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Data from: Nutritional control of body size through FoxO-Ultraspiracle mediated ecdysone biosynthesis

Despite their fundamental importance for body size regulation, the mechanisms that stop growth are poorly understood. In Drosophila melanogaster, growth ceases in response to a peak of the molting hormone ecdysone that coincides with a nutrition-dependent checkpoint, critical weight. Previous studies indicate that insulin/insulin-like growth factor signaling (IIS)/Target of Rapamycin (TOR) signaling in the prothoracic glands (PGs) regulates ecdysone biosynthesis and critical weight. Here we elucidate a mechanism through which this occurs. We show that Forkhead Box class O (FoxO), a negative regulator of IIS/TOR, directly interacts with Ultraspiracle (Usp), part of the ecdysone receptor. While overexpressing FoxO in the PGs delays ecdysone biosynthesis and critical weight, disrupting FoxO–Usp binding reduces these delays. Further, feeding ecdysone to larvae eliminates the effects of critical weight. Thus, nutrition controls ecdysone biosynthesis partially via FoxO–Usp prior to critical weight, ensuring that growth only stops once larvae have achieved a target nutritional status.

Data from: Biophysical model of water economy can explain geographic gradients in body size in anurans

Geographical gradients of body size express climate-driven constraints on animals, but whether they exist and what cause them in ectotherms remains contentious. For amphibians, the water conservation hypothesis posits that larger bodies reduce evaporative water loss (EWL) along dehydrating gradients. To address this hypothesis mechanistically, we build upon well-established biophysical equations of water exchange in anurans to propose a state-transition model that predicts an increase of either body size or resistance to EWL as alternative specialization along dehydrating gradients. The model predicts that species whose water economy is more sensitive to variation in body size than to variation in resistance to EWL should increase in size with response to increasing potential evapotranspiration (PET). To evaluate the model predictions, we combine physiological measurements of resistance to EWL with geographic data of body size for four different anuran species. Only one species, Dendropsophus minutus, was predicted to exhibit a positive body size–PET relationship. Results were as predicted for all cases, with one species – Boana faber – showing a negative relationship. Based on a mathematical model verified empirically, we show that clines of body size among anurans depend on the current values of those traits and emerges as an advantage for water conservation. Our model offers a mechanistic and compelling explanation for the cause and the variation of gradients of body size in anurans.

Data from: Convergence in reduced body size, head size, and blood glucose in three island reptiles

Many oceanic islands harbor diverse species that differ markedly from their mainland relatives with respect to morphology, behaviour, and physiology. A particularly common morphological change exhibited by a wide range of species on islands worldwide involves either a reduction in body size, termed island dwarfism, or an increase in body size, termed island gigantism. While numerous instances of dwarfism and gigantism have been well documented, documentation of other morphological changes on islands remains limited. Furthermore, we lack a basic understanding of the physiological mechanisms that underlie these changes, and whether they are convergent. A major hypothesis for the repeated evolution of dwarfism posits selection for smaller, more efficient body sizes in the context of low resource availability. Under this hypothesis we would expect the physiological mechanisms known to be downregulated in model organisms exhibiting small body sizes due to dietary restriction or artificial selection would also be downregulated in wild species exhibiting dwarfism on islands. We measured body size, relative head size, and circulating blood glucose in three species of reptiles—two snakes and one lizard—in the California Channel Islands relative to mainland populations. Collating data from six years of study, we found that relative to mainland population the island populations had smaller body size (i.e. island dwarfism), smaller head sizes relative to body size, and lower levels of blood glucose, though with some variation by sex and year. These findings suggest that the island populations of these three species have independently evolved convergent physiological changes (lower glucose set-point) corresponding to convergent changes in morphology that are consistent with a scenario of reduced resource availability and/or changes in prey size on the islands. This provides a powerful system to further investigate ecological, physiological, and genetic variables to elucidate the mechanisms underlying convergent changes in life history on islands.

Data from: Body size and allometric shape variation in the molly Poecilia vivipara along a gradient of salinity and predation

Background: Phenotypic diversity among populations may result from divergent natural selection acting directly on traits or via correlated responses to changes in other traits. One of the most frequent patterns of correlated response is the proportional change in the dimensions of anatomical traits associated with changes in growth or absolute size, known as allometry. Livebearing fishes subject to predation gradients have been shown to repeatedly evolve larger caudal peduncles and smaller cranial regions under high predation regimes. Poecilia vivipara is a livebearing fish commonly found in coastal lagoons in the north of the state of Rio de Janeiro, Brazil. Similar to what is observed in other predation gradients, lagoons inhabited by P. vivipara vary in the presence of piscivorous fishes; contrary to other poeciliid systems, populations of P. vivipara vary greatly in body size, which opens the possibility of strong allometric effects on shape variation. Here we investigated body shape diversification among six populations of P. vivipara along a predation gradient and its relationship with allometric trajectories within and among populations.ResultsWe found substantial body size variation and correlated shape changes among populations. Multivariate regression analysis showed that size variation among populations accounted for 66% of shape variation in females and 38% in males, suggesting that size is the most important dimension underlying shape variation among populations of P. vivipara in this system. Changes in the relative sizes of the caudal peduncle and cranial regions were only partly in line with predictions from divergent natural selection associated with predation regime.ConclusionsOur results suggest the possibility that adaptive shape variation among populations has been partly constrained by allometry in P. vivipara. Processes governing body size changes are therefore important in the diversification of this species. We conclude that in species characterized by substantial among-population differences in body size, ignoring allometric effects when investigating divergent natural selection?s role in phenotypic diversification might not be warranted.

Data from: Bison body size and climate change

The relationship between body size and temperature of mammals is poorly resolved, especially for large keystone species such as bison (Bison bison). Bison are well-represented in the fossil record across North America, which provides an opportunity to relate body size to climate within a species. We measured the length of a leg bone (calcaneal tuber, DstL) in 849 specimens from 60 localities that were dated by stratigraphy and 14C decay. We estimated body mass (M) as: M = (DstL/11.49) 3. Average annual temperature was estimated from δ18O values in the ice cores from Greenland. Calcaneal tuber length of Bison declined over the last 40,000 years, that is, average body mass was 37% larger (910 ± 50 kg) than today (665 ± 21 kg). Average annual temperature has warmed by 6°C since the Last Glacial Maximum (~24-18 kya) and is predicted to further increase by 4°C by the end of the 21st century. If body size continues to linearly respond to global temperature, Bison body mass will likely decline by an additional 46%, to 357 ± 54 kg, with an increase of 4°C globally. The rate of mass loss is 41 ± 10 kg per °C increase of global temperature. Changes in body size of Bison may be a result of migration, disease, or human harvest but those effects are likely to be local and short-term and not likely to persist over the long-time-scale of the fossil record. The strong correspondence between body size of bison and air temperature is more likely the result of persistent effects on the ability to grow and the consequences of sustaining a large body mass in a warming environment. Continuing rises in global temperature will likely depress body sizes of bison, and perhaps other large grazers, without human intervention.

opencc-zeroDec 2011View details →

Data from: Body-size scaling of metabolic rate in the trilobite Eldredgeops rana

We infer the body-size scaling slope of metabolic rate in a trilobite by applying a cell-size model that has been proposed to explain metabolic scaling in living organisms. This application is especially tractable in fossil arthropods with well-preserved compound eyes because the number and size of eye facets appear to be useful proxies for the relative number and size of cells in the body. As a case study, we examined the ontogenetic scaling of facet size and number in a ∼390-Myr-old local assemblage of the trilobite Eldredgeops rana, which has well-preserved compound eyes and a wide body-size range. Growth in total eye lens area resulted from increases in both facet area and number in relatively small (presumably young) specimens, but only from increases in facet area in large (presumably more mature) specimens. These results suggest that early growth in E. rana involved both cell multiplication and enlargement, whereas later growth involved only cell enlargement. If the cell-size model is correct, then metabolic rate scaled allometrically in E. rana, and the scaling slope of log metabolic rate versus log body mass decreased from ∼0.85 to 0.63 as these animals grew. This inferred age-specific change in metabolic scaling is consistent with similar changes frequently observed in living animals. Additional preliminary analyses of literature data on other trilobites also suggest that the metabolic scaling slope was <1 in benthic species, but ∼1 in pelagic species, as has also been observed in living invertebrates. The eye-facet size (EFS) method featured here opens up new possibilities for examining the bioenergetic allometry of extinct arthropods.

opencc-zeroDec 2016View details →

Data from: Body size and evolution of motion dazzle coloration in lizards

'Motion dazzle' patterns are a form of defensive coloration suggested to prevent successful capture during motion by causing predators to misjudge the direction or speed of prey movement. Several studies have found results supporting this idea, but little is known about the factors that favour the evolution of these antipredator colorations. A recent experimental study has suggested that the longitudinal striped patterns on body of lizards can redirect attacks to the tail via the motion dazzle effect. Using a virtual predation experiment with humans and a phylogenetic comparative analysis, we show that evolution of longitudinal striped coloration is associated with prey size. Experiments showed that longitudinal stripes located at the anterior reduced lethal attacks (i.e. attacks directed to the anterior and centre), but this benefit was greater for shorter prey. Our comparative analysis revealed a negative association between stripe occurrence and body length, but no association between stripes and body width. Overall, our results suggest that the dazzle effect produced by stripes is more advantageous in shorter lizards than in longer ones, and that the error induced by stripes might be distributed along the axis parallel to the prey trajectory. We discuss reasons why dazzle coloration could be associated with evolution of smaller body size in animals.

Data from: Body size response to warming: time of the season matters in a tephritid fly

Whether shrinking body size is a universal response to climate change remains controversial. Moreover, the mechanisms underlying body size shifts are poorly understood. Here, assuming that life history traits evolve to maximize fitness according to life history plasticity theory, we hypothesized that under global warming temperate multivoltine insects should emerge earlier with a smaller body mass in the early growing season, but emerge later with a larger body mass in the late season. We tested this hypothesis by conducting two field artificial warming experiments in an alpine meadow: 1) with one pre-dispersal seed predator species (tephritid flies, Tephritis femoralis) and its two host-plant species flowering in early and late growing seasons, respectively, and 2) with the tephritid flies and one host species with a flowering season that occupies parts of both the early and late growing seasons. These experiments were complemented by a microcosm chamber warming experiment, in which increasing and decreasing temperature trends were set to simulate temperature variation pattern in early and late growing seasons, respectively, but photoperiod was held constant. Warming generally advanced the adult emergence and decreased the body size (adult body mass) in the early season but delayed the emergence and increased the size in the late season in both field experiments, indicating that the seasonally different effects of warming on the fly body size was constant regardless of host-plant identity. The chamber warming resulted in consistent responses of emerging timing and body size to the simulated seasonal warming, demonstrating that the temperature increase per se and its interaction with direction of temperature change, but not other correlated effects, should be primarily responsible for the observed contrasting shifts of body size at different times of the season. Our results indicate that taking into account temperate seasonal patterns of temperature variation could be of general importance for predicting animal body size changes in the warmed future.

Data from: Geographic variation of body size in new world anurans: energy and water in a balance

The validity of Bergmann's rule, perhaps the best known ecogeographical rule, has been questioned for ectothermic species. Here, we explore the interspecific version of the rule documenting body size gradients for anurans across the whole New World and evaluating which environmental variables best explain the observed patterns. We assembled a dataset of body sizes for 2761 anuran species of the Western Hemisphere and conducted assemblage-based and cross-species analyses that consider the spatial and phylogenetic structure in the data. In accordance with heat and water-related explanations for body size clines, we found a consistent association of median body size and potential evapotranspiration across the New World. A relevant role of water availability also emerges, suggesting the joint importance of body size for thermoregulation and hydroregulation in anurans. Anurans do not follow a simple Bergmannian pattern of increasing size towards high latitudes. Consistent with previous regional findings, our Hemisphere-wide analyses detect that the geographic variation in anuran body sizes is highly dependent on a trade-off between heat and water balance. The observed size-climate relationships possibly emerge from the interplay between thermoregulatory abilities and the benefits inherent to reduced surface-to-volume ratios in larger species, which decrease the rates of evaporative water loss and favour heat retention. Our results also show how temperature becomes important for species that are directly in contact with the substrate and water, like burrowing and terrestrial anurans, while arboreal species exhibit a body size cline linked with potential evapotranspiration.

opencc-zeroDec 2015View details →

Data from: Exceptional body sizes but typical trophic structure in a Pleistocene food web

In this study, we focused on the exceptionally large mammals inhabiting the Americas during the Quaternary period and the paramount role of body size in species ecology. We evaluated two main features of Pleistocene food webs: the relationship between body size and (i) trophic position and (ii) vulnerability to predation. Despite the large range of species sizes, we found a hump-shaped relationship between trophic position and body size. We also found a negative trend in species vulnerability similar to that observed in modern faunas. The largest species lived near the boundary of energetic constraints, such that any shift in resource availability could drive these species to extinction. Our results reinforce several features of megafauna ecology: (i) the negative relationship between trophic position and body size implies that large-sized species were particularly vulnerable to changes in energetic support; (ii) living close to energetic imbalance could favour the incorporation of additional energy sources, for example, a transition from a herbivorous to a scavenging diet in the largest species (e.g. Megatherium) and (iii) the interactions and structure of Quaternary megafauna communities were shaped by similar forces to those shaping modern fauna communities.

opencc-zeroDec 2015View details →

Data from: Revising traditional theory on the link between plant body size and fitness under competition: evidence from old-field vegetation

The selection consequences of competition in plants have been traditionally interpreted based on a "size-advantage" hypothesis – that is, under intense crowding/competition from neighbors, natural selection generally favors capacity for a relatively large plant body size. However, this conflicts with abundant data, showing that resident species body size distributions are usually strongly right-skewed at virtually all scales within vegetation. Using surveys within sample plots and a neighbor-removal experiment, we tested: (1) whether resident species that have a larger maximum potential body size (MAX) generally have more successful local individual recruitment, and thus greater local abundance/density (as predicted by the traditional size-advantage hypothesis); and (2) whether there is a general between-species trade-off relationship between MAX and capacity to produce offspring when body size is severely suppressed by crowding/competition – that is, whether resident species with a larger MAX generally also need to reach a larger minimum reproductive threshold size (MIN) before they can reproduce at all. The results showed that MIN had a positive relationship with MAX across resident species, and local density – as well as local density of just reproductive individuals – was generally greater for species with smaller MIN (and hence smaller MAX). In addition, the cleared neighborhoods of larger target species (which had relatively large MIN) generally had – in the following growing season – a lower ratio of conspecific recruitment within these neighborhoods relative to recruitment of other (i.e., smaller) species (which had generally smaller MIN). These data are consistent with an alternative hypothesis based on a 'reproductive-economy-advantage' – that is, superior fitness under competition in plants generally requires not larger potential body size, but rather superior capacity to recruit offspring that are in turn capable of producing grand-offspring – and hence transmitting genes to future generations – despite intense and persistent (cross-generational) crowding/competition from near neighbors. Selection for the latter is expected to favor relatively small minimum reproductive threshold size and hence – as a tradeoff – relatively small (not large) potential body size.

opencc-zeroDec 2011View details →

Data from: Body size evolution on islands: are adult size variations in tiger snakes a non-adaptive consequence of selection on birth size?

Mean adult size has been used as the traditional measure of body size to explain trends of insular gigantism and dwarfism in a wide array of taxa. However, patterns of variation in body size at birth have received surprisingly little attention, leaving open the possibility that adult body-size differences are nonadaptive consequences of selection acting on neonate body size. Here I used an empirical and correlative approach to test this hypothesis in a mosaic of 12 island and mainland snake populations in Australia. Data collected on 597 adult and 1,084 neonate tiger snakes showed that (1) both adult and neonate mean body sizes varied strongly across populations; (2) prey diversity and size convincingly explained birth-size variations: birth size—notably, gape size—correlated with prey size; (3) neonate snout-vent length was significantly correlated with neonate gape size; and (4) neonate snout-vent length was significantly correlated with adult snout-vent length. Postnatal growth rates recorded under common-garden conditions differed across populations and were correlated with mean prey size. These data collectively suggest that (1) prey size is the main driver for the evolution of body size at birth in gape-limited predators, (2) adult size variations may reflect selective forces acting on earlier life stages, and (3) adult size variations may also reflect resource availability during ontogeny (notably, prey diversity).

opencc-zeroDec 2012View details →

Data from: Mammals across the K/Pg boundary in northeastern Montana, U.S.A.: dental morphology and body-size patterns reveal extinction selectivity and immigrant-fueled ecospace filling

The Cretaceous/Tertiary (K/Pg) mass extinction has long been viewed as a pivotal event in mammalian evolutionary history, in which the extinction of non-avian dinosaurs allowed mammals to rapidly expand from small-bodied, generalized insectivores to a wide array of body sizes and ecological specializations. Many studies have used global- or continental-scale taxonomic databases to analyze this event on coarse temporal scales, but few studies have documented morphological diversity of mammalian paleocommunities on fine spatiotemporal scales in order to examine ecomorphological selectivity and ecospace filling across this critical transition. Focusing on well-sampled and temporally well-constrained mammalian faunas across the K/Pg boundary in northeastern Montana, I quantified dental-shape disparity and morphospace occupancy via landmark- and semilandmark-based geometric morphometrics and mean body size, body-size disparity, and body-size structure via body-mass estimates. My results reveal several key findings: (1) latest Cretaceous mammals, particularly metatherians and multituberculates, had a greater ecomorphological diversity than is generally appreciated, occupying regions of the morphospace that are interpreted as strict carnivory, plant-dominated omnivory, and herbivory; (2) the decline in dental-shape disparity and body-size disparity across the K/Pg boundary shows a pattern of constructive extinction selectivity against larger-bodied dietary specialists, particularly strict carnivores and taxa with plant-based diets, that suggests the kill mechanism was related to depressed primary productivity rather than a globally instantaneous event; (3) the ecomorphological recovery in the earliest Paleocene was fueled by immigrants, namely three multituberculate families (taeniolabidids, microcosmodontids, eucosmodontids) and to a lesser extent archaic ungulates; and (4) despite immediate increases in the taxonomic richness of eutherians, their much-celebrated post-K/Pg ecomorphological expansion had a slower start than is generally perceived and most likely only began 400,000 to 1 million years after the extinction event.

Data from: The trade-off between clutch size and egg mass in tree swallows (Tachycineta bicolor) is modulated by female body mass

Egg production is a costly component of reproduction for female birds in terms of energy expenditure and maternal investment. Because resources are typically limited, clutch size and egg mass are expected to be constrained, and this putative trade-off between offspring number and size is at the core of life history theory. Nevertheless, empirical evidence for this trade-off is equivocal at best, as individual heterogeneity in resource acquisition and allocation may hamper the detection of the negative correlation between egg number and mass within populations. Here, we investigated how female body mass and landscape composition influences clutch size, egg mass, and the relationship between these two traits. To do so, we fitted linear mixed models using data from tree swallows Tachycineta bicolor breeding in a network of 400 nestboxes located along a gradient of agricultural intensity between 2004 and 2011. Our dataset comprised 1463 broods for clutch size analyses and 4371 eggs (from 847 broods laid between 2005–2008) for egg mass analyses. Our results showed that agricultural intensity negatively impacted clutch size, but not egg mass nor the relationship between these two traits. Female mass, on the other hand, modulated the trade-off between clutch size and egg mass. For heavier females, both traits increased jointly, without evidence of a trade-off. However, for lighter females, there was a clear negative relationship between clutch size and egg mass. This work shows that accounting for individual heterogeneity in body mass allows the detection of a clutch size/egg mass trade-off that would have remained undetected otherwise. Identifying habitat and individual effects on resource allocation towards reproductive traits may help bridging the gap between predictions from theory and empirical evidence on life history trade-offs.

Data from: Bergmann's Rule rules body size in an ectotherm: heat conservation in a lizard along a 2200-meter elevational gradient

Bergmann's Rule predicts larger body sizes in colder habitats, increasing organisms' ability to conserve heat. Originally formulated for endotherms, it is controversial whether Bergmann's Rule may be applicable to ectotherms, given that larger ectotherms show diminished capacity for heating up. We predict that Bergmann's Rule will be applicable to ectotherms when the benefits of a higher conservation of heat due to a larger body size overcompensate for decreased capacity to heating up. We test this hypothesis in the lizard Psammodromus algirus, which shows increased body size with elevation in Sierra Nevada (SE Spain). We measured heating and cooling rates of lizards from different elevations (from 300 to 2500 meters above sea level) under controlled conditions. We found no significant differences in the heating rate along an elevational gradient. However, the cooling rate diminished with elevation and body size: highland lizards, with larger masses, have a higher thermal inertia for cooling, which allows them to maintain heat for more time and keep a high body temperature despite the lower thermal availability. Consequently, the net gaining of heat increased with elevation and body size. This study highlights that the heat conservation mechanism for explaining Bergmann's Rule works and is applicable to ectotherms, depending on the thermal benefits and costs associated with larger body sizes.

Data from: Energy expenditure and body size are targets of natural selection across a wide geographic range, in a terrestrial invertebrate

One of the central questions in evolutionary ecology is how different functional capacities impact fitness, and how it varies across populations. For instance, do phenotypic attributes influence fitness similarly across geographic gradients? Which traits (physiological, morphological, life history) are most likely to be targets of natural selection? Do particular combinations of traits maximize fitness? In a semi-natural experiment, we analyzed introduced populations of an invasive species, the garden snail (Cornu aspersum) in Chile, which show low levels of genetic differentiation in spite of the distance. Specifically, we addressed whether the magnitude, sign and form of selection in snail populations could explain the differentiation (or its absence) among populations. A common garden/reciprocal transplant experiment was performed in three populations (La Serena, Constitución and Valdivia) that span a 1300 km latitudinal gradient and differ markedly in climate (semiarid North to humid South). We then released ca. 450 individuals per population (two generations after field-captured snails) in replicated enclosures at the range extremes (La Serena and Valdivia). Morphological (size and shell darkness), physiological (standard metabolic rate and digestive efficiency) and life history (growth rate) traits were measured in all snails before the release. Survival was recorded monthly during one year. However, we only detected significant selection on body size (MB), residual standard metabolic rate (SMRR) and growth rate. Survival was significantly higher in snails from La Serena than in snails from Constitución and Valdivia, when raised at La Serena. However, at Valdivia survival was not different among source populations. Interestingly, we found negative correlational selection in MB and SMRR at La Serena whereas at Valdivia, we only found directional selection on growth rate and MB, and stabilizing selection on SMRR. These results suggest that selection on physiological traits related with energy allocation is pervasive, irrespective of climate and distance.

opencc-zeroDec 2016View details →

Data from: Reversal of response to artificial selection on body size in a wild passerine

A general assumption in quantitative genetics is the existence of an intermediate phenotype with higher mean individual fitness in the average environment than more extreme phenotypes. Here we investigate the evolvability and presence of such a phenotype in wild bird populations from an eleven-year experiment with four years of artificial selection for long and short tarsus length, a proxy for body size. The experiment resulted in strong selection in the imposed directions. However, artificial selection was counteracted by reduced production of recruits in offspring of artificially selected parents. This resulted in weak natural selection against extreme trait values. Significant responses to artificial selection were observed at both the phenotypic and genetic level, followed by a significant return towards pre-experimental means. During artificial selection, the annual observed phenotypic response closely followed the predicted response from quantitative genetic theory (ryear = 0.96, rcohorts = 0.56). The rapid return to pre-experimental means was induced by three interacting mechanisms: selection for an intermediate phenotype, immigration and recombination between selected and unselected individuals. The results of this study demonstrates the evolvability of phenotypes and that selection may favour an intermediate phenotype in wild populations.

Data from: Heterogeneity of genetic architecture of body size traits in a free-living population

Knowledge of the underlying genetic architecture of quantitative traits could aid in understanding how they evolve. In wild populations, it is still largely unknown whether complex traits are polygenic or influenced by few loci with major effect, due to often small sample sizes and low resolution of marker panels. Here, we examine the genetic architecture of five adult body size traits in a free-living population of Soay sheep on St Kilda using 37 037 polymorphic SNPs. Two traits (jaw and weight) show classical signs of a polygenic trait: the proportion of variance explained by a chromosome was proportional to its length, multiple chromosomes and genomic regions explained significant amounts of phenotypic variance, but no SNPs were associated with trait variance when using GWAS. In comparison, genetic variance for leg length traits (foreleg, hindleg and metacarpal) was disproportionately explained by two SNPs on chromosomes 16 (s23172.1) and 19 (s74894.1), which each explained >10% of the additive genetic variance. After controlling for environmental differences, females heterozygous for s74894.1 produced more lambs and recruits during their lifetime than females homozygous for the common allele conferring long legs. We also demonstrate that alleles conferring shorter legs have likely entered the population through a historic admixture event with the Dunface sheep. In summary, we show that different proxies for body size can have very different genetic architecture and that dense SNP helps in understanding both the mode of selection and the evolutionary history at loci underlying quantitative traits in natural populations.

Data from: Male body size and condition affects sperm number and production rates in mosquitofish, Gambusia holbrooki

Sperm number is an important predictor of paternity when there is sperm competition. Sperm number is often measured as maximum sperm reserves, but in species where mating is frequent, males will often be replenishing their reserves. Thus, variation in how quickly males can produce sperm is likely to be important in determining male success in sperm competition. Despite this, little is known about how male size, body condition or diet affects sperm production rates. We counted sperm number in large and small Gambusia holbrooki (eastern mosquitofish) after 3 weeks on either a high or low food diet. Sperm number was significantly higher in both larger males and in well-fed males. We then stripped ejaculates again either 1, 2, 3, 4 or 5 days later to investigate subsequent sperm production. The rate of sperm replenishment was influenced by an interaction between size and diet. Large, well-fed males had consistently high levels of sperm available over the 5 days (i.e. rapid replenishment), whereas small poorly fed males showed consistently low levels of sperm availability over the 5 days (i.e. slow replenishment). In contrast, large, poorly fed and small, well-fed males increased their sperm numbers over the first 3 days (i.e. intermediate replenishment). Our study highlights that when mating is frequent and sperm competition is high, size and condition dependence of maximal sperm number and of sperm production rate might both contribute to variation in male reproductive success.