Geometric craniometric analysis of sexual dimorphism and ontogenetic variation: A case study based on two geographically disparate species, Aethomys ineptus from southern Africa and Arvicanthis niloticus from Sudan (Rodentia: Muridae)

Non-geographic morphometric variation, particularly at the level of sexual dimorphism and ontogenetic (age-related) variation, has been documented in rodents, and useful for establishing whether to analyse sexes separately or together, and for selecting adult specimens for subsequent data recording and analysis. However, such studies have largely been based on traditional morphometric analyses of linear measurements that mainly focus on overall size, rather than shape-related morphometric variation. Unit-free, landmark/outline-based geometric morphometric analyses are considered to offer a more appropriate tool for assessing shape-related morphometric variation. In this study, we used geometric cranial morphometric analysis to assess the nature and extent of sexual dimorphism and age variation within the Tete veld rat, Aethomys ineptus (Thomas and Wroughton, 1908) from southern Africa and the African Nile rat, Arvicanthis niloticus (Desmarest, 1822) from Sudan. The results obtained were in turn compared with previously published results based on independent geometric and traditional cranial morphometric data from the same sampled populations examined in the present study. While our geometric morphometric results detected statistically significant sexual dimorphism in cranial shape within Ar. niloticus only, previously published results based on traditional morphometric data failed to detect significant sexual dimorphism within this species. However, similar to previously published traditional morphometric data, our geometric morphometric results detected statistically significant age-related variation in cranial shape and size within both Ae. ineptus and Ar. niloticus, with individuals of age classes 5 and 6 being considered to represent adult specimens. Our results highlight the importance of carefully evaluating both size- and shape-related non-geographic morphometric variation prior to the analysis of geographic variation and the delineation of species. Erroneous conclusions of non-geographic variation may have implications in the interpretation of geographic and evolutionary processes that may be responsible for morphological differences at both the inter- and intra-specific levels.     

CRANIAL SEXUAL DIMORPHISM AND GEOGRAPHIC VARIATION IN ERASER'S DOLPHIN, LAGENODELPHIS HOSEI

Knowledge of geographic variation is important to questions of population assessment and management. Fraser's dolphins have been exploited in two regions in the western Pacific. Analysis of 137 skulls from the Philippines, Japan, Taiwan, Australia, Indonesia, Malaysia, South Africa, France, the U.S., St. Vincent and the Grenadines, and the eastern tropical Pacific revealed sexual dimorphism in 5 of 26 measurements (difference of 1.9%-5.8% between males and females), similar to levels of cranial dimorphism in other small pelagic delphinids. Males had a larger braincase and temporal fossae and smaller external nares than females. Sexually dimorphic characters were excluded, and male and females samples were pooled to examine geographic differences in the remaining characters. Multivariate analyses yielded significant differences between the Philippine and Japanese series within the North Pacific and between a pooled North Pacific series and a North Atlantic series. The Japanese skulls were on average broader and had a wider rostrum, larger orbit, larger internal nares, and longer braincase than the Philippine skulls. These differences suggest that Fraser's dolphins exploited in Japanese and Philippine waters in directed fisheries or as by catch belong to different populations and should be assessed and managed separately.     

昆虫体型及性体型二型性的地理变异

体型是昆虫基本的形态特性,它会影响到昆虫几乎所有的生理和生活史特性。同种昆虫不同地理种群在体型上常表现出明显的渐变,导致这些渐变的环境因素包括温度、湿度、光照、寄主植物、种群密度等,并且多种环境因素也会对昆虫种群内个体体型产生影响。雌雄个体的体型存在差异,称性体型二型性。性体型二型性也显示了地理差异。这些差异形成的途径已经得到详细的分析,其形成机制导致多个假说的提出,这些假说又在多种昆虫中得到验证。本文从同一种昆虫不同种群间、同一种群内、雌雄虫个体间3个水平,对种内昆虫体型变异的方式,影响昆虫种群间体型变异和种群内昆虫体型的变异的环境因素,以及昆虫性体型二型性及其地理变异的现象等方面的研究进行了综述,并对未来的相关研究提供了建议。     

Isolation by distance and gene flow in the Eurasian badger (Meles meles) at both a local and broad scale

Eurasian badgers, Meles meles, have been shown to possess limited genetic population structure within Europe; however, field studies have detected high levels of philopatry, which are expected to increase population structure. Population structure will be a consequence of both contemporary dispersal and historical processes, each of which is expected to be evident at a different scale. Therefore, to gain a greater understanding of gene flow in the badger, we examined microsatellite diversity both among and within badger populations, focusing on populations from the British Isles and western Europe. We found that while populations differed in their allelic diversity, the British Isles displayed a similar degree of diversity to the rest of western Europe. The lower genetic diversity occurring in Ireland, Norway and Scotland was more likely to have resulted from founder effects rather than contemporary population density. While there was significant population structure (F ST = 0.19), divergence among populations was generally well explained by geographic distance (P < 0.0001) across the entire range studied of more than 3000 km. Transient effects from the Pleistocene appear to have been replaced by a strong pattern of genetic isolation by distance across western Europe, suggestive of colonization from a single refugium. Analysis of individuals within British populations through Mantel tests and spatial autocorrelation demonstrated that there was significant local population structure across 3-30 km, confirming that dispersal is indeed restricted. The isolation by distance observed among badger populations across western Europe is likely to be a consequence of this restricted local dispersal.     

Latitudinal variation in cranial dimorphism in Macaca fascicularis

This study examines latitudinal and insular variation in the expression of sexual dimorphism in cranial length in three geographical groupings of Macaca fascicularis. In addition, the relationship between cranial length dimorphism (CLD) and sex‐specific size is examined. The results of the study identified a significant relationship between CLD and latitude for only one of the three geographic groupings. Sex‐specific relationships between cranial length and CLD were detected. The pattern of these relationships varied by geographic grouping. This study is important because it demonstrates that despite very similar levels of CLD in a single primate species, there exists important geographic variability in the correlates of that dimorphism. I suggest that geographically varying ecological factors may influence sex‐specific natural selection and the intensity of CLD in M. fascicularis. Gaining a better understanding of this geographical variability will require that future research examines morphological variation, including CLD, within its corresponding ecological and social contexts. Such research should be comparative, and incorporate multiple geographically separated populations with disparate environmental settings. Am. J. Primatol. 72:152–160, 2010. © 2009 Wiley‐Liss, Inc.     

Genetic influences on cranial form: variation among ranch and feral American mink Mustela vison (Mammalia: Mustelidae)

Multivariate statistical techniques were used to examine craniometric variation within and between ranch and feral populations of American mink (Mustela vison). An examination of variation between ranches revealed that differences are greater between ranches than within ranches, although all comparisons were statistically significant. There is highly significant variation within a population of mink sampled from a single ranch during a short time period; female mink of different pelt colour are differentiable by their skull shape but not by skull size. This offers evidence for a genetic background to cranial variation in ranch mink. Cranial sexual dimorphism is reduced in ranch mink, when compared to feral populations, and size accounts for a lesser proportion of the variation between the sexes in ranched populations. In addition, the skulls of ranched mink are larger, have a relatively shorter palate and a relatively narrower postorbital constriction compared to their feral counterparts. We believe this reduced dimorphism to be a product of relaxed sexual selection, lack of resource competition and selective breeding for larger specimens of both sexes within ranch populations.     

Is sexual size dimorphism variable? Data on species of the genus <Emphasis Type="Italic">Martes</Emphasis> in the Urals

Sexual dimorphism in cranial measurements was studied in Ural populations of two species of the genus Martes, M. martes and M. zibellina. The index of sexual dimorphism averaged 8.39% in the former and 9.03% in the latter, being subject to geographic variation. For the first time, temporal variability of sexual dimorphism in dimensional characters was revealed in mammals by analyzing M. zibellina samples collected in different decades of the second half of the 20th century. The majority of corresponding microtrends had reciprocating dynamics.     

Detection of mustelid herpesvirus-1 infected European badgers (Meles meles) in the British Isles

The aim of this study was to assess the frequency of mustelid herpesvirus-1 (MusHV-1) infection in free-ranging badgers (Meles meles) in the British Isles. A polymerase chain reaction assay was developed that detected MusHV-1 DNA in 95% (18/19) and 100% (10/10) of anticoagulant-treated blood samples collected from free-ranging badgers sampled in the southwest of England and the Republic of Ireland, respectively. An indirect immunoassay was also developed to detect MusHV-1-specific immunoglobulin-G in serum samples. Using an arbitrary cutoff of twice the optical density obtained with a virus-negative preparation, 32.7% (36/110) of sera sampled from badgers were positive. The conclusion drawn from these data is that infection with MusHV-1 is common among free-ranging badgers in the British Isles.     

Variation in cranial form and sexual dimorphism among five European populations of the otter Lutra lutra

Craniometric variation between otters Lutra lutra (L.) from Ireland, Scotland, Shetland, Norway, and Czechoslovakia was examined using multivariate statistical analyses. Significant sexual dimorphism in cranial size and shape occurred in all samples, although the degree of dimorphism varied between samples, with the Irish sample exhibiting the most. Morphological differences were also observed across samples. Three groups were observed; Irish, Scottish, (Mainland and Shetland), and Mainland European, though within the two composite groups the samples were still differentiable. There was a good agreement between the patterns of variation between male and female samples, but morphological differentiation was not significantly correlated with geographic separation. These results are discussed in the light of our current knowledge of the ecology of the species in Europe. It is argued that, while there is morphological evidence for subspecific status of the Irish otter (as L. l. roensis ), further investigations of morphological and genetic variation across Eurasia are clearly required.     

Sexual dimorphism of craniological characters in Eurasian badgers, Meles spp. (Carnivora, Mustelidae)

An analysis of 30 craniological characters of Eurasian badgers (Meles spp.) revealed different levels of sexual size dimorphism (SSD) and geographic variation in the three different species. SSD is displayed mostly in the general size of the skull (condylobasal length, zygomatic width, width of rostrum, and cranial height) and mandible (height of the vertical mandibular ramus, total length of the mandible, and length between the angular process and infradentale), and in some dental characters (length of the upper molar M¹). The most stable size dimorphism is manifested in the size of the canines, which is pronounced in all studied samples. SSD is not apparent in the length of the auditory bulla, the postorbital width, the minimum palatal width, the length of the lower molar M₂, and the talonid length of the lower carnassial tooth M₁.In both the European badgers, Meles meles, and the Asian badgers, Meles leucurus, two geographic forms that differ in the degree of SSD have been found. The most pronounced SSD of cranial characters is found in the Transcaucasian form of Meles meles and the Far-Eastern form of Meles leucurus. In the large European form of Meles meles, SSD is less pronounced in both absolute and relative criteria than in the smaller Transcaucasian form. An analogous situation is observed in the larger Siberian and smaller Far-Eastern forms of Meles leucurus. In the Japanese badgers, Meles anakuma, a downsizing of the skull is accompanied by a decrease in SSD (except in canine size) in comparison to the continental species. The significant differences in the manifestation of SSD in the Eurasian badgers support an earlier hypothesis (Abramov, 2001. Proc. Zool. Inst. Russ. Acad. Sci. 288, 221-233; Abramov, 2002. Russ. J. Theriol. 1 (1), 57-60; Abramov, 2003. Small Carnivore Conserv. 29, 5-7) that Meles meles, Meles leucurus, and Meles anakuma are separate species. The differences in the level of SSD among and within these three species of badgers may be attributed not to differences in the diet or social structure, but to the history of speciation events and formation of intraspecific distribution ranges.     

SEXUAL DIMORPHISM IN SPOTTED DOLPHINS (STENELLA ATTENUATA) IN THE EASTERN TROPICAL PACIFIC OCEAN

We analyzed 4 meristic and 32 morphometric cranial traits of 612 adult specimens of spotted dolphins ( Stenella attenuata ) from the eastern tropical Pacific Ocean for sexual dimorphism. Eleven 5" latitude/longitude blocks with five or more of each sex were assessed with a two-way ANOVA for sex and geographic differences. Interaction between these two factors was found for six measurements, suggesting that the degree of sexual dimorphism varies geographically for a few characters (although in no discernible geographic pattern). Sexual dimorphism was demonstrated for 23 of the 36 characters, with differences ranging from 0.00 to 5.88 percent. Females characteristically had a longer rostrum, while males generally had larger skulls overall. In terms of number of characters, the extent of sexual dimorphism demonstrated for skulls of spotted dolphins goes considerably beyond that shown for any other small delphine. A discriminant function involving a combination of 10 characters enabled us to identify correctly the sex of more than 75 percent of the specimens. A procedure for correcting specimen measurements is outlined that would enable an investigator to combine male and female specimens in geographic variation studies. A term (zwitter) is proposed for use when referring to specimens where measurements have been corrected to take into account differences between the sexes.     

Intrapopulational body size variation and cranial capacity variation in middle pleistocene humans: The Sima de los Huesos sample (Sierra de Atapuerca,Spain)

A sexual dimorphism more marked than in living humans has been claimed for European Middle Pleistocene humans, Neandertals and prehistoric modern humans. In this paper, body size and cranial capacity variation are studied in the Sima de los Huesos Middle Pleistocene sample. This is the largest sample of non-modern humans found to date from one single site, and with all skeletal elements represented. Since the techniques available to estimate the degree of sexual dimorphism in small palaeontological samples are all unsatisfactory, we have used the bootstraping method to asses the magnitude of the variation in the Sima de los Huesos sample compared to modern human intrapopulational variation. We analyze size variation without attempting to sex the specimens a priori. Anatomical regions investigated are scapular glenoid fossa; acetabulum; humeral proximal and distal epiphyses; ulnar proximal epiphysis; radial neck; proximal femur; humeral, femoral, ulnar and tibial shaft; lumbosacral joint; patella; calcaneum; and talar trochlea. In the Sima de los Huesos sample only the humeral midshaft perimeter shows an unusual high variation (only when it is expressed by the maximum ratio, not by the coefficient of variation). In spite of that the cranial capacity range at Sima de los Huesos almost spans the rest of the European and African Middle Pleistocene range. The maximum ratio is in the central part of the distribution of modern human samples. Thus, the hypothesis of a greater sexual dimorphism in Middle Pleistocene populations than in modern populations is not supported by either cranial or postcranial evidence from Sima de los Huesos. Am J Phys Anthropol 106:19–33, 1998. © 1998 Wiley-Liss, Inc.     

Sexual dimorphism in the skull of minks Mustela vison, badgers Meles meles and otters Lutra lutra

Multiple-group principal component analysis and discriminant analysis were used to investigate the morphological differences between adult skulls of male and female minks, badgers and otters from Norway. The first principal component axis, calculated from the variance-covariance matrix of log-transformed data, was interpreted as a growth-free size axis in all three species, while the other components were interpreted as representing shape. Having largely separated size and shape variation, these two aspects of sexual dimorphism could be studied. The standardized component scores were subjected to an analysis of variance and discriminant analyses were performed on size-in and size-out data. Sexual dimorphism was disclosed on eight of the 12 components in minks and on seven of the 12 components in badgers and otters. In mink the multivariate differences were more due to size than to shape, whereas in badgers and otters most of the multivariate differences were due to shape, but the differences in size were also significant. The shape dimorphism was shown to be functionally related to jaw and neck muscles. The results were discussed in relation to recent theories to explain the evolutionary significance of sexual dimorphism in body size of mustelids. It was concluded that these theories do not fully explain the dimorphism found in the skulls of the moderately dimorphic badger and otter.     

Sex determination from the occipital condyle: discriminant function analysis in an eighteenth and nineteenth century British sample

Fragmentary human remains compromised by different types of inhumation, or physical insults such as explosions, fires, and mutilations may frustrate the use of traditional morphognostic sex determination methods. The basicranium is protected by a large soft tissue mass comprising muscle, tendon, and ligaments. As such, the occipital region may prove useful for sex identification in cases of significantly fragmented remains. The aims of this paper are to (1) evaluate sexual dimorphism in British cranial bases by manually recorded unilateral and bilateral condylar length and width as well as intercondylar measurements and (2) develop discriminant functions for sex determination for this cranial sample. The crania selected for this study are part of the 18th-19th century documented skeletal collection of St. Bride's Church, Fleet Street, London. Adult human skulls (n = 146; male75/female71) were measured to derive statistical functions. Results indicated that expression of sexual dimorphism in the occipital condylar region within the St. Bride's population is demonstrable but low. Crossvalidated classification accuracy ranged between 69.2 and 76.7%, and sex bias ranged from 0.3 to 9.7%. Therefore, the use of discriminant functions derived from occipital condyles, especially in British skeletal populations, should only be considered in cases of fragmented cranial bases when no other morphognostic or morphometric method can be utilized for sex determination.     

Allozyme variation in Atlantic salmon from the British Isles: associations with geography and the environment

Data on geographical variation in allele frequencies at enzyme coding loci in Atlantic salmon from the British Isles were collated from published and unpublished sources. Statistically significant differences in allele frequencies were found among samples both within and among river systems, suggesting that the Atlantic salmon in the British Isles is not a panmictic population and that even within major river systems it cannot be treated as a single genetic stock for fisheries management purposes. Although there was some evidence of regional differences in the frequency of some rare alleles, most single‐locus variation did not show strong geographic patterns, with the exception of the AAT‐4 * locus at which allele frequencies had a significant latitudinal cline. There was some evidence for the existence of genetically‐distinct celtic and boreal races of Atlantic salmon in the British Isles as previously has been suggested. Multiple regression analyses revealed associations between genetic variation and local environmental conditions ( i.e . between variation at MEP‐2 * and both temperature and local river gradient), providing additional evidence for adaptive population divergence in the species.     

Sexual dimorphism and speciation on two ecological coins: patterns from nature and theoretical predictions

Adaptive divergence of phenotypes, such as sexual dimorphism or adaptive speciation, can result from disruptive selection via competition for limited resources. Theory indicates that speciation and sexual dimorphism can result from identical ecological conditions, but co-occurrence is unlikely because whichever evolves first should dissipate the disruptive selection necessary to drive evolution of the other. Here, we consider ecological conditions in which disruptive selection can act along multiple ecological axes. Speciation in lake populations of threespine sticklebacks (Gasterosteus aculeatus) has been attributed to disruptive selection due to competition for resources. Head shape in sticklebacks is thought to reflect adaptation to different resource acquisition strategies. We measure sexual dimorphism and species variation in head shape and body size in stickleback populations in two lakes in British Columbia, Canada. We find that sexual dimorphism in head shape is greater than interspecific differences. Using a numerical simulation model that contains two axes of ecological variation, we show that speciation and sexual dimorphism can readily co-occur when the effects of loci underlying sexually dimorphic traits are orthogonal to those underlying sexually selected traits.     

Factors influencing the degree of sexual size dimorphism within and among calanoid copepod species

Populations of Diaptomus leptopus (Copepoda: Calanoida) and other calanoid copepods exhibit varying degrees of sexual size dimorphism. We examined whether intraspecific or interspecific variation in dimorphism could be explained by allometry, and we examined the relationship between adult size attained and development rate to determine any relationship between the two. We compared the degree of sexual size dimorphism in D. leptopus and in other calanoid copepods inhabiting temporary and permanent habitats. Allometry did not explain variation in sexual size dimorphism within or among populations or among species. Permanence of habitat affected the degree of dimorphism: dimorphism was greater within and among species inhabiting temporary environments. Non-significant differences in development rate were found among populations and significant differences were found between sexes of D. leptopus when reared under identical laboratory conditions: males developed more rapidly than females but there was no general relationship between development rate and adult size. Potential adaptive hypotheses to explain the differences between populations inhabiting temporary and permanent habitats are discussed.     

Craniometric variation in a population of mantled howler monkeys (Alouatta palliata): evidence of size selection in females and growth in dentally mature males

A large body of work on monkey cranial metrics (involving conclusions about interspecific variation, sexual dimorphism, and ontogeny) depends on the assumptions that growth effectively ceases with dental maturity and that intraspecific variation is negligible. We test these assumptions by examining variation in 39 measurements of 166 dentally mature Alouatta palliata skulls from animals found dead on Barro Colorado Island (BCI), Panama. We also investigate whether this population is under size-based selection, since our found-dead sample reflects the natural mortality in this population. The sample was divided into three age stages by occlusal wear (A-C, least to most wear). Female stage A means are significantly smaller than female stage B means for three cranial measures. Female stage B means are significantly smaller than female stage C means for five cranial measures. Male stage A means are significantly smaller than male stage B means for 21 cranial measures. Multivariate analyses confirm this trend of expansion between adult age stages. The dental metric and suture closure data suggest that the cranial expansion in females is due to size-based selection, while the cranial expansion in males is due to significant growth after dental maturity. Sexual dimorphism ratios are highly variable across different samples of A. palliata, indicating that dimorphism varies between populations of this species. These results provide insight into the selective forces operating on the BCI howlers and challenge the validity of the many studies which pool subspecies and assume growth ceases with maturity.     

Phenotypic divergence and inter-specific trait correlation in a plant-pollinator/seed predator mutualism

Plant-pollinator interactions have been suggested as key drivers of morphological divergence and speciation of the involved taxa. These interactions can also promote sexual dimorphism in both the plant and pollinator, particularly if the pollinator is also a seed-eater and/or exerts different selection pressures on male and female plants. Here we tested the hypotheses that plant-pollinator interactions can be reflected in trait variation and sexual dimorphism in both organisms within and across populations. Across nine European populations, we examined intraspecific variation and sexual dimorphism in phenotypic traits potentially involved in the plant–insect interaction of the dioecious white campion Silene latifolia (Caryophyllaceae) and its specialist pollinator Hadena bicruris (Noctuidae). This interaction is expected to entail sex-specific selective pressures, as female moths lay eggs on female plants and the larvae predate on the seeds during their development. We compared divergence in phenotypic traits among populations and between sexes within populations, examined correlations between plant and pollinator traits, and between phenotypic distances and genetic distances among co-occurring populations for both plants and insects. We found key differences in phenotypic traits across populations of both the plant and moth, though only in the moth were these differences correlated with geographic distances. We also found evidence for sexual dimorphism in the plant but not in the pollinator. Evolution of floral sexual dimorphism in S. latifolia most likely results from the joint contribution of different selective forces, including biotic interactions with H. bicruris moths.     

Among population differentiation at nuclear genes in native Scots pine (Pinus sylvestris L.) in Scotland

In the Scottish Highlands, Scots pine is at the north-western extreme of its wide natural distribution. Here, the remaining native populations are patchily distributed in highly variable environments, from the more continental, drier eastern Highlands to the milder, wetter Atlantic Ocean coast. As these pinewoods are the remnants of a naturally established forest, they form a valuable system for analysis of genetic and adaptive variation in heterogeneous environments. Using samples from across the Scottish population, we analysed data from nuclear and mitochondrial genes to assess patterns of within and between population genetic variation. Within population diversity levels were high, and significant genetic differentiation among pairs of Scottish populations at relatively small spatial scales was present at several nuclear loci. At these loci, no differentiation had been found among continental populations, even those separated by large geographic distances. Overall, no clear clustering of Scottish samples was found in population structure analysis suggesting that geographically distant populations with high intra-population nucleotide diversity are not strongly isolated or diverged from each other. Scottish populations lacked a mitotype that is widespread in eastern and north-eastern Europe, indicating that pines from that area may not have participated in the most recent colonisation of the British Isles.