Sex Differences In Size Research Articles

Sex Differences in the Dorsolateral Telencephalon Correlate with Home Range Size in Blenniid Fish

Blenniid fish exhibit a polygynandric mating system with parental care restricted to males. Nest-holder males defend a breeding territory centered on their nest, usually a crevice or hole in a rocky substrate, to which they attract females to spawn. Females, on the other hand, must search for nests in order to spawn and usually are the choosy sex, producing several sequential egg batches and broods during the breeding season. Therefore, male blennies are more site-attached than females. This situation offers an opportunity to investigate potential neural correlates of intraspecific differences in selective pressures for different spatial abilities in these species. Since the dorsolateral telencephalon has been considered a teleost homologue of the mammalian hippocampus, we predicted that the spatial abilities required for females to locate and return accurately to nests of males may have produced a sex difference in the size of the telencephalic nuclei involved in spatial abilities, biased towards females. To test this hypothesis, we assessed the home ranges and measured the size of the dorsolateral telencephalon of both sexes during the breeding season in two blenniid species, the shanny (Lipophrys pholis) and the Azorean rock-pool blenny (Parablennius parvicornis). We chose these two species because they differ in the degree of chemical communication they use, and this could also lead to differences in telencephalic areas. As predicted, in both species females present considerably larger home ranges paralleled by larger dorsolateral ventral telencephalic nuclei (DLv) than males. Other telencephalic nuclei that were measured did not show any sex difference in size. These results suggest that the DLv is involved in spatial abilities in blenniid fish and that sexual selection may be promoting this divergence as already described for mammals and birds.

Relationship between canine dimorphism and mandibular morphology in the hamadryas baboon and the Japanese monkey

To examine morphological interrelationships between canine size and mandibular corpus shape, inter-sex comparisons were made in the hamadryas baboon and the Japanese monkey, known to display extreme and moderate canine dimorphism, respectively. Results of adult comparisons showed that all mandibular dimensions were significantly larger in the males than in the females in both species. In the hamadryas baboon, the males also exhibited a higher ratio of anterior to posterior corpus height than the females. This sex difference in corpus shape was not significant in the Japanese monkey, indicating lack of involvement of canine dimorphism. Analysis of mandibular growth patterns in the hamadryas baboon demonstrated that significant sexual size difference did not occur before incisor eruption, and that the anteriorly high corpus of the adult male mandible was associated with a rapidly increasing symphyseal height after incisor eruption. It was also shown that the female canine started to erupt shortly after incisor eruption, while the forming male canine continued to stay near the mandibular base and developed further in size until eruption. The relative positions of the incisors kept shifting upwards even after eruption in the males, while they hardly changed in the females. It is therefore suggested that the prolonged development and size increase of the male canine is accompanied by further enlargement of the symphysis, resulting in the higher anterior corpus of the adult males compared to the adult females. The results thus indicate the importance of understanding the spatial relationships of the developing teeth in interpreting mandibular morphology.

A developmental perspective on the evolution of sexual size dimorphism of a moth

Males and females of almost all organisms exhibit sexual differences in body size, a phenomenon called sexual size dimorphism (SSD). How the sexes evolve to be different sizes, despite sharing the same genes that control growth and development, and hence a common genetic architecture, has remained elusive. Here, we show that the genetic architecture (heritabilities and genetic correlations) of the physiological mechanism that regulates size during the last stage of larval development of a moth, differs between the sexes, and thus probably facilitates, rather than hinders, the evolution of SSD. We further show that the endocrine system plays a critical role in generating SSD. Our results demonstrate that knowledge of the genetic architecture underlying the physiological process during development that ultimately produces SSD in adults can elucidate how males and females of organisms evolve to be of different sizes.

Song Control Nuclei in Male and Female Large-Billed Crows (Corvus macrorhynchos)

We show that the learned vocalizations of male and female large-billed crows (Corvus macrorhynchos) are similar and that their functions and physical features show significant differences from those of other oscine species. We investigate whether the song control nuclei of crows show any sexual differences in size, reflecting differences in their singing behavior, and whether these nuclei are different from those of other songbirds in terms of neural connectivity size and relative to the forebrain. Our Nissl staining results reveal that 1) of the four song nuclei examined (HVC; the robust nucleus of the arcopallium [RA]; Area X; and the dorsolateral medial nucleus [DLM]), HVC, RA, and Area X volumes are significantly larger in males than in females, but DLM volume and body and brain weights show no significant gender differences; and 2) the sizes of song nuclei relative to the forebrain are within the range of other oscines. By injecting a neural tract tracer (DiI) into various song nuclei in brain slices, we found that, as in other songbirds, HVC projects to RA and Area X, while Area X projects to the lateral magnocellular nucleus of the anterior nidopallium (IMAN) and DLM, DLM to IMAN, and IMAN to RA. Our results Indicate that, although the crow has songs very different from those of other oscine species, Its song nuclei and the connections between them are not obviously different.

Habitat- and sex-specific life history patterns of yellow tang Zebrasoma flavescens in Hawaii, USA

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 389:245-255 (2009) - DOI: https://doi.org/10.3354/meps08114 Habitat- and sex-specific life history patterns of yellow tang Zebrasoma flavescens in Hawaii, USA Jeremy T. Claisse1,*, Marco Kienzle2, Megan E. Bushnell1, David J. Shafer1, James D. Parrish1 1University of Hawaii, Department of Zoology, 2538 McCarthy Mall, Edmondson 165, Honolulu, Hawaii 96822, USA 2CSIRO, Marine and Atmospheric Research Division, 233 Middle Street, Cleveland, Queensland 4163, Australia *Email: claisse@hawaii.edu ABSTRACT: Yellow tang Zebrasoma flavescens is the primary coral reef fish species taken in Hawaii for the aquarium trade. As part of an extensive adaptive management effort that included a network of marine protected areas that prohibited commercial aquarium fishing, an emphasis was placed on obtaining habitat- and sex-specific life history information for this valuable species. Using otolith and capture-mark-recapture methods we examined sexual differences in ontogenetic patterns of habitat use, growth rate, size dimorphism and longevity. Age validation using tetracycline to mark otoliths provided evidence that a single annulus formed each year. Yellow tang are a long-lived species (the oldest individual collected was 41 yr old) and display an asymptotic growth pattern typical of the family Acanthuridae. Median size and age at the transition between deeper coral-rich and shallow turf-dominated habitat use were about 20 mm longer and about 2 yr older for males than females and coincided with an increase in reproductive output. The sexual difference in size at habitat transition, combined with sexual size dimorphism (mean asymptotic maximum length—male: 179 mm; female 156 mm) results in differences in the size distributions of both sexes in the 2 habitats. Sexual size dimorphism resulted from a higher growth rate for males through the juvenile period. KEY WORDS: Ontogenetic habitat use · Hawaiian aquarium fishery · Acanthuridae · Model selection· Otolith · Capture-mark-recapture · Growth · Coral reef fish Full text in pdf format Supplementary material PreviousNextCite this article as: Claisse JT, Kienzle M, Bushnell ME, Shafer DJ, Parrish JD (2009) Habitat- and sex-specific life history patterns of yellow tang Zebrasoma flavescens in Hawaii, USA. Mar Ecol Prog Ser 389:245-255. https://doi.org/10.3354/meps08114Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 389. Online publication date: September 04, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Flight Morphology and Foraging Behavior of Male and Female Cory's Shearwaters

In some seabirds sexually dimorphic in size, males and females segregate at sea or diverge in other aspects of foraging behavior. We examined factors influencing foraging strategies of Cory's Shearwater and compared the sexes' flight morphology and activity patterns. Trip duration, incubation-shift length, total mass gain, and rate of mass gain at sea of birds from our two study colonies differed. The colonies are situated in regions of contrasting oceanographic conditions: Selvagem Grande, a remote subtropical oceanic island, and Berlengas, an island on the Portuguese continental shelf. Although the wing loading and wing span of males and females breeding at Selvagem Grande differed significantly, sex did not consistently influence activity patterns of Cory's Shearwaters foraging at sea during the incubation period. Moreover, both sexes breeding at Selvagem Grande foraged in areas with similar sea-surface temperatures. Our study suggests that sexual differences in size and shape may be poor predictors of differentiation in the ways male and female pelagic seabirds use the marine environment.

Home Range Size and Use in Allocebus trichotis in Analamazaotra Special Reserve, Central Eastern Madagascar

No information is currently available on the space needs of hairy-eared dwarf lemurs (Allocebus trichotis), classified as Data Deficient. The data are crucial for their conservation and comparison with other nocturnal primates. I conducted the first radiotracking study of the species from January to December 2007 in the Analamazaotra Special Reserve of Central Eastern Madagascar. I used nocturnal focal individual follows and daytime nest locations to determine home ranges. I followed 1 full sleeping group (4 adults) for 8 mo and 1 partial sleeping group (2 females) for 3 mo. Group home ranges, calculated via 100% minimum convex polygons (MCP), were 35.5 ha and 16.0 ha, respectively. The 95% kernel method of analysis yielded group home ranges of 15.2 ha and 7.1 ha respectively. The mean home range size for individuals was 15.4 ha (MCP) and 5.4 ha (kernel). This is much larger than for other Cheirogaleidae and could be due to a more insectivorous diet or the use of patchily distributed gum-producing trees. There were small nonsignificant monthly variations in home range size. The mean home range size per individual per month was 5.2 ha (MCP) and 2.2 ha (kernel). Important individual differences in overall and monthly home range size could be due to variations in the individual reproductive cycles and survival strategies. Overlap analyses and the lack of sexual difference in home range size suggest the social unit is a family or multimale/multifemale sleeping group with monogamous or promiscuous mating. The Analamazaotra Special Reserve probably holds ca. 100 adult individuals. Additional research is urgently needed to clarify the habitat needs of this rare species.

Body Size and Shape of the Large Carpenter Bee, Xylocopa virginica (L.) (Hymenoptera: Apidae)

In bees, both body size and shape may show considerable variation associated with differences between females and males, among populations, and due to local environmental variation. We studied multiple parameters of body size and shape of the large carpenter bee Xylocopa virginica, to understand how body size and shape are influenced by sex, and by seasonal and annual variation. In addition, we compared bees from a population near the northern edge of the range (southern Ontario, ON) which experience relatively severe winters, to bees from the central portion of the range (Maryland, MD), which experience milder winters. Overall, males and females differed in linear dimensions but were more or less the same mass. Seasonal variability was investigated using ON bees. In winter, females and males had the same overall mass, but male thoracic volume and linear dimensions such as head capsule width, intertegular width, and costal vein length, were all larger for a given mass. In summer, males weighed less than females, due to loss of mass from the abdomen. Year-to-year differences in size and shape were indicated in MD bees, which exhibited significant differences in linear dimensions and dry mass, but not wet mass. In addition, northern bees were smaller than southern bees in terms of linear dimensions, but it is not entirely clear if overwintering mass differs. We suggest that seasonal and sex differences in size are related to the different flight activity patterns of males and females in summer, and that geographic differences between the two populations are related to the length and duration of winters.

Does phenotypic integration constrain sexual size dimorphism in eastern lubber grasshoppers (Romalea microptera)

Conspicuous sex differences in size, shape, color and behavior are pervasive throughout both the animal and plant kingdoms. Although previous work has shown that sexual dimorphism can evolve purely as the result of either natural (intersexual resource competition) or sexual selection (fecundity selection, male-male combat), a handful of studies have also shown that these mechanisms need not be mutually exclusive and may even act synergistically to enhance dimorphisms. More recent studies have focused on the factors that act to constrain the evolution of sexual dimorphism. For example, a high genetic covariance among traits between the sexes is thought to retard adaptive sexual differentiation or place an upper limit on the degree of intersexual divergence. However, high genetic correlations themselves are often the result of selection for functional coherence amongst traits (integration), and thus divergent selection between the sexes could theoretically drive sexual dimorphism in the genetic variance-covariance matrix (G). To address this issue, we consider the phenotypic variance-covariance matrix (P), within the context of the evolution of sexual dimorphism in the eastern lubber grasshopper (Romalea microptera). We show that: 1) adult female lubbers have relatively wider heads than adult males; 2) most morphological traits show significant integration with one another, but males show a higher degree of integration than females; 3) P in lubbers is not strongly correlated across the sexes. These results suggest that phenotypic integration does not constrain the evolution of sexual dimorphism in R. microptera. Hence, sex-related traits can evolve separately from other traits.

Sexual size dimorphism in Orthoptera (sens. str.) — a review

Sexual size dimorphism (SSD) is a common phenomenon in animal taxa. While males are the larger sex in many birds and mammals, female-biased SSD predominates among insects, including Orthoptera. We analyzed size differences of 1503 Orthoptera species, suggesting that SSD is rather uniform in Ensifera, with the females being on average 9% larger than the males (ranging from −20 to 40%). In contrast, SSD is usually much stronger in Caelifera (37%) and also more variable (ranging from −20 to 140%). Caelifera with larger females exhibit stronger size differences than smaller species, whereas in Ensifera SSD decreases with male body size, but is not related to female size. Sexual size differences in Orthoptera are usually associated with a higher number of nymphal instars in females, leading to an earlier emergence of adult males (protandry). Both growth rates and the number of instars seem to be affected by genetic and environmental cues. Two major hypotheses have been proposed to explain the ultimate causes for SSD: the intersexual competition hypothesis and the differential equilibrium hypothesis. The first suggests that sexual dimorphism is a mechanism to reduce intraspecific competition, enabling the sexes to specialize on different food items. The differential equilibrium hypothesis proposes that the different body sizes represent sex-specific fitness optima, which are caused by their specific life-history strategies. Females may maximize their reproductive success by increasing the number (or size) of eggs (fecundity selection), whereas males may maximize their reproduction by being more mobile and fertilizing many females in a short period of time. These fundamental differences in the life-history strategies of the sexes may also lead to sexual selection, which has sometimes been referred to as an additional hypothesis. There is still a need for more empirical research on the ultimate causes for SSD. At present, there is much more support for the differential equilibrium hypothesis, but the intersexual competition hypothesis has rarely been tested. We propose some experimental approaches to test both hypotheses in micro- and macroevolutionary contexts.

Sexual dimorphism in bill morphology and feeding ecology in Cory's shearwater ( Calonectris diomedea)

The bill is a sexually dimorphic structure in many bird species and implicated in numerous functions. Sexual differences may arise from sexual selection or ecological divergence. Here, we examined differences in bill size and shape between males and females and explored to what extent these relate to feeding ecology of each sex in Cory's shearwater ( Calonectris diomedea). We applied linear measurements and geometric morphometric methods to examine sexual differences in bill size and shape. We investigated feeding ecology by tracking foraging movements during the breeding period and by analysing stable isotope signatures in blood during the breeding period and in feathers grown during the non-breeding period. Bill traits were all sexually dimorphic, both in absolute and relative terms, and scaled hypermetrically with body mass in several characters in males. However, males and females did not differ in their feeding areas or isotopic signatures and no significant correlation was observed between these traits and bill dimorphism. Therefore, we discard the foraging-niche divergence hypothesis, and suggest that sexual dimorphism in bill size in this species is more likely driven by sexual selection related to antagonistic interactions.

Size matters: cerebral volume influences sex differences in neuroanatomy.

Biological and behavioral differences between the sexes range from obvious to subtle or nonexistent. Neuroanatomical differences are particularly controversial, perhaps due to the implication that they might account for behavioral differences. In this sample of 200 men and women, large effect sizes (Cohen's d > 0.8) were found for sex differences in total cerebral gray and white matter, cerebellum, and gray matter proportion (women had a higher proportion of gray matter). The only one of these sex differences that survived adjustment for the effect of cerebral volume was gray matter proportion. Individual differences in cerebral volume accounted for 21% of the difference in gray matter proportion, while sex accounted for an additional 4%. The relative size of the corpus callosum was 5% larger in women, but this difference was completely explained by a negative relationship between relative callosal size and cerebral volume. In agreement with Jancke et al., individuals with higher cerebral volume tended to have smaller corpora callosa. There were few sex differences in the size of structures in Broca's and Wernicke's area. We conclude that individual differences in brain volume, in both men and women, account for apparent sex differences in relative size.

Características morfométricas, genéticas, alimenticias y vectoriales de Panstrongylus herreri procedentes de Jaén (Cajamarca) y Cajaruro (Amazonas), Perú

Objective. To compare the morphology, feeding and the genetic profile of specimens of Panstrongylus herreri from districts of Jaen (Cajamarca) and Cajaruro (Amazonas). Material and methods. Adult specimens of P. herreri were collected, 42 from Jaen and 68 from Cajaruro. Geometric morphometrics was used to evaluate the sexual differences in size and form. The genetic profile was done using the PCR amplification, second internal transcribed spacer (ITS-2) of the ribosomal ADN. The molecular characterization of the isolated trypanosomes was made by the amplification the parasite. We used the precipitin test for identification the feeding sources and thus be able to associate them with the potential reservoir of the parasite. Results. The morphometric analyses showed no difference (p<0,05) in sex between the insects from both departments, but there was difference in the form between females. The genetic profile showed by ITS-2 similar bands of 960 pb in both groups (five insects) from Jaen and Cajaruro. The trypanosoma isolated from P. herreri (only in Cajaruro) was identify as Trypanosoma cruzi TCI (350 pb). Guinea pig blood was the main source of food for the insects. Conclusions. P. herreri from both localities no showed important differences under geometric morphometry and genetic studies. These results indicate the same population with similar origin. The guinea pig blood was the main source of food of P. herrei, may be the principal reservoir for the Chagas disease from this Peruvian area.

Sexual differences in size and shape of the Mosor rock lizard [Dinarolacerta mosorensis (Kolombatovic, 1886)] (squamata: lacertidae): A case study of the Lovcen mountain population (Montenegro)

Sexual differences in size and shape of the Mosor rock lizard, Dinarolacerta mosorensis (Kolombatovic, 1886), from Lovcen Mountain (Montenegro) were examined on the basis of the intersex variation pattern of nine morphometric, eight pholidotic, and four qualitative traits. Sexual dimorphism was apparent for all morphometric characters except snout-vent length, while scalation and dorsal pattern exhibited small differences between sexes. The value of the sexual size difference (SSD) index based on snout-vent length was 1.028. The sex-specific allometric slopes for head dimensions and interlimb distance significantly diverged. Head dimensions, especially head height, showed strong positive allometry in males, while interlimb distance was the only character which showed positive allometry in females. Generally, males had significantly greater body size than females. This was true of all body measurements except interlimb distance. The influence of sexual and natural selection on the examined traits is discussed.

Sexual dimorphism in America: geometric morphometric analysis of the craniofacial region.

One of the four pillars of the anthropological protocol is the estimation of sex. The protocol generally consists of linear metric analysis or visually assessing individual skeletal traits on the skull and pelvis based on an ordinal scale of 1-5, ranging from very masculine to very feminine. The morphologic traits are then some how averaged by the investigator to estimate sex. Some skulls may be misclassified because of apparent morphologic features that appear more or less robust due to size differences among individuals. The question of misclassification may be further exemplified in light of comparisons across populations that may differ not only in cranial robusticity but also in stature and general physique. The purpose of this study is to further examine the effect of size and sex on craniofacial shape among American populations to better understand the allometric foundation of skeletal traits currently used for sex estimation. Three-dimensional coordinates of 16 standard craniofacial landmarks were collected using a Microscribe-3DX digitizer. Data were collected for 118 American White and Black males and females from the W.M. Bass Donated Collection and the Forensic Data Bank. The MANCOVA procedure tested shape differences as a function of sex and size. Sex had a significant influence on shape for both American Whites (F = 2.90; d.f. = 19, 39; p > F = 0.0024) and Blacks (F = 2.81; d.f. = 19, 37; p > F = 0.0035), whereas size did not have a significant influence on shape in either Whites (F = 1.69; d.f. = 19, 39; p > F = 0.08) or Blacks (F = 1.09; d.f. = 19, 37; p > F = 0.40). Therefore, for each sex, individuals of various sizes were statistically the same shape. In other words, while significant differences were present between the size of males and females (males on average were larger), there was no size effect beyond that accounted for by sex differences in size. Moreover, the consistency between American groups is interesting as it suggests that population differences in sexual dimorphism may result more from human variation in size than allometric variation in craniofacial morphology.

REVIEW. Sex with the lights on? A review of bioluminescent sexual dimorphism in the sea

The reflected patterns and colours of terrestrial animals often show a sexual dimorphism associated with visual display, mate recognition and selection. In the sea the structures associated with bioluminescence may also show a marked sexual dimorphism. Some apparent bioluminescent dimorphisms (e.g. differences in photophore numbers) are probably secondary functions of sexual differences in size. A role in sexual communication is much more likely where specific photophores are developed or enlarged in mature specimens of one sex only but the presence of light organs in female anglerfishes (but not in males) is complicated by a significant size dimorphism. Dimorphisms in dragonfishes and lanternfishes primarily involve the relative enlargement of particular photophores in the males. A sexual role is assumed, but the morphological differences are often small. Most male ponyfishes have enlarged light organs; behavioural observations of free-swimming animals have clearly demonstrated that the males use them to generate bioluminescent sexual signals.

The development of sexual differences in body size in Odonata in relation to mating systems

Adult body size is the result of important environmental, maternal and/or genetic effects acting on animals during devel- opment. Here we investigate how sexual size dimorphism (SSD) develops in seven species of Odonata: Anax imperator, Cordule- gaster boltonii, Onychogomphus uncatus, Oxygastra curtisii (Anisoptera), Cercion lindeni, Ischnura graellsii and Platycnemis acutipennis (Zygoptera). SSD of both the last larval and adult stages of the same individuals, which were reared under laboratory conditions, was measured. The aims were to investigate (i) whether SSD develops during the larval stage, (ii) the direction of larval and adult SSD, and (iii) whether the direction of adult SSD can be predicted by the mating system of a given species (e.g. males of territorial species being larger than females and the opposite for non-territorial species). We found that although larval differences in size may be present between the sexes, these are not necessarily shown in the adult stage (they may change or disappear). Also, the mating system was not related to patterns of adult SSD. Differences in SSD in larvae may be caused by differential use of resources via differential niche-utilisation or sex-specific growth patterns. We highlight the fact that sexual selection favouring large male size and fecundity selection, which selects for large females may be acting on the observed patterns in SSD in adults.

Sexual size dimorphism in the American rubyspot: male body size predicts male competition and mating success

Sexual differences in body size are widespread among animals, and various explanations for the evolution and maintenance of sexual size dimorphism have been proposed. We investigated the effects of sexual selection and fecundity selection on the sizes of males and females, respectively, in American rubyspots, Hetaerina americana. Males are larger than females and have large red spots at the base of each wing that are sexually selected via male-male contests. Mating success is determined by the ownership of a territory. Large males held territories for longer and sustained longer territorial fights than small males. Territorial males obtained more copulations than nonterritorial ones. Large males also had more wing pigmentation and mated with large females. Large territorial males had high energy reserves, whereas nonterritorial males appeared to have depleted reserves. Selection analyses of body size showed disruptive selection acting on male body size, suggesting that both small and large males may be favoured in terms of mating success. We also tested whether fecundity selection acts on female size. However, female body size was unrelated to the number of eggs carried. Taken together, our results suggest that in this territorial damselfly species male-biased size dimorphism is driven by large male size in male-male competition being selectively advantageous in territory acquisition and/or maintenance. We also suggest that small size is advantageous in nonterritorial males to improve their agility in courting (or subduing) females.

Allometry, bilateral asymmetry and sexual differences in the vocal tract of common eidersSomateria mollissimaand king eidersS. spectabilis

Intraspecific sexual differences, high variation, and positive allometry of sexually‐selected external display structures are common. Many sexually‐selected anatomical specializations occur in the avian vocal tract but intraspecific variation and allometry have been investigated little. The tracheal bulla bulla syringealis occurs in males of most duck species. We quantified variation and size‐scaling of the bulla, plus sexual differences in size of trachea, bronchi, and vocal muscles, for 62 common eiders Somateria mollissima and 51 king eiders S. spectabilis. Trends were similar in both species. Bullar ossification and definitive size occurred early in life: bullar size did not differ between first‐year and older males. Bullar size did not vary more than size of other body parts (CVs of 3.4–7.0% for bullar length and breadth). Bullar size scaled to body size with negative allometry or isometry. Vocal muscles were 10–50% thicker in males than females, a much greater sexual difference than in body size (CVs of 3–6% on linear body‐size variables). Vocal muscles were larger on the left side in both sexes and bilateral asymmetry was slightly more pronounced in males. Low variation and a trend towards negative allometry suggest that bullar size is under stabilizing selection; if bullar size affects vocal attributes of voice, then the latter cannot be condition‐dependent. We recommend comparative research on vocal communication, vocal individuality and vocal‐tract anatomy and function in eiders and other ducks.

Sex and age influence intestinal parasite burden in three boreal grouse species

Parasite infections are often more common in male vertebrates than in females. Sexual selection leading to dimorphism can cause sexual differences in immune defence, behaviour or body size. Possible proximate explanations for male bias in parasitism are the immunosuppressive effect of male sex hormones and the large body size of males which increases the likelihood of being parasitized. To evaluate these hypotheses, we studied the prevalence and abundance of intestinal helminth parasites in three boreal grouse species, the capercaillie Tetrao urogallus, the black grouse Tetrao tetrix, and the hazel grouse Bonasa bonasia. The first two are sexually dimorphic polygynous species while the latter species is sexually monomorphic and monogamous. We found a male‐bias in the prevalence and abundance of the nematode Ascaridia compar. The bias was most pronounced in the polygynous black grouse and capercaillie. In the monogamous hazel grouse, there was a slight male‐bias in occurrence of ascarids, but no bias in abundance. In juvenile grouse, the male‐bias was larger than in adult grouse. No sexual bias was found in regard to the cestodes (Skrjabinia cesticillus, Paroniella urogalli and Hymenolepis spp.). However, age was a factor in cestode prevalence: juvenile grouse were more commonly infected than adults. Differences in growth rates and body size are potential factors that may lead to male‐biased parasitism in these grouse species, and their impact requires further studies.