Increase In Female Size Research Articles

Foraging strategy as a route for sexual size dimorphism evolution.

Female-biased sexual size dimorphism stands as a widespread evolutionary pattern. Fecundity selection, favouring larger females with greater reproductive output, is a leading explanation. However, larger body sizes demand greater energy intake, potentially hindering the evolution of extreme female sizes. Thus, the evolution of more lucrative foraging tactics may allow for an increase in size. Hence, coupled with selection against larger males, fecundity selection should result in larger SSD in species with more lucrative foraging strategies. Crab spiders are sit-and-wait predators that hunt in several plant substrata. Species that forage on flowers or employ prey-luring strategies likely have access to higher food intake than other species. We extracted body size measurements of 614 crab spider species from 43 genera and classified them based on their foraging strategy. Our findings show that foraging strategies that provide higher energy input (EFS) result in larger SSD. Statistical estimates indicate that females have a cephalothorax width 91% larger than males in EFS species, compared to 26% larger females than males in non-EFS species. These differences possibly arise due to larger females and smaller males. The effects on male size reduction might result from scramble competition, whereas the increase in female size is likely due to fecundity selection. These results suggest that the shift towards more lucrative foraging strategies may have been a key event in body size and SSD evolution in crab spiders.

Influence of microhabitat, fecundity, and parental care on the evolution of sexual size dimorphism in Caribbean Eleutherodactylus frogs.

Rensch's rule suggests that sexual size dimorphism (SSD) increases with species size when males are the larger sex, whereas it decreases when females are the larger sex. However, the process responsible for this pattern remains obscure. SSD can result from sexual selection, such as intrasexual competition for access to mates, or from natural selection, due to resource partitioning or fecundity selection. We studied SSD in Caribbean Eleutherodactylus frogs using phylogenetic comparative methods to investigate the influence of microhabitat, fecundity, and parental care. Our results show that in Caribbean Eleutherodactylus females tend to be larger and, contrary to Rensch's rule, dimorphism increases with species size. SSD was not related to microhabitat use. However, SSD was positively correlated with fecundity, mediated by a greater increase in female size. SSD was also influenced by parental care, suggesting that male care promotes larger male size and reduces the female bias in SSD. As suggested for other anurans, female-biased SSD in Caribbean Eleutherodactylus results from fecundity selection, although the magnitude is countered by increased male size in species with paternal care. Our results highlight the importance of considering various selective forces that may act in concert to influence the evolution of SSD.

Reproduction and recruitment of sympatric fish species on an intertidal rocky shore.

The reproductive cycle of seven common species (Gobius paganellus, Gobius bucchichi, Gobius cobitis, Parablennius sanguinolentus, Salaria pavo, Tripterygion tartessicum and Symphodus roissali) on rocky shores in the Gulf of Cadiz and their relationship with the sea surface temperature (SST) is analysed. Partial data on Scorpaena porcus are also given. Fecundity of these short lifespan species shows a positive linear correlation between the number of oocytes and an increase in female size. Spawnings are concentrated in the first 7 months of the year. An analysis of reproductive growth with respect to SST shows that water temperature in the winter months affects the timing of the onset of reproduction in most species. Recruitment is takes place throughout the year, but a temporal segregation within different families occurs in their spawning as well as recruitment.

Phenotypic Variation and Sexual Size Dimorphism in Dichroplus elongatus (Orthoptera: Acrididae).

Patterns of body size evolution are of particular interest because body size can affect virtually all the physiological and life history traits of an organism. Sexual size dimorphism (SSD), a difference in body size between males and females, is a widespread phenomenon in insects. Much of the variation in SSD is genetically based and likely due to differential selection acting on males and females. The importance of environmental variables and evolutionary processes affecting phenotypeic variation in both sexes may be useful to gain insights into insect ecology and evolution. Dichroplus elongatus Giglio-Tos is a South American grasshopper widely distributed throughout Argentina, Uruguay, most of Chile, and southern Brazil. In this study, we analyzed 122 adult females of D. elongatus collected in eight natural populations from central-east Argentina. Females show large body size variation among the analyzed populations and this variation exhibits a strong relationship with fecundity. Our results have shown that larger females were more fecund than smaller ones. We found that ovariole number varied along a latitudinal gradient, with higher ovariole numbers in populations from warmer locations. A considerable female-biased SSD was detected. SSD for three analyzed morphometric traits scaled isometrically. However, SSD for thorax length displayed a considerable variation across the studied area, indicating a larger relative increase in female size than in male size in warmer environmental conditions.

Higher reproductive success of small males and greater recruitment of large females may explain strong reversed sexual dimorphism (RSD) in the northern goshawk.

Reversed sexual dimorphism (RSD), which occurs when the female of a species is larger than the male, is the rule for most birds of prey but the exception among other bird and mammal species. The selective pressures that favour RSD are an intriguing issue in animal ecology. Despite the large number of hypotheses proposed to explain the evolution of RSD, there is still no consensus about the mechanisms involved and whether they act on one or both sexes, mainly because few intrapopulation studies have been undertaken and few raptor species have been investigated. Using the strongly size-dimorphic northern goshawk (Accipiter gentilis L.) as a model, we studied a population with one of the highest densities of breeding pairs reported in the literature in order to understand selective pressures that may favour RSD. We evaluated life-history processes, including recruitment of adult breeders and reproductive success, and we explored the mechanisms thought to act on each sex, including hunting efficiency, diet, body condition and mate choice. We found that smaller males produced more fledglings than larger ones, but there was no relationship between size and reproductive success for females. The mean body size of female breeders was larger than that of female fledglings, but male fledglings and breeders did not differ in size. Male body size was related to the type but not to the amount of prey captured during the nestling stage. We conclude that RSD may be favoured in this goshawk population because small males tend to enjoy higher reproductive success and large females greater recruitment. Our results do not support the hypotheses that evolutionary reduction in male size is driven by hunting efficiency, at least during the nestling stage, or the hypotheses that it is driven by greater recruitment. Our findings also suggest that increase in female size is driven by recruitment, rather than by reproductive success as previously postulated.

Implications of Male and Female Contributions to Sexual Size Dimorphism for Inferring Behavior in the Hominin Fossil Record

Sexual dimorphism is commonly used to directly infer or support reconstructions of social behavior in early hominins. This is often done by comparing the magnitude of sexual size dimorphism to that seen in extant primates and extrapolating a likely social behavior. Such comparisons are of limited value, though, allowing only the inference of strong male–male competition when dimorphism is strong. Recent studies have begun to focus on the selective factors that impact female body size, and thereby size dimorphism. Considerations of changes in male and female size in the fossil record potentially allow insight into the meaning of changes in sexual dimorphism through time. To illustrate, I compare estimates of body mass dimorphism for four hominin taxa to assess changes in male and female size. Assuming that early Homo represents a single taxon, sexual size dimorphism increased in early Homo through an increase in male size, but was subsequently reduced through an increase in female size in Homo erectus. This would imply a significant increase in sexual selection acting on males in early Homo. An increase in female size with a loss of dimorphism in Homo erectus would imply a simultaneous shift in female optimal body size through selection for increased female fecundity, and/or an increase in female resource abundance, coupled with a shift in selection acting on male size. Although none of these inferences are certain, the exercise illustrates the potential for considering how dimorphism changes through time, rather than simply focusing on the magnitude of size dimorphism in isolation.

Sexual size and shape evolution in European newts (Amphibia: Caudata: Salamandridae) on the Balkan Peninsula

We used a phylogenetic perspective in an examination of the direction and extent of sexual dimorphism in body size and body shape in European newts from the Balkan Peninsula (alpine newts, Mesotriton alpestris; crested newts, Triturus cristatus superspecies; smooth newts, Lissotriton vulgaris). We found a strong, female-biased sexual size dimorphism (SSD) in the analysed clades of alpine newt, whereas within crested newts we found a less stringent female-biased SSD in Triturus carnifex, Triturus macedonicus and Triturus karelinii, and no significant SSD in T. cristatus or Triturus dobrogicus. Among the smooth newts, we found male-biased SSD in Lissotriton vulgaris vularis and Lissotriton vulgaris greacus and no SSD in Lissotriton vulgaris meridionalis. Most of these newts also exhibit a significant sexual dimorphism in body shape, which varied more randomly than body size, regardless of SSD level. Female and male body size as well as the degree of SSD displayed statistically significant phylogenetic signal, while sexual dimorphism in body shape was phylogenetically independent. The relationship between independent contrast data for female size and male size indicated that SSD in European newts could be driven by a disproportionate increase in female size as increase in female size was not accompanied by a proportional increase in male size.

EVOLUTION OF BODY SIZE IN THE MAP TURTLES AND SAWBACKS (EMYDIDAE: DEIROCHELYINAE: GRAPTEMYS)

Map turtles and sawbacks (Emydidae: Deirochelyinae: Graptemys) are a diverse group of turtles that are of ecological interest due to their diversity in trophic morphology, particularly in females, and their extreme sexual size dimorphism (with females larger). I used comparative analyses (independent contrasts in correlation analyses and GLM analyses) to examine hypotheses regarding the evolution of body size in the genus. Evolutionary changes in body size of both males and females were positively related to inferred shifts in latitude. Trophic morphology (relative head width, expressed either as a continuous or a discrete variable) was not an additive source of variation in male body size, but was for female body size, primarily due to the large body sizes exhibited by the megacephalic southern clade (G. pulchra, G. ernsti, G. gibbonsi, and G. barbouri). Status as allopatric or sympatric to other species of Graptemys was not an additive source of variation in female body size. These results argue against the hypothesis that character displacement of body size in females was important in the radiation of map turtle and sawback species and instead suggest a functional relationship whereby degree of molluscivory in females covaries with body size. Sexual size dimorphism was found to increase with body size of females, also due primarily to the large females of the megacephalic clade; this result means Graptemys is an exception to Rensch's Rule. In a latitude-corrected analysis of body size evolution in deirochelyine turtles, the exceptional degree of sexual size dimorphism in Graptemys appears to result more from reduction in male size than increase in female size. Decreased male size may indicate relaxed selection for large body size in Graptemys males as a consequence of the fact that they rarely leave the water for terrestrial excursions, because selection for larger male body size in other deirochelyines may be mediated by (a) predation pressure imposed by terrestrial predators, (b) enhancement of overland mobility for mate searching, and (c) the need for resistance to desiccation during overland excursions. Alternatively, male body size reduction in Graptemys may be explained by energetic requirements of searching for mates in the fluvial environment or unknown differences in social structure among deirochelyine turtles.

Site -2548 of the leptin gene is associated with gender-specific trends in newborn size and cord leptin levels.

Circulating leptin levels positively correlate with adult BMI and size at birth. Previous studies found gender-specific associations between polymorphisms in the leptin gene and postnatal obesity-related traits or circulating leptin levels. We examined the relationships among leptin gene polymorphisms, size for gestational age, umbilical cord leptin, and gender. Six single nucleotide polymorphisms (SNPs) were genotyped in the leptin gene in 261 newborns (72 low birth weight Caucasians, 189 randomly-selected African-Americans). In African-Americans, umbilical cord leptin and free testosterone levels were measured. Linear regression was used to identify significant predictors of size for gestational age or cord leptin levels and gender x genotype interaction effects. There is a significant interaction between gender and genotype at site -2548 (A/G). Among low birth weight Caucasians, the A allele was associated with an increase in female size for gestational age, while the A allele was associated with decreased male birth size. Among African-Americans, the A allele was associated with a decrease in umbilical cord leptin in females and with an increase in cord leptin in males. Cord testosterone levels were not a significant predictor of cord leptin levels either among all African-American newborns or among strata of -2548 genotypes and gender. In male and female fetuses, site -2548 of the leptin gene may differently affect the expression level of the leptin gene or the rate of fetal growth. This gender-specific effect does not appear to be mediated by the level of free testosterone at delivery.

Sexually antagonistic selection on primate size

Abstract Male intrasexual selection in haplorhine primates has previously been shown to increase male size and to a lesser degree also female size. I address the following questions: (1) why does female size increase when the selection is on males, and (2) why does female size not increase to the same extent as that of males. The potential for correlational selection on females through increased resource competition was analysed with independent contrasts analyses. No such effect was found, nor did matched pairs comparisons reveal females to increase in size because of selection to bear larger male offspring. Instead further matched pairs analyses revealed higher female postpartum investment, as indicated by a longer lactation period, in more sexually selected species, also after correcting for body weight. Concerning the second question, independent contrast analyses showed that large size has had negative effects on female reproductive rate across the primate order. Matched-pairs analyses on haplorhines revealed that females of species in more polygynous clades have lower reproductive rates than females of species in less polygynous clades. This is also true after the effects of body weight are removed. These results, both when correcting for body weight and when not, suggest that sexual selection has shifted female size from one favouring female lifetime fecundity to one favouring male success in competition. This depicts antagonistic selection pressures on female size and a trade-off for females between the ecologically optimal size of their foremothers and the larger size that made their forefathers successful.

The evolution of egg size in socially polyandrous shorebirds

In classical and multi‐clutch polyandry, females lay multiple clutches during a breeding season for more than one mate. The production of multiple clutches may be energetically demanding. We used comparative analyses to investigate three possible ways of reducing such egg‐laying costs in polyandrous shorebirds: (1) reduction in egg size, (2) reduction in clutch size, and (3) evolutionary increase in female size. Paired comparisons of polyandrous and non‐polyandrous taxa showed that females of polyandrous shorebirds lay smaller eggs than females of closely related monogamous and polygynous species. Directional analyses corroborated this result by indicating a significant decrease in egg size after phylogenetically independent origins of polyandry. The comparative analyses uniformly rejected the two alternatives, i.e. neither clutch size nor female size is related to social mating pattern. We also tested and rejected three alternative explanations for reduced egg size in polyandrous taxa. First, we found no evidence that polyandrous females have evolved smaller egg sizes in response to selection to match smaller size of males, which provide the parental care in these species. Second, reduction in egg size was not related to longer breeding seasons (and hence more opportunity for re‐nesting). Third, reduced egg sizes were also not related to rates of clutch predation (another potential correlate of multiple clutch production). Our results are thus consistent with the hypothesis that selection for reducing laying costs explains small egg size in socially polyandrous shorebirds.

The phylogenetic basis of sexual size dimorphism in orb-weaving spiders (Araneae, Orbiculariae).

Extreme sexual body size dimorphism (SSD), in which males are only a small fraction of the size of the females, occurs only in a few, mostly marine, taxonomic groups. Spiders are the only terrestrial group in which small males are relatively common, particularly among orb-weavers (especially in the families Tetragnathidae and Araneidae) and crab spiders (Thomisidae). We used a taxonomic sample of 80 genera to study the phylogenetic patterns (origins and reversals) of SSD in orb-weaving spiders (Orbiculariae). We collected and compiled male and female size data (adult body length) for 536 species. Size data were treated as a continuous character, and ancestral sizes, for males and females separately, were reconstructed by using Wagner parsimony on a cladogram for the 80 genera used in this study. Of these 80 genera, 24 were female-biased dimorphic (twice or more the body length of the male); the remaining 56 genera were monomorphic. Under parsimony only four independent origins of dimorphism are required: in the theridiid genus Tidarren, in the distal nephilines, in the "argiopoid clade," and in the araneid genus Kaira. Dimorphism has reversed to monomorphism at least seven times, all of them within the large "argiopoid clade." The four independent origins of dimorphism represent two separate instances of an increase in female size coupled with a decrease of male size (involving only two genera), and two separate instances of an increase in female size with male size either remaining the same or increasing, but not as much as females (involving 30 genera). In orb-weaving spiders, far more taxa are sexually dimorphic as a result of female size increase (22 genera) than as a result of male size decrease (two genera). SSD in orb-weaving spiders encompasses several independent evolutionary histories that together suggest a variety of evolutionary pathways. This multiplicity strongly refutes all efforts thus far to find a general explanation for either the origin or maintenance (or both) of SSD, because the different pathways very likely will require distinctly different, possibly unique, explanations. Each pattern must be understood historically before its origin and maintenance can be explained in ecological and evolutionary terms. The most frequently cited example of male dwarfism in spiders, the golden orb-weaving spider genus Nephila (Tetragnathidae), is in fact a case of female giantism, not male dwarfism.

Indoor Spawning and Egg Development of the Red Claw Crayfish Cherax quadricarinatus

AbstractIn 1991 and 1992, indoor hatchery experiments were conducted with red claw crayfish Cherax quadricarinatus a large tropical crayfish from northern Australia. Six fiberglass tanks were stocked with 40–190 g mature red claw crayfish at 10/m2 and a 1:2 male to female ratio. Water temperature was maintained at 28 ± 1 C. Photoperiod was provided by natural daylight in 1991 and controlled lighting in 1992.With natural lighting, monthly spawning rates (% of females spawning/month) ranged from an average of 15% during months with less than 12 h of light per day to over 35% in months with more than 12 h of light per day. Peak spawning occurred from May to July, too late to obtain juveniles for spring stocking. During the second year, photoperiod was increased from 10L:14D in December to 14L:10D in February and maintained at this level until July. Red claw spawning increased with increasing day length. Peak spawning rates of 50% per month and higher occurred from March to May. Spawning rates decreased dramatically after May, even though photoperiod and temperature remained constant. Eggs per spawn increased with an increase in female size. Fecundity average 7.3 juveniles per gram of body weight for females from 40 to 190 g, regardless of size. Egg color was useful for predicting stage of development.

The Survival-Reproduction Selection Equilibrium and Reversed Size Dimorphism in Raptors

A variety of avian taxa contain groups with 'reversed' (i.e. males smaller than females) sexual size dimorphism (Jehl and Murray 1986). Of all these taxa, its occurrence in the Falconiformes (hawks, eagles, falcons), the Strigiformes (owls) and the Stercorariidae (skuas) has received particular attention in the literature. These three independently evolved taxa have very different biologies but share predatory habits and, to a lesser degree, species in each show partial or complete sexual role specialization during breeding with males provisioning and females nest-tending. In other avian taxa reversed sexual dimorphism seems largely attributable to female-female competition for mates, but this explanation has seldom been applied to the hawks, owls and skuas (cf. Olsen and Olsen 1987). The diverse hypotheses that have been proposed (see reviews in Andersson and Norberg 1981, Mueller and Meyer 1985) are usually linked to the predatory nature of these birds. Many consider only a single selection pressure, operating only on one sex, without considering opposing selective forces. Arak (1988) has criticized this approach, pointing out that sexual dimorphism must arise from selection pressures for and against changes in body size acting differently on the two sexes (Ralls 1976, Price 1984). In this paper we use the framework for the evolution of sexual size dimorphism set out by Arak (1988) in an attempt to identify how different selection pressures acting on the two sexes could have resulted in the evolution of reversed sexual dimorphism in hawks, owls and skuas. We do not attempt a comprehensive review. Rather, we hope to convince the reader of the usefulness of Arak's approach: it easily accommodates the existing hypotheses, but helps make them more comprehensive, and it aids in identifying testable predictions not previously evident. We also modify and extend an existing hypothesis and show how, when used in the context laid out here, it provides explanations for broad patterns, as well as some of the exceptions, of sexual dimorphism in the groups considered. Hypothesis for reversed sexual dimorphism in predatory birds can be sorted into three categories, based on an evolutionary increase in female size, a decrease in male size, or a divergence in male and female size. The 'large female' hypotheses are (using the terminology in Andersson and Norberg's (1981) Table 1): the anticannibalism hypothesis, the male subordination hypothesis, the pair formation hypothesis, the egg size hypothesis, the incubation efficiency hypothesis, the nest protection hypothesis, the follicle protection hypothesis, and the starvation hypothesis. All these hypotheses assume that selection on breeding success favors large body size in females. In contrast, other hypotheses assume that the breeding success of pairs is enhanced when males are small. The 'small male', hypotheses are the energy savings hypothesis, the pyramid of prey hypothesis, the territorial defense hypothesis, and the courtship hypothesis. The 'dimorphic niche' hypotheses maintain that breeding pairs divergent in size are better able to exploit the prey base. They include the food competition and female supplementary feeding hypotheses. Descriptions of all these hypotheses, plus their sources, can be found in Andersson and Norberg (1981), with the exception of the starvation (see Lundberg 1986) and courtship (see Jehl and Murray 1986) hypotheses. Arak's (1988, see also Hedrick and Temeles 1989) framework for the evolution of sexual dimorphism considers natural selection acting on the survival and the reproductive consequences of body size. At the equilibrium body size in either sex, the fitness consequences of

The Energetics of Plant Parasitic Nematodes-a Review

The mean size of tylenchid species that lack saccate females is significantly less than that of the bacterial-feeding Rhabditida. This suggests that the body size of these plant parasites is limited by a factor other than that imposed on all nematodes by the soil environment. The trophic mode of feeding with a stomatostylet may limit the food intake of some species because a limited maximum number of cells can be attacked each day and each possesses a relatively small volume of extractable contents. The major adaptation overcoming this limitation is the ability of several genera with saccate females to modify and enlarge the plant cells on which they feed. The associated increase in female size results in enhanced fecundity rather than an increase in egg size, which seems unnecessary for either prolonged dormancy or successful invasion of plants after hatching. The modification of plant cells by saccate genera raises feeding rates to an extent that growth of large females does not overprolong the developmental period. Feeding by these saccate females at high density may have a pathogenic effect by reducing the energy available for root growth and maintenance. Further study of the energetics of tylenchids may contribute to our understanding of the pathogenicity of these important plant parasites.

Reproductive Tactics and Female Body Size in Butler's Garter Snake, Thamnophis butleri

Reproductive strategies of snakes are complicated by the fact that these animals have indeterminate growth. Larger females can produce more offspring, but whether female size affects the size of individual offspring has not been carefully examined. In the viviparous garter snake, Thamnophis butleri, measures of neonate size (snout-vent length, mass or mass/SVL) were posi- tively correlated with measures of female size (length, mass, or mass/SVL) and negatively correlated to clutch size. Relative clutch mass (RCM: total clutch mass/female mass, an operational measure of reproductive effort) did not change with increase in female size. However, RCM per progeny decreased with female size, which would allow larger females to produce clutches of larger numbers with less effect on neonate size.