Reverse Sexual Size Dimorphism Research Articles

Long-term female bias in sex ratios across life stages of Harpy Eagle, a large raptor exhibiting reverse sexual size dimorphism.

The primary (PSR), secondary (SSR) and adult (ASR) sex ratios of sexually reproducing organisms influence their life histories. Species exhibiting reversed sexual size dimorphism (RSD) may imply a higher cost of female production or lower female survival, thus generating biases in PSR, SSR and/or ASR towards males. The Harpy Eagle is the world's largest eagle exhibiting RSD. This species is found in the Neotropical region and is currently threatened with extinction. We used molecular markers to determine the sex of 309 Harpy Eagles spanning different life stages-eaglets, subadults and adults-from 1904 to 2021 within the Amazon Rainforest and Atlantic Forest. Sex ratios for all life stages revealed a female-biased deviation across all periods and regions. Our results suggest that the population bias towards females is an evolutionary ecological pattern of this species, and SSR and ASR likely emerged from the PSR. This natural bias towards females may be compensated by an earlier sexual maturation age of males, implying a longer reproductive lifespan and a higher proportion of sexually active males. A better understanding of the Harpy Eagle's life history can contribute to understanding sex-role evolution and enable more appropriate conservation strategies for the species.

Egg Incubation Mechanics of Giant Birds

Simple SummaryExtinct giant birds have been a source of imagination, and knowledge of their incubation mechanics is crucial to our understanding of the evolution of avian reproduction. Despite the extensive studies on avian eggs, our understanding of the eggshell mechanics of giant birds, particularly the extinct ones, remains incomplete—most of these prior works were based on empirical or allometric relationships with limited quantitative analysis. In the present study, with the help of advanced three-dimensional computer simulation using data from published fossil records, we obtain more comprehensive quantitative analysis to answer important questions related to contact incubation of giant birds. Specifically, how much safety margin does the reversed sexual size dimorphism (RSSD) of moas provide? What is the theoretical upper limit of body mass for contact incubation? Is the Williams’ egg, or the putative Genyornis oological material (PGOM), really the egg of the extinct giant bird Genyornis newtoni, as commonly accepted since its discovery in 1981?Finite element analysis (FEA) was used to conduct mechanical analyses on eggshells of giant birds, and relate this to the evolution and reproductive behavior of avian species. We aim to (1) investigate mechanical characteristics of eggshell structures of various ratite species, enabling comparisons between species with or without reversed sexual size dimorphism (RSSD); (2) quantify the safety margin provided by RSSD; (3) determine whether the Williams’ egg can have been incubated by an extinct giant bird Genyornis newtoni; (4) determine the theoretical maximum body mass for contact incubation. We use a dimensionless number C to quantify relative shell stiffness with respect to the egg size, allowing for comparison across wide body masses. We find that RSSD in moas significantly increases the safety margin of contact incubation by the lighter males. However, their safety margins are still smaller than those of the moa species without RSSD. Two different strategies were adopted by giant birds—one is RSSD and thinner shells, represented by some moa species; the other is no RSSD and regular shells, represented by the giant elephant bird. Finally, we predicted that the upper limit of body mass for contact incubation was 2000 kg.

Higher reversed sexual size dimorphism among nesting pairs of Northern Pygmy-Owls (Glaucidium gnoma) in northwestern Oregon than among specimens collected at the range-wide scale

I report on the strength of reversed sexual size dimorphism (RSD) in a local sub-population of Northern Pygmy-Owls (Glaucidium gnoma) in northwestern Oregon (2007–2012) in comparison to range-wide museum specimens examined by Earhart and Johnson (1970). Using Storer's Dimorphism Index (DI), RSD was higher among local breeding pairs (DImass = 6.64, DIwing = 4.90, n = 20 pairs) and among all local specimens (DImass = 6.20, DIwing = 4.39, n = 31 females, n= 23 males) than for specimens range-wide (DImass = 5.45, DIwing = 4.30, n= 10 females, n= 42 males). RSD was also more distinct locally, because mass alone was diagnostic of the sexes at a threshold of 69 g and wing chord was nearly diagnostic, whereas range-wide there was greater intrasexual variation and intersexual overlap in both metrics. Locally, females weighed 75.3 ± 5.6 g (range 69.0–94.2 g), and males weighed 62.5 ± 2.5 g (range 58.5–68.5 g); all local males had wing chords <92 mm (89.2 ± 2.0 mm; range 85.0–91.5 mm), whereas for 90% of females wing chord was ≥92 mm (93.2 ± 1.3 mm; range 89.0–96.0 mm). Clutch size (F1,4 = 1.822, P= 0.248, n= 20) and the number of fledged young (F1,8 = 0.00, P= 0.619, n= 19) were independent of RSD among local breeding pairs. Vertebrate prey composition shifted seasonally and was similar at the 2 scales, with agile avian prey taken most frequently during the breeding season, when the evolutionary consequences of diet on RSD matter most. I found intersexual variation in prey composition at the local scale where males took avian prey more frequently than females. I concluded that RSD in pygmy-owls was stronger at the local scale and that intersexual differences in prey composition may be related to RSD in this species. This study highlights the value of examining multiple scales and seasonal variation in prey composition when evaluating the merits of RSD hypotheses. It also contributes to our understanding of one of the least studied owls in North America.

Brood sex ratio modulates the effects of extra food on parental effort and sibling competition in a sexually dimorphic raptor

Conflicts are inherent to family systems and may occur at three levels. First, each parent benefits if its mate takes the greater share of parental investment. Second, offspring try to manipulate their parents into devolving more resources than it is optimal for them. Third, siblings compete for resources. Food availability can affect the dynamics of each level of interaction. By means of a food supplementation experiment, we assessed how the initial availability of extra food during breeding affects later parental effort, sibling competition, and parent-offspring interactions in a small dimorphic raptor, the lesser kestrel (Falco naumanni). Being the larger sex, female nestlings are likely to have higher energy requirements. Female-biased broods had a higher rate of aggressive interactions and were fed more frequently in control nests. Food was not shared evenly among broodmates, and daughters tended to receive more feedings than sons. In young broods, parents controlled food allocation by entering the nest, whereas in older broods, offspring controlled food allocation by monopolizing the nest site entrance. Extra food induced male parents to reduce the rate of feedings delivered by entering the nest. Additionally, extra food improved nestling growth in male-biased broods, leading to an increase in the frequency of parental feedings and aggressive interactions, likely due to faster growth rates. These findings reveal a key effect of brood sex ratio in determining family interactions in a species with reverse sexual size dimorphism and suggest that all levels of conflict between family members should be considered simultaneously when investigating the evolution of parental care. Breeding poses energetic costs on parents, and environmental resources are usually limited. To successfully breed, parents coordinate their efforts and manage the allocation of resources to offspring, while offspring communicate their needs. Conflict can arise among family members: each parent benefits if the other takes on most of the workload, offspring ask for more care than what parents can provide, and siblings compete for food. Food availability affects these interaction levels separately, but they have rarely been integrated. By providing extra food to lesser kestrel (Falco naumanni) nests, we show that parental provisioning behaviour and sibling competition are simultaneously affected by initial resource availability during breeding. Although lesser kestrel nestlings show only moderate sex differences in body size (and therefore energy requirements), the effect of extra food on family interactions primarily depended on the sex ratio of the brood.

Reversed Sexual Size Dimorphism in the Ryukyu Scops Owl Otus elegans on Minami-Daito Island

Abstract One of the consequences of selections acting on body size is the difference in body size between the sexes or sexual size dimorphism. Although many hypotheses have been proposed for reversed sexual size dimorphism (RSD) in raptors, ornithologists have rarely paid attention to temporal aspects of RSD when testing their hypotheses. Because selection pressures may vary temporally, describing temporal variation in RSD is a first step towards understanding evolutionary mechanism which shape and maintain the dimorphism. Here, we describe RSD in the population of Ryukyu Scops Owls Otus elegans on Minami-daito Island using a dataset of the external measurements of 770 individuals obtained during a 17-year long-term population monitoring project. Females were larger than males in body mass, culmen length, bill depth, bill width, tail length and flattened wing length, whereas males were larger than females in tarsus and head lengths. Among these traits, the degree and direction of RSD of body mass, tarsus length, bill depth and flattened wing length varied across years. There were neither increasing nor decreasing trends in RSD. This is a rare study which addressed temporal variation in RSD in a raptor species.

Geographic variation in ringed seal (Pusa hispida) growth rate and body size

We summarize geographical patterns in ringed seal (Pusa hispida (Schreber, 1775)) body length and girth growth using 3012 samples collected by Inuit hunters in the eastern Canadian Arctic from 1990 to 2016. Spatial structure was detected using cluster analysis of environmental variables separating a northern region in the eastern Canadian High Arctic and a southern region in Hudson Bay. The north was characterized by more fast ice, multiyear ice, greater snow depth, colder temperatures, and greater sea-ice concentration in the spring seal breeding season. Hierarchical Bayesian models described length and axillary girth growth of northern seals as slower than in the south, reaching asymptotic size 5–7 years later. Northern females were larger than males (asymptotic length of 149 versus 140 cm, respectively) and both were larger than southern seals (males and females 126 cm). We conclude that environmental variation was best represented by regions rather than latitude, regional body size differences were driven by differential growth rates, and northern ringed seals may be characterized by reverse sexual size dimorphism.

Age and growth of Franciscana dolphins from northern Argentina

Knowledge of growth patterns of an animal species is fundamental to understand their life history. This information is also used to help define population boundaries of threatened cetaceans, such as the Franciscana dolphin, Pontoporia blainvillei. A total of 108 Franciscana dolphins incidentally captured in artisanal fishing nets in estuarine and marine coastal waters of the northern part of the Franciscana Management Area IV (North FMA IV), Argentina, were studied. The objective of the present paper was to study age and growth parameters of Franciscanas from North FMA IV and to compare these parameters between estuarine and marine potential populations within this area. We used von Bertalanffy and Gompertz growth curves to model growth trajectories. The estimated asymptotic lengths demonstrated that Franciscana dolphins from this area were smaller (females: 136.3 cm and males, 122.1 cm) than southern FMA IV values previously published. They also showed the reverse sexual size dimorphism that is known across their range. However, the estimated asymptotic length was not statistically different between the estuarine and marine females within the study area. In spite of this, the fact that the northern forms of the FMA IV were smaller than the southern specimens supports the hypothesis of more than one population of the species within this management area. The North FMA IV has the highest reported mortality levels of Franciscana dolphins within the FMA IV; these results are relevant to the knowledge base of Franciscana dolphins in the region.

Determinants and Patterns of Reproductive Success in the Greater Horseshoe Bat during a Population Recovery

An individual's reproductive success will depend on traits that increase access to mates, as well as the number of mates available. In most well-studied mammals, males are the larger sex, and body size often increases success in intra-sexual contests and thus paternity. In comparison, the determinants of male success in species with reversed sexual size dimorphism (RSD) are less well understood. Greater horseshoe bats (Rhinolophus ferrumequinum) exhibit RSD and females appear to exert mate choice when they visit and copulate with males in their underground territories. Here we assessed putative determinants of reproductive success in a colony of greater horseshoe bats during a 19-year period of rapid population growth. We genotyped 1080 bats with up to 40 microsatellite loci and assigned maternity to 99.5% of pups, and paternity to 76.8% of pups. We found that in spite of RSD, paternity success correlated positively with male size, and, consistent with our previous findings, also with age. Female reproductive success, which has not previously been studied in this population, was also age-related and correlated positively with individual heterozygosity, but not with body size. Remarkable male reproductive skew was detected that initially increased steadily with population size, possibly coinciding with the saturation of suitable territories, but then levelled off suggesting an upper limit to a male's number of partners. Our results illustrate that RSD can occur alongside intense male sexual competition, that male breeding success is density-dependent, and that male and female greater horseshoe bats are subject to different selective pressures.

Morphometrics and Sexual Dimorphism of Chinese Goshawks (Accipiter soloensis)

Sex determination is a key component in many ecological and evolutionary studies, and morphological measurements, based on reversed sexual size dimorphism (RSD), are the most widely used means of sexing raptors. We studied morphometrics and sexual dimorphism of the poorly documented Chinese Goshawk (Accipiter soloensis) in the Republic of Korea, where we measured 54 birds and developed discriminant functions to allow sexing. The species showed apparent, but small, RSD in all of eight measured parameters, with the dimorphic index ranging from 1.39 for tarsus to 16.09 for body mass. A stepwise discriminant procedure, followed by discriminant function analysis (DFA), indicated that tail, bill, and head lengths were all reliable parameters for sex determination of goshawks with an accuracy of 100% among 30 individuals. Head length is often unavailable from museum specimens; however, a discriminant function using tail and bill lengths classified the sex of 44 goshawks with an accuracy of 95.5%. Our morphometric data and discriminant function may benefit further studies on the Chinese Goshawk by allowing easy sexing of most birds.La determinación del sexo es un componente clave en diversos estudios ecológicos y evolucionarios, y las medidas morfométricas, basadas en el dimorfismo sexual invertido (DSI) son los medios más utilizados para determinar el sexo de las rapaces. Estudiamos las medidas morfométricas y el dimorfismo sexual de la especie rapaz poco documentada Accipiter soloensis en la República de Corea, donde medimos 54 aves y desarrollamos funciones discriminantes para permitir la determinación del sexo. La especie evidenció un DSI aparente pero pequeño en los ocho parámetros medidos, con el índice de dimorfismo variando de 1.39 para el tarso a 16.09 para la masa corporal. Un proceso discriminante paso a paso, seguido de un análisis de función discriminante (AFD), indicaron que las longitudes de la cola, el pico y la cabeza son todos parámetros confiables para la determinación del sexo de individuos de A. soloensis con una precisión del 100% en 30 individuos. El largo de la cabeza a menudo no se encuentra disponible en especímenes de museo; sin embargo, una función discriminante usando las longitudes de la cola y del pico clasificó el sexo de 44 individuos de A. soloensis con una precisión del 95.5%. Nuestros datos morfométricos y la función discriminante pueden beneficiar a otros estudios de A. soloensis al permitir una fácil determinación del sexo de la mayoría de las aves.

Sexual size monomorphism in the crested porcupine (Hystrix cristata)

Amongst mammals, female-biased sexual size dimorphism (SSD) is rare and it occurs mostly in species where reduced male intrasexual competition is present. Reverse SSD has been reported for Old World porcupines Hystrix spp. We compared weight and six metric body measurements of 40 male and 42 female crested porcupines from Southern Tuscany, Italy. No significant difference was observed between sexes. The monogamous mating system of porcupines, sharing parental care, together with no evidence of territoriality, militate against previous claims of SSD presence, probably due to small sample size and inappropriate statistical analyses.

The evolution of sexual dimorphism in New Zealand giant moa (Dinornis) and other ratites

The extinct giant moa Dinornis is one of the most remarkable known examples of reversed sexual size dimorphism (RSD), with males weighing 34-85 kg, but females weighing up to 240 kg. However, there has been little consideration of the evolutionary mechanism that produced this level of dimorphism, and most living palaeognaths also exhibit varying levels of RSD. Using male and female body mass data for extant ratites and tinamous and four extinct moa genera, and tests of phylogenetic dependence (λ) of body size evolution among these species, we investigated whether Dinornis was truly unusual with respect to RSD relative to other palaeognaths, which sex was under greater pressure to change in size over evolutionary time, and which candidate hypotheses explaining the presence and variability of RSD in the genus are most plausible. We demonstrate that the extreme level of RSD exhibited by Dinornis represents a straightforward consequence of positive allometric scaling of body size. However, Dinornis females have undergone more evolutionary change than males, and larger females from high-productivity environments are associated with greater differentiation, possibly driven by intraspecific competition and female-biased selection for increased offspring investment.

Nestling Sex Ratio in a Critically Endangered Dimorphic Raptor, Ridgway's Hawk (Buteo ridgwayi)

Variation in nestling sex ratio is an important concept in population ecology, and has particular implications for the conservation status of small populations. Research on species exhibiting Reversed Sexual Size Dimorphism (RSSD), in which females are larger than males, have shown that significant biases in nestling sex ratios can result from demographic and environmental conditions experienced by parents during the breeding episode. We collected morphometric measurements from the critically endangered Ridgway's Hawk (Buteo ridgwayi) over a 4-yr period, verifying that the species exhibits RSSD. Females weighed 25% more than males and were significantly larger for 7 of 12 body measurements. Nestling sex-ratios were determined by PCR amplification of the CHD1 gene. The results revealed a weak but consistent trend toward female-biased broods for the small remaining population. Parents may potentially be producing more females, the more costly sex, due to an ample food supply and adaptive allocation of parental care. A female-bias sex ratio may also signal management concern for the species if it is caused by inbreeding; however, currently the bias is not significant enough to warrant immediate intense nest intervention or nest management for this critically endangered species.La variación en la relación de sexos en pichones es un concepto importante en la ecología de poblaciones y tiene implicancias particulares para el estado de conservación de poblaciones pequeñas. Investigaciones en especies que presentan Dimorfismo Sexual Invertido (DSI), en el cual las hembras son más grandes que los machos, han demostrado que los sesgos significativos en la relación de sexos de los pichones pueden ser el resultado de condiciones demográficas y ambientales experimentadas por los padres durante el episodio de cría. Colectamos medidas morfométricas de individuos de Buteo ridgwayi, una especie en peligro crítico, durante un periodo de cuatro años, verificando que la especie presenta DSI. Las hembras pesaron 25% más que los machos y fueron significativamente más grandes en 7 de 12 medidas corporales. La relación de sexos de los pichones se determinó por amplificación de RCP del gen CHD1. Los resultados revelaron una tendencia débil pero consistente hacia nidadas sesgadas hacia las hembras para la pequeña población restante. Los padres pueden estar produciendo más hembras, que representa el sexo más costoso, debido a un amplio suministro de alimento y a una distribución flexible del cuidado parental. La relación de sexos sesgada hacia las hembras puede ser una señal de preocupación en el manejo de la especie, si es que ésta es causada por endogamia; sin embargo, actualmente el sesgo no es lo suficientemente significativo como para requerir intervención de nidos de forma inmediata o manejo de nidos para esta especie en peligro crítico.

Sexual size dimorphism in greater mouse‐eared bat Myotis myotis (Chiroptera: Vespertilionidae) from a Mediterranean region

AbstractAlthough sexual size dimorphism (SSD) is common among mammals, there is no clear explanation for its maintenance in nature. Bats are one of the few groups of mammals where reverse SSD appears. In this group, the size of individuals may have very important ecological consequences related with flight. In this study, we examine sexual dimorphism in the wing measurements of 195 adult individuals (141 males and 54 females) of the greater mouse‐eared bat Myotis myotis in the south‐east of the Iberian Peninsula. We also investigated size differences between paired and single males in a swarming roost. The results indicate that there are significant differences in the wing measurements between sexes, females being bigger than males in this respect. While no significant differences in the wing measurements of paired and single males were observed, significant differences were found in their relative weight and fitness, single males being significantly heavier and having a better physical condition. We discuss the implications of SSD for the female of M. myotis in terms of reproductive advantages, trophic niche segregation and a greater ability to move, which may favour gene flow between populations.

Phylogenetic relationships of the mustache toads inferred from mtDNA sequences

Species of the genus Vibrissaphora are unique among all anurans in that males exhibit external cornified spines in the maxillary region during the breeding season. They were separated from species of the genus Leptobrachium based on this unique character. We construct a phylogeny using the 16S, ND4, and cytochrome b mitochondrial genes of 42 individuals from eight species of Vibrissaphora and five species of Leptobrachium from mainland China, Southeast Asia, and Hainan Island. Species of both Oreolalax and Scutiger were used as outgroups. The results indicate that: L. huashen and L. chapaense form a clade that is nested within Vibrissaphora, and L. hainanense is the sister taxon to the clade comprising all Vibrissaphora plus L. chapaense and L. huashen; V. boringiae is grouped with a clade consisting of V. leishanensis, V. liui, and V. yaoshanensis; and V. yaoshanensis is a species separate from V. liui. We propose taxonomic changes that reflect these findings. Also based on the resulting phylogenetic trees, we propose that the mustache toads originated in the trans-Himalayan region of southwest China, and that the evolution of maxillary spines, large body size, and reverse sexual size dimorphism in these frogs was influenced by intrasexual selection due to adopting a resource-defense polygyny mating system.

A phylogeny of the high‐elevation Tibetan megophryid frogs and evidence for the multiple origins of reversed sexual size dimorphism

AbstractA molecular phylogeny of the high‐elevation Tibetan megophryid frogs was reconstructed using mitochondrial and nuclear gene sequences. Both parsimony and Bayesian analysis produced similar tree topologies, which identified the genusLeptobrachiumas a sister group toVibrissaphora, and the generaOreolalaxandScutigeras monophyletic groups. Within the latter two genera, several species groups were also clearly recognized. At least 13 megophryids display reversed sexual size dimorphism (RSSD), where males are equal to or larger than females. According to our phylogenetic hypothesis, RSSD may have independently evolved at least five times or as many as seven times in this group. The multiple origins of RSSD within this group provide an excellent opportunity for studying the relationship between body size sexual dimorphism and other life‐history traits and for determining the costs and benefits of RSSD.

Voices of the dead: complex nonlinear vocal signals from the larynx of an ultrasonic frog

Most anurans are highly vocal but their vocalizations are stereotyped and simple with limited repertoire sizes compared with other vocal vertebrates, presumably because of the limited mechanisms for fine vocal motor control. We recently reported that the call of the concaveeared torrent frog (Amolops tormotus Fei) is an exception in its seemingly endless variety, musical warbling quality, extension of call frequency into the ultrasonic range and the prominence of subharmonics, chaos and other nonlinear features. We now show that the major spectral features of its calls, responsible for this frog's vocal diversity, can be generated by forcing pressurized air through the larynx of euthanized males. Laryngeal specializations for ultrasound appear to include very thin portions of the medial vocal ligaments and reverse sexual size dimorphism of the larynx--being smaller in males than in females. The intricate morphology of the vocal cords, which changes along their length, suggests that nonlinear phenomena probably arise from complex nonlinear oscillatory regimes of separate elastically coupled masses. Amolops is thus the first amphibian for which the intrinsic nonlinear dynamics of its larynx--a relatively simple and expedient mechanism--can account for the species' call complexity, without invoking sophisticated neuromuscular control.

Voices of the dead: Complex nonlinear vocal signals from the larynx of an ultrasonic frog

Most anurans are highly vocal but their vocalizations are stereotyped and simple with limited repertoire sizes compared to other vocal vertebrates, due presumably to the limited mechanisms for fine vocal motor control. We recently reported that the call of the concave-eared torrent frog (Amolops tormotus) is an exception in its seemingly endless variety, musical warbling quality, extension of call frequency into the ultrasonic range, and the prominence of nonlinear features such as period doubling. We now show that the major spectral features of its calls, responsible for this frog’s vocal diversity, can be generated by forcing pressurized air through the larynx of euthanized males. Laryngeal specializations for ultrasound appear to include very thin portions of the medial vocal ligaments and the reverse sexual size dimorphism of the larynx being smaller in males than in females. The intricate morphology of the vocal cords, which changes along their length, suggests that nonlinear phenomena likely arise from complex nonlinear oscillatory regimes of separate elastically coupled masses. Amolops is thus the first amphibian for which the intrinsic nonlinear dynamics of its larynx, a relatively simple and expedient mechanism, can account for the species call complexity, without invoking sophisticated neuromuscular control.

Do snakes exhibit shifts in feeding ecology associated with the presence or absence of potential competitors? A case study from tropical Africa

Two species of rain-forest snakes, the colubrids Natriciteres fuliginoides and Natriciteres variegata, are potential competitors, because they attain, on average, the same body size (20–30 cm long), have similar habits and ecological requirements (they are both diurnal and nocturnal, often semi-aquatic), and have similar dietary spectra ( including both small vertebrates and invertebrates). I studied prey type and predator size – prey size relationships of these snakes in four swamp-forest areas of southeastern Nigeria (West Africa): only N. fuliginoides was found in two of these areas, whereas both species were common in the other two areas. The feeding ecology of N. fuliginoides shifted greatly in the presence of its potential competitor, N. variegata, in terms of prey-type preferences and predatory behaviour as well as in terms of predator size – prey size relationships. In particular, the effects of the potential competitor on the feeding ecology of N. fuliginoides were (i) a shift toward eating many invertebrates from a diet based on many small vertebrates, (ii) a shift toward eating many terrestrial organisms from a diet based on many aquatic organisms, and (iii) a shift in mean prey size for females toward relatively larger prey from a diet based on smaller prey. The resulting effect was a clear partitioning of food resources between the two predator species in both the areas where they are sympatric, whereas N. fuliginoides had a wider dietary spectrum (covering that of the two species together) in the areas where its competitor is not found. The role of intraspecific competition seems less strong, as no apparent resource partitioning in prey type or prey size was observed between males and females of either species or in any study area. However, mean prey size was significantly greater for females than for males of both species and in all study areas, but depended on the significant reverse sexual size dimorphism observed in both species.

Sexual selection and sexual size dimorphism in the eastern mosquitofishGambusia holbrooki(Pisces Poeciliidae)

We have previously shown (BISAZZA & MARIN 1991) that female eastern mosquitofish, Gambusia holbrooki, did not discriminate among males of different size, although they showed a preference for the dominant male in a group of males. In this study we examined the relationship between body size and the success of gonopodial thrusting, a mating tactic characterisitc of the poeciliids that enables a male to achieve copulation without the female's cooperation. Without competitors the probability that a mating attempt was successful decreased exponentially with male length while it increased with the size of the female. When tested in a group, males competed for access to females. The largest individual monopolized access to females and accomplished the majority of mating attempts, whatever the number of its competitors. We used these results and demographic data (length distributions and composition of groups) of a natural population to simulate how the different mechanisms of sexual selection may interact under natural condition. The results of computer simulations indicated that the size of successful males always ranged well below the average length of females. Small male advantage in copulation was generally more effective than intrasexual competition and female choice in determining male mating success. A simulation which accounted for seasonal variation in body size and in the pattern of male aggregation indicated that small males had a reproductive advantage for most of the time, and that the correlation between body size and mating success became positive only at the end of the reproductive season, when population density peaks and the sex-ratio becomes male-biased. We suggest that the mating advantage of small males may be the cause of the reverse sexual size dimorphism of the poeciliids.

Reversed Sexual Size Dimorphism in Tengmalm's Owl: Is Small Male Size Adaptive?

Reversed sexual size dimorphism (RSD) of Tengmalm's owls was studied during 1981-1990 under fluctuating food conditions in western Finland. Dimorphism indices of 379 breeding pairs were calculated for body mass, wing, tail and beak lengths. Breeding sucess of pairs was approximated by the number of eggs and fledglings produced. The degree of RSD within pairs was not of great importance to the breeding success. No clear evidence for the starvation, female reproductive effort, female dominance and sexual selection hypotheses was found. The selective pressure on male size varied depending on the phase of vole cycle