Variation In Male Size Research Articles

The effect of genetics, diet, and social environment on adult male size in a sexually dimorphic spider

The role of developmental plasticity in the evolution and maintenance of sexual size dimorphism (SSD) has recently received more attention. We experimentally investigated the effects of genetics (pedigree), social cues, and food availability on developmental time and adult male size in Nephilingis cruentata, an extremely female-biased sexually size dimorphic spider with notable male size variation. In a split-brood design, we exposed spiderlings of known pedigrees to either a high or low feeding regime. We tested the males’ ability to match the sub-adult growth and time of maturation to the perceived female availability and male competition by exposing them to silk cues of either males or females during the subadult stage. We recorded male size at maturation and total developmental time, the duration of the sub-adult stage, and the growth during the sub-adult stage. Poorly fed males had a longer development and matured at extremely small sizes compared to well-fed males. The social cues did not influence the duration of the sub-adult stage nor the male size at maturation. However, males exposed to male cues grew more and were heavier at reaching maturity than those exposed to female cues, which implies that sub-adult males respond to perceived male–male competition by investing more in growth. Furthermore, variation in male size has been explained by low additive genetic variability but high maternal effects. Our results highlight the role of maternal effects and/or common environment in shaping male body size. Future studies with a scope for maternal effects on SSD are warranted.

Seasonal Reproductive Dynamics of a Lek-Breeding Neotropical Treefrog is not Organized by Male Size (Anura, Hylidae)

Social structure in anurans is directly related to reproductive traits such as chorus organization, male territoriality, pair formation, breeding site choice, and temporal breeding pattern. The Neotropical treefrog Dendropsophus sanborni is an excellent model for studies on reproductive dynamics because it is a prolonged-breeder and usually occurs in high densities. Here, our goals were to better understand the factors that lead to social organization and influence seasonal reproductive dynamics of males of the species. We addressed the hypotheses that (1) the seasonal dynamics of D. sanborni is locally delimited by climatic conditions throughout its breeding season and (2) territorial males are structured spatially and temporally based on body size. We carried out monthly field sampling between October 2015 and September 2016 in an ecotone region of Brazil that originally comprised Semideciduous Seasonal Forest and Cerrado (savannah vegetation). We specifically analyzed environmental conditions, population abundance, size of calling males, and microhabitat use. Our results showed that environmental conditions drive the seasonal reproductive pattern and social structure of calling males is not organized by size. We did not find significant variation in male size among months of the breeding season or among perch heights and types. Our findings suggest a prolonged seasonal reproductive pattern associated with climatic factors without a social structure based on body size.

Male preference for conspecific females depends on male size in the splendid darter, Etheostoma barrenense

Mating preferences vary among individuals in response to a number of factors, such as the age and phenotype of the choosing individual. However, studies addressing variation in mating preferences generally focus on female preferences and usually are limited to choice within a species. Variation in preferences likely also exists at the species boundary, affecting an individual’s propensity to mate with heterospecific individuals. In darters (Percidae: Etheostoma), male mating preferences are thought to prevent interbreeding between species, with male preferences for conspecifics sometimes as strong as, or stronger than, females’. In the current study, we tested whether male size and coloration predicts preference for conspecific females in Etheostoma barrenense. Previous studies have shown that males of this species strongly prefer conspecific females relative to females of the sympatric congener, the banded darter, Etheostoma zonale; however, those studies did not consider whether variation in male size or coloration affected preferences. Comparing large and small males of E. barrenense in dichotomous choice assays, we found that larger males exhibited stronger preferences for conspecific females and had proportionally more red coloration than smaller males. The proportion of red coloration did not significantly predict male preferences, suggesting that male size rather than male coloration predicts variation in male mating preferences for conspecific females in E. barrenense. Male size is often considered an indicator of individual quality. Alternatively, size might better reflect age in the focal species, suggesting that experience and learning may shape male preferences for conspecific females.

Variation in body size and sexual size dimorphism in the most widely ranging lizard: testing the effects of reproductive mode and climate.

Reproductive mode, ancestry, and climate are hypothesized to determine body size variation in reptiles but their effects have rarely been estimated simultaneously, especially at the intraspecific level. The common lizard (Zootoca vivipara) occupies almost the entire Northern Eurasia and includes viviparous and oviparous lineages, thus representing an excellent model for such studies. Using body length data for >10,000 individuals from 72 geographically distinct populations over the species' range, we analyzed how sex‐specific adult body size and sexual size dimorphism (SSD) is associated with reproductive mode, lineage identity, and several climatic variables. Variation in male size was low and poorly explained by our predictors. In contrast, female size and SSD varied considerably, demonstrating significant effects of reproductive mode and particularly seasonality. Populations of the western oviparous lineage (northern Spain, south‐western France) exhibited a smaller female size and less female‐biased SSD than those of the western viviparous (France to Eastern Europe) and the eastern viviparous (Eastern Europe to Far East) lineages; this pattern persisted even after controlling for climatic effects. The phenotypic response to seasonality was complex: across the lineages, as well as within the eastern viviparous lineage, female size and SSD increase with increasing seasonality, whereas the western viviparous lineage followed the opposing trends. Altogether, viviparous populations seem to follow a saw‐tooth geographic cline, which might reflect the nonmonotonic relationship of body size at maturity in females with the length of activity season. This relationship is predicted to arise in perennial ectotherms as a response to environmental constraints caused by seasonality of growth and reproduction. The SSD allometry followed the converse of Rensch's rule, a rare pattern for amniotes. Our results provide the first evidence of opposing body size—climate relationships in intraspecific units.

A biogeographic reversal in sexual size dimorphism along a continental temperature gradient

The magnitude and direction of sexual size dimorphism (SSD) varies greatly across the animal kingdom, reflecting differential selection pressures on the reproductive and/or ecological roles of males and females. If the selection pressures and constraints imposed on body size change along environmental gradients, then SSD will vary geographically in a predictable way. Here, we uncover a biogeographical reversal in SSD of lizards from Central and North America: in warm, low latitude environments, males are larger than females, but at colder, high latitudes, females are larger than males. Comparisons to expectations under a Brownian motion model of SSD evolution indicate that this pattern reflects differences in the evolutionary rates and/or trajectories of sex‐specific body sizes. The SSD gradient we found is strongly related to mean annual temperature, but is independent of species richness and body size differences among species within grid cells, suggesting that the biogeography of SSD reflects gradients in sexual and/or fecundity selection, rather than intersexual niche divergence to minimize intraspecific competition. We demonstrate that the SSD gradient is driven by stronger variation in male size than in female size and is independent of clutch mass. This suggests that gradients in sexual selection and male–male competition, rather than fecundity selection to maximize reproductive output by females in seasonal environments, are predominantly responsible for the gradient.

Relatively large males lower reproductive success in female zebrafish

Females can adjust their reproductive effort in relation to their partner’s perceived fitness value. In zebrafish (Danio rerio), large males are typically preferred mating partners. However, females have been observed to reduce their reproductive output with exceptionally large males but it remains unknown whether it is due to sexual harassment or aggressive behavior to establish and maintain dominance. Here, we study the association between relative male size, sexual harassment and dominance behavior, female stress status (stress behaviors and whole-body cortisol concentration), and reproductive success during a 4 day spawning trial. We found female cortisol to correlate negatively with female body size and positively with female dominance behavior. However, male and female behavior as well as female cortisol level were not related to relative male size. Females mating with relatively large males produced more and most of their eggs during the first spawning day, while females with smaller males produced few eggs during the first day but then increased egg production. Despite females produced more eggs when mating with relatively larger males, their eggs had substantially lower fertilization rates compared to females mating with relatively smaller males. Hence, overall, the reproductive fitness was lowest when females mated with a relatively large male. These findings could help to explain the maintenance of male size variation under natural conditions.

Multiple mechanisms of cryptic female choice act on intraspecific male variation in Drosophila simulans

Postcopulatory sexual selection can arise when females mate with multiple males and is usually mediated by an interaction between the sexes. Cryptic female choice (CFC) is one form of postcopulatory sexual selection that occurs when female morphology, physiology, or behavior generates a bias in fertilization success. However, its importance in nonrandom reproductive success is poorly resolved due to challenges distinguishing the roles of females and males in generating patterns of fertilization bias. Nevertheless, two CFC mechanisms have recently been documented and characterized in Drosophila simulans within the context of gametic isolation in competitive hybrid matings with Drosophila mauritiana: sperm ejection and nonrandom use of sperm storage organs for fertilization. Here, we explore if and how female D. simulans employ these two mechanisms of CFC in response to intraspecific male size variation. We used transgenic males expressing green (GFP) or red fluorescent protein (RFP) in sperm heads to document postcopulatory processes, in conjunction with a probabilistic analytical model. We unexpectedly found that differential reproductive success was also a function of male population (GFP or RFP), suggesting that females use different CFC mechanisms to select for different male traits. Moreover, concordance of selection at the precopulatory (as measured by mating latency) and postcopulatory stages depends on both the male trait and the CFC mechanism examined. Larger males were more successful both before and after mating, but we unexpectedly found that females also mated more quickly with males with GFP-labeled sperm, while fertilization bias favored RFP-labeled sperm.

Equal temperature-size responses of the sexes are widespread within arthropod species.

Sexual size dimorphism (SSD) is often affected by environmental conditions, but the effect of temperature on SSD in ectotherms still requires rigorous investigation. We compared the plastic responses of size-at-maturity to temperature between males and females within 85 diverse arthropod species, in which individuals of both sexes were reared through ontogeny under identical conditions with excess food. We find that the sexes show similar relative (proportional) temperature-body size (T-S) responses on average. The high degree of similarity occurs despite an analysis that includes a wide range of animal body sizes, variation in degree of SSD and differences in the sign of the T-S response. We find no support for Rensch's rule, which predicts greater variation in male size, or indeed the reverse, greater female size variation. SSD shows no systematic temperature dependence in any of the 17 arthropod orders examined, five of which (Diptera, Orthoptera, Lepidoptera, Coleoptera and Calanoida) include more than six thermal responses. We suggest that the same proportional T-S response may generally have equivalent fitness costs and benefits in both sexes. This contrasts with effects of juvenile density, and food quantity/quality, which commonly result in greater size plasticity in females, suggesting these variables have different adaptive effects on SSD.

Differential investment and size-related mating strategies facilitate extreme size variation in contesting male spiders

Sexually selected variation in male body size is often associated with alternative reproductive strategies that may persist under frequency-dependent selection or result from ‘making the best of a bad job’ in inferior individuals. The spider genus Nephila is well known for female gigantism and male dwarfism, serving as an established model system to investigate selection on male size and the still enigmatic sexual size dimorphism (SSD). Less attention has been paid to the enormous size variation among males that exists in some Nephila species despite broad evidence for large male advantages in contest competition. As the existing variation contradicts strongly biased fitness returns among different phenotypes, detailed investigations of fitness consequences related to different body sizes and their associated mating strategies are required. We used the African golden-silk spider Nephila senegalensis to test whether size-dependent alternative mating strategies yield equivalent reproductive success in dyads of size-mismatched males competing for a single female. Our results confirm that differently sized males achieve similar paternity and reveal complex interrelations between competing strategies, which mutually influence each other. Males integrate their own competitive ability, the intensity of sperm competition and female reproductive value to adjust their mating investment accordingly. Size-related mating strategies, combined with behavioural flexibility, may thus promote the maintenance of male size variation by balancing fitness returns and exemplify how changes in selective context can dissolve the principal large-size benefits. We suggest that flexible adaptive responses may significantly contribute to stabilizing SSD.

Differential Allocation by Female Zebrafish (Danio rerio) to Different-Sized Males – An Example in a Fish Species Lacking Parental Care

Organisms allocate resources to reproduction in response to the costs and benefits of current and future reproductive opportunities. According to the differential allocation hypothesis, females allocate more resources to high-quality males. We tested whether a fish species lacking parental care (zebrafish, Danio rerio) expresses male size-dependent differential allocation in monogamous spawning trials. In addition, we tested whether reproductive allocation by females is affected by previous experience of different-quality males, potentially indicating plasticity in mate choice. To that end, females were conditioned to large, small or random-sized males (controls) for 14 days to manipulate females' expectations of the future mate quality. Females showed a clear preference for large males in terms of spawning probability and clutch size independent of the conditioning treatment. However, when females experienced variation in male size (random-sized conditioning treatment) they discriminated less against small males compared to females conditioned to large and small males. This might suggest that differential allocation and size-dependent sexual selection is of less relevance in nature than revealed in the present laboratory study.

Individual Variation in Male Size and Behavioral Repertoire in the Sailfin Molly Poecilia latipinna

AbstractVariation among individuals in the expression of behaviors and associations of behaviors in different contexts can lead to the maintenance of behavioral polymorphisms. Individual variation in morphology is often associated with behavioral polymorphism, yet the degree to which morphology predicts behavioral phenotype is not well understood. We measured individual variation in size and behaviors in the sailfin molly, Poecilia latipinna, by comparing the behavior of individual males of different sizes across four different contexts (mating, exploratory tendency, sociability, and predator inspection). We also investigated the degree to which male size, a fixed genetic trait, influenced the expression of each behavior and associations between behaviors. We found that male mollies show strong associations between certain behaviors and only some of these are predicted by male size. For example, size has a strong influence on the courtship‐boldness axis with larger males showing higher rates of courtship displays and being bolder in predator inspection. A positive association was found between exploratory tendency, sociability, and gonopodial thrusting rates, yet the expression of these behaviors was independent of male size. Thus, sailfin mollies, like many fish species, show associations of behaviors that contribute to differences among males in personality type. The fixed genetic effect of male size at maturity influences courtship and boldness, but individual variation in exploratory tendency, sociability, and sneak copulation attempts through gonopodial thrusts is independent of male size. Such variation among males in behavioral associations within and between different contexts may slow the rate at which populations of P. latipinna can diverge in individual behaviors.

Testing ecological and developmental hypotheses of mean and variation in adult size in nephilid orb-weaving spiders

Fecundity selection has been hypothesized to drive the evolution of female gigantism in the orb-weaving family Nephilidae. Several species of these spiders also exhibit large amounts of variation in size at maturity in one or both sexes. In this article, we attempt to detect correlations of mean and variation in adult size at a phylogenetic scale between the sexes and with latitude. We tested six predictions derived from three broad developmental, ecological, and age structure hypotheses, using independent contrasts and a recent species-level nephilid phylogeny as well as least squares and other conventional statistics: 1. In both sexes, species with larger mean size will have greater variation in size; 2. Males and females will show correlated changes in mean size and of variation in size; 3. In both sexes, mean size will be negatively correlated with the midpoint of the latitudinal range; 4. In both sexes, tropical species will be more variable; 5. In both sexes, more widespread species will be more variable; 6. Variation in male size will be positively correlated with mean female size. In no cases were male and female development correlated, suggesting that in this lineage male and female body size evolve independently. The only significant trend detected was a positive phylogenetic correlation between variation in female size and latitude, the opposite of prediction 4. Power tests showed that in all tests of the ecological hypothesis, sample sizes were more than adequate to detect significant trends, if present. Our results suggest that evolutionary trends in juvenile development among species are too weak to be detectable in such data sets.

Does divergence in female mate choice affect male size distributions in two cave fish populations?

Sexual selection by female choice can maintain male traits that are counter selected by natural selection. Alteration of the potential for sexual selection can thus lead to shifts in the expression of male traits. We investigated female mate choice for large male body size in a fish (Poecilia mexicana) that, besides surface streams, also inhabits two caves. All four populations investigated, exhibited an ancestral visual preference for large males. However, only one of the cave populations also expressed this female preference in darkness. Hence, the lack of expression of female preference in darkness in the other cave population leads to relaxation of sexual selection for large male body size. While P. mexicana populations with size-specific female mate choice are characterized by a pronounced male size variation, the absence of female choice in one cave coincides with the absence of large bodied males in that population. Our results suggest that population differences in the potential for sexual selection may affect male trait variation.

Size-dependent mating strategies and the risk of cannibalism

The role of sexual selection in determining the nature and direction of sexual size dimorphism may depend upon the timing of sexual selection, and this may also influence the variation in male size. For example, selection through sperm competition favours smaller males in the highly sexually size dimorphic orb-weaving spider Nephila edulis, whereas larger males are better able to exclude their smaller rivals from the central hub of the web where mating takes place. We investigate experimentally the role of body size and hub tenure in determining male fertilization success when males of different sizes compete for a single female over a 24-h period that includes a period of darkness. Our results confirm that small and large males obtain similar paternity share but that, in contrast with previous studies, hub tenure does not translate into greater paternity share. Unexpectedly, smaller males are at greater risk of postmating sexual cannibalism than larger males, suggesting that natural selection through sexual cannibalism may place a lower limit on male size.

Within and Between Species Variation in Male Mating Behaviors in the Mexican Sailfin Mollies Poecilia velifera and P. petenensis

AbstractPolymorphism in male morphology is often correlated with the expression of alternate behavioral tactics. This relationship between behavioral and morphological polymorphisms, however, is less well understood. We characterized male mating behaviors and morphological variation within and between Poecilia velifera and P. petenensis to understand mating signal evolution in the sailfin molly lineage. In addition, we examined whether differences between these species in the size range of mature males and the strength of allometry between dorsal fin size and body length could explain the variation observed in their expression of different mating behaviors. We determined each male's mating behavior profile by observing the behavior of a single male in the presence of a receptive female. We found that P. velifera showed evidence of an alternate male mating strategy, with small males generally performing only gonopodial thrusts (forced insemination attempts) towards receptive females, while large males performed courtship displays as well as gonopodial thrusts. Males of P. petenensis performed similar rates of courtship displays and gonopodial thrusts regardless of body length. Little variation existed among populations of P. velifera in mating behaviors, while males from different populations of P. petenensis showed population‐specific average rates of each mating behavior. Population differences in P. petenensis may be driven, in part, by its occurrence in more variable habitats than those occupied by P. velifera. Variation among individuals in the mating repertoire of P. velifera, but not P. petenensis, suggests that the greater range of variation in male size at maturity, as well as considerably stronger allometry between dorsal fin size and body length, may explain why males of P. velifera show the greatest degree of expression of alternate male mating behaviors when compared to other sailfin species. These results also suggest an important role of morphological polymorphisms in predicting the expression of alternate male mating behaviors.

Genetic and environmental effects on secondary sex traits in guppies (Poecilia reticulata)

Male guppies (Poecilia reticulata) exhibit extreme phenotypic and genetic variability for several traits that are important to male fitness, and several lines of evidence suggest that resource level affects phenotypic expression of these traits in nature. We tested the hypothesis that genetic variation for male secondary sex traits could be maintained by genotype-specific effects of variable resource levels (genotype-environment interaction). To do this, we measured genetic variation and covariation under two environmental conditions--relatively low and relatively high food availability. We found high levels of genetic variation for most traits, but we only found a significant G x E interaction across food levels for one trait (body size) for one population. The across-environment correlations for size were large and positive, indicating that the reaction norms for size did not cross. We also found that male colour pattern elements had nearly an order of magnitude more genetic variation than did male size. Heritability estimates indicated that Y-linked genes are responsible for some of the genetic variation in male size and colour traits. We discuss implications of these results for theories of the maintenance of genetic variation in male secondary sexual traits in guppies.

Ecological context and the importance of body and gnathopod size for pairing success in two amphipod ecomorphs

Ecological context generates interspecific variation in mating behavior by imposing differential constraints on the action of sexual selection, but operation of these constraints in nature is not well understood. We used field and laboratory studies to examine the importance of body size and size of sexually dimorphic appendages, the gnathopods, for pairing success in two freshwater amphipod species within the Hyalella azteca species complex. We focused on a large-bodied species found in habitats where ecological factors tend to favor large body size, and a small-bodied species in habitats where small body size is favored by size-selective predation. A field study indicated that although male pairing success was greater for larger males in both species, pairing success increased throughout the range of variation in male size in the large species, whereas, in the small species, pairing success was low for smaller individuals, but roughly constant across intermediate to larger sizes. A laboratory mate choice experiment was consistent with the field study, finding that females of the large species exhibited a preference for larger males that was independent of absolute male size, but females of the small species were indifferent when choosing between males of intermediate to larger size. Species also differed in the direction of sexual size dimorphism in the field, with males being the larger sex in the large species but the smaller sex in the small species, a pattern consistent with the species differences in mate preference. Large gnathopod size relative to body size was associated with enhanced pairing success across all body sizes for the large species, but, in the small species, large gnathopod size enhanced pairing success only in smaller males. Species differences in mating behavior appear consistent with change driven by differing forms of the interaction between sexual and natural selection.

Male copulation behaviour and the risk of sperm competition

When females mate with more than one male during their reproductive cycle, males may increase their share of paternity by copulating repeatedly with the same female. Accordingly, males should mate repeatedly with the same female more frequently when the risk of sperm competition is greater. We examined this idea experimentally in the orb-web spiderNephila edulis , which is characterized by both extreme sexual size dimorphism and extreme male size variation. Comparison of the mating behaviour of solitary and pairs of males on the webs of virgin and mated females revealed that males adjust the frequency and duration of copulation according to the mating history of the female and the presence of rival males. Males copulated more frequently and for longer with virgin than mated females. The copulation behaviour of males in the presence of rivals depended upon their relative size. Typically, larger males prevented smaller rivals from gaining access to the female and therefore were able to copulate more frequently. Smaller males copulated less frequently, but for longer periods, which may have increased their share of paternity. The size of male N.edulis can vary by an order of magnitude, and our results suggest that this variation may be maintained by the alternative size-dependent strategies of preventing or winning sperm competition. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.

Male choice, willingness to mate and body size in seaweed flies (Diptera: Coelopidae)

The mating system of seaweed flies is characterized by scramble competition and a premating struggle during which females vigorously attempt to remove mounted males and prevent copulation. Here we investigate factors affecting the willingness of males to mate in five species ofCoelopa:C.frigida, C.nebularum, C.ursina, C.pilipesandC.vanduzeei. The data from 2000 individual observations of pairs of flies revealed no association between female size and the willingness of males to mate, suggesting that males do not exhibit mate choice for larger, more fecund females. Male willingness to mount was associated with male size inC.frigida, C.nebularumandC.ursina, although not inC.pilipesandC.vanduzeei. There is considerable intraspecific variation in male size in the former three species, including the presence of two almost discrete size classes withinC.nebularum. We suggest that males of different sizes in these species may be adopting alternative reproductive strategies. Large males may benefit from increased longevity and their ability to withstand female rejection responses, whereas small males are more active and thus likely to encounter and mount more females. In addition small males develop faster and thus gain earlier access to females.

The Influence of Male Body Size and Social Environment on the Mating Behavior of Phallichthys quadripunctatus (Pisces: Poeciliidae)

AbstractIn some poeciliid fishes, variation in male size is accompanied by differences in mating behavior. Large males are preferred by females and perform courtship displays followed by copulatory thrusts, whereas small males perform copulatory thrusts with few or no displays. This phenomenon has been described in only a few genera and little is known about mating behavior in other poeciliids. Although Phallichthys quadripunctatus males display size dimorphism that has a genetic component, mating behavior of this species has not been documented. We conducted experiments using socially experienced and socially naive males to characterize the mating behavior of this species and to evaluate potential size‐dependent differences in behavior. Males were tested with postpartum (presumably receptive) and midcycle (presumably unreceptive) females in different social environments. Whereas neither size class of P. quadripunctatus males performed courtship displays or altered behavior based on female receptivity, large males performed several reproductive behaviors more frequently than small males. This trend was repeatable and occurred in all social environments examined. Some males also attempted to mate with other males, with small males showing a greater tendency to perform this behavior than large males. The manner in which differences in reproductive activity translate into differences in reproductive success must be examined before inferring sexual selection favoring large males in P. quadripunctatus.