Bilateral Characters Research Articles

Wing asymmetry of a butterfly community: is altitude a source of stress?

We present a novel analysis of fluctuating asymmetry (FA) for butterfly wings at the community-level, along an altitudinal gradient. FA is an important biomonitoring tool that detects deviations of perfect symmetry in bilateral characters, assuming that genetic and/or environmental factors can be a source of stress. This study evaluated the effects of increased altitude on the symmetry of butterfly wings, testing the hypothesis that FA should increase with increased elevation in a tropical mountain. Butterflies were sampled along an altitudinal gradient of 800 to 1.400 m and forewings were detached, scanned and evaluated for symmetry. Length, width and area of the right and left forewings were measured as surrogates for FA and then combined into an index taking into account the variability of wing sizes of the whole butterfly community. We observed true patterns of FA in the length, width and area of the wings, and wing FA area increased with increased altitude. This study pioneered the analysis of FA for a community of butterflies and FA was efficient to detect developmental instability indicated by imperfections in butterfly wings.

Evaluating Morphological Asymmetry in Three Species of the Genus <i>Hydrocynus</i> from the White Nile in Sudan

The present study aimed to determine and evaluate the level of fluctuating asymmetry (FA) of morphological characters for three species of Hydrocynus from the White Nile. In H. vittatus 41% of morphological characters fluctuated to the right side and 59% to left side. In H. brevis 50% fluctuated to right and 50% to left side, while in H. forskalii 34% fluctuated to right and 66% fluctuated to left side. More scales were recorded along the lateral line on the right side of H. vittatus and H. brevis compared to H. forskalii , while more gill rakers were recorded on the left side of H. vittatus . The smallest asymmetry index was found for PVFL of the three species and the largest index was found for POL of H. brevis . Mean asymmetry index was (3.5%) in H. forskalii , (5.1%) and (4.9%) in H. vittatus and H. brevis , respectively. Absolute asymmetry values were significantly correlated with character size for the number of lateral line scales, head length, pectoral fin length and ventral fin length (P>0.05) and highly correlated for snout length and caudal peduncle length (P>0.001) in the three species. Correlations between means of bilateral characters and body size were significant (P>0.05) for the head length, pelvic fin length and pectoral spine length of the three species. This study is the first of its kind in species of the Nile fish. The present result can be a base line for further study of fluctuating asymmetry in fish and as indicators to detect the level of pollutants in the Nile and the condition of the environment and the organisms.

Low temperatures during ontogeny increase fluctuating asymmetry and reduce maternal aggression in the house mouse, Mus musculus.

Maternal aggression is behavior displayed by post-partum lactating female mice toward unfamiliar conspecifics, presumably as a defense against infanticide. A variety of perinatal stressors can impair maternal care in adulthood. Previous studies on associations between developmental perturbations and maternal aggression have produced mixed results. To address this issue, we employed a proxy for developmental instability, fluctuating asymmetry (FA) to further elucidate the relationship between low temperature stress and maternal aggression. FA, small, random deviations from perfect symmetry in bilateral characters is used as a quantitative measure of stress during ontogeny. Dams were either maintained in standard laboratory temperatures (21 ± 2 °C), or cold temperatures (8 ± 2 °C) during gestation. During lactation, their progeny either remained in the temperature condition in which they were gestated or were transferred to the other temperature condition. Four individual measures of FA, a composite of these measures, and three measures of maternal aggression were assessed in the female progeny in adulthood. Exposure to low temperatures during both pre- and early post-natal development increased composite FA and reduced all three measures of maternal aggression compared to controls. Exposure to low temperatures during the pre- or postnatal period alone did not induce either high FA or altered maternal aggression. Certain measures of FA and nest defense were negatively correlated. Our results suggest that low temperatures experienced during gestation and lactation may have important fitness costs. Low maternal aggression towards infanticidal conspecifics is likely to limit the number of offspring surviving into adulthood. Overall, FA appears to be a reliable indicator of chronic developmental stress with implications for fitness.

Bilateral asymmetry in certain morphological characters of Sarotherodon melanotheron Rüppell 1852 and Coptodon guineensis (Günther 1862) collected from Lake Ahémé and Porto-Novo Lagoon Bénin, West Africa

Asymmetry analysis was carried out for three bilateral characters of two cichlid fish species, Sarotherodon melanotheron and Coptodon guineensis collected from two localities, Lake Ahémé and Porto-Novo lagoon, Republic of Bénin, West Africa. For both S. melanotheron and C. guineensis snout length (SL), eye diameter (ED) and number of scales on lateral line (LLS), bilateral asymmetry was higher in the fish specimens collected from Porto-Novo lagoon compared with that of specimens from Lake Ahémé. Bilateral asymmetry in S. melanotheron and C. guineensis increased with the fish size at all localities. The possible causes of bilateral asymmetry in the two cichlid species are discussed in relation to different pollutants in both water bodies.

Fluctuating and Directional Asymmetry of the Blue Mussel (Mytilus edulis): Improving Methods of Morphological Analysis to Explore Species Performance at the Northern Border of Its Range

In this work we examined morphological variation at different levels to study performance and population structuring of the blue mussel Mytilus edulis. Our objectives were: (i) to develop an integrated technique for analyzing morphological variation in blue mussels and, based on this technique; (ii) to perform a morphometric description of mussels from the northern part of their range; and (iii) to verify the hypothesis that populations at the outer range of their distribution have reduced performance due to harsh climatic conditions. Means, directional asymmetry (i.e., systematic differences between left and right structures), fluctuating asymmetry (random deviations from perfect symmetry, a measure of developmental instability), factorial variation (difference between total variance and variance of fluctuating asymmetry, an indirect index of genotypic variation), and measurement error were examined for 14 bilateral characters of muscle scars on mussel shells. Also, we analyzed one non-bilateral character. For the first time directional asymmetry—approaching 13% of the right:left ratio—was described in blue mussels. Measurement error, often ignored in morphometric studies, contributed 26% of total variance. Accurately addressing these methodical challenges is key to further and improved ecological interpretations. Morphological differences were found between (i) samples from estuarine areas in the White and Pechora Seas and (ii) samples from Barents Sea areas with oceanic salinity. Samples from the first group exhibited lower fluctuating asymmetry, indicating higher developmental stability likely due to better feeding conditions and lower factorial variation, which may result from lower genotypic variation. Absence of correlation between fluctuating asymmetry and temperature suggests that low temperatures in the northern border of their range do not degrade the performance of adult blue mussels in this region.

Karakter bilateral simetri ikan betutu (Oxyeleotris sp.): Kajian keragaman morfologi sebagai dasar pengembangan budidaya

Abstract. Genetic diversity information of Gobiidae is needed as a basis for management of its culture or conservation efforts. Mean while, the stability development of the paired organs in animals is closely related to the level of their genetic diversity. In this regard, a study has been conducted to determine of bilateral symmetry characteristics of marble gobies from Jenderal Soedirman reservoir of Banjarnegara and Rawapening Swamp of Salatiga. A survey method was used with purposive sampling technique. The observed variables were abnormalities and meristic bilateral characters. The characters including the number of soft rays on pectoral fins and pelvic fins, and the number of scales on lateral lines. The results showed that both marble gobies from Jenderal Soedirman Reservoir and Rawapening Swamp did not show abnormalities. However, their bilateral meristic characters had been fluctuated, especially for pectoral fins and lateral lines; whereas pelvic fins seem to be symmetry. According to their observed bilateral symmetry characters, in general, it could be concluded that marble gobies from Jenderal Soedirman Reservoir had a better genetic diversity compare to those from Rawapening Swamp. Therefore, it can be used as local genetic resources.Key words. Bilateral symmetry character, marbel gobies, Jenderal Soedirman Reservoirs, Rawa pening Swamp

Asymmetry in certain morphological characters of the carangid species Decapterus russelli, collected from the Lemah coastal area, on the northern part of Oman Sea

Asymmetry in certain morphological characters of the carangid species Decapterus russelli, collected from the Lemah coastal area, on the northern part of Oman Sea Asymmetry analysis has been carried out for some bilateral characters of Decapterus russelli (Family: Carangidae) collected from Oman Sea. The results showed that postorbital length demonstrated the highest incidence of asymmetry among the characters studied. The lowest asymmetry value was for the number of pectoral fin rays. An increasing trend in the asymmetry value with fish length is also obtained for preorbital length, number of pectoral fins and number of lateral line scutes. A similar trend is also evident in the postorbital length and eye lens diameter of individuals with body sizes <160 mm. The possible causes of asymmetry in this species are herein discussed in relation to different pollutants and their presence in the area.

Intracranial hypertension: A rare presentation of lupus nephritis

A 14-year-old male presented with bilateral papilledema, growth retardation and absent secondary sexual characters. He had a past history of fever, headache and fatigue of 6 months duration. The diagnosis of intracranial hypertension (IH) was confirmed by an increased intracranial pressure and normal neuroimaging studies of the brain, except for partial empty sella, prominent perioptic cerebrospinal fluid (CSF) spaces and buckling of optic nerves. Evaluation showed erythrocyte sedimentation rate (ESR) of 150 mm/hr, positive antinuclear antibody (ANA), anti dsDNA and anti ribosomal P protein. Renal biopsy revealed diffuse segmental proliferative lupus nephritis (LN) class IV S (A) confirming the diagnosis of systemic lupus erythematosus (SLE). Treatment of LN with intravenous pulse methyl prednisolone and cyclophosphamide was effective in normalizing the CSF pressure, resulting in express and dramatic resolution of symptomatology. In a case of IH, SLE must be considered. IH, growth retardation and absence of sexual characters may be presenting manifestations of a chronic systemic inflammatory disease like SLE. These manifestations may act as a pointer to associated advanced grades of LN, which can be totally asymptomatic and missed without a renal biopsy.

Effect of directional selection for body size on fluctuating asymmetry in certain morphological traits in Drosophila ananassae

Variation in the subtle differences between the right and left sides of bilateral characters or fluctuating asymmetry (FA) has been considered as an indicator of an organism's ability to cope with genetic and environmental stresses during development. However, due to inconsistency in the results of empirical studies, the relationship between FA and stress has been the subject of intense debate. In this study, we investigated whether stress caused by artificial bidirectional selection for body size has any effect on the levels of FA of different morphological traits in Drosophila ananassae. The realised heritability (h2) was higher in low-line females and high-line males, which suggests an asymmetrical response to selection for body size. Further, the levels of FA were compared across 10 generations of selection in different selection lines in both sexes for sternopleural bristle number, wing length, wing-to-thorax ratio, sex combtooth number and ovariole number. The levels of FA differed significantly among generations and selection lines but did not change markedly with directional selection. However, the levels of FA were higher in the G10 generation (at the end of selection) than G0 (at the start of selection) but lower than the G5 generation in different selection lines, suggesting that the levels of FA are not affected by the inbreeding generated during the course of selection. Also, the levels of FA in the hybrids of high and low lines were signifi cantly lower than the parental selection lines, suggesting that FA is influenced by hybridisation. These results are discussed in the framework of the literature available on FA and its relationship with stress.

Developmental Stability in Chickens Local to Warm Climate 1. Variation in Internal Organs and Bilateral Traits of Lines Selected Short-Term for Growth

This study aimed at adopting the developmental changes in the internal organs and bilateral traits of growing chickens selected five generations for increased 6-week body weight and genotyping for normal feathering (line F) or naked-necks (line N), compared to their corresponding genetic controls (lines CF and CN). Line N had the smallest weight percentages of total lungs and liver (0.54 and 2.38%) and biggest weight percentages of right and left shanks (2.46 and 2.48%). Line F had the smallest spleen weight percentage (0.20%) and tallest right and left shanks (8.50 and 8.57 cm) and widest right and left shanks (3.74 and 3.75 cm). Gizzard weight percentages of lines N and CN (3.37 and 3.55%) were significantly more than those of lines F and CF (2.74 and 3.00%). The results indicated that neither short-term selection nor Na allele have influenced the development of heart. Na allele showed variable pleiotropic effects, where the weight percentages of lungs and liver were decreased and the weight percentages of gizzard and spleen were increased. A reduction in lung and liver weight percentages characterized the naked-neck selected line, revealing an interaction between selection and Na allele. The pressure of short-term selection for body weight was early noticeable on the development of liver and spleen that could initiate possible metabolic disorders or susceptibility to disease infection later with the continuity of selection scheme. Short-term selection albeit boosted the bilateral shank measurements, but did not influence the bilateral developmental stability. The developmental stability characterized the bilateral characters in chickens carrying Na allele, suggesting a role of Na allele in the developmental stability of the birds pertaining the natural heating waves. The results of correlation coefficients indicated that the bilateral asymmetries of different characters are not significantly associated.

Fluctuating asymmetry, abnormalities and parasitism as indicators of environmental stress in cultured stocks of goldfish and carp

Knowledge about the effects of captivity and environmental stress on fish populations is basic to culture them for ornamental, conservation or exploitation purposes. The aim of this study was to test how captivity affect morphological development of bilateral characters and whether levels of fluctuating asymmetry, morphological abnormalities and ectoparasites reflect developmental stress caused by adverse environmental conditions in two species of cyprinids, goldfish Carassius auratus and two varieties of carp Cyprinus carpio, scale and mirror carp. Samples from captivity were caught in an artificial pond (Madrid City, Spain), which presents poor environmental conditions; while samples from nature were captured in the Ebro River basin (NE Spain). Six meristic and four morphometric bilateral characters were measured and compared between the two samples. Additionally, the incidence of morphological abnormalities and ectoparasites was also recorded. Captive goldfish presented significant lower number of scales of the lateral line, scale rows above lateral line and length of pectoral and ventral fins. Captive carps showed significant shorter first barbels and longer ventral fins. Ornamental goldfish showed significant higher levels of fluctuating asymmetry than wild samples for number of branched rays of pectoral fin, lateral line, scale rows below lateral line and length of pectoral and ventral fins. Ornamental carps presented a similar trend for the same characters, except for branched rays and length of pectoral fins, together with the length of second barbels. In the artificial pond, the incidence of abnormalities was higher in carp, whereas goldfish showed a higher occurrence of ectoparasites. These facts support the hypothesis of a different species response to environmental stress. The overall results indicate that fish from the artificial pond are subjected to environmental stress caused by adverse conditions such as overcrowding and lower water quality. Moreover, estimated parameters in this study mean a useful tool to detect developmental instability in confined fish.

Behavioural laterality in the shrimp-eating cichlid fish Neolamprologus fasciatus in Lake Tanganyika

Behavioural laterality has been observed in various vertebrates, including fish, but its significance is little known. This study investigated behavioural laterality, corresponding to morphological asymmetry, in individuals of the cichlid Neolamprologus fasciatus as they hunted shrimp. This species shows lateralized hunting; when aiming at prey, individuals bend with either the left or the right side of the body abutting a rock. In field observations, the numbers of leftward and rightward hunts were recorded during 1-h periods for each of 44 individuals. The frequency distribution of the proportion of rightward hunting was bimodal, and approximately one-third of the observed individuals showed a significant leftward or rightward bias. The degree of behavioural laterality of each fish was associated with that of morphological asymmetry of the mouth but not with any of four bilateral characters (outer teeth in upper jaw, gill rakers, upper lateral line scales and lower lateral line scales); ‘lefties’ (‘righties’) showed more rightward (leftward) hunting. Furthermore, it was suggested that hunts corresponding to each individual's mouth laterality achieved higher hunting success than did reverse hunts. Antisymmetrical hunting behaviour in a population may affect predation efficiency on the basis of frequency-dependent selection.

The relationship between developmental instability of gudgeonGobio gobioand abundance or morphology of its ectoparasiteParadiplozoon homoion(Monogenea)

GudgeonGobio gobio(Cyprinidae) were collected monthly from the Vlara Stream (Czech Republic) during 2004 and examined for the presence of the ectoparasiteParadiplozoon homoion(Diplozoidae; Monogenea). Over 26% ofP. homoionshowed abnormalities of the attachment sclerites, indicating either a stressful environment or that the quality of the fish host was suboptimal for parasite development. Here, the hypotheses that parasite abundance and the occurrence of morphological abnormalities are correlated with fluctuating asymmetry (FA, the random deviation from the perfect bilateral symmetry) in its fish host were tested. The FA and morphological abnormality are estimators of developmental instability. To estimate fish developmental instability nine meristic and two morphometric bilateral characters on the right and the left sides of the fish were measured. In general, there was no correlation between mean fish FA and parasite abundance or the proportion of parasites with abnormalities. There was, however, a significant relationship between the proportion of abnormalP. homoionand the number of asymmetric meristic traits per individual fish. This study thus indicates that developmental instability ofP. homoion, measured as morphological abnormality of the attachment apparatus, is only weakly dependent on the fish quality as measured by FA.

Asymmetries in the attachment apparatus of a gill parasite

AbstractDiplozoids are fish ectoparasites with a direct life cycle. Adults live attached to fish gills, using two haptors with four pairs of clamps for attachment. Three planes of symmetry (the individual, haptor and clamp planes) can be identified in adult diplozoids. In this study, the haptor and clamp planes of symmetry are used to study different asymmetries (fluctuating asymmetry FA, directional asymmetry DA, antisymmetry) in the attachment apparatus of Paradiplozoon homoion (Diplozoidae; Monogenea). DA and antisymmetry may be adaptive asymmetries, whereas FA – measured as the random deviation from perfect symmetry in bilateral characters – is suggested as a useful tool for estimating developmental instability. Fifty individuals of P. homoion were collected from 25 gudgeon Gobio gobio. Asymmetries were estimated for nine morphometric traits in the haptor plane and three morphometric traits in the clamp plane on all four pairs of clamps from each individual. FA did not differ between traits on the individual pairs of clamps. However, significant DA was detected for most traits in the clamp plane.

Directional asymmetry and correlation of tail injury with left-side dominance occur in Serpentes (Sauropsida)

Recent evidence on directional asymmetry (DA) in species of the paraphyletic assemblage “Lacertilia” indicates a possible biological significance of this phenomenon, and we tested its occurrence in Serpentes by examining bilateral morphological characters, such as the numbers of supralabial and infralabial scutes and the diameter of the eyes, on both sides of museum specimens of seven species: Coronella austriaca, Elaphe longissima, Hierophis viridiflavus, Natrix natrix, Natrix tessellata, Platyceps saharicus, and Vipera aspis. Significant DA (not antisymmetry) occurred in at least one sex of four species and in two characters: H. viridiflavus—the number of infralabial scutes in males; N. tessellata—the diameter of the eyes in males; P. saharicus—the number of infralabial scutes in males and the diameter of the eyes in females; and V. aspis—the number of infralabial scutes in the pooled sexes. In N. tessellata and P. saharicus the DA varied geographically. Rank correlation between the DA of different characters, positive or negative, was significant in one character pair in each of three species. In the largest sample (N. tessellata males from the Levant), tail injury correlated with morphological left-dominant asymmetry (a greater number on the left) of infralabial scutes. Four of ten morphologically left-dominant males, but only 1 of 37 symmetrical or right-dominant males, had an injured tail. The syndrome of morphological DA with functional handicap may be genotypical or phenotypical. Our results support three earlier conclusions: bilateral characters should be examined on both sides of an animal; asymmetry should be analyzed separately in each sex; and the accident proneness of left-handed Homo sapiens merits re-evaluation in view of the apparently wide-spread accident proneness in left-biased Sauropsida (exclusive of Aves).

Neonatal exposure to short days and low temperatures blunts stress response and yields low fluctuating asymmetry in Siberian hamsters

Fluctuating asymmetry (FA) refers to small, non-directional deviations from perfect bilateral symmetry in morphological characters. Individuals with low FA presumably either developed in a relatively stable environment and/or were better able to buffer against developmental stressors. The present study investigated the effects of seasonal factors measured by day length and ambient temperature manipulations on the development of bilateral characters and concomitant changes in stress responses. Siberian hamsters were exposed to either long days (16 h of light per day) or short days (8 h of light per day) combined with either standard temperatures (21 ± 2 °C) or low temperatures (8 ± 2 °C) on the day of birth until weaning. Cortisol concentrations at baseline and following acute restraint stress, and FA values were measured in adulthood. Females reared in winter-like conditions with short day lengths and low temperatures had low FA and low cortisol concentrations following restraint stress compared to other females. Females reared in long day lengths and standard temperatures had the highest rate of increase in cortisol concentrations after restraint among other female groups. No group effects were observed in males regarding day length and temperature manipulations. Baseline and post-restraint cortisol concentrations were higher in females than males for all groups except in animals reared in short day lengths and low temperatures. Our results suggest that winter-like conditions during neonatal period evoke hyposensitivity to stress in adult females and this blunted response to stress is a key factor in achieving ideal growth patterns.

Developmental instability, hybridization and heterozygosity in stick insects of the genus Bacillus (Insecta; Phasmatodea) with different modes of reproduction

Several genetic factors are assumed to influence developmental instability (DI). One is the level of heterozygosity, with higher levels often being associated with decreased DI; another is genetic incompatibility in hybrids, which in several cases has been shown to increase DI. The genus Bacillus includes species which have both amphigonic heterozygous reproducing populations and homozygous parthenogenetic reproducing populations (B. rossius rossius and B. r. redtenbacheri). Furthermore, Bacillus includes hybrid parthenogenetic species, which have very high levels of almost fixed heterozygosities (B. atticus, B. whitei, B. lynceorum). We investigated the phenotypic variance (σ2p) and the impact of hybridization and level of heterozygosity on DI in females from these populations and species of Bacillus. DI was estimated as fluctuating asymmetry (FA) for three bilateral traits: the labial palpus, the maxillary palpus and the antenna. For the labial palpus and maxillary palpus we found, in general, a lower level of DI in the amphigonic females compared with parthenogenetic counterparts from the same species and with parthenogenetic females from the three hybrid species. A higher DI of the antenna was found in the hybrid species when compared with both parthenogenetic and amphigonic populations of the nonhybrid species, suggesting that the genes controlling antenna development are located on the sex chromosomes. The development of the investigated bilateral characters in the hybrid species seemed to be affected more by factors relating to genetic incompatibilities as a consequence of hybridization than by the stabilizing force of increased heterozygosity. Only few differences in σ2p were observed, supporting the possibility that the observed differences in DI are related mainly to internal genetic factors. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 87, 249–259.

The Genetics and Evolution of Fluctuating Asymmetry

▪ Abstract Variation in the subtle differences between right and left sides of bilateral characters, or fluctuating asymmetry (FA), has long been considered to be primarily environmental in origin, and this has promoted its use as a measure of developmental instability (DI) in populations. There is little evidence for specific genes that govern FA per se. Numerous studies show that FA levels in various characters are influenced by dominance and especially epistatic interactions among genes. An epistatic genetic basis for FA may complicate its primary use in comparisons of DI levels in outbred or wild populations subjected or not subjected to various environmental stressors. Although the heritability of FA typically is very low or zero, epistasis can generate additive genetic variation for FA that may allow it to evolve especially in populations subjected to bottlenecks, hybridizations, or periods of rapid environmental changes caused by various stresses.

An epistatic genetic basis for fluctuating asymmetry of tooth size and shape in mice

Although there typically is little additive genetic variation for fluctuating asymmetry (FA), or variation in nondirectional differences between left and right sides of bilateral characters, several investigators have hypothesized that FA may have an epistatic genetic basis. We tested this hypothesis by conducting a whole genome scan of FA of size and shape of the mandibular molars in house mice from an F2 intercross population generated from crossing the Large (LG/J) and Small (SM/J) inbred strains. Although no individual genes (QTLs=quantitative trait loci) on any of the 19 autosomes significantly affected FA for centroid size, and only two affected shape FA, a number of pairwise combinations of QTLs exhibited significant epistasis for FA in both molar size and shape. The QTLs involved in these interactions differed for FA in molar size versus FA in molar shape, but their epistatic contributions to the total variance was nearly the same (about 20%) for FA in both molar characters. It was noted that the genetic architecture of FA in the molar characters, consisting of little or no additive genetic variance but an abundance of epistatic genetic variance, is consistent with that of other typical fitness components such as litter size.

The ontogeny of fluctuating asymmetry.

We tested seven hypotheses regarding the mechanisms by which fluctuating asymmetry (FA) originates. We did this by analyzing data on four bilateral characters measured repeatedly during the development of individual domestic fowl. Immediately posthatching, there was substantial directional asymmetry, which rapidly decreased. We detected FA at significant levels in all characters in the majority of our measurements over the remainder of development. We also examined the effects of known environmental stressors (food and density stress) on levels of FA. At the levels we examined, changes in these stressors did not alter the degree of asymmetry we found in fowl. Time series of asymmetry for individuals did not exhibit regular oscillations, as much of the relevant literature predicts. Asymmetry levels reflected the combined effects of developmental noise, which was random in degree and direction, and feedback processes, which decreased asymmetry by altering growth rates on both sides of the body. Our findings best fit the predictions of the residual asymmetry and compensatory growth hypotheses, which suggest that levels of asymmetry reflect only recent growth history.