Strong Sexual Size Dimorphism Research Articles

Dynamics of a goshawk population across half a century is driven by the variation of first-year survival.

Population dynamics are driven by stochastic and density-dependent processes acting on demographic rates. Individuals differ demographically, and to capture these differences, models of population dynamics are usually structured by age and stage, rarely by sex. An effect of sex on population dynamics is expected if the dynamics of males and females differ, requiring an unequal sex ratio at birth and/or sex-specific survival probabilities. Goshawks (Accipiter gentilis) show large sexual size dimorphism and differential survival, but it is unknown whether males and females contribute differently to population dynamics. We studied a goshawk population in northern Germany over 47 years using brood monitoring data, collected feathers and nestling ringing data. We jointly analyzed the data using a two-sex integrated population model and performed retrospective and prospective population analyses to understand whether the demographic drivers of population change differ between the sexes. The population showed large fluctuations, during which the number of breeding pairs doubled, but the long-term trend of the population was slightly negative. Female survival exceeded male survival during the first year of life. Females started to reproduce at a younger age than males, productivity increased with female age, the sex ratio of nestlings was male biased and there was moderate male immigration. Despite these differences, temporal variation in sex ratio did not contribute to population dynamics and the contribution of temporal variation in survival was similar for both sexes. Variation in first-year survival was the strongest driver in this population, regulated by a weak density-dependent feedback acting through female first-year survival. Overall, the contributions of the two sexes to population dynamics were similar in this monogamous species with strong sexual size dimorphism.

Morphological Characteristics and DNA Barcoding of the Rare Blanket Octopus Tremoctopus violaceus (Cephalopoda: Tremoctopodidae) in the Adriatic Sea

Tremoctopods are epipelagic argonautoid octopods characterized by their expanded dorsal webs and strong sexual size dimorphism, with dwarfed males. The scarcity of taxonomic features attributed to this genus presents a challenge, and there is growing evidence of species misidentification in Tremoctopus genus on a molecular level. In this study, we investigated four female specimens of blanket octopus Tremoctopus violaceus caught by purse seine fishing in the Central Eastern Adriatic Sea in 2019. Individuals had smooth, firm and muscular bodies, dark bluish purple on the dorsal and iridescent silvery on the ventral side, with dorsal mantle lengths of 113, 82, 80 and 78 mm. The constructed phylogenetic trees based on the 16S ribosomal RNA and cytochrome c oxidase subunit I sequences of investigated Adriatic specimens and publicly available sequences showed strong support for the T. violaceus clade, consisting of individuals collected from the Adriatic Sea, Mediterranean Sea and Atlantic Ocean, with the exclusion of Indo-Pacific clade most probably corresponding to T. gracilis. To fully understand the life-history traits of Tremoctopus species, future research should focus on DNA-based methods for correct species identification combined with morphological characters, geographic distribution and ecological information.

Population size and density in two European pond turtle populations of central Italy

Abstract Two populations of the European pond turtle were studied employing capture, marking and recapture (CMR) techniques to determine population density and abundance in natural and protected areas within the Regional Natural park Migliarino San Rossore Massaciuccoli in north western Tuscany, Italy. The results indicated strong sexual size dimorphism in both populations, with males smaller than females. In one area Emys orbicularis was in general smaller in size with a very low reproductive rate (three out of 25 adult females were gravid), which contrasted with a second population where 20 out of 42 females were gravid. Population estimates indicated 79 turtles were present in the first area (0.34 ha) and 61 turtles in the second area (0.58 ha), giving an overall density estimate of 183.6 turtles/ha and 146.7 turtles/ha respectively. Whilst our results are similar to those found in other areas of Europe, they diverge from other regions, suggesting that sampling and monitoring techniques should be standardized to obtain comparable data that will enable conservation and protection measures to be recommended.

Loss of largest and oldest individuals of the Montpellier snake correlates with recent warming in the southeastern Iberian Peninsula.

The effects of climate change on organisms are now being extensively studied in many different taxa. However, the variation in body size, usually shrinkage in response to increasing temperature, has received little attention regarding to reptiles. During past periods of global warming, many organisms shrank in size, and current evidence and experiments manipulating temperature have shown a biomass decrease in some organisms with increasing temperatures. Here we test whether the body size of the Montpellier snake Malpolon monspessulanus from the southeastern Iberian Peninsula is changing and correlated with the increasing temperature in this region during a 39-year period (1976–2014). We measured the snout–vent length (SVL) of vouchers in scientific collections to check for trends in adult body size at the population level in relation with temperature, while controlling for the age of the individuals (estimated by skeletochronology, n =141). Given the great ontogenetic variation in body size of the study species, we categorized age in 3 classes: “young adults” (under 5 years old), “intermediate adults” (from 5 to 7 years old), and “old adults” (from 8 to 14 years old). By means of linear mixed models, we found a negative relationship between SVL of “old adults” and average annual temperature in the region during the lifetime of each individual. Our results indicate that largest and oldest individuals of the Montpellier Snake, that is, males because of strong sexual size dimorphism in this species, disappeared from the study population, and suggest that it occurred in response to rising environmental temperature.

A test of the size-constraint hypothesis for a limit to sexual dimorphism in plants.

In flowering plants, many dioecious species display a certain degree of sexual dimorphism in non-reproductive traits, but this dimorphism tends to be much less striking than that found in animals. Sexual size dimorphism in plants may be limited because competition for light in crowded environments so strongly penalises small plants. The idea that competition for light constrains the evolution of strong sexual size dimorphism in plants (the size-constraint hypothesis) implies a strong dependency of the expression of sexual size dimorphism on the neighbouring density as a result of the capacity of plants to adjust their reproductive effort and investment in growth in response to their local environment. Here, we tested this hypothesis by experimentally altering the context of competition for light among male-female pairs of the light-demanding dioecious annual plant Mercurialis annua. We found that males were smaller than females across all treatments, but sexual size dimorphism was diminished for pairs grown at higher densities. This result is consistent with the size-constraint hypothesis. We discuss our results in terms of the tension between selection on size acting in opposite directions on males and females, which have different optima under sexual selection, and stabilizing selection for similar sizes in males and females, which have similar optima under viability selection for plasticity in size expression under different density conditions.

Dietary divergence in the most sexually size-dimorphic bird

ABSTRACT In sexually size-dimorphic species, physiological constraints derived from differences in body size may determine different food requirements and thus a trophic niche divergence between males and females. These relationships between sexual size dimorphism (SSD) and dietary overlap are not well understood in birds. We analyzed differences between the sexes in diet composition, dietary diversity, diet selection, and volume and density of droppings, as well as the dietary overlap between sexes, in the Great Bustard (Otis tarda), the species showing the highest SSD among birds. We discuss the differences that we found in relation to various predictions derived from ecological and physiological differences between the sexes, under the hypothesis that these differences are ultimately determined by the strong SSD of this species. As expected, our best linear mixed-effects food selection models included sex as a main factor explaining differences in diet composition and dietary diversity of Great Bustard...

The functional morphology of male courtship displays in the Pectoral Sandpiper (Calidris melanotos)

ABSTRACT The Pectoral Sandpiper (Calidris melanotos) is one of a few highly polygynous shorebirds with strong sexual size dimorphism. The vocal part of the male courtship display has received some attention, but how the sound is generated is largely unknown. To fill this gap, we analyzed video and sound recordings collected on the breeding grounds at Barrow, Alaska, USA. The anatomy of 2 males was investigated by macroscopic and histological dissection. Synchronized wing movements and a closed beak accompany hooting calls during flight displays. Courtship vocalizations on the ground include stereotypic beak and hyoid movements. We found a symmetric bipartite syrinx with songbird-like adduction and abduction mechanisms. Lateral and medial labia consisted of homogeneous extracellular matrix containing collagen fibers, which were only loosely organized, few elastin fibers, and a high proportion of hyaluronan. The upper vocal tract includes the trachea and an inflatable esophagus supported by thick and heavy ...

Theridion zonulatum Thorell 1890, a senior synonym of Theridion zebrinusum Zhu 1998

Abstract — Examination of the type material and description of Theridion zonulatum Thorell 1890 shows that this name is a senior synonym of Theridion zebrinusum Zhu 1998.Key words — Araneae, spiders, Theridion , nomenclature Introduction During a visit to Khao Sok National Park, Thailand (8° 55fa, 98°35,E), on 12 November 2013 one of us [RE] ob­served a couple of strikingly colored theridiid spiders, with a strong sexual size dimorphism. The specimens, which were found in their web on a leaf in low shrubbery along a rainforest trail, were photographed (Fig. 1), but not col­lected. Nonetheless, the unique color pattern on the female abdomen, consisting of a series of 7 narrow black stripes on a white background, allowed initial identification as Theridion zebrinusum. This was confirmed by examining the epigyne of a second female collected on 22 March the following year at the same location (Fig. 2). T. zebrinusum was first described by Zhu (1998:165) based on a single fe­male collected on 11 November 1979 in Menglun, Mangla County, Yunnan, Southern China, close to the border to Myanmar and Laos (21°55fa, 101°15,E [21°25fa, 100°36' E, according to the original publication]), but is widely dis­tributed in Southeast Asia, with records from Brunei, Singapore, peninsular Malaysia and Thailand (Koh & Ming 2014).Another specimen was photographed by one of us [GA] in riverine forest in the Mulu National Park, below 250 m a.s.l., Sarawak, Borneo, East Malaysia (4°8fa, 114°55,E; Fig. 3) on 14 February 2008.The fact that the species is widespread, apparently quite common and visually striking caused the suspicion that it might have been reported earlier from the region. A search of the classical literature describing spiders from Southeast Asia, in particular the works of Tord Tamerlan Teodor Thorell, quickly revealed a strong candidate:

Mating patterns of Gammarus lacustris (Amphipoda: Gammaridae) from three populations across a gradient on the Tibetan Plateau

The large-male mating advantage and size-assortative mating are two different size-based patterns which deviate from random mating in Gammarus species. The size-assortative pairing in gammarid amphipods may have arisen for microhabitat segregation, loading constraints, sexual selection, or a combination of them. This study investigated the mating patterns of Gammarus lacustris from three populations along an altitudinal gradient in the northeastern Tibetan plateau. We demonstrated that the species shows a size-related non-random mating pattern. There is a strong sexual size dimorphism (SSD) in G. lacustris, which does not vary along the altitudinal gradient. The ultimate cause of male-biased SSD is likely the stronger selection acting on male than on female body size, either because large males have a higher mating success than smaller males (large male advantage at the highest elevation population), or because they mate with larger females with more fecundity in three populations, or both, which results in size-assortative mating of G. lacustris. Additionally, the loading constraint hypothesis (size-assortative patterns potentially increased their fitness in fast-flowing water) cannot be excluded.

Body Size, Size Variation, and Sexual Size Dimorphism in EarlyHomo

Size variation provides important clues about the taxonomy, morphology, behavior, and life history of extinct species. Body size variation in living species is commonly attributed to Bergmann’s rule, resource availability, nutrition, local selection pressures, and sexual size dimorphism. While our understanding of the mechanisms producing size variation in living species has grown more sophisticated in recent years, our ability to apply this knowledge to the fossil record is limited by the quality of the available fossil and extant comparative samples. New discoveries of fossil Homo have expanded the known range of size variation and provide hints of geographic and temporal variation in size within and between named taxa and possible strong sexual size dimorphism. Even so, the range of size variation in Homo habilis/rudolfensis and Homo erectus matches or even is less than that seen in geographically restricted samples of living anthropoid primates. These observations dictate caution in interpreting the meaning of variation in early Homo but also underscore the critical necessity of improving comparisons of size among fossils and establishing an adequate comparative database of living species that allows us to discriminate between the effects of epigenetic and selective factors on the expression of variation.

Skewed soldier sex ratio in termites: testing the size-threshold hypothesis

Social insect colonies contain multiple phenotypes, i.e. castes, and this caste polyphenism is often linked to sexual dimorphism. Unlike social hymenopterans, both termite sexes are diploid and contribute to helper-type tasks within the colony. Nonetheless, a biased sex ratio is frequently observed in termites, especially in soldiers. To explain this bias in soldier sex ratio, Matsuura (Evol. Ecol. 20: 565–574, 2006) postulated the existence of a size threshold for workers molting into soldiers. Under the influence of sexual size dimorphism (SSD), such a threshold was considered to indirectly favor one sex. We conducted a literature survey of sex ratio among termite soldiers and tested this size-threshold hypothesis using data for 67 termite species from a variety of termite lineages. We demonstrated the existence of a size threshold for individuals molting into soldiers, resulting in the acquisition of soldiers of only one sex in species exhibiting strong SSD. In species exhibiting weak SSD, the size threshold skews the sex ratio of soldiers, but does not necessarily cause the loss of one sex. Finally, we observed a prevalence of single-sex soldiers in the Termitidae, regardless of SSD, suggesting that the ancestral developmental mechanisms that constrain soldier differentiation from one sex are maintained in certain extant species.

Age-related male reproductive effort in two mountain ungulates of contrasting sexual size dimorphism

In polygynous ungulates, the reproductive effort of adult males peaks during a short period in which feeding activities are sacrificed for mating activities. Hence, both fat reserves and body mass are predicted to decline markedly during this period. The decline is also predicted to be greater in fat reserves than in body mass because fat is catabolized before muscle, and to increase with the intensity of sexual selection. In contrast, no specific patterns are expected in females for which late gestation and lactation rather than mating are the energetically most demanding periods. We tested these hypotheses in two mountain ungulates of contrasting sexual size dimorphism (SSD): Himalayan tahr ( Hemitragus jemlahicus (H. Smith, 1826)) (SSD = 123%) and alpine chamois ( Rupicapra rupicapra (L., 1758)) (SSD = 26%). As expected, kidney fat declined more rapidly than body mass in adult males of both species. Kidney fat declined faster in adult male tahr compared with adult male chamois. There was no consistent pattern of changes in body mass or kidney fat in female tahr or female chamois. Our results suggest that adult males of species with strong SSD allocate more energy to mating than males of less dimorphic species.

An additional field method to sex adult Barn Swallows during the non-breeding season in Zambia: white spot length in the outer tail feather

Adult Barn Swallows Hirundo rustica exhibit strong sexual size dimorphism in the length of the outermost tail feathers, which are longer in males compared with females. This trait is traditionally used to sex adult Barn Swallows in the field. However, due to the wear and breakage of the tips of the outer tail feather and tail moult during the non-breeding season, sexing becomes unreliable or even impossible. We therefore tested whether the length of the white spot on the outer tail feather is sexually dimorphic, and whether it can be used as an additional sexing method for adult Barn Swallows. The white spot length was sexually dimorphic, based on DNA analysis of 101 adult individuals caught at their roost during the non-breeding season in Zambia. Accuracy in sex determination of 95% could be obtained by classifying individuals with a white spot length <17.5 mm as females and those with a white spot length >29.5 mm as males. When applying the length of the white spot as an additional method to sex adult Barn Swallows on all birds caught in Zambia during the study period (N = 759), the percentage of birds that could successfully be sexed increased to more than 55%. Therefore we emphasise the importance of measuring the white spot length in addition to the tail fork depth and tail length to sex adult Barn Swallows in the non-breeding season.

Predicting Biomass from Snout–Vent Length in New World Frogs

In this study, we determined how well the snout–vent length (SVL) of anurans estimated their mass for 36 species in the New World. Linear regressions of log-mass on log-SVL were highly significant for all species, explaining more than 75% of the mass variation in most species, and over 50% of the mass variation in all species. We also investigated differences in the mass/SVL relationship within species, comparing juveniles to adults, females to gravid females, and males to females, to determine the importance of developing separate regressions for sex or life-stage classes. Three of six tests between juveniles and adults, and two of nine tests between females and gravid females, indicated statistically significant differences, although these differences had only minor effects on mass estimates. More statistical differences in regression equations occurred between males and females; again, these differences were unimportant for estimates of mass in some cases, but they were important where there was strong sexual size dimorphism within a species. Continued collection of both SVL and mass data in new field studies of anurans will provide broader analyses of mass/SVL regressions. These species regressions along with data on density can be used to determine anuran community biomass.

Seasonal decline of body mass in adults of the wood-dwelling caddisfly Lype phaeopa (Psychomyiidae)

Larvae of the psychomyiid caddisfly Lype phaeopa (Stephens 1836) were reared under close to outdoor temperature conditions to investigate the seasonal fluc- tuations in body masses of emergent adults. The adult emergence period lasted from late February until late September, with two emergence peaks of males (March and August) and one for females (September). Females were always larger than males, illustrating a strong sexual size dimorphism. Decreasing body mass of emergent adults coincided with increasing temperature during the seasons until the end of the emer- gence period. Both sexes showed a significant relationship between body mass and wing length, which allowed me to estimate the loss of female body mass due to oviposition (55.7% ± 4.9). Egg size remained constant throughout the study (ovoid shape, mean length 216 µm, width 186 µm) but number of eggs decreased. The results imply that increasing temperature could be responsible for the decline in size and body mass of emergent adults (both sexes). A reason for this size reduction could be a constant growth rate (biomass increment) but an accelerated developmental rate induced by increasing tem- perature.

The observations on the biometry and the reproductive biology of the Leopard snake, Zamenis situlus, in Italy

Museum specimens of the Leopard snake, Zamenis situlus, were considered for these observations. Strong sexual size dimorphism was evident when raw data were considered. Males had longer body sizes and longer tails than females; also head length and width, but not interorbital distance, were greater than those of females. Subcaudal scales were higher in number in males, while ventral and total scales were similar among sexes. When influence of sex and covariation of snout to vent length were controlled for log transformed data, it was found that head size features varied according to snout to vent length and no effect of sex was detected. Growth trajectories are likely similar between sexes and suggest that male maturity is reached at older age than that of females.

Effects of sex-associated competitive asymmetries on foraging group structure and despotic distribution in Andean condors

Phenotype-limited interference models assume competitive asymmetries among conspecifics and unequal sharing of resources. Their main prediction is a correlation between dominance status and patch quality: dominant individuals should preferentially exploit better-quality habitats. We tested assumptions and predictions of the phenotype-limited interference model in Andean condors (Vultur gryphus), a New World vulture with strong sexual size dimorphism (males are 30–40% heavier than females). We recorded searching birds in habitats differing in quality: mountains and plains. We also observed scavenging behaviour at 20 sheep carcasses, and videotaped 5 of them. Intraspecific hierarchy at carcasses was based on size: males dominated females and, within each sex, older birds dominated younger ones. Adult males and juvenile females occupied extreme positions in the feeding hierarchy. Aggression was directed at those individuals belonging to lower hierarchical levels. In high-quality areas (mountains), more condors arrived at carcasses. Juvenile females were more often observed searching in low-quality areas (plains), far from breeding areas and main roost sites. GLM analyses of individual behaviour showed that the hierarchy did not influence time of arrival, but low-ranking individuals spent more time at carcasses, especially if the number of condors at arrival was high. Additionally, low-ranking condors spent less time feeding at carcasses when individuals of higher hierarchical levels were present. On the other hand, the number of condors present had a positive effect on feeding rates of dominant individuals, probably because of a reduction in individual vigilance. These results support most of the assumptions and predictions of the phenotype-limited distribution model, although a spatial truncated distribution between phenotypes was not observed. Asymmetric feeding pay-off, unequal parental roles and sexual selection constraints could favour sexual divergence in body size in Andean condors.

Body Size and Sexual Size Dimorphism in Marine Iguanas Fluctuate as a Result of Opposing Natural and Sexual Selection: An Island Comparison

Body size is often assumed to represent the outcome of conflicting selection pressures of natural and sexual selection. Marine iguana (Amblyrhynchus cristatus) populations in the Galapagos exhibit 10-fold differences in body mass between island populations. There is also strong sexual size dimorphism, with males being about twice as heavy as females. To understand the evolutionary processes shaping body size in marine iguanas, we analyzed the selection differentials on body size in two island populations (max. male mass 900 g in Genovesa, 3500 g in Santa Fe). Factors that usually confound any evolutionary analysis of body sizes-predation, interspecific food competition, reproductive role division-are ruled out for marine iguanas. We show that, above hatchlings, mortality rates increased with body size in both sexes to the same extent. This effect was independent of individual age. The largest animals (males) of each island were the first to die once environmental conditions deteriorated (e.g., during El Ninos). This sex-biased mortality was the result of sexual size dimorphism, but at the same time caused sexual size dimorphism to fluctuate. Mortality differed between seasons (selection differentials as low as -1.4) and acted on different absolute body sizes between islands. Both males and females did not cease growth when an optimal body size for survival was reached, as demonstrated by the fact that individual adult body size phenotypically increased in each population under favorable environmental conditions beyond naturally selected limits. But why did marine iguanas grow too for survival? Due to lek mating, sexual selection constantly favored large body size in males (selection differentials up to +0.77). Females only need to reach a body size sufficient to produce surviving offspring. Thereafter, large body size of females was less favored by fertility selection than large size in males. Resulting from these different selection pressures on male and female size, sexual size dimorphism was mechanistically caused by the fact that females matured at an earlier age and size than males, whereafter they constantly allocated resources into eggs, which slowed growth. The observed allometric increase in sexual size dimorphism is explained by the fact that the difference between these selective processes becomes larger as energy abundance in the environment increases. Because body size is generally highly heritable, these selective processes are expected to lead to genetic differences in body size between islands. We propose a common-garden experiment to determine the influence of genetic factors and phenotypic reaction norms of final body size.

BODY SIZE AND SEXUAL SIZE DIMORPHISM IN MARINE IGUANAS FLUCTUATE AS A RESULT OF OPPOSING NATURAL AND SEXUAL SELECTION: AN ISLAND COMPARISON.

Body size is often assumed to represent the outcome of conflicting selection pressures of natural and sexual selection. Marine iguana (Amblyrhynchus cristatus) populations in the Galápagos exhibit 10-fold differences in body mass between island populations. There is also strong sexual size dimorphism, with males being about twice as heavy as females. To understand the evolutionary processes shaping body size in marine iguanas, we analyzed the selection differentials on body size in two island populations (max. male mass 900 g in Genovesa, 3500 g in Santa Fé). Factors that usually confound any evolutionary analysis of body sizes-predation, interspecific food competition, reproductive role division-are ruled out for marine iguanas. We show that, above hatchlings, mortality rates increased with body size in both sexes to the same extent. This effect was independent of individual age. The largest animals (males) of each island were the first to die once environmental conditions deteriorated (e.g., during El Niños). This sex-biased mortality was the result of sexual size dimorphism, but at the same time caused sexual size dimorphism to fluctuate. Mortality differed between seasons (selection differentials as low as -1.4) and acted on different absolute body sizes between islands. Both males and females did not cease growth when an optimal body size for survival was reached, as demonstrated by the fact that individual adult body size phenotypically increased in each population under favorable environmental conditions beyond naturally selected limits. But why did marine iguanas grow "too large" for survival? Due to lek mating, sexual selection constantly favored large body size in males (selection differentials up to +0.77). Females only need to reach a body size sufficient to produce surviving offspring. Thereafter, large body size of females was less favored by fertility selection than large size in males. Resulting from these different selection pressures on male and female size, sexual size dimorphism was mechanistically caused by the fact that females matured at an earlier age and size than males, whereafter they constantly allocated resources into eggs, which slowed growth. The observed allometric increase in sexual size dimorphism is explained by the fact that the difference between these selective processes becomes larger as energy abundance in the environment increases. Because body size is generally highly heritable, these selective processes are expected to lead to genetic differences in body size between islands. We propose a common-garden experiment to determine the influence of genetic factors and phenotypic reaction norms of final body size.

Species and sexual isolating mechanisms in sibling species of giant petrels Macronectes

The two sibling species of giant petrels Macronectes halli and M. giganteus are the dominant avian scavengers in the Southern Ocean ecosystem. They breed sympatrically at a number of sub-Antarctic sites. This paper synthesises data from a detailed study at South Georgia to examine the importance of various interspecific and intersexual differences between these closely related species. Morphological, breeding, dietary, feeding and moult characteristics were investigated. The most significant interspecific differences are a six week separation in the onset of breeding and the importance of seal carrion to male M. halli, the earlier breeding species. There is a strong sexual size dimorphism with females being only 75–80% of the weight of males in both species. Intersexual dietary differences are stronger than interspecific ones and females also have later primary moult schedules. Reproductive isolation and ecological adaptations are discussed in relation to the present distribution of giant petrels, comprising a more restricted sub-Antarctic species (M. halli) and a widespread Antarctic species (M. giganteus). It is suggested that the marked sexual dimorphism evolved before the two taxa became specifically distinct.