Clutch and egg size of Tree Swallows along an elevational gradient

How avian reproductive behavior changes at high elevations, and whether changes are the result of adaptation or constraint remains unclear. We compared clutch and egg sizes in two populations of Tree Swallows (Tachycineta bicolor), one at an elevation of ~2500 m a.s.l. in Wyoming's Bighorn Mountains and the other at ~1350 m at the base of these mountains. Females at the high‐elevation site began laying 10 d later, on average, than females at the lower site. Females at the high site laid an average of 0.4 fewer eggs than those at the low site, a significant difference. Eggs were also slightly (2.3%), but significantly, smaller at the high site. Smaller clutches and eggs at the high site may have resulted in part from greater energetic constraints on females. Females at this site faced colder, windier weather during egg formation which would have increased thermoregulatory costs while simultaneously reducing the abundance of prey, i.e., flying insects. Laying a relatively small clutch at the high‐elevation site may also be adaptive, however. Having a smaller brood could help ensure there is an adequate supply of food for each offspring during bouts of inclement weather that are more severe at higher elevations. Also, if the delay in breeding inherent at high elevations reduces the survival prospects of the young, then producing fewer young could enhance a female's chances of breeding again, perhaps at a different location.     

Proximate causes of altitudinal differences in body size in an agamid lizard

Body size is directly linked to key life history traits such as growth, fecundity, and survivorship. Identifying the causes of body size variation is a critical task in ecological and evolutionary research. Body size variation along altitudinal gradients has received considerable attention; however, the underlying mechanisms are poorly understood. Here, we compared the growth rate and age structure of toad‐headed lizards (Phrynocephalus vlangalii) from two populations found at different elevations in the Qinghai‐Tibetan Plateau. We used mark‐recapture and skeletochronological analysis to identify the potential proximate causes of altitudinal variation in body size. Lizards from the high‐elevation site had higher growth rates and attained slightly larger adult body sizes than lizards from the low‐elevation site. However, newborns produced by high‐elevation females were smaller than those by low‐elevation females. Von Bertalanffy growth estimates predicted high‐elevation individuals would reach sexual maturity at an earlier age and have a lower mean age than low‐elevation individuals. Relatively lower mean age for the high‐elevation population was confirmed using the skeletochronological analysis. These results support the prediction that a larger adult body size of high‐elevation P. vlangalii results from higher growth rates, associated with higher resource availability.     

Patterns of seasonal activity in a Mediterranean lizard along a 2200 m altitudinal gradient

Ectothermy in reptiles involves strong environmental temperature dependence to maintain body temperature. High mountains are extreme habitats which, because of their marked temperature decrease with altitude, impose difficulties for reptile thermoregulation, as the number of optimum-temperature days diminish in altitude. We studied the activity patterns of the lizard Psammodromus algirus, which has a wide altitudinal distribution, at six sampling stations that spanned an altitudinal range from 300 to 2500 m asl in Sierra Nevada (SE Iberian Peninsula). We performed censuses of the species during the seasonal and daily activity period, and measured lizard body temperature and environmental temperature in the six sampling plots along the altitude gradient. The body temperature of the lizards remained closely the same even though environmental temperature decreased around 9°C along that gradient. Seasonal activity peaked at the beginning of the activity period in highlands, but in the middle of the season in middle and lowland regions. However, newborns appeared in the field almost at the same time along the altitude gradient. We suggest that the thermoregulatory strategies of the lizards must vary along this altitudinal gradient, permitting lizards to keep their body temperature close to a very similar optimum and thus, develop very similar circa-annual activity patterns.     

Variation in home range size along an elevational gradient in the iguanid lizard Sceloporus merriami

Summary Variation in home range size among three populations of the saxicolous iguanid lizard Sceloporus merriami was studied in relation to density, food availability, behavioral patterns, and daily activity periods. These populations, separated by a maximum distance of 50 km, occur along an elevational gradient which spans the elevational range of the species in west Texas. The population with the smallest home ranges had the highest density, the lowest food availability, and the most restricted daily activity period. Within populations, males consistently had larger home ranges than females. No significant between-year variation in home range size was noted within a given area or sex. Intraspecific variation in home range size is not explained by any single factor, such as food availability or density, but appears to be complexly determined by the interaction of energetic, breeding, and activity constraints.     

Among‐species overlap in rodent body size distributions predicts species richness along a temperature gradient

Temperature is widely regarded as a major driver of species richness, but the mechanisms are debated. Niche theory suggests temperature may affect richness by filtering traits and species in colder habitats while promoting specialization in warmer ones. However, tests of this theory are rare because niche dimensions are challenging to quantify along broad thermal gradients. Here, we use individual‐level trait data from a long‐term monitoring network spanning a large geographic extent to test niche‐based theory of community assembly in small mammals. We examined variation in body size among 23 communities of North American rodents sampled across the National Ecological Observatory Network (NEON), ranging from northern hardwood forests to subtropical deserts. We quantified body size similarity among species using a metric of overlap that accounts for individual variation, and fit a structural equation model to disentangle the relationships between temperature, productivity, body size overlap, and species richness. We document a latitudinal gradient of declining similarity in body size among species towards the tropics and overall increase in the dimensions of community‐wide trait space in warmer habitats. Neither environmental temperature nor net primary productivity directly affect rodent species richness. Instead, temperature determines the community‐wide niche space that species can occupy, which in turn alters richness. We suggest a latitudinal gradient of trait space expansion towards the tropics may be widespread and underlie gradients in species diversity.     

Bergmann's rule does not apply to geometrid moths along an elevational gradient in an Andean montane rain forest

Aim Bergmann's rule generally predicts larger animal body sizes with colder climates. We tested whether Bergmann's rule at the interspecific level applies to moths (Lepidoptera: Geometridae) along an extended elevational gradient in the Ecuadorian Andes. Location Moths were sampled at 22 sites in the province Zamora‐Chinchipe in southern Ecuador in forest habitats ranging from 1040 m to 2677 m above sea level. Methods Wingspans of 2282 male geometrid moths representing 953 species were measured and analysed at the level of the family Geometridae, as well as for the subfamily Ennominae with the tribes Boarmiini and Ourapterygini, and the subfamily Larentiinae with the genera Eois, Eupithecia and Psaliodes. Results Bergmann's rule was not supported since the average wingspan of geometrid moths was negatively correlated with altitude (r = ?0.59, P < 0.005). The relationship between body size and altitude in Geometridae appears to be spurious because species of the subfamily Larentiinae are significantly smaller than species of the subfamily Ennominae and simultaneously increase in their proportion along the gradient. A significant decrease of wingspan was also found in the ennomine tribe Ourapterygini, but no consistent body size patterns were found in the other six taxa studied. In most taxa, body size variation increases with altitude, suggesting that factors acting to constrain body size might be weaker at high elevations. Main conclusions The results are in accordance with previous studies that could not detect consistent body size patterns in insects at the interspecific level along climatic gradients.     

Geographic gradients in body size: a clarification of Bergmann's rule

1997 marked the sesquicentenary of the publication by Carl Bergmann of the observation that, in general, large-bodied animal species tend to live further north than their small-bodied relatives. This has been dubbed Bergmann's rule in his honour. However, more than 150 years on, we appear to be little closer to a general understanding of the rule, or even to any consensus as to whether it exists. This is due in large part to confusion about the taxonomic level at which the rule is considered to operate, and to the conflation of pattern and mechanism. In this paper, we attempt to resolve this confusion by highlighting its sources, and by providing a definition of Bergmann's rule that is practical and useful, yet that retains the essential features of its original formulation. We conclude by briefly reviewing the mechanisms proposed to explain Bergmann's rule as we define it.     

Energy flux, body size and density in relation to bird species richness along an elevational gradient in Taiwan

Aim To examine the species richness of breeding birds along a local elevational gradient and to test the following assumptions of the energy limitation hypothesis: (1) the energy flux through birds is positively correlated with above‐ground net primary productivity, (2) bird density is positively correlated with total energy flux, and (3) bird species richness is positively correlated with bird density. Location An elevational gradient from 1400 to 3700 m on Mt. Yushan, the highest mountain in Taiwan (23°28′30″ N, 120°54′00″ E), with a peak of 3952 m a.s.l. Methods We established 50 sampling stations along the elevational gradient. From March to July 1992, we estimated the density of each bird species using the variable circular‐plot method. Above‐ground net primary productivity was modelled using monthly averages from weather data for the years 1961–90. Results Bird species richness had a hump‐shaped relationship with elevation and with net primary productivity. Bird energy flux was positively correlated with net primary productivity and bird species richness was positively correlated with bird density. The relationship between bird density and energy flux was hump‐shaped, which does not support one assumption of the energy limitation hypothesis. Main conclusions The results supported two essential assumptions of the energy limitation hypothesis. However, when energy availability exceeded a certain level, it could decrease species richness by increasing individual energy consumption, which reduced bird density. Thus, energy availability is a primary factor influencing bird species richness at this scale, but other factors, such as body size, could also play important roles.     

The combined use of raw and phylogenetically independent methods of outlier detection uncovers genome‐wide dynamics of local adaptation in a lizard

Local adaptation is a dynamic process by which different allele combinations are selected in different populations at different times, and whose genetic signature can be inferred by genome‐wide outlier analyses. We combined gene flow estimates with two methods of outlier detection, one of them independent of population coancestry (CIOA) and the other one not (ROA), to identify genetic variants favored when ecology promotes phenotypic convergence. We analyzed genotyping‐by‐sequencing data from five populations of a lizard distributed over an environmentally heterogeneous range that has been changing since the split of eastern and western lineages ca. 3 mya. Overall, western lizards inhabit forest habitat and are unstriped, whereas eastern ones inhabit shrublands and are striped. However, one population (Lerma) has unstriped phenotype despite its eastern ancestry. The analysis of 73,291 SNPs confirmed the east–west division and identified nonoverlapping sets of outliers (12 identified by ROA and 9 by CIOA). ROA revealed ancestral adaptive variation in the uncovered outliers that were subject to divergent selection and differently fixed for eastern and western populations at the extremes of the environmental gradient. Interestingly, such variation was maintained in Lerma, where we found high levels of heterozygosity for ROA outliers, whereas CIOA uncovered innovative variants that were selected only there. Overall, it seems that both the maintenance of ancestral variation and asymmetric migration have counterbalanced adaptive lineage splitting in our model species. This scenario, which is likely promoted by a changing and heterogeneous environment, could hamper ecological speciation of locally adapted populations despite strong genetic structure between lineages.     

Flowering phenology and reproduction of the Solidago virgaurea L. complex along an elevational gradient on Mt Norikura,central Japan

This study examined the flowering phenology and reproductive traits of the Solidago virgaurea complex at four elevations in the subalpine zone in Japan using a bagging experiment. Flowering started earlier at higher elevations. Syrphid flies mainly visited flowers of the S. virgaurea complex, and the number of visits was considerably lower at the lowest elevation than at the three other elevations. Although the number of seeds per individual did not differ among the four elevations, total achene weight per individual was lower at the lowest elevation than at the three other elevations. The weight of an achene and seed germination rate of the control were much greater at higher elevations than at the lowest elevation. The weight of an achene and germination rate for the control were as low as the bagging treatment at the lowest elevation with infrequent flower visitors, which indicates that the S. virgaurea complex is a facultative outcrosser. The S. virgaurea complex is thought to produce seeds during a short growing season at high elevations by starting to flower earlier, and the large seed size is advantageous for seedling establishment at high elevations. Therefore, elevational changes in flowering phenology and reproductive traits are thought to be an adaptation to the short growing season at high elevations.     

Bergmann's rule in the ant lion Myrmeleon immaculatus DeGeer (Neuroptera: Myrmeleontidae): geographic variation in body size and heterozygosity

Aim Geographic variation in body size and heterozygosity were surveyed for discrete populations of the ant lion, Myrmeleon immaculatus DeGeer, collected from the central and northeastern United States. Location Collection sites were located in the central and eastern United States ranging from western Oklahoma to northern New York. Methods We collected 872 M. immaculatus larvae from thirty-four collecting sites. At each site, we randomly sampled ant lion pits and collected between fifteen and fifty-two larvae in total. Larvae were preserved in 95% ETOH for morphological analysis and frozen in a ?80°C freezer for protein electrophoresis. We measured the body size of eighty-five preserved adult M. immaculatus obtained from museum collections using head width as an indicator of body size. Five enzymes [GPI (glucose phosphate isomerase), MDH (malate dehydrogenase), PEP (peptidase), DIA (diaphorase) and SOD (superoxide dismutase)] were used in the heterozygosity analyses. Results Larval and adult body size increased with latitude, but decreased with elevation. Average heterozygosity, measured at five polymorphic loci, also increased significantly with latitude. Minimum temperature variance was the best predictor of body size, whereas precipitation and maximum temperature were the best predictors of heterozygosity. Populations were genetically differentiated from one another and showed a pattern of isolation by distance, as measured by Wright's F_st values and Nei's genetic distances. Main conclusions Sampling artifacts, heat conservation, character displacement, cell-size variation, density-dependent mortality, and differential dispersal probably cannot account for latitudinal variation in ant lion body size. Our results implicate the importance of diurnal photoperiod, which varies with latitude, but not with elevation. Because photoperiod often controls growth, diapause, and metamorphosis, it may be an important determinant of latitudinal clines in body size and life history of insects.     

Amplicon‐pyrosequencing‐based detection of compositional shifts in bryophyte‐associated fungal communities along an elevation gradient

Although bryophytes are a dominant vegetation component of boreal and alpine ecosystems, little is known about their associated fungal communities. HPLC assays of ergosterol (fungal biomass) and amplicon pyrosequencing of the ITS2 region of rDNA were used to investigate how the fungal communities associated with four bryophyte species changed across an elevational gradient transitioning from conifer forest to the low‐alpine. Fungal biomass and OTU richness associated with the four moss hosts did not vary significantly across the gradient (P > 0.05), and both were more strongly affected by host and tissue type. Despite largely constant levels of fungal biomass, distinct shifts in community composition of fungi associated with Hylocomium, Pleurozium and Polytrichum occurred between the elevation zones of the gradient. This likely is a result of influence on fungal communities by major environmental factors such as temperature, directly or indirectly mediated by, or interacting with, the response of other components of the vegetation (i.e. the dominant trees). Fungal communities associated with Dicranum were an exception, exhibiting spatial autocorrelation between plots, and no significant structuring by elevation. Nevertheless, the detection of distinct fungal assemblages associated with a single host growing in different elevation zones along an elevational gradient is of particular relevance in the light of the ongoing changes in vegetation patterns in boreal and alpine systems due to global climate warming.     

Metabolic costs of capital energy storage in a small‐bodied ectotherm

Reproduction is energetically financed using strategies that fall along a continuum from animals that rely on stored energy acquired prior to reproduction (i.e., capital breeders) to those that rely on energy acquired during reproduction (i.e., income breeders). Energy storage incurs a metabolic cost. However, previous studies suggest that this cost may be minimal for small‐bodied ectotherms. Here I test this assumption. I use a laboratory feeding experiment with the European green crab Carcinus maenas to establish individuals with different amounts of energy storage. I then demonstrate that differences in energy storage account for 26% of the variation in basal metabolic costs. The magnitudes of these costs for any individual crab vary through time depending on the amount of energy it has stored, as well as on temperature‐dependent metabolism. I use previously established relationships between temperature‐ and mass‐dependent metabolic rates, combined with a feasible annual pattern of energy storage in the Gulf of Maine and annual sea surface temperature patterns in this region, to estimate potential annual metabolic costs expected for mature female green crabs. Results indicate that energy storage should incur an ~8% increase in metabolic costs for female crabs, relative to a hypothetical crab that did not store any energy. Translated into feeding, for a medium‐sized mature female (45 mm carapace width), this requires the consumption of an additional ~156 mussels annually to support the metabolic cost of energy storage. These results indicate, contrary to previous assumptions, that the cost of energy storage for small‐bodied ectotherms may represent a considerable portion of their basic operating energy budget. An inability to meet these additional costs of energy storage may help explain the recent decline of green crabs in the Gulf of Maine where reduced prey availability and increased consumer competition have combined to hamper green crab foraging success in recent years.     

Quantitative trait loci for growth and body size in the nine‐spined stickleback Pungitius pungitius L.

Body size is an ecologically important trait shown to be genetically variable both within and among different animal populations as revealed by quantitative genetic studies. However, few studies have looked into underlying genetic architecture of body size variability in the wild using genetic mapping methods. With the aid of quantitative trait loci (QTL) analyses based on 226 microsatellite markers, we mapped body size and growth rate traits in the nine‐spined stickleback (Pungitius pungitius) using an F₂‐intercross (n = 283 offspring) between size‐divergent populations. In total, 17 QTL locations were detected. The proportion of phenotypic variation explained by individual body size‐related QTL ranged from 3% to 12% and those related to growth parameters and increments from 3% to 10%. Several of the detected QTL affected either early or late growth. These results provide a solid starting point for more in depth investigations of structure and function of genomic regions involved in determination of body size in this popular model of ecological and evolutionary research.     

Variation in the species composition and mean body size of an avian foliage-gleaning guild along an elevational gradient: correlation with arthropod body size

The composition of an avian foliage-gleaning guild was analyzed with respect to body size at nine sites along an elevational gradient in the Oregon Cascades. Mean body size decreased from 20.5 g near the lower forest boundary where it meets the grassland at about 775 m to 9.3 g near timberline at about 1720 m. Both the loss of larger species and the gain of smaller species contributed to the change. Mean volume of the foliage-dwelling arthropods also decreased with increasing elevation by two orders of magnitude along the same gradient. A significant decrease in body size occurred in three arthropod groups, larval Lepidoptera, Homoptera, and spiders, and of these, larval Lepidoptera dominated the overall size trend among arthropods. Both developmental differences (higher elevation sites are delayed seasonally on the same calendar date) and taxonomic differences contributed to the change in mean arthropod size. Mean bird size was positively correlated (r=0.93) with the body size of foliage-dwelling arthropods. A similar pattern was suggested for other avian guilds dependent directly or indirectly upon foliage-dwelling arthropods, but not for guilds independent of foliage-dwelling arthropods.     

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.     

Prey‐associated head‐size variation in an invasive lizard in the Hawaiian Islands

Biological invasions are recognized as a primary driver of large‐scale changes in global ecosystems. This study addresses ecomorphological variation in head size within and among populations of an ecologically destructive invasive predator, and evaluates the potential roles of environmental components in phenotypic differentiation. We used four size‐corrected measurements of head morphology in Jackson's chameleons, Trioceros jacksonii xantholophus (N = 319), collected from multiple Hawaiian Islands to assess phenotypic variation among and within islands. Results of analysis of variance (ANOVA) comparing chameleon head size (PC1) among islands revealed significant differences (mean difference > 5%) associated with variation in both rainfall and diet composition using Mann–Whitney U‐tests and chi‐squared analyses. These results suggest that morphological differentiation among populations from different islands has occurred over a relatively short ecological timescale, and is likely the result of ecomorphological adaptation to differences in exploited prey hardness. Intra‐island allopatric population variation, however, was also detected in this study. Although we might expect that genetic change is the more likely explanation for differences between islands than within, and that plasticity may be more likely an explanation for the within‐ than the between‐island differences, it is also possible that both within‐ and between‐island patterns are the results of genetic change, or of plasticity.     

Geographic variation of life‐history traits in the sand lizard,Lacerta agilis: testing Darwin's fecundity‐advantage hypothesis

The fecundity‐advantage hypothesis (FAH) explains larger female size relative to male size as a correlated response to fecundity selection. We explored FAH by investigating geographic variation in female reproductive output and its relation to sexual size dimorphism (SSD) in Lacerta agilis, an oviparous lizard occupying a major part of temperate Eurasia. We analysed how sex‐specific body size and SSD are associated with two putative indicators of fecundity selection intensity (clutch size and the slope of the clutch size–female size relationship) and with two climatic variables throughout the species range and across two widespread evolutionary lineages. Variation within the lineages provides no support for FAH. In contrast, the divergence between the lineages is in line with FAH: the lineage with consistently female‐biased SSD (L. a. agilis) exhibits higher clutch size and steeper fecundity slope than the lineage with an inconsistent and variable SSD (L. a. exigua). L. a. agilis shows lower offspring size (egg mass, hatchling mass) and higher clutch mass relative to female mass than L. a. exigua, that is both possible ways to enhance offspring number are exerted. As the SSD difference is due to male size (smaller males in L. a. agilis), fecundity selection favouring larger females, together with viability selection for smaller size in both sexes, would explain the female‐biased SSD and reproductive characteristics of L. a. agilis. The pattern of intraspecific life‐history divergence in L. agilis is strikingly similar to that between oviparous and viviparous populations of a related species Zootoca vivipara. Evolutionary implications of this parallelism are discussed.