OBSERVATIONS ON THE BREEDING OF THE GREATER FLAMINGO PHOENICOPTERUS RUBER LINNAEUS IN THE BREDASDORP DISTRICT,SOUTH AFRICA

Clark, R. A. 1979. Seasonal levels of body fat, protein, ash and moisture in the Sacred Ibis. Ostrich 50:129-133.

Sacred Ibises Threskiornis aethiopicus in Pretoria, Transvaal, were analyzed for body composition throughout the year. Body fat levels in adults and immatures were approximately 7% of feather-free empty body mass in the winter. Just prior to breeding the levels of fat were approximately 12% in adults and 6% in immatures. Lowest levels were recorded for adults and immatures in January (3,5%). In contrast to fat, levels of moisture, protein and ash did not appear to vary significantly between seasons.     

The timing of gonadal development and moult in three raptors with different breeding seasons: effects of gender, age and body condition

Differences between species in breeding seasons are thought to be mediated through differences in their reproductive physiology. Little is known about how the timing and duration of gonadal maturation varies between raptor species, how the timing of moult relates to the gonadal cycle, whether the timing and degree of sexual maturation varies between juveniles and adults or whether body condition has a significant effect. To address these questions, data on gonadal size and moult for adults and juveniles of both sexes of three raptor species were extracted from the Predatory Bird Monitoring Scheme (based on birds found dead by members of the public). The three species, Sparrowhawk Accipiter nisus, Kestrel Falco tinnunculus and Barn Owl Tyto alba, have different ecologies – diurnal bird predator, diurnal mammal predator and nocturnal mammal predator, respectively. All are single‐brooded but have different breeding seasons. The duration of gonadal maturation was markedly different between the species. Barn Owls showed the earliest maturation and the latest gonad regression, and Sparrowhawks the latest maturation and earliest gonad regression. Kestrels were intermediate. In males of all species, the testes remained fully mature throughout their respective breeding seasons. In females, the ovaries remained partially mature throughout the breeding season. Moult started slightly earlier in Sparrowhawks than in Kestrels and coincided with gonadal regression in the two species. Although females of the two species started to moult earlier than males, moult duration was similar between the sexes. Barn Owls showed no distinct annual pattern of moult. In juveniles of all three species, the gonads were smaller than in adults throughout spring and started to mature later. Gonad size in birds that had starved tended to be smaller than in birds dying from other causes, but did not influence the difference in gonad mass between adults and juveniles and between seasons. Body condition had no effect on moult. Whilst ecology has led to the evolution of different breeding seasons, differences between species, and between adults and juveniles, are mediated through adaptive differences in their reproductive physiology.     

THE BIRDS OF LIVINGSTONE TOWNSHIP,NORTHERN RHODESIA, 1942–1950

Summary

Clark, R. A. 1979. DDT contamination of the Sacred Ibis. Ostrich 50:134-138.

Samples of fat and brain from Sacred Ibises Threskiornis aethiopicus in Pretoria, Transvaal, were analyzed for organochlorine insecticides. Except for traces of p,p'-DDT only p,p'-DDE was found to occur (means of 28,35 mg/kg wet fat and 0,22 mg/kg wet brain). There was a significant correlation between the concentrations of DDE in the fat and brain of birds (r = 0,53; p ? 0,01). In the period when Sacred Ibises mobilized their fat reserves there was no significant correlation between the level of body fat and the concentration of DDE in either the brain or the remaining body fat.     

Size,plumage, moult and supposed hybrids of African Goshawks (Accipiter tachiro/toussenelii group) in DR Congo

Parapatric A. tachiro sparsimfasciatus and A. toussenelii canescens are in a size cline from large east African to small west African birds. While their plumage colour is different, the pattern of spotting (juvenile) and barring (adult) of the breast feathers is similar. No general hybridisation is found in the contact region in Kivu: the plumage of some aberrant individuals can be due to great age or to individual variation. One supposed hybrid was obtained far from the contact region. I consider them as paraspecies. Based on direct evidence and on annual moult I conclude that the breeding period is prolonged in both taxa in equatorial DR Congo, and that it is seasonal in tachiro sparsimfasciatus in southern DR Congo. In the latter population, the postjuvenile moult starts probably just before the age of one year but it lasts many months, leaving the juvenile upper tail coverts in place for one more year. Plumage characteristics are related to habitat. The female of the woodland sparsimfasciatus is cryptically coloured, with individual variation, possibly helpful for 'image avoidance'. The male is even more variable in colour, in part age related: a 'sepia' morph is described for the first adult plumage. Older birds are grey with reddish flanks, becoming darker with age. The adult evergreen forest canescens shows enforcement of colourful advertising plumage and loss of sexual plumage dimorphism. The first adult is advertised by the late moult of the barred juvenile flank feathers in both sexes. Its juvenile is peculiar; it lacks breast spots, suggesting image avoidance, but possibly also character displacement or mimicry.     

BIOLOGY OF THE BANK CORMORANT,PART 2: MORPHOMETRICS,PLUMAGE, BARE PARTS AND MOULT

Cooper J. 1985. Biology of the Bank Cormorant, Part 2: Morphometrics, plumage, bare parts and moult. Ostrich 56: 79–85.

The Bank cormorant Phalacrocorax neglectus is a medium-sized, heavily built but relatively short-tailed cormorant. Males average 2 107 g, females 1794 g. There are significant differences, but overlaps, in mass, wing length, bill, tarsus and tail length between the sexes of breeding adults. The species is all black; adults have a white rump patch and white filoplumes on the head and neck in prenuptial plumage. The white rump patch is lost during breeding. Bare parts are black except for the iris (the only useful aging character) which is dark brown in juveniles, green in subadults and horizontally divided orange-brown above, green below, in adults. Partial albinism is common but leucism is not. Adults can moult while breeding but it seems likely that most moult occurs outside the breeding season. Adults and a single subadult examined exhibited Staffelmauser or stepwise moult of the primaries.     

Determination of growth and maturation in the common frog, Rana temporaria, by skeletochronology

Growth and maturation in a Swiss population of Rana temporaria were studied in 1983 and 1984 by means of skeletochronology. Resting line (growth ring) diameters were used to back-calculate individual body sizes in previous years; these permitted establishment of an average growth curve and determination of individual ages and sizes at first reproduction. Growth was rapid up to maturation, but continued thereafter at a decreased rate. Males were larger than females at age two but females grew faster thereafter, causing sexual dimorphism in adult body sizes. Body size distributions for both years and for frogs recaptured and first captured in 1984 were established. Growth in immatures was positively, but in adults negatively correlated with body size, with considerable variation at all sizes. Individual adult sizes were positively correlated with body sizes at the end of the first year. Average individual age at first reproduction was 2.8 years in males and 3.1 years in females (range in both sexes two to four years). There is no evidence for a two-year-cycle of reproduction.     

Selection and inheritance of sexually dimorphic juvenile plumage coloration

Sexually dimorphic plumage coloration is widespread in birds and is generally thought to be a result of sexual selection for more ornamented males. Although many studies find an association between coloration and fitness related traits, few of these simultaneously examine selection and inheritance. Theory predicts that sex‐linked genetic variation can facilitate the evolution of dimorphism, and some empirical work supports this, but we still know very little about the extent of sex linkage of sexually dimorphic traits. We used a longitudinal study on juvenile Florida scrub‐jays (Aphelocoma coerulescens) to estimate strength of selection and autosomal and Z‐linked heritability of mean brightness, UV chroma, and hue. Although plumage coloration signals dominance in juveniles, there was no indication that plumage coloration was related to whether or not an individual bred or its lifetime reproductive success. While mean brightness and UV chroma are moderately heritable, hue is not. There was no evidence for sex‐linked inheritance of any trait with most of the variation explained by maternal effects. The genetic correlation between the sexes was high and not significantly different from unity. These results indicate that evolution of sexual dimorphism in this species is constrained by low sex‐linked heritability and high intersexual genetic correlation.     

Prebreeding moult, plumage and evidence for a presupplemental moult in the Great Knot Calidris tenuirostris

We studied the prebreeding moult and resulting plumage in a long-distance migrant sandpiper (Scolopacidae), the Great Knot Calidris tenuirostris , on the non-breeding grounds (northwest Australia), on arrival at the staging grounds after the first migratory flight (eastern China) and on or near the Russian breeding grounds (Russian data from museum specimens). We show that breeding plumage scores and breast blackness were affected not only by the increase in moulted feathers but also in the wearing down of overlaying pale tips of fresh feathers. Birds migrating from Australia and arriving in China had completed or suspended moult, but more moult must occur in Asia as Russian specimens had moulted more of their mantle and scapular feathers. Russian birds had significantly more red feathering on their upperparts than had birds in Australia or those arriving in China. The increase in reddish feathers cannot by accounted for simply by continuation of the prealternate moult. Instead, a third, presupplemental moult must occur, in which red-marked feathers replace some scapular and especially mantle feathers that were acquired in a prealternate moult only 1–3 months earlier. Great Knot sexes show little size and plumage dimorphism, whereas two other sandpipers that have supplemental plumages (Ruff Philomachus pugnax and Bar-tailed Godwit Limosa lapponica ) are thought to be highly sexually selected. Bidirectional sexual selection may therefore be involved in the evolution of a supplemental plumage in Great Knots.     

Sources of distinctness of juvenile plumage in Western Palearctic passerines

Juveniles of many avian species possess a spotted or mottled body plumage that is visually distinct from the plumage of adults. In other species, however, juveniles fledge with a body plumage that is just a pale representation of adult female plumage. The reasons for this variation are poorly understood. Several hypotheses concerning social (parent–offspring, adult–juvenile, juvenile–juvenile), ecological (predation risk) and physiological (costs of plumage development) implications of juvenile body plumage are presented in relation to predictions concerning associations with certain ecological and life‐history attributes of avian species. In the present study, we conduct a phylogenetically corrected comparative analysis of Western Palearctic passerines looking for sources of variation in the incidence of distinct and adult‐like juvenile body plumages. We scored plumages based on plates in the Handbook of the Birds of the Western Palearctic (Cramp & Perrins, 1988–1994; Oxford University Press) (HBWP) and entered body mass, migratory habits, habitat, nestling diet, breeding dispersion, gregariousness, duration of the nestling period, type of nest, conspicuousness of female plumage, and sexual dimorphism as explanatory variables, as presented in HBWP, in phylogenetic generalized least square regression analyses. One‐third of the species presented distinct juvenile body plumages, which lasted on average for the first 2 months of life. Body mass, conspicuousness of female plumage, migratory habits, and habitat were significantly associated with interspecific variation in distinctness of juvenile plumage, with smaller species, more conspicuous species, migrants, and species from forested habitats showing distinct juvenile plumages with higher frequency. The phylogenetic signal was moderately high. Assuming that conspicuous adult plumage is costlier to produce than distinct juvenile body plumage (pigments, conspicuousness), the need to acquire social status among juveniles before the winter may explain the more adult‐like plumage in resident species because juveniles will probably compete with individuals that they may have known during their first months of life. On the other hand, migrant juveniles may compete with a different set of individuals in winter quarters and can use savings in resources necessary for developing adult‐like plumages to improve migration capacity by allocating resources to other functions. The association with habitat could be related to juveniles in open habitats participating in more extended interactions with other juveniles than in forested habitats where lower visibility may reduce the capacity to detect or respond to signals from juvenile conspecifics. More studies on this possibly crucial life stage are needed. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 440–454.     

山地麻蜥个体发育过程中头部两性异形和食性的变化

研究了山地麻蜥(Eremias brenchleyi)个体发育过程中头部两性异形和食性的变化．成体个体大小(SVL)无显著的两性差异,但雄体具有较大的头部(头长和头宽)．头部两性异形在孵出幼体就已存在,成体头部两性异形比幼体(包括孵出幼体)更为显著,雄性较大的头部与其头部随SVL的增长速率大于雌性有关．两性头部总体上随SVL呈异速增长,表现为个体发育过程中头长和头宽与SVL的线性回归方程斜率有显著的变化．孵出幼体有相对较大的头部,这种形态特征是胚胎优先保证生态学意义更为显著的头部生长的结果,有利于孵出幼体的早期生存和生长．相对头部大小在个体发育过程中有显著的变化．不同性别和大小的山地麻蜥摄入食物的种类及各种食物在摄入食物中所占的比例有一定程度的差别,食物生态位宽度和重叠度因此有一定的差别．然而,没有直接的证据表明头部两性异形能导致两性食物生态位的明显分离,并有利于减缓两性个体对食物资源的竞争。     

5. LIVINGSTONE IN NORTH-EASTERN RHODESIA

Sugg, M. St. J. 1974. Mensural and moult data from a breeding colony of Pied Kingfishers. Ostrich 45: 227–234.

Pied Kingfishers were ringed over two breeding seasons at a breeding colony on the Kenyan shore of Lake Victoria. Data were collected on weight, wing and bill length, injury and moult. Bill abrasion and breakage from nest excavation was found in both sexes and regeneration of worn and broken bills was recorded.

Adult birds of both sexes returned to the colony to breed but no juvenile was recaptured the year after hatching. Females are slightly larger than males (wing length) and both sexes showed a weight increase in the evening prior to roosting. Juveniles had shorter wings and bills than adults but their weights were similar. Their bills were short, soft, weak and the gape was salmon pink. No juveniles showed any moult. Adult moult records support the sequence and duration of moult suggested by Douthwaite (1971) though no overall decrease of moult activity was recorded during the breeding season.     

TRAPPING AND RINGING OF EGYPTIAN GEESE AND AFRICAN SHELDUCK AT VOGELVLEI,CAPE

Many small plovers Charadrius spp. have sexually monomorphic plumage and cryptic sexual size dimorphism. The objective of our study was to assess the variation in body sizes between male and female plovers breeding in Madagascar. We collected blood samples and data on adult body sizes of four small plovers (Madagascar Plover Charadrius thoracicus, Kittlitz's Plover C. pecuarius, White-fronted Plover C. marginatus and Three-banded Plover C. tricollaris), and used molecular genetic markers to sex the adults. We found significant differences in body size among the four species, and between sexes. Furthermore, individuals from the southern ecoregion tended to be larger than in the western ecoregion. The Madagascar Plover's body size was significantly more dimorphic than the Kittlitz's and White-fronted Plovers. Breeding Malagasy plovers' show significant sexual size dimorphism (SSD): Madagascar Plover females were heavier and had longer wings than males, whereas the males had longer tarsi; in White-fronted Plover only wing length was different between the sexes. Taken together, our work reports SSD in small African plovers that exhibit monomorphic plumage, and we propose that SSD may be more common than currently acknowledged; we term this 'cryptic sexual size dimorphism'. Our results also suggest sexual selection and/or natural selection exert different pressures on body size in different Malagasy plover species.     

The effects of long‐distance migration on the evolution of moult strategies in Western‐Palearctic passerines

Although feathers are the unifying characteristic of all birds, our understanding of the causes, mechanisms, patterns and consequences of the feather moult process lags behind that of other major avian life‐history phenomena such as reproduction and long‐distance migration. Migration, which evolved in many species of the temperate and arctic zones, requires high energy expenditure to endure long‐distance journeys. About a third of Western‐Palearctic passerines perform long‐distance migrations of thousands of kilometres each year using various morphological, physiological, biomechanical, behavioural and life‐history adaptations. The need to include the largely non‐overlapping breeding, long‐distance migration and feather moult processes within the annual cycle imposes a substantial constraint on the time over which the moult process can take place. Here, we review four feather‐moult‐related adaptations which, likely due to time constraints, evolved among long‐distance Western‐Palearctic migrants: (i) increased moult speed; (ii) increased overlap between moult and breeding or migration; (iii) decreased extent of plumage moult; and (iv) moult of part or all of the plumage during the over‐wintering period in the tropics rather than in the breeding areas. We suggest that long‐distance migration shaped the evolution of moult strategies and increased the diversity of these strategies among migratory passerines. In contrast to this variation, all resident passerines in the Western Palearctic moult immediately after breeding by renewing the entire plumage of adults and in some species also juveniles, while in other species juvenile moult is partial. We identify important gaps in our current understanding of the moult process that should be addressed in the future. Notably, previous studies suggested that the ancestral moult strategy is a post‐breeding summer moult in the Western Palearctic breeding areas and that moult during the winter evolved due to the scheduling of long‐distance migration immediately after breeding. We offer an alternative hypothesis based on the notion of southern ancestry, proposing that the ancestral moult strategy was a complete moult during the ‘northern winter’ in the Afro‐tropical region in these species, for both adults and juveniles. An important aspect of the observed variation in moult strategies relates to their control mechanisms and we suggest that there is insufficient knowledge regarding the physiological mechanisms that are involved, and whether they are genetically fixed or shaped by environmental factors. Finally, research effort is needed on how global climate changes may influence avian annual routines by altering the scheduling of major processes such as long‐distance migration and feather moult.     

Pathways to elaboration of sexual dimorphism in bird plumage patterns

Patterns, such as bars and spots, are common in birds. Some patterns can function in camouflage and/or communication and can benefit both males and females, paving the way for elaboration in sexual dimorphism. Historically, sexual dichromatism was predominantly considered to be a consequence of mating systems. However, the distribution of traits between the sexes is not always indicative of function; genetic correlation may cause traits to evolve in both sexes and traits may serve a social function in males and/or females. In addition, sexual dichromatism in bird plumage patterns can be composed of multiple types of patterns within and/or between the sexes. Therefore, there can be more than one type of dimorphism and some are more elaborate than others. Under classical models of genetic correlation, patterns evolve in both sexes followed by a loss of patterning in one sex. Elaborate types of sexual dimorphism in plumage patterns may be due to selection acting on existing patterns and are perhaps derived. Waterfowl (Anseriformes) and gamebirds (Galliformes) arguably have the most striking plumage patterns. Using 288 species from these orders I reconstructed the evolutionary history of plumage pattern dimorphism. There was little support for genetic correlation but elaborate types of dimorphism are probably derived. Backward and forward evolutionary transitions between different types of dimorphism can occur by loss or elaboration. These results demonstrate that plumage patterns are evolutionary labile and current forms may represent shifting adaptations to a changing environment. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 262–273.     

Which proximate factor determines sexual size dimorphism in tiger snakes?

Diverse interactions between factors that influence body size complicate the identification of the primary determinants of sexual size dimorphism. Using data from a long‐term field study (1997–2009), we examined the contributions of the main proximate factors potentially influencing sexual size dimorphism from birth to adulthood in tiger snakes (Notechis scutatus). Data on body size, body mass and body condition of neonates, juveniles and adults were obtained by mark–recapture. Frequent recaptures allowed us to monitor reproductive status, diet and food intake, and to estimate survival and growth rates in age and sex classes. Additional data from females held briefly in captivity enabled us to assess reproductive output and the body mass lost at parturition (proxies for reproductive effort). From birth to maturity, individuals of both sexes experienced similar growth and mortality rates. We found no difference in diet, feeding and survival rates between the sexes, nor between juveniles and adults. On maturity, despite comparable diet and food intake by both sexes, the high energy requirements of vitellogenesis and gestation were responsible for a depletion of body reserves and probably resulted in a marked decrease in growth rates. Males were largely exempt from such costs of reproduction, and so could grow faster than females and attain larger body sizes. The absence of niche divergence between the sexes (uniformity of habitat, lack of predators) suggests that the impact of differential energetic investment for reproduction on growth rate is probably the main proximate factor influencing sexual size dimorphism in this species. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 668–680.     

Sexual dimorphism in traits related to locomotion: ontogenetic patterns of variation in Podarcis wall lizards

Sexual dimorphism in body size and shape in animals is normally linked to sexual selection mechanisms that modify the morphological properties of each sex. However, sexual dimorphism of ecologically relevant traits may be amplified by natural selection and result in the ecological segregation of both sexes. In the present study, we investigated patterns of sexual dimorphism of morphological traits relevant for locomotion in two lacertid lizards, Podarcis bocagei and Podarcis carbonelli, aiming to identify ontogenetic sources of variation. We analysed trunk and limb variation in relation to total body size, as well as the covariation of different traits, aiming to shed light on the proximate causation of adult sexual dimorphism. We find that, although immatures are generally monomorphic, adult females have a longer trunk, and adult males have longer fore and hind limbs. Both sexes differ substantially with respect to their growth trajectories and relationships between traits, whereas, in some cases, there are signs of morphological constraints delimiting the observed patterns. Because of the direct connection between limb size/shape and locomotor performance, which is relevant both for habitat use and escape from predators, the observed patterns of sexual dimorphism are expected to translate into ecological differences between both sexes. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 530–543.     

Morphology of the Hooded crow Corvus corone cornix in relation to age, sex and latitude

The morphological variation of the Hooded crow at Trondheim, Norway, was studied, based on a sample of 734 birds collected during a six year period. Mouth colour, plumage colour, skull thickness and feather length were found to be characters which could readily be used to separate juveniles from adults. Females aged 15–19 months had a thinner skull roof than older female birds. Low coefficients of variation were found for the lengths of the third primary and of the tail feathers. A discriminant analysis showed that of the various body dimensions studied bill height and bill length distinguished the sexes most precisely. A high degree of sexual dimorphism was also found to exist in body weight and in the thickness of the skull roof.
Those body structures which develop at about the same stage during the juvenile growth period were associated with the same principal component, viz. the lengths of bony structures, parts that develop early on in life, were intercorrelated (wing bones, tarsus, bill basis and the width of foramen magnum). The lengths of the primaries and of tail feathers were also intercorrelated, structures which develop late.
The mean body weight of the Hooded crow population studied in Norway was intermediate between that of the Hooded crow in Germany and of the Carrion and Hooded crows in England and Scotland. No such differences were found in wing length. Norwegian Hooded crows have shorter tails than German ones, but their bills are much larger, in particular for the females. Therefore, the degree of sexual dimorphism in bill size seems to be reduced at high latitudes.     

The evolution of sexual dimorphism in parasitic cuckoos: sexual selection or coevolution?

Sexual dimorphism is ubiquitous in animals and can result from selection pressure on one or both sexes. Sexual selection has become the predominant explanation for the evolution of sexual dimorphism, with strong selection on size-related mating success in males being the most common situation. The cuckoos (family Cuculidae) provide an exceptional case in which both sexes of many species are freed from the burden of parental care but where coevolution between parasitic cuckoos and their hosts also results in intense selection. Here, we show that size and plumage differences between the sexes in parasitic cuckoos are more likely the result of coevolution than sexual selection. While both sexes changed in size as brood parasitism evolved, we find no evidence for selection on males to become larger. Rather, our analysis indicates stronger selection on parasitic females to become smaller, resulting in a shift from dimorphism with larger females in cuckoos with parental care to dimorphism with larger males in parasitic species. In addition, the evolution of brood parasitism was associated with more cryptic plumage in both sexes, but especially in females, a result that contrasts with the strong plumage dimorphism seen in some other parasitic birds. Examination of the three independent origins of brood parasitism suggests that different parasitic cuckoo lineages followed divergent evolutionary pathways to successful brood parasitism. These results argue for the powerful role of parasite-host coevolution in shaping cuckoo life histories in general and sexual dimorphism in particular.     

The evolution of sexual size dimorphism in the house finch. IV. Population divergence in ontogeny

Differences among taxa in sexual size dimorphism of adults can be produced by changes in distinct developmental processes and thus may reflect different evolutionary histories. Here we examine whether divergence in sexual dimorphism of adults between recently established Montana and Alabama populations of the house finch (Carpodacus mexicanus) can be attributed to population differences in growth of males and females. In both populations, males and females were similar at hatching, but as a result of sex-specific growth attained sexual size dimorphism by the time of independence. Timing and extent of growth varied between the sexes: Females maintained maximum rates of growth for a longer time than males, whereas males had higher initial growth rates and achieved maximum growth earlier and at smaller sizes than females. Ontogeny of sexual dimorphism differed between populations, but in each population, sexual dimorphism in growth parameters and sexual dimorphism at the time of nest leaving were similar to sexual dimorphism of adults. Variation in growth of females contributed more to population divergence than did growth of males. In each population, we found close correspondence between patterns of sexual dimorphism in growth and population divergence in morphology of adults: Traits that were the most sexually dimorphic in growth in each population contributed the most to population divergence in both sexes. We suggest that sex-specific expression of phenotypic and genetic variation throughout the ontogeny of house finches can result in different responses to selection between males and females of the same age, and thus produce fast population divergence in the sexual size dimorphism.     

Clinal variation in body size and sexual dimorphism in an Indian fruit bat, Cynopterus sphinx (Chiroptera: Pteropodidae)

Geographic variation in body size and sexual dimorphism of the short‐nosed fruit bat (Cynopterus sphinx) was investigated in peninsular India. Bats were sampled at 12 localities along a 1200 km latitudinal transect that paralleled the eastern flanks of the Western Ghats. The geographic pattern of variation in external morphology of C. sphinx conforms to the predictions of Bergmann's Rule, as indicated by a steep, monotonic cline of increasing body size from south to north. This study represents one of the first conclusively documented examples of Bergmann's Rule in a tropical mammal and confirms that latitudinal clines in body size are not exclusively restricted to temperate zone homeotherms. Body size was indexed by a multivariate axis derived from principal components analysis of linear measurements that summarize body and wing dimensions. Additionally, length of forearm was used as a univariate index of structural size to examine geographic variation in a more inclusive sample of bats across the latitudinal transect. Multivariate and univariate size metrics were strongly and positively correlated with body mass, and exhibited highly concordant patterns of clinal variation. Stepwise multiple regression on climatological variables revealed that increasing size of male and female C. sphinx was associated with decreasing minimum temperature, increasing relative humidity, and increasing seasonality. Although patterns of geographic size variation were highly concordant between the sexes, C. sphinx also exhibited a latitudinal cline in the magnitude and direction of sexual size dimorphism. The size differential reversed direction across the latitudinal gradient, as males averaged larger in the north, and females averaged larger in the south. The degree of female‐biased size dimorphism across the transect was negatively correlated with body size of both sexes. Canonical discriminant analysis revealed that male‐ and female‐biased size dimorphism were based on contrasting sets of external characters. Available data on geographic variation in the degree of polygyny in C. sphinx suggests that sexual selection on male size may play a role in determining the geographic pattern of sexual size dimorphism.