Consequences Of Sexual Size Dimorphism Research Articles

Penny and penny laid up will be many: large Yellow anacondas do not disregard small prey

AbstractThe study of diet may help to predict the consequences of ontogeny and sexual size dimorphism in resource use. Although diet changes are expected in dimorphic species, ontogeny can be a factor in determining the degree of diet variation within a species. We studied large sexually dimorphic predator, the Yellow anaconda Eunectes notaeus, to learn how influences of sex and size on diet might lead to intersexual niche divergence, therefore avoiding intraspecific competition. We tested the consequences of sexual size dimorphism via two foraging metrics: prey size and feeding frequency. To test the consequences of ontogeny on trophic niche metrics, we related changes in feeding frequency and maximum prey size to increase in anaconda body size. Finally, we tested whether diet composition changed between sexes to the point where it could lead to reduced competition. While females (the larger sex) did eat larger prey compared to males, this effect disappeared when we removed the effect of body size. Females ate more frequently than males, even with body size effect was removed. Predator‐prey size ratios were positively affected by maximum prey size, and as expected from foraging theory, did not increase minimum prey size. Feeding frequency did not display any ontogenetic effects. While diet composition varied between sexes, overlap is high. This indicates that variations in resource use as a product of sex‐based differences in size are negligible in Yellow anacondas. Although females feed more frequently, this may be an effect of the greater energetic costs of reproduction. Ontogeny has a positive effect on maximum prey size, though this is a general trend, and has already been demonstrated for several other species. Finally, understanding of sex‐based changes in resource use will be improved if it can be determined whether such phenomena are consequences rather than causes for sexual size dimorphism.

Habitat- and sex-related differences in a small carnivore’s diet in a competitor-free environment

The alien invasive American mink Neovison vison is fully established in the low species richness and competitor-free environment of Iceland. This study documents the diversity as well as seasonal and sexual variation in the diet of mink in Iceland based on stomach contents. Seasonal changes mainly reflected variation in abundance of migratory birds and wood mice Apodemus sylvaticus. In comparison with mink elsewhere in similar habitats, the mink in Iceland consumed more fish and birds and fewer mammals, which is in accordance with local availability. This reinforces evidence of opportunistic foraging. Females generally ate more fish and fewer birds than males and this might be explained by their smaller body size and possible limitation in catching larger birds. Mink in coastal habitats showed greater sexual differences in diet than mink in riparian habitats, probably reflecting less prey diversity in riparian habitats than coastal ones. This study is an input towards explaining the ecological consequences of sexual size dimorphism and supports the hypothesis that generalist species might be successful invaders due to their capability in utilising new and diverse resources. The mink in Iceland can be regarded as a model for a small-bodied semi-aquatic carnivore away from the confounding effects of inter-specific competition.

Male body size varies with latitude in a temperate lizard

To examine the evolution of sexual size dimorphism (SSD), interspecific studies are often performed to generate hypotheses for the origin and maintenance of SSD. Although these methods are invaluable to our understanding of the evolution of SSD, they often quantify SSD for a species based on few populations. We found a significant sex-specific latitudinal cline in Plestiodon fasciatus (L., 1758) (= Eumeces fasciatus (L., 1758)), a species that was previously considered to be monomorphic for body size. Male body size significantly increased with increasing latitude, whereas female body size was relatively constant. Our findings argue for the importance of increased understanding and appreciation of intraspecific variation in SSD. We suggest that a more integrated approach to SSD be employed, where both intraspecific and interspecific variation is considered. We provide a foundation for posing hypotheses of the causes and consequences of SSD in P. fasciatus and perhaps other members of the species group.

Behavioral Causes and Consequences of Sexual Size Dimorphism

(Invited Article)AbstractSexual size dimorphism (SSD) is widespread and variable among animals. According to the differential equilibrium model, SSD in a given species is expected to result if opposing selection forces equilibrate differently in both sexes. Here I review the factors that affect the evolution of SSD specifically as they relate to behavior. Taking the approach that SSD results as an epiphenomenon from separate but related selection on male and female body size, the advantages and disadvantages of large size in terms of the standard components of individual fitness (mating success, fecundity, viability, growth, foraging success) are discussed to help guiding future research on the subject. This includes a discussion of intra‐SSDs. The main conclusions are: (1) Evidence for disadvantages of large body size is still sparse and requires more research. In contrast, evidence for sexual or fecundity selection favoring large body size is overwhelming, so these mechanisms do no longer require special attention, but need to be documented nonetheless to acquire a complete picture. (2) Some hypotheses suggesting that small size is favored are not well investigated at all, because they apply only to some species or restricted situations, may be difficult to study, or have simply been disregarded. Evidence for these cryptic hypotheses is best revealed using experiments under multiple environmental (food, temperature, etc.) stresses with particularly well‐suited model species. (3) The evolution of SSD ultimately depends on processes generating variation within as well as between the sexes, so studies should always investigate and report effects on both sexes separately, in addition to size‐dependent effects within each sex; within sexes the key issue is whether small individuals under, over‐ or perfectly compensate their general fitness disadvantage. (4) Tests of several hypotheses should be integrated in case studies of well‐suited model species to investigate selection on body size comprehensively. For example, all episodes of sexual selection (mate search, competition, pre‐ and post‐copulatory choice) should be addressed in conjunction. Investigations of size‐selective and sex‐dependent predation should take the viewpoint of the prey rather than the predator to permit integration of effects throughout prey ontogeny generated by various predators with differing preferences. Comparative studies should also test multiple alternative hypotheses at the same time to permit stronger inference. (5) Experimental behavioral studies of sexual and natural selection should provide selection differentials using the available standard methods. This would allow integration with phenomenological studies of selection and facilitate subsequent meta‐analyses, which are very valuable in evaluating general patterns. (6) Comparative phylogenetic studies identifying patterns and phenomenological and experimental studies of model species that investigate particular mechanisms should be integrated, so that macro‐evolutionary patterns can be linked to micro‐evolutionary processes, which is the central paradigm of evolutionary ecology. (7) A major problem is the general difficulty of separating causes generating a particular body size and SSD over evolutionary time and its consequences for behavior and ecology today, i.e. today's researchers cannot completely avoid this ‘ghost of SSD evolution past’.

Potential causes and life‐history consequences of sexual size dimorphism in mammals

ABSTRACT1. Male‐biased sexual size dimorphism (SSD) in mammals has been explained by sexual selection favouring large, competitive males. However, new research has identified other potential factors leading to SSD. The aim of this review is to evaluate current research on the causes of SSD in mammals and to investigate some consequences of SSD, including costs to the larger sex and sexual segregation.2. While larger males appear to gain reproductive benefits from their size, studies have also identified alternative mating strategies, unexpected variance in mating success and found no clear relationship between degree of polygyny and dimorphism. This implies that sexual selection is unlikely to be the single selective force directing SSD.3. Latitude seems to influence SSD primarily through variation in overall body size and seasonal food availability, which affect potential for polygyny. Likewise, population density influences resource availability and evidence suggests that food scarcity differentially constrains the growth of the sexes. Diverging growth patterns between the sexes appear to be the primary physiological mechanism leading to SSD.4. Female‐biased dimorphism is most adequately explained by reduced male–male competition resulting in a decrease in male size. Female–female competition for dominance and resources, including mates, may also select for increased female size.5. Most studies found that sexual segregation arises through asynchrony of activity budgets between the sexes. The larger sex can suffer sex‐biased mortality through increased parasite load, selective predation and the difficulty associated with sustaining a larger body size under conditions of resource scarcity.6. None of the variables considered here appears to contribute a disproportionate amount to SSD in mammals. Several promising avenues of research are currently overlooked and long‐term studies, which have previously been biased toward ungulates, should be carried out on a variety of taxa.

SEXUAL SIZE DIMORPHISM OF THE MUSK DUCK

We examined sexual size dimorphism of a lek-displaying diving duck from Australia, the Musk Duck (Biziura lobata). Like other lek-displaying species, Musk Ducks exhibit extreme sexual size dimorphism in addition to structural dimorphism. Body mass ratios (male:female) for Musk Ducks are among the highest reported for birds (more than 3:1). Multivariate analyses of 16 anatomical measurements indicated that body plans of male and female Musk Ducks have diverged isometrically except for the addition of a pendant lobe on lower mandibles of males. Within males, pendant lobe length, depth, and breadth were positively correlated with center rectrix length and bill width. Lobe area also was positively related to bill width, but not to center rectrix length. Lobe breadth and center rectrix length were positively related to overall body mass. Our results suggested that information about male physical quality may be conveyed to other Musk Ducks by parts of the anatomy most conspicuously exposed during sexual advertising displays. In contrast, anatomical features that function in foraging activity showed no sexual differences in anatomical shape relative to other parts of the anatomy that do not serve obvious foraging functions. We argue that foraging niche divergence or use of different food resources, if they have occurred, probably are secondary consequences of sexual size dimorphism.

Evaluating the consequences of sexual size dimorphism—reply to Sodhi

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Fluctuating asymmetry in the skeleton of the American kestrel, Falco sparverius: a test of the consequences of sexual size dimorphism

Fluctuating asymmetry, i.e., random, subtle departures from perfect bilateral symmetry, is believed to be a measure of developmental noise. The more physical stress an organism has undergone, the greater the asymmetry. We investigated the degree of fluctuating asymmetry in bones of the American kestrel (Falco sparverius) as a test of the developmental consequences of sexual dimorphism in size. The "cost of rearing" hypothesis predicts that females, because of their larger size, will suffer food shortages more often than males and hence show greater asymmetry. In contrast, the "female dominance" hypothesis predicts that females will be under less developmental stress because their size gives them an advantage when they compete with their brothers for limited food while in the nest. Significant fluctuating asymmetry was detected for all characters of both sexes; however, differences between males and females only approached statistical significance. We suggest that the degree of among-broods variation is so great that the role of gender in influencing developmental stability may be relatively small.

Energetic Consequences of Sexual Size Dimorphism in White Ibises (Eudocimus albus)

Energetic Consequences of Sexual Size Dimorphism in White Ibises (Eudocimus albus)

Energetic Consequences of Sexual Size Dimorphism in Nestling Red‐Winged Blackbirds

The energy budget of nestling Red—winged Blackbirds (Agelaius phoeniceus) was determined using doubly labeled water (3HH18O) to measure field metabolic rate (FMR) and body component data to measure growth energy. Sex—specific measurements permitted the evaluation of the effects of this species' substantial sexual size dimorphism on FMR and total energetics. FMR averaged CO2 release of 5.12 mL°g—1°h—1, or 0.129 kJ°g—1°h—1, with no significant differences between the sexes. Daytime FMRs of CO2 production (5.34 mL°g—1°h—1) were higher, but not significantly so, than nighttime FMRs (4.45 mL°g—1°h—1). Water influx averaged 0.95 mL°g—1°d—1, with daytime rates (1.22 mL°g—1°d—1) significantly higher than nighttime (0.40 mL°g—1°d—1) rates. Total assimilated energy from hatching to fledging was 1014 and 797 kJ for male and female nestlings, respectively. The sexual differences in total energetics reflected differences in body size of the nestlings and suggest that there is a greater cost to the parents in raising males than in raising females.