the expression of sexually-selected traits among closely-related species. We draw particular attention to the Kinda baboon (Papio kindae), a comparatively less-studied baboon species, by providing a primer to Kinda baboon morphology, genetics, physiology, and behavior. Based on observations of large group sizes, combined with low degrees of sexual dimorphism and large relative testis size relative to other baboon species, we test the hypothesis that Kinda baboons have evolved under reduced direct, and increased indirect, male–male competition. We present the first long-term data on wild Kinda baboons in Zambia. Kinda baboon females show seasonal peaks in births and reproductive receptivity, and males exhibit a queing-rather than contest-based dominance acquisition with long alpha-male tenure lengths. We finish by making a number of explicit testable predictions about Kinda baboon sexual signals and behaviors, and suggest that Kinda baboons have potential to offer new insights into the selective environments that may have been experienced during homininization.

Behaviour before beauty: signal weighting during mate selection in the butterfly Papilio polytes.

Mating displays often contain multiple signals. Different combinations of these signals may be equally successful at attracting a mate, as environment and signal combination may influence relative signal weighting by choosy individuals. This variation in signal weighting among choosy individuals may facilitate the maintenance of polymorphic displays and signalling behaviour. One group of animals known for their polymorphic patterning are Batesian mimetic butterflies, where the interaction of sexual selection and predation pressures are hypothesized to influence the maintenance of polymorphic wing patterning and behaviour. Males in the female-limited polymorphic Batesian mimetic butterfly Papilio polytes use female wing pattern and female activity levels when determining whom to court. They court stationary females with mimetic wing patterns more often than stationary females with non-mimetic, male-like wing patterns, and active females more often than inactive females. It is unclear whether females modify their behaviour to increase (or decrease) their likelihood of receiving male courtship, or whether non-mimetic females spend more time in cryptic environments than mimetic females, to compensate for their lack of mimicry-driven predation protection (at the cost of decreased visibility to males). In addition, relative signal weighting of female wing pattern and activity to male mate selection is unknown. To address these questions, we conducted a series of observational studies of a polymorphic P. polytes population in a large butterfly enclosure. We found that males exclusively courted active females, irrespective of female wing pattern. However, males did court active non-mimetic females significantly more often than expected given their relative abundance in the population. Females exhibited similar activity levels, and selected similar resting environments, irrespective of wing pattern. Our results suggest that male preference for non-mimetic females may play an active role in the maintenance of the non-mimetic female form in natural populations, where males are likely to be in the presence of active, as well as inactive, mimetic and non-mimetic females.

Penis size and sperm quality, are all bats grey in the dark?

Penises play a key role in sperm transport and in stimulating female genitals. This should impact post-copulatory competition, and expose penis characteristics to sexual selective pressures. Studies of male genitalia have repeatedly reported negative static allometries, which mean that, within species, large males have disproportionally small genitals when compared with smaller individuals. Males of some sperm-storing bat species may stand as an exception to such a pattern by arousing from hibernation to copulate with torpid females. The selection for large penises might take place, if a long organ provides advantages during post-copulatory competition and/or if females have evolved mechanisms allowing the choice of sire, relying on characters other than pre-copulatory traits (e.g., penis size). In this study, we measured dimensions of the erected penis in 4 sperm-storing bat species. Furthermore, we collected sperm and evaluated the link between penis dimensions and sperm velocity. Our results revealed steep allometric slopes of the erected penis length in Barbastella barbastellus and an inverse allometry of penis head width in Myotis nattereri. More detailed studies of copulatory behavior are urgently needed to explain the range of observed scaling relations. Furthermore, penis head width correlates with sperm velocity in Plecotus auritus. For this last species, we propose that penis shape might act as a marker of male fertility.

Adaptive changes in sexual signalling in response to urbanization.

Urbanization can cause species to adjust their sexual displays, because the effectiveness of mating signals is influenced by environmental conditions. Despite many examples that show that mating signals in urban conditions differ from those in rural conditions, we do not know whether these differences provide a combined reproductive and survival benefit to the urban phenotype. Here we show that male túngara frogs have increased the conspicuousness of their calls, which is under strong sexual and natural selection by signal receivers, as an adaptive response to city life. The urban phenotype consequently attracts more females than the forest phenotype, while avoiding the costs that are imposed by eavesdropping bats and midges, which we show are rare in urban areas. Finally, we show in a translocation experiment that urban frogs can reduce risk of predation and parasitism when moved to the forest, but that forest frogs do not increase their sexual attractiveness when moved to the city. Our findings thus reveal that urbanization can rapidly drive adaptive signal change via changes in both natural and sexual selection pressures.

The relationship between latitude, migration and the evolution of bird song complexity

For the past several decades it has been proposed that birds show latitudinal variation in song complexity. How universal this variation may be and what factors generate it, however, are still largely unknown. Furthermore, while migration is confounded with latitude, migratory behaviour alone may also be associated with variation in song complexity. In this paper we review the literature to assess current ideas on how latitude and migratory behaviour may drive large‐scale geographical patterns of song complexity. At least seven distinct hypotheses have been proposed in 29 studies of the topic. Four of these hypotheses posit that sexual selection pressures co‐vary with latitude and/or migration, resulting in concordant changes in song. Other hypotheses suggest that mechanisms other than sexual selection, such as large‐scale changes in environmental sound transmission properties, may be at play. Sixteen studies found support for increased song complexity with increased latitude and/or migration, whereas 13 did not. Relatively few studies exist on this topic, and methodological differences between them and variable definitions of ‘complexity’ make it difficult to determine whether results are comparable and concordant. At a minimum, it is possible to conclude there is no strong evidence that song complexity increases with latitude and/or migration in all birds. Future work should focus on examining multiple hypotheses at once to further advance our understanding of how latitude, migration and song complexity may or may not be related.

Female-biased population divergence in the venom of the Hentz striped scorpion (Centruroides hentzi)

Sex-biased genes are expressed at higher levels in one sex and contribute to phenotypic differences between males and females, as well as overall phenotypic variation within and among populations. Venom has evolved primarily for predation and defense, making venom expression a highly variable phenotype as a result of local adaptation. Several scorpion species have shown both intraspecific and intersexual venom variation, and males have been observed using venom in courtship and mating, suggesting the existence of venom-specific, sex-biased genes that may contribute to population divergence. We used reversed-phase high-performance liquid chromatography (RP-HPLC), Agilent protein bioanalyzer chips, nano-liquid chromatography mass spectrometry (nLC/MS/MS), and median lethal dose (LD50) assays in fruit flies (Drosophila melanogaster) and banded crickets (Gryllodes sigillatus) to investigate proteomic and functional venom variation within and among three Florida populations of the Hentz striped scorpion (Centruroides hentzi). We found significant venom variation among populations, with females, not males, being responsible for this divergence. We also found significant variation in venom expression within populations, with males contributing more to within population variation than females. Our results provide evidence that male and female scorpions experience different natural and sexual selective pressures that have led to the expression of sex-biased venom genes and that these genes may be consequential in population divergence.

Climate, isolation and intraspecific competition affect morphological traits in an endangered steppe bird, the Dupont’s Lark Chersophilus duponti

ABSTRACTCapsule: We assess biometric variation in the threatened Dupont’s Lark along a wide fraction of its distribution area analysing the largest data set for this bird species available to date, comprising a 28-year period and including birds captured in Spain, Morocco and Tunisia.Aims: To analyse Dupont’s Lark morphology evaluating five potential sources of divergence: (I) sexual dimorphism, (II) macro-regional differences, (III) climate, (IV) isolation and (V) intraspecific competition.Methods: Multivariate analysis was used to summarize biometric data. Sexual dimorphism and macro-regional divergence were assessed by generalized linear mixed models. Climate, isolation and intraspecific competition effects on phenotype were explored by means of model averaging.Results: Sexes differed in wing shape suggesting a sexual selection pressure on males for aerial displays. Males showed longer bills after controlling for body size. We found an increasing Spain < Morocco < Tunisia trend in body size and bill length and volume. Tunisian populations showed more pointed wings than Moroccan and Iberian ones. Maximum temperature increased bill size as predicted by Allen’s rule. We found support for Bergmann’s rule in relation to temperature in the breeding season and water availability. Intraspecific competition was associated with more rounded wing shapes suggesting a pressure related to breeding display performance. Isolation reduced wingtip pointedness.Conclusion: We found convincing results for wing morphology variation in relation to intrasexual competition, as males seem to be subjected to a sexual selection pressure for aerial display and this adaptation strengthens when intraspecific competition increases. We also found solid support for Allen’s rule, with results suggesting that bill size plays an important role in the bird’s thermoregulation, which does not occur in the case of tarsus. Although Bergmann’s rule is not supported in relation to minimum annual temperature, we found a relationship with thermal conditions in the breeding season, as well as water availability.

An analysis of white-handed gibbon male song reveals speech-like phrases.

Our goal was to document song phrases of the white-handed gibbon (Hylobates lar), an Asian ape that produces elaborate songs, often in well-coordinated male/female duets. We focused on the male coda, which is produced during vocal turn-taking with one's mate, and particularly its phrases containing rapid spectral and temporal variation, to investigate if modulation rates resemble those of lip-smacking in other nonhuman primates and human speech rhythm. We produced recordings from a large population of wild gibbons. Using terminology consistent with that used to describe vocalizations in other singing species, we analyzed coda phrases, overall coda properties, coda distinctiveness across individuals, and flexibility of phrase production within song bouts. Our song phrase-level analysis showed that male codas differed between individuals and increase in complexity within song bouts by the addition of the only two male-specific phrases of the species' repertoire. These phrases differ from all others of the species and from vocalizations typical of the larger, nonhuman great apes, in that they contain rapid within-phrase modulation. Their modulation rates (6.82 and 7.34 Hz) are similar to that of lip-smacking in other nonhuman primates and speech in humans and, like human speech, are produced exclusively during exhalation. One phrase type (trills) contains multiple notes per exhalation, another characteristic similar to speech but not most primate vocalizations. Our data highlight the complexity and flexibility of gibbon song, and show that particular phrase features likely arose from sexual selection pressures and possess similarities to human speech rhythm.

Does male preference play a role in maintaining female limited polymorphism in a Batesian mimetic butterfly?

Female-limited polymorphism occurs in multiple butterfly species with Batesian mimicry. While frequency-dependent selection is often argued as the driving force behind polymorphism in Batesian mimicry systems, male preference and alternative female mating strategies may also influence the maintenance of multiple female forms. Through a series of behavioural assays with the female-limited Batesian mimetic butterfly Papilio polytes, we show that males prefer stationary mimetic females over stationary non-mimetic females, but weigh female activity levels more heavily than female wing pattern when choosing between active mimetic and active non-mimetic females. Male preference for mimetic vs. non-mimetic females is independent of male genotype at the locus responsible for the female wing pattern, the autosomal gene doublesex. However male genotype does influence their response to active females. Male emphasis on female behaviour instead of appearance may reduce sexual selection pressures on female morphology, thereby facilitating frequency-dependent natural selection due to predation risk and toxic model abundance.

Environmental conditions limit attractiveness of a complex sexual signal in the t\xfangara frog

Animals choosing particular display sites often balance sexual and natural selection pressures. Here we assess how physical properties of display sites can alter this balance by influencing signal production and attractiveness of the túngara frog (Physalaemus pustulosus). Males that call from very shallow water bodies (few mm depth) benefit from reduced predation risk, but by manipulating water levels, we show that this comes at a cost of reduced attractiveness to females. Our data show that calling from shallower water reduces a male’s ability to float, limits the inflation of his vocal sac, and consequently reduces signal conspicuousness in terms of amplitude and complexity. Our results demonstrate that display site properties can set limits on signal production and attractiveness and may hence influence signal evolution. Signallers may shift between sites or engineer their display location, which can play a crucial role in signal divergence and speciation, particularly in a rapidly changing world.

SEXUAL SELECTION IN BIRDS: INFLUENCE OF AMBIENT LIGHT AND CONSPICUITY ON THE REPRODUCTIVE SUCCESS OF MALES

Sexual selection postulates that the evolution of certain conspicuous physical traits in animals may lead to a greater success in obtaining mates. The majority of bird species are under strong sexual selection pressure which promotes the generation of flashy, ornamented plumage. In addition, males may also use ambient light to become more conspicuous to females as a way of increasing their chances of being selected. We reviewed sexual selection regarding birds and discussed how ambient light and background colors could affect the reproductive success of males. We recorded 17 bird species belonging to 10 families that display in periods of the day that increase their conspicuity to conspecifics, or actively modify the background to enhance plumage contrast during the display. We recommend further investigation, particularly with a behavioral and phylogenetic approach, on the influence of abiotic factors, such as ambient light, on bird courtship displays and its consequent reproductive success, in order to find patterns.

Sex-specific cognitive-behavioural profiles emerging from individual variation in numerosity discrimination in Gambusia affinis.

The relationship between an individual's cognitive abilities and other behavioural attributes is complex, yet critical to understanding how individual differences in cognition arise. Here we use western mosquitofish, Gambusia affinis, to investigate the relationship between individual associative learning performance in numerical discrimination tests and independent measures of activity, exploration, anxiety and sociability. We found extensive and highly repeatable inter-individual variation in learning performance (r=0.89; ICC=0.89). Males and females exhibited similar learning performance, yet differed in sociability, activity and their relationship between learning and anxiety/exploration tendencies. Sex-specific multivariate behaviour scores successfully predicted variation in individual learning performance, whereas combined sex analyses did not. Female multivariate behaviour scores significantly predict learning performance across females (ρ=0.80, p=0.005) with high-performing female learners differentiated from female non-learners and low-performing learners by significant contributions of activity and sociability measures. Meanwhile, males of different learning performance levels (high-, low- and non-learners) were distinguished from each other by unique behavioural loadings of sociability, activity and anxiety/exploration scores, respectively. Our data suggest that despite convergence on learning performance, the sexes diverge in cognitive-behavioural relationships that are likely products of different sexual selection pressures.

Intraspecific variation in reproductive characters is associated with the strength of sexual selection in the hermaphroditic land snail Cornu aspersum

Intraspecific variation in sexual traits may arise from a variety of processes such as genetic drift, sexual selection, or spatial heterogeneity of environmental conditions. Recent empirical studies suggest that sexual selection may vary with environmental conditions accelerating differentiation of sexual traits among populations. Land snails are simultaneously hermaphroditic animals characterized by low mobility and limited dispersal, so that life cycle characteristics of their populations are strongly influenced by local environmental conditions. In this study, we report results on intraspecific variation in sexual selection intensity as well as in behavioral and anatomical reproductive traits in the land snail Cornu aspersum. We performed a common garden experiment of adult snails originating from four populations from either of two regions in Greece differing in habitat temperature and predictability of rainfall during their reproductive season. Our results indicate substantial intraspecific variation in the opportunity for sexual selection, which was associated with differences in both behavioral and anatomical reproductive characters. Snails from populations that experienced unpredictable habitat conditions expanded their reproductive activity period, exhibited higher mating frequency, and showed longer copulation duration, relative to snails living in more predictable habitats. Additionally, snails originating from populations with unpredictable habitat conditions showed a distinct reproductive morphology with a shorter bursa tract length, which we interpret as indicative of an enhanced ability to hydrolyze the received spermatophores. We conclude that local environmental conditions experienced by the different populations could have triggered differences in the opportunity for sexual selection, which may ultimately lead to allopatric speciation through sexual selection. Environment-dependent sexual selection is considered to promote allopatric speciation of distant populations. However, empirical studies testing for geographic differences in the strength of sexual selection and associated reproductive characters are scarce. In C. aspersum, variation in both behavioral and anatomical reproductive traits among populations is well-documented, but the extent to which this is driven by diverse pressures of sexual selection under heterogeneous environmental conditions among populations remains unexplored. We demonstrate that snails from two geographically distant regions of Greece, with contrasting natural habitats, differ mainly in their behavioral traits and in several morphological traits, which are associated with differences in the opportunity for pre- and post-copulatory sexual selection.

The extension of biology through culture

Biology is the study of life. How our understanding of the nature and evolution of living systems is being enriched and extended through new discoveries about social learning and culture in human and nonhuman animals is the subject of the collection of articles we introduce here. Recent decades have revealed that social learning and the transmission of cultural traditions are much more widespread in the animal kingdom than earlier suspected, affecting numerous forms of functional behavior and creating a secondary form of evolution, built onto the better-known primary, genetically based form. New scientific approaches to the study of human cultural evolution have also emerged and become productive. However, these developments in the study of cultural phenomena in both human and nonhuman animals have yet to be seriously integrated into mainstream evolutionary biology. Here we offer an introductory overview of the background and scope of a collection of articles that report recent progress in these fields, and outline their proposed significance for biology at large. The theoretical backbone of the life sciences, its central organizing principle, is of course evolution, by now rich in both theory and empirical support (1⇓–3). The great synthesis of Darwin’s and Wallace’s evolutionary insights and early 20th century understanding of genetics that became known as the “Modern Synthesis” was achieved by a brilliant set of biologists mainly in the period 1938–1946 (4), and its principles have provided the core of evolutionary theory since that time (5). Thus, contemporary texts on “evolution” focus on such topics as mutation, genetically based inheritance, population genetics, genomics, and the natural and sexual selection pressures that shape gene frequencies, genotypes, and phenotypes (1, 2, 6, 7). Genes and their role in inheritance have come to be celebrated as the pivotal elements in evolution (8). … [↵][1]1To whom correspondence should be addressed. Email: a.whiten{at}st-andrews.ac.uk. [1]: #xref-corresp-1-1

Sexual shape dimorphism and selection pressure on males in fossil ostracodes

AbstractSexual dimorphism is thought to have evolved via selection on both sexes. Ostracodes display sexual shape dimorphism in adult valves; however, no previous studies have addressed temporal changes on evolutionary timescales or examined the relationships between sexual shape dimorphism and selection pressure and between sexual shape dimorphism and juvenile shape. Temporal changes in sexually dimorphic traits result from responses of these traits to selection pressure. Using the Gaussian mixture model for the height/length ratio, a valve-shape parameter, we identified sexual differences in the valve shape ofKrithe dolichodeiras.l. from deep-sea sediments of the Paleocene (62.6–57.6 Ma) and estimated the proportion of females in the fossil populations at 11 time intervals. Because the proportion of females in a population is altered by the mortality rate of adult males, it is reflective of selection pressure on males. We attempted to correlate the height/length ratios between the sexes with the proportion of females, taking into consideration that the valve shape was not linked with the selection pressure on males. In time-series data of the height/length ratio, both sexes indicate no significant changes on evolutionary timescales, even though the sex ratio of the population changed from female skewed to male skewed during the late Paleocene. The sexual shape dimorphism was not driven by sexual selection. The static allometry between the height/length ratio and length indicates that the sexual shape dimorphism did not function for sexual display. The absence of change over time in the female allometric slope suggests that the evolution of valve shape was constrained by stasis.