Male-biased Sexual Dimorphism Research Articles

Morphological diversification with emphasis on the structural and homology patterns of male genitalia in genus Limnebius (Leach 1815; Hydraenidae: Coleoptera)

Abstract Limnebius exhibits uniform external appearance but variable male genitalia: from differently curved rod shapes in the subgenus Bilimneus to up to seven longitudinally separated folds or appendages in Limnebius s.s.. Among nonsexual traits, two subgenera diverge only in the wing. The aedeagal complexity in Limnebius s.s. is associated with the secondary sexual structures on the abdomen and metatibia; larger body size, which forms two clusters; and male-biased sexual dimorphism. To cluster the subdivisions with phylogenetically unresolved relationships, we examined the coincidence of the traits in both sexes, based on the premise of the independent and monophyletic origin of histologically unsimilar elements. The folds and additional appendages on the median lobe, abdominal paired rows of setae, and protuberances show that the L. parvulus and L. nitidus groups have maximum aedeagal complexity in the L. parvulus and L. nitiduloides subgroups, respectively. The L. parvulus group exhibits a gradual increase in the number of elements of the aedeagi. The histology of connections of the parameres and additional appendages to the median lobe, along with that of the flagellum opening, reveal the subgroups’ relationship in the L. nitidus group, as well as a decrease in the complexity of the male genitalia in their recent branches.

Patterns of sexual dimorphism in the armoured tardigrades.

Sexual dimorphism is widespread among animals, with diverse patterns and proposed explanations observed across the Tree of Life. Here we present the first formal analysis of the patterns of sexual dimorphism in body size and cephalic sensory appendages across 40 species (from 10 genera) of armoured tardigrades (Echiniscidae). Phylogenetic signal was found for body size traits and the cephalic papilla relative size, indicating that the association between these traits between the sexes has high evolutionary persistence. The Echiniscidae body size dimorphism is generally female-biased, which would be in accordance with the fecundity hypothesis. No strong evidence of allometric patterns of body size sexual dimorphism was found. In contrast, some of the cephalic appendages show male-biased sexual dimorphism, particularly those that, by being more innervated, are thought to function as chemodetection organs used by males during mate search. The latter is consistent with the sexual selection hypothesis. As the first systematic quantification and analysis of the patterns of sexual dimorphism in the phylum Tardigrada, this study provides important insights into their ecology and evolution, such as corroborating the suggestion that cephalic appendages evolved for mate searching.

Intersexual relationships in mandrills: dominance, sexual conflict and the influence of social integration

The conflict of evolutionary interests between the sexes is commonly imprinted in social dynamics and physical asymmetries between the sexes. In species that live in polygynandrous groups, male-biased sexual size dimorphism often allows males to socially dominate and sexually coerce females. Yet, smaller female size might allow females to escape from males and female philopatry might promote female-female social support allowing females to resist or retaliate against males. The study of such intersexual conflicts is highly relevant in mandrills (Mandrillus sphinx) as they form large polygynandrous groups and exhibit extreme male-biased sexual dimorphism in size and ornaments, higher social integration of females than males, and female coalitions against males. The set up of Mandrillus Project over a decade ago has offered a unique opportunity to study intersexual relationships of mandrills in their natural habitat. Here, I present a synthesis of some recent studies that used the long-term behavioural and life-history dataset of Mandrillus Project and documented patterns of sexual coercion and intersexual dominance in mandrills. These studies show that male mandrills are generally dominant over females and use sexual coercion to increase their mating success. However, female social bonds and social integration permit females to occasionally outrank certain males. Finally, and surprisingly, higher-ranking and more socially integrated females receive higher rates of male sexual coercion, highlighting the need for future research to shed light on the trade-offs between costs and benefits of mandrill sociality.

Strongly sexually dimorphic forelegs are not more condition-dependent than less dimorphic traits in Drosophila prolongata

Directional sexual selection drives the evolution of traits that are most closely linked to reproductive success, giving rise to trait exaggeration and sexual dimorphism. Exaggerated structures are often costly and, therefore, thought to be expressed in a condition-dependent manner. Sexual selection theory thus predicts a direct link between directional sexual selection, sexual dimorphism, and sex-specific condition dependence. However, only a handful of studies investigate the relationship between sexual dimorphism and condition dependence. Using 21 genetic lines of Drosophila prolongata, we here compared the degree of sexual dimorphism and sex-specific condition dependence, measured as allometric slopes, in sexually selected and non-sexual traits. Our data revealed male-biased sexual dimorphism in all traits examined, most prominently in the sexually selected forelegs. However, there was no relationship between the degree of sex-specific condition dependence and sexual dimorphism across traits and genetic lines. Our results contradict theoretical predictions and highlight the importance of understanding the role of exaggerated traits in the context of both sexual and natural selection.

Age Determination of <i>Ophisops elegans</i> (Ménétriés, 1832) (Squamata: Lacertidae) Populations from Different Altitudes in Turkey

In this study, comparison of body size and age of two populations of Ophisops elegans from different altitudes was carried out (Kepez, 180 m; Paşayaylası, 1250 m). We applied phalangeal skeletochronology to obtain the age of juveniles and adults by counting the lines of arrested growth (LAGs) in phalangeal cross-sections. Male-biased sexual dimorphism was observed in Kepez, while female-biased sexual dimorphism was determined in the Paşayaylası population. Both male and female individuals in Kepez and Paşayaylası populations reached sexual maturity at the age of 3. The maximum age or longevity was calculated as 7 years in males and 9 years in females. Considering all individuals, the Kepez population was found to be older than Paşayaylası population. For both populations, a significant positive correlation was found between age and size in males and females.

Male intrasexual aggression and partial dominance of females over males in vervet monkeys

Females dominate a subset of the males in a minority of mammalian species despite male-biased sexual dimorphism. How this may arise is suggested by a computational model, DomWorld. The model represents male-biased sexual dimorphism through the males’ greater initial dominance and higher intensity of aggression, meaning that fights initiated by males have a greater impact than those by females. The model shows that female dominance over males increases with a greater proportion of males in the group. This happens because when males are involved in a larger fraction of fights this results in greater hierarchical differentiation (i.e., steepness). This causes rank overlap between the sexes (i.e., partial female dominance). We test the validity of these processes in vervet monkeys (Cercopithecus pygerythrus), a primate species with partial female dominance. We confirm that the proportion of males in the group is significantly positively correlated with the degree of dominance by females over males and with the steepness of the hierarchy among males exclusively, but not with the steepness of the hierarchy among all adults of the group. The steepness in male hierarchies correlated positively with female dominance over males in these groups. We show that steeper hierarchies among vervet males resulted from male-to-male fights being a larger proportion of the fights among all adults of the group. We conclude that the higher frequency of male intrasexual aggression favors female dominance in vervet monkeys. We also show that females received coalitionary support when they were in conflict with a male, mainly from other females, and that this favors female dominance in this species, but this does not explain why partial female dominance increased with the proportion of males in the group. We advocate further investigation of the influence of male intrasexual aggression on the degree of female dominance over males in other species with partial female dominance.

Beyond facial width-to-height ratios: bizygomatic width is highly sexually dimorphic when adjusting for allometry

A large and ever-growing literature implicates male facial width-to-height ratio (bizygomatic width divided by facial height) as a secondary sexual trait linked to numerous physical and psychological perceptions. However, this research is based entirely on the premise that bizygomatic width is sexually dimorphic, which recent research has called into question. Unfortunately, statisticians for the last 125 years have noted that morphological ratio measurements may engender spurious correlations and biased effect-size estimates. In the current study, we find that bizygomatic width is highly sexually dimorphic (equivalent d = 1.39), even after adjusting for 92 allometric measurements, including multiple facial height and other craniofacial measurements (equivalent d = 1.07) in a sample of 6068 men and women. By contrast, facial width-to-height ratio (fWHR) measurements demonstrated a statistical pattern consistent with the age-old argument that morphological ratio measurements may engender spurious correlations and biased effect-size estimates. Thus, when avoiding facial ratio measurements and adjusting for allometry in craniofacial measures, we found strong support for a key premise in the human evolutionary and behavioural sciences that bizygomatic width exhibits male-biased sexual dimorphism.

Extensive variation in feather ornaments of Whiskered Auklets in the Aleutian Islands reflects age, sex, condition, and geography

Abstract Both sexes of Whiskered Auklets (Aethia pygmaea) display the most elaborate feather ornaments of any seabird: a slender black forehead crest, and 3 bilaterally symmetrical pairs of white facial plumes (superorbital, suborbital, and auricular). We studied patterns of ornament variation in 796 banded individuals (147 of known sex, 254 of known age from 1 to 16 years) during 1992–2009 at Buldir Island (principally), and 3 other Aleutian Islands (Davidof, Ulak, and Egg) in Alaska, USA. As expected for socially selected traits, ornaments were more variable across individuals than anatomical traits in size but with only slightly male-biased sexual dimorphism. Body condition index increased from age 1 to 3 years but changed little thereafter. Even within birds ≥4 years old, ornament size was positively related to body condition index. Subadults (one-year-olds) had smaller ornaments than adults (age 2–16 years) but there was no further change in ornament size as adults aged and no evidence of senescence even in the oldest birds (>8 years old). Nonetheless, overall ornament size varied from year-to-year at Buldir and was correlated with indices of both ocean climate and auklet productivity in the preceding 2–5 years. From Buldir to Egg Island (1,266 km), the size of both anatomical and ornamental traits increased by 5–15% except for bill depth, which was largest in birds from Buldir and Egg at opposite ends of the Aleutian breeding range. This study is one of few to examine patterns of ornament variation in a long-lived, socially monogamous bird, even though such patterns are crucial to understanding the relationship between sexual selection and life history.

Sexual dimorphism in skeletal shape in voles (Arvicolinae): disparate selection on male bodies and female heads

AbstractSexual dimorphism evolves as a response to different selective pressures on males and females. In mammals, sexual selection on traits that improve a male’s ability to compete for access to mates is a common cause of sexual dimorphism. In addition to body mass, adaptations in specific components of the musculoskeletal system that increase strength, stability, and agility, may improve male fighting performance. Here we test the hypotheses that males, when compared to females, are more specialized for physical competition in their skeletal anatomy and that the degree of this sexual dimorphism increases with the intensity of male–male competition. In three species of voles (Cricetidae: Arvicolinae: Microtus), we found partial support for these hypotheses. Male-biased sexual dimorphism in a set of functional indices associated with improved fighting performance was identified in the postcranial anatomy. This dimorphism was greatest in the polygynous Microtus californicus, absent in the monogamous M. ochrogaster, and intermediate in the promiscuous or socially flexible M. oeconomus. However, in the skull, we found results opposite to our predictions. Females had larger skulls relative to overall skeletal size than did males. This may be associated with selection for increased food processing efficiency, which should be highly important because of the compounding effects of increased caloric requirements during gestation and lactation, and the generally low-quality diet of voles. In addition, larger heads in females may be associated with selection for greater digging ability or for defending offspring. These results suggest disparate selective pressures on the postcranial skeletons and skulls of male and female voles.

Mating system of Thrichomys fosteri in the Brazilian Pantanal: spatial patterns indicate promiscuity

Animal mating systems provide key insights into the relationship between evolutionary processes and ecological factors such as the spatio-temporal fluctuation of resource abundance. Characteristics of mating systems can be inferred from the spatial distribution of conspecifics and the arrangement of reproductive pairs. Here we used home-range size and overlap for Thrichomys fosteri in the Brazilian Pantanal to infer the mating system on this echimyid rodent. Our aims were to verify the existence of sexual dimorphism, to test whether home-range size varied with the sex and body weight of the individuals, to evaluate the degree of home-range overlap, to estimate mean population density, and to infer individuals’ mating system. Twenty one individuals (15 males and six females) were radio tracked from 14 to 349 days, with the number of locations by individuals ranging from 19 to 193 locations. There was a male-biased sexual dimorphism in body weight where males were 1.36 times heavier than females. Males’ home-range size increased with their body weight, while for females there was no relationship. There was extensive home-range overlap between both sexes, and no evidence of territoriality. Mean population density ranged from 0.9 to 3.03 individuals/ha. Our results indicate that multiple mates were available for both sexes, characterizing a promiscuous mating system.

Insect harem polygyny: when is a harem not a harem?

In the context of animal behaviour, a harem is generally recognised as a mating system where a single dominant male defends and mates with a group of females. Examples of harem polygyny are best known from mammals. A small number of insects, though, have been described as being harem polygynous but information on insects which display this mating system is scarce, and it remains poorly studied. Here we review the mating systems of these “harem polygynous” insects. We identify four main behavioural characteristics that, apart from individual males mating with multiple females in a group, are often associated with harem polygyny in vertebrates: exclusive maternal care, monandry, male-biased sexual dimorphism and temporal continuity of harem composition where the dominant male guards females from intruding males over a prolonged period. All four characteristics are commonly seen in mammals with this mating system, but no insect described as harem polygynous consistently displays all of them. Further, the mating systems of these insects can change within a breeding season, which suggests that they adapt to the changes in their environment by switching between polygyny and monogamy or female defence and resource defence polygyny. Thus, the occasional occurrence of harems in such insect species represents temporary “marriages of convenience”—with individuals (male and female) maximising their reproductive potential by adjusting their behaviour to fit the current situation.

Sexual dimorphism in human arm power and force: implications for sexual selection on fighting ability.

Sexual dimorphism often arises from selection on specific musculoskeletal traits that improve male fighting performance. In humans, one common form of fighting includes using the fists as weapons. Here, we tested the hypothesis that selection on male fighting performance has led to the evolution of sexual dimorphism in the musculoskeletal system that powers striking with a fist. We compared male and female arm cranking power output, using it as a proxy for the power production component of striking with a fist. Using backward arm cranking as an unselected control, our results indicate the presence of pronounced male-biased sexual dimorphism in muscle performance for protracting the arm to propel the fist forward. We also compared overhead pulling force between males and females, to test the alternative hypothesis that sexual dimorphism in the upper body of humans is a result of selection on male overhead throwing ability. We found weaker support for this hypothesis, with less pronounced sexual dimorphism in overhead arm pulling force. The results of this study add to a set of recently identified characters indicating that sexual selection on male aggressive performance has played a role in the evolution of the human musculoskeletal system and the evolution of sexual dimorphism in hominins.

Age structure and body size of a critically endangered species, Acanthodactylus harranensis (Squamata: Lacertidae) and its demography

Abstract Within reptiles, lizards cover less area across the globe than snakes do. One out of every seven known species of lizards in the world is found only in its type locality. Acanthodactylus harranensis Baran, Kumlutaş, Lanza, Sindaco, Ilgaz, Avcı & Crucitti, 2005, is one of these species. It is an endemic lizard species in Turkey with a very small area of occupancy and is listed in the critically endangered category (CR) by the IUCN. Here we document the age structure and body size of A. harranensis using skeletochronological methods for the first time and aim to point out current problems and contribute to an understanding of its demography. The mean age of males was found to be significantly higher than that of females. The maximum life span was 10 years in males while it was 9 years in females. The 8-year-old age group contained a higher number of individuals than any other age group. The mean snout-vent length of specimens was not significantly different between the sexes. As in many other lizards, A. harranensis exhibits a low-level male-biased sexual dimorphism. Acanthodactylus harranensis also displayed a considerably bigger body size than other studied lacertids from Şanlıurfa province. The data presented in this study may contribute to future conservation efforts for this endangered species.

Further Evidence that Facial Width-to-Height Ratio and Global Facial Masculinity Are Not Positively Associated with Testosterone Levels

Facial masculinity, as for example measured by the facial width-to-height ratio (fWHR) or the global facial masculinity index, has been associated with a vast range of behavioural traits, including dominance and aggression. Further, facial masculinity is thought to be influenced by testosterone (T) levels as an underlying mechanism. However, a recent meta-analysis on fWHR and T levels provided non-significant associations in men, which we wanted to examine further in men and additionally in women. We examined whether fWHR and global facial masculinity are positively associated with salivary baseline T and T reactivity in 140 men (age 18–34 years), as well as with salivary baseline T and hair T concentrations in 151 women (age 18–35 years). No associations of salivary baseline T, T reactivity or hair T levels with fWHR or global facial masculinity were observed. Additional analyses revealed sex differences in sexual dimorphism in fWHR and global facial masculinity: men had generally higher global facial masculinity compared to women, but unexpectedly a lower fWHR. Overall, our results provide further evidence that neither fWHR nor global facial masculinity are related to T levels and question earlier findings on male-biased sexual dimorphism in fWHR.

Comparison of the body size and age structure of Lebanon lizard, Phoenicolacerta laevis (Gray, 1838) at different altitudes in Turkey

In this study, comparison of a life history traits (e.g. body size, age at maturity, longevity) of two populations of Phoenicolacerta laevis from different altitudes is being carried out (Anamur, 22 m a.s.l.; Andırın, 1.083 m a.s.l.) for the first time. We applied phalangeal skeletochronology to obtain the age of juveniles and adults. Age was determined by counting the lines of arrested growth (LAGs) in cross-sections. Males in both populations were the larger of the two sexes and a male biased sexual dimorphism was determined for both populations. Body size (SVL) was similar in both sexes and populations. Age of maturity was calculated to be 3 years of age for males and females in both populations. The average age of males and females was calculated as 6.62 ± 0.37 (Mean±SE) and 6.11 ± 0.26 years in Anamur, and 6.15 ± 0.51 and 5.26 ± 0.24 years in Andırın. There was statistically significant variation between sexes, but no significant difference in populations was found in relation to age. For both populations, a significant positive correlation was found between age and SVL in males and females.

Global patterns of sexual dimorphism in Amblypygi

Sexual dimorphism is a common feature of life. Researchers have noted that some species of Amblypygi (Class Arachnida) exhibit male-biased sexual dimorphism, but a broad overview of the prevalence of sexual dimorphism in the order does not exist. In order to provide such an overview, we imaged and measured morphological characteristics of 390 amblypygids from 36 species. Our aim was to gain insight into general patterns of sexual dimorphism across the order. We found that, generally, males have larger pedipalps, longer antenniform legs, but smaller carapace widths relative to conspecific females for nearly all species studied. Species ranged from no sexual dimorphism (Phrynus exsul and Charinus insularis) to male pedipalps almost double the length of females in some species (Charon grayi, Euphrynichus amanica). Phylogenetic patterns remain unclear in Amblypygi and our data better match geographic patterns than evolutionary history. We thus suggest that sexual dimorphism is ancestral for the order, but has been reduced or lost in some species.

Demographic life history traits in a population of a critically endangered species, Darevskia dryada (Darevsky & Tuniyev, 1997)

Abstract Knowing the age structure of endangered species is important in order to contribute to future conservation studies for such species. In this context, we investigated age structure, age at sexual maturity, potential reproductive lifespan and longevity in a population of the Charnali lizard, Darevskia dryada, an endangered species from Turkey. The results show that the Charnali lizard has a longer life span than other lizards of the genus Darevskia that live in the same region. We estimated that these lizards attain their sexual maturity at the age of one or two years and the potential reproductive life span for males and females is six and five years, respectively. As in many other lizards, the Charnali lizard exhibited a low-level male-biased sexual dimorphism in terms of increased size. We believe that this study, in which prior information related to the Charnali lizard is shared, will contribute to future conservation activities for this critically endangered species.

Sexual behavior-associated c-Fos induction in the sagittalis nucleus of the hypothalamus in male rat.

The sagittalis nucleus of the hypothalamus (SGN) is a small nucleus located in the interstitial area between the arcuate and ventromedial nuclei of the hypothalamus in rats. The SGN exhibits male-biased sexual dimorphism and expresses estrogen receptor α and calbindin-D28K. This suggests a contribution of the SGN to sexually differentiated brain function, but its functional role is unknown. In this study, neuronal activation in the SGN during sexual behavior in male rats was examined by c-Fos immunohistochemistry. The number of c-Fos-immunoreactive (c-Fos-ir) cells was elevated with only exposure to chemosensory cues of estrous females and significantly increased after the first mount. The first intromission and ejaculation did not induce further increases in the number of c-Fos-ir cells in the SGN. These findings suggest that the SGN is involved in regulation of the early phase of male sexual behavior, including motivation.

Age Structure and Body Size of Mertensiella caucasica (Waga, 1876) (Caudata: Salamandridae) in a Population from Turkey

Here we present the age structure of a breeding Caucasian Salamander population from a high-altitude locality (Kazikbeli Plateau, Kurtun, Gumushane; 2213 m a.s.l.). Age structure was analyzed by counting lines of arrested growth (LAG). Endosteal resorption raised difficulties of counting LAGs. Males were found to be average older than females. Range of age was from 5 to 11 years in males, 4 to 8 years in females and 1 to 3 years in juveniles. Snout-vent length (SVL) of each individual was used as body size and mean values were recorded as 64.24 mm in males, 58.45 mm in females, and 44.60 mm in juveniles. Difference of SVL between sexes was found statistically significant. Meanwhile, male-biased sexual dimorphism was calculated (SDI = –0.099). The SVL was correlated with age in both sexes as strongly positive.

Age Structure and Body Size of <i>Mertensiella caucasica</i> (Waga, 1876) (Caudata: Salamandridae) in a Population from Turkey

Here we present the age structure of a breeding Caucasian Salamander population from a high-altitude locality (Kazıkbeli Plateau, Kürtün, Gümüşhane; 2213 m a.s.l.). Age structure was analyzed by counting lines of arrested growth (LAG). Endosteal resorption raised difficulties of counting LAGs. Males were found to be average older than females. Range of age was from 5 to 11 years in males, 4 to 8 years in females and 1 to 3 years in juveniles. Snout-vent length (SVL) of each individual was used as body size and mean values were recorded as 64.24 mm in males, 58.45 mm in females, and 44.60 mm in juveniles. Difference of SVL between sexes was found statistically significant. Meanwhile, male-biased sexual dimorphism was calculated (SDI = –0.099). The SVL was correlated with age in both sexes as strongly positive.