Contrasting latitudinal patterns of life‐history divergence in two genera of new world thrushes (Turdinae)

Several long‐standing hypotheses have been proposed to explain latitudinal patterns of life‐history strategies. Here, we test predictions of four such hypotheses (seasonality, food limitation, nest predation and adult survival probability) by examining life‐history traits and age‐specific mortality rates of several species of thrushes (Turdinae) based on field studies at temperate and tropical sites and data gathered from the literature. Thrushes in the genus Catharus showed the typical pattern of slower life‐history strategies in the tropics while co‐occuring Turdus thrushes differed much less across latitudes. Seasonality is a broadly accepted hypothesis for latitudinal patterns, but the lack of concordance in latitudinal patterns between co‐existing genera that experience the same seasonal patterns suggests seasonality cannot fully explain latitudinal trait variation in thrushes. Nest‐predation also could not explain patterns based on our field data and literature data for these two genera. Total feeding rates were similar, and per‐nestling feeding rates were higher at tropical latitudes in both genera, suggesting food limitation does not explain trait differences in thrushes. Latitudinal patterns of life histories in these two genera were closely associated with adult survival probability. Thus, our data suggest that environmental influences on adult survival probability may play a particularly strong role in shaping latitudinal patterns of life‐history traits.     

Sources of variation in the nesting success of understory tropical birds

Survival of offspring is a key fitness component and, for birds, the threat of predation on nests is especially influential. Data on rates of nest success from tropical regions are comparatively few, conservation‐relevant, and essential for assessing the validity of models comparing the life histories and behavior or birds across latitudinal gradients. We monitored over 2 000 nests in the lowland forests of central Panama and, using the logistic exposure to model the fate of nests, explored the importance of variation in rate of nest success according to type of nest, height of nests, among years, in early versus late nests, and at different stages of the nest cycle. Analyses of over 1 400 nests for 18 species revealed considerable variation among species in the daily survival rate of nests (range among 18 species=0.91 to 0.98), but nest type and stage of the nesting cycle were generally influential on the probability of nest success. Cavity or enclosed nesters experienced greater nest success than open cup nesters and rates of nest loss were generally greatest in the nestling stage. We found limited evidence that height of nests affected probability of success, but no indication that timing of nesting effort was influential. Despite the occurrence of a severe ENSO event during our sampling, annual variation in nest success was not consistent among species. Interspecific variation in the rates and patterns of nest predation in our study, coupled with reports of high rates of nest loss at temperate latitudes, lead us to question long standing assumptions about latitudinal trends in rates of nest loss. We urge further work to understand the implications of nest predation on the evolutionary ecology of tropical birds.     

Extra‐pair paternity in a population of Chilean Swallows breeding at 54 degrees south

Geographic patterns of variation in life‐history traits have puzzled researchers for decades. However, the widely accepted idea that mating systems exhibit a tropical–temperate latitudinal trend, with extra‐pair mating systems being the norm among temperate species and genetic monogamy the norm among tropical species, is supported by sparse data, particularly for birds breeding in the tropics and even more so for birds that breed in the southern hemisphere temperate zone. Our objective was to examine the genetic mating system of Chilean Swallows (Tachycineta meyeni) breeding at 54°S in Tierra del Fuego, Argentina. From 2006 to 2009, we examined the paternity of young in 52 broods. Contrary to predictions based on their congeners that breed at equivalent latitudes in the northern hemisphere, Chilean Swallows in our study had low rates of extra‐pair paternity (EPP), with 13.5% of nests (N = 52) having at least one extra‐pair young and 6.8% of all nestlings (N = 161) fathered by extra‐pair males. These rates are also lower than those reported for species of Tachycineta swallows that breed at tropical latitudes. We found no support for a tropical–south temperate latitudinal cline in EPP rates. The highly unpredictable weather of the island of Tierra del Fuego might be influencing parental investment at this site; small clutch sizes and low EPP rates are expected if biparental attention is crucial for chick survival and reproductive success for these aerial insectivores. We argue that the sparse sampling of mating systems in birds worldwide may have contributed to a misapprehension of a global pattern. More studies of tropical and south‐temperate systems are needed to build on theories based on a wider set of taxa.     

An alternative explanation for global trends in thermal tolerance

Ectotherms from higher latitudes can generally perform over broader temperature ranges than tropical ectotherms. This pattern is thought to reflect trends in temperature variability: tropical ectotherms evolve to be ‘thermal specialists’ because their environment is thermally stable. However, the tropics are also hotter, and most physiological rates increase exponentially with temperature. Using a dataset spanning diverse ectotherms, we show that the temperature ranges ectotherms tolerate (the difference between lower and upper critical temperatures, and between optimum and upper critical temperatures) generally represents the same range of equivalent biological rates (e.g. metabolism) for cool‐ and warm‐adapted species, and independent of latitude or elevation. This suggests that geographical trends in temperature variability may not be the ultimate mechanism underlying latitudinal and elevational trends in thermal tolerance. Rather, we propose that tropical ectotherms can perform over a narrower range of temperatures than species from higher latitudes because the tropics are hotter.     

Latitudinal variation in subspecific diversification of birds

Patterns of evolution are believed to vary latitudinally, but our understanding of this variation remains limited. Here we examine how patterns of subspecific diversification vary within species of birds, specifically addressing three questions: (1) Are subspecies more numerous at lower latitudes within species, consistent with greater phenotypic differentiation at lower latitudes? (2) If there are more subspecies at lower latitudes within species, can area of breeding range explain this relationship? and (3) how do latitudinal differences in subspecies within species vary geographically across the globe? Using all species with five or more subspecies from 12 of the most diverse families of birds in the world, we found consistently more subspecies at lower latitudes across all families, both hemispheres, and all continents examined. Despite the positive influence of area on the number of subspecies within species, area did not explain the greater number of subspecies at lower latitudes within species. Global patterns of subspecies support the idea that phenotypic differentiation of populations is greater at lower latitudes within species. If subspecies density provides an index of rates of incipient speciation, then our results support evolutionary hypotheses for the latitudinal diversity gradient that invoke higher tropical speciation rates.     

Intrinsic vs. extrinsic influences on life history expression: metabolism and parentally induced temperature influences on embryo development rate

Intrinsic processes are assumed to underlie life history expression and trade‐offs, but extrinsic inputs are theorised to shift trait expression and mask trade‐offs within species. Here, we explore application of this theory across species. We do this based on parentally induced embryo temperature as an extrinsic input, and mass‐specific embryo metabolism as an intrinsic process, underlying embryonic development rate. We found that embryonic metabolism followed intrinsic allometry rules among 49 songbird species from temperate and tropical sites. Extrinsic inputs via parentally induced temperatures explained the majority of variation in development rates and masked a relationship with metabolism; metabolism explained a minor proportion of the variation in development rates among species, and only after accounting for temperature effects. We discuss evidence that temperature further obscures the expected interspecific trade‐off between development rate and offspring quality. These results demonstrate the importance of considering extrinsic inputs to trait expression and trade‐offs across species.     

Population divergence in plant species reflects latitudinal biodiversity gradients

The trend for increasing biodiversity from the poles to the tropics is one of the best-known patterns in nature. This latitudinal biodiversity gradient has primarily been documented so far with extant species as the measure of biodiversity. Here, we evaluate the global pattern in biodiversity across latitudes based on the magnitude of genetic population divergence within plant species, using a robust spatial design to compare published allozyme datasets. Like the pattern of plant species richness across latitudes, we expected the divergence among populations of current plant species would have a similar pattern and direction. We found that lower latitudinal populations showed greater genetic differentiation within species after controlling for geographical distance. Our analyses are consistent with previous population-level studies in animals, suggesting a high possibility of tropical peaks in speciation rates associated with observed levels of species richness.     

Repeated intraspecific divergence in life span and aging of African annual fishes along an aridity gradient

Life span and aging are substantially modified by natural selection. Across species, higher extrinsic (environmentally related) mortality (and hence shorter life expectancy) selects for the evolution of more rapid aging. However, among populations within species, high extrinsic mortality can lead to extended life span and slower aging as a consequence of condition‐dependent survival. Using within‐species contrasts of eight natural populations of Nothobranchius fishes in common garden experiments, we demonstrate that populations originating from dry regions (with short life expectancy) had shorter intrinsic life spans and a greater increase in mortality with age, more pronounced cellular and physiological deterioration (oxidative damage, tumor load), and a faster decline in fertility than populations from wetter regions. This parallel intraspecific divergence in life span and aging was not associated with divergence in early life history (rapid growth, maturation) or pace‐of‐life syndrome (high metabolic rates, active behavior). Variability across four study species suggests that a combination of different aging and life‐history traits conformed with or contradicted the predictions for each species. These findings demonstrate that variation in life span and functional decline among natural populations are linked, genetically underpinned, and can evolve relatively rapidly.     

Assessing the latitudinal gradient in herbivory

Plant–herbivore interactions occur in all ecosystems and provide a major avenue for energy flow to higher trophic levels. A long‐standing hypothesis to explain the latitudinal gradient in species diversity proposes that the relatively stable and frost‐free climate of the tropics should lead to more intense biotic interactions in tropical compared with temperate environments, giving rise to a greater diversity of plants and herbivores. Herbivory rates have been compared across latitudes to test this biotic interactions hypothesis, with herbivory typically being measured from observable leaf damage. However, we argue that a measure of percentage leaf damage alone does not straightforwardly reflect the cost of herbivory to the plant, and on its own does not constitute an appropriate test of the biotic interactions hypothesis. For a given amount of herbivory, the impact of herbivory is dependent upon many factors, such as the construction cost of the leaf, the growth and replacement rates and leaf life span. We investigate the latitudinal gradient in herbivory by analysing a large dataset of herbivory rates for 452 tree species and separating the species into those with short and long leaf life spans. We show that annual herbivory rates tend to be greater at lower latitudes for evergreen species (which have long‐lived leaves), but no trend in herbivory rate with latitude was found for species with short leaf life spans. Phylogenetic least squares regression assuming Ornstein‐Uhlenbeck processes also showed a negative effect of latitude on herbivory rate for evergreen trees, but we caution that viewing herbivory as a species trait is problematic. An integrative approach that incorporates leaf life span, as well as the costs of investment in growth and potential costs of losing leaf tissue, is needed to further our understanding of the ecological and evolutionary dynamics of herbivory.     

Energy and the tempo of evolution in amphibians

Aim The evolutionary speed hypothesis (ESH) attempts to explain global patterns of species richness on the basis that rates of molecular evolution and speciation in warmer climates have led to a greater accumulation of taxa at lower latitudes. A substantial alternative hypothesis to the ESH is the tropical conservatism hypothesis (TCH). However, recent tests of the TCH, using amphibians as the model taxon, have relied on the assumption that rates of molecular evolution are stable across latitudes and elevations. Here, we test for the first time for systematic variation in rates of molecular evolution across latitude and elevation among amphibians. Location The dataset is geographically diverse with samples from all continents except Antarctica and also from many of the earth's major tropical–warm temperate archipelagos. Methods We tested for substitution rate heterogeneity across climatically varying habitats with the mitochondrial RNA genes 12S and 16S. Thus, we report here on our findings for amphibians – a taxon whose phylogenetic and trophic contexts are remote from those previously tested – using genes that have also not been examined before. The study utilized paired contrasts of sister species (188 species across 18 families, including both caudates and anurans) that are spatially separated in either latitudinal or elevational dimensions. Results We found substantially faster substitution rates for species living in warmer habitats (P= 0.001–0.002) at both lower latitudes (P < 0.02) and lower elevations (P < 0.01). Main conclusions The consistency of these results with the previous studies that used quite different organisms – and in this instance also using different genes – suggests that this is a ubiquitous pattern in nature consistent with the predictions of the ESH. Recent tests of the TCH that, in estimating diversification rates, have relied on the assumption that DNA evolution occurs at a constant rate across latitudes and elevations, require reconsideration in light of the findings presented here. Our results indicate that greater caution is required when estimating dates of divergence using DNA sequence data.     

High rates of extra‐pair paternity in two equatorial populations of rufous‐collared sparrow,Zonotrichia capensis

The latitudinal increase in extra‐pair paternity (EPP) rates in birds suggests broad selective benefits to low EPP rates in the tropics. However, we have few EPP data from tropical birds, particularly from species with close relatives at high latitudes. Here, we report EPP rates in two resident equatorial populations of rufous‐collared sparrow Zonotrichia capensis, a genus well‐represented at high latitudes. We found 64% and 60% of broods contained extra‐pair offspring, and 42% and 52% of all young were extra‐pair. EPP rates were similar in these populations, despite clear differences in elevation, temperature, rainfall, and breeding season length. These findings provide evidence that EPP rates in tropical birds can be as high as those observed in temperate birds, and suggest that the selective pressures acting on EPP rates vary markedly across tropical birds.     

Latitudinal variation in herbivory: influences of climatic drivers,herbivore identity and natural enemies

Although a number of investigations have concluded that lower latitudes are associated with increases in herbivore abundance and plant damage, the generality of this pattern is still under debate. Multiple factors may explain the lack of consistency in latitude–herbivory relationships. For instance, latitudinal variation in herbivore pressure may be shaped entirely or not by climatic variables, or vary among herbivore guilds with differing life‐history traits. Additionally, the strength of top–down effects from natural enemies on herbivores might also vary geographically and influence latitude–herbivory patterns. We carried out a field study where we investigated the effects of latitude and climate on herbivory by a seed‐eating caterpillar and leaf chewers, as well as parasitism associated to the former across 30 populations of the perennial herb Ruellia nudiflora (Acanthaceae). These populations were distributed along a 5° latitudinal gradient from northern Yucatan (Mexico) to southern Belize, representing one‐third of the species' latitudinal distribution and the entirety and one‐third of the precipitation and temperature gradient of this species' distribution (respectively). We found opposing latitudinal gradients of seed herbivory and leaf herbivory, and this difference appeared to be mediated by contrasting effects of climate on each guild. Specifically, univariate regressions showed that seed herbivory increased at higher latitudes and with colder temperatures, while leaf herbivory increased toward the equator and with wetter conditions. Multiple regressions including temperature, precipitation and latitude only found significant effects of temperature for seed herbivory and latitude for leaf herbivory. Accordingly, that latitudinal variation in seed herbivory appears to be driven predominantly by variation in temperature whereas latitudinal variation in leaf herbivory was apparently driven by other unexplored correlates of latitude. Parasitism did not exhibit variation with latitude or climatic factors. Overall, these findings underscore that the factors driving latitudinal clines in herbivory might vary even among herbivore species coexisting on the same host plant.     

Density dependence in forests is stronger in tropical and subtropical climates among closely related species

Density dependence has long been considered an important mechanism for species coexistence in forests. Density‐dependent processes can be important mechanisms driving differences in species diversity across latitudes. Here we examined the decline in strength of density dependence with increasing latitude, and particularly how density dependence affected both conspecifics and heterospecifics. Conspecific individuals within a species were predominantly aggregated at the three different latitudes of the three study sites in China. The percentage of aggregated species declined with increasing spatial scale and growth stages, which confirmed the overall importance of density dependence. Compared with a latitudinal gradient, the intensity of aggregation in the most northerly temperate (Changbaishan) plot was significantly higher than that in the tropical (Bawangling) or subtropical (Heishiding) plots. This showed that the strength of density dependence among conspecific individuals at low latitudes was stronger than that for high latitudes. We found that the more closely related species were more spatially adjacent in the temperate plot, while the opposite was true in the tropical and subtropical plots at most scales. After calculating the recruitment probability of all species of mature trees, we found that 19 of the 32 species in the tropical plot and 7 of the 12 species in the subtropical plot were less likely to recruit near closely related species. In the northern temperate plot, only one species demonstrated this phenomenon. These results therefore suggest that latitudinal variation in the intensity of negative density‐dependent recruitment resulting from specialist natural enemies (the Janzen–Connell hypothesis) may contribute to the latitudinal gradient of diversity in trees. The strength of density dependence at low latitudes was stronger than at high latitudes, regardless of whether this dependence was measured between only conspecific individuals or between individuals of closely related species.     

Seasonality of peak metabolic rate in non‐migrant tropical birds

Birds that are year‐round residents of temperate and tropical regions have divergent life histories. Tropical birds have a slower ‘pace of life’, one characteristic of which includes lower peak metabolic rate and daily activity levels. Temperate resident birds are faced with seasonal variation in thermogenic demand. This challenge is met with seasonally increased peak metabolic rate during winter. These thermogenic demands are much lower in birds that are year‐round tropical residents. By measuring peak (summit) metabolic rate in tropical and temperate resident bird species during summer and winter, we asked whether tropical birds exhibit seasonality in peak metabolic rate, and if the direction of seasonality differs between tropical and temperate species. We measured summit metabolism in seven tropical and one temperate species during the winter and during the summer breeding season to test the hypothesis that summit metabolism of tropical residents would change seasonally. We consider whether metabolic seasonality is associated with breeding season for tropical species. We found that summit metabolism was significantly greater during the summer for most tropical residents, while the temperate resident matched several previous reports with higher summit metabolism in winter. We conclude that metabolic seasonality occurs in tropical residents and differs from temperate residents, suggesting that breeding during the summer may be driving relatively higher metabolism as compared to winter thermogenesis in temperate birds.     

Growth rate variation among passerine species in tropical and temperate sites: an antagonistic interaction between parental food provisioning and nest predation risk

Causes of interspecific variation in growth rates within and among geographic regions remain poorly understood. Passerine birds represent an intriguing case because differing theories yield the possibility of an antagonistic interaction between nest predation risk and food delivery rates on evolution of growth rates. We test this possibility among 64 Passerine species studied on three continents, including tropical and north and south temperate latitudes. Growth rates increased strongly with nestling predation rates within, but not between, sites. The importance of nest predation was further emphasized by revealing hidden allometric scaling effects. Nestling predation risk also was associated with reduced total feeding rates and per-nestling feeding rates within each site. Consequently, faster growth rates were associated with decreased per-nestling food delivery rates across species, both within and among regions. These relationships suggest that Passerines can evolve growth strategies in response to predation risk whereby food resources are not the primary limit on growth rate differences among species. In contrast, reaction norms of growth rate relative to brood size suggest that food may limit growth rates within species in temperate, but not tropical, regions. Results here provide new insight into evolution of growth strategies relative to predation risk and food within and among species.     

Latitudinal gradients in some,but not all,avian life history traits extend into the Arctic

Latitudinal variation in avian life history strategies is well documented. Clutch size and nest success tend to increase with latitude, whereas longevity and developmental periods have been argued to decrease with latitude. However, these patterns are largely based on interspecific comparisons of species breeding at tropical and temperate latitudes. We compared the life history of Yellow Warblers Setophaga petechia breeding in arctic habitat at the northern extent of their range, in Inuvik, NWT (68°N), Canada, with those breeding in temperate habitat in Revelstoke, BC (50°N), and use data from 21 populations spanning 0–68°N to evaluate latitudinal trends in life history traits from tropical to arctic habitats. Females breeding in Inuvik laid first clutches that were slightly (although not significantly) larger and had higher nest success, which resulted in higher annual productivity compared with their low- latitude counterparts. Apparent adult survival rates were only marginally lower in Inuvik than in Revelstoke, whereas incubation and nestling periods in the arctic were similar to our temperate site. When comparing life history traits across the Yellow Warbler breeding range, we observed increases in clutch sizes and nest success with increasing latitude that appeared to be associated with declines in adult survival, though this relationship was weakened by the addition of our arctic site. We detected more moderate declines in incubation and nestling periods with increasing latitude. As we observed latitudinal variation in some life history traits, but not a consistent transition of traits associated with a shift from a slow to fast life history from tropical to arctic latitudes, our study suggests that the expectation for a general shift in life history traits may be over-simplified.     

Thermal tolerance, climatic variability and latitude

The greater latitudinal extents of occurrence of species towards higher latitudes has been attributed to the broadening of physiological tolerances with latitude as a result of increases in climatic variation. While there is some support for such patterns in climate, the physiological tolerances of species across large latitudinal gradients have seldom been assessed. Here we report findings for insects based on published upper and lower lethal temperature data. The upper thermal limits show little geographical variation. In contrast, the lower bounds of supercooling points and lower lethal temperatures do indeed decline with latitude. However, this is not the case for the upper bounds, leading to an increase in the variation in lower lethal limits with latitude. These results provide some support for the physiological tolerance assumption associated with Rapoport's rule, but highlight the need for coupled data on species tolerances and range size.     

Functional linkages for the pace of life, life-history, and environment in birds

For vertebrates, body mass underlies much of the variation in metabolism, but among animals of the same body mass, metabolism varies six-fold. Understanding how natural selection can influence variation in metabolism remains a central focus of Physiological Ecologists. Life-history theory postulates that many physiological traits, such as metabolism, may be understood in terms of key maturational and reproductive characteristics over an organism's life-span. Although it is widely acknowledged that physiological processes serve as a foundation for life-history trade-offs, the physiological mechanisms that underlie the diversification of life-histories remain elusive. Data show that tropical birds have a reduced basal metabolism (BMR), field metabolic rate, and peak metabolic rate compared with temperate counterparts, results consistent with the idea that a low mortality, and therefore increased longevity, and low productivity is associated with low mass-specific metabolic rate. Mass-adjusted BMR of tropical and temperate birds was associated with survival rate, in accordance with the view that animals with a slow pace of life tend to have increased life spans. To understand the mechanisms responsible for a reduced rate of metabolism in tropical birds compared with temperate species, we summarized an unpublished study, based on data from the literature, on organ masses for both groups. Tropical birds had smaller hearts, kidneys, livers, and pectoral muscles than did temperate species of the same body size, but they had a relatively larger skeletal mass. Direct measurements of organ masses for tropical and temperate birds showed that the heart, kidneys, and lungs were significantly smaller in tropical birds, although sample sizes were small. Also from an ongoing study, we summarized results to date on connections between whole-organism metabolism in tropical and temperate birds and attributes of their dermal fibroblasts grown in cell culture. Cells derived from tropical birds had a slower rate of growth, consistent with the hypothesis that these cells have a slower metabolism. We found that dermal fibroblasts from tropical birds resisted chemical agents that induce oxidative and non-oxidative stress better than do cells from temperate species, consistent with the hypothesis that birds that live longer invest more in self-maintenance such as antioxidant properties of cells.     

Latitudinal variation in growth and survival of juvenile corals in the West and South Pacific

Reef-building corals are found across > 30° of latitude from tropical to temperate regions, where they occupy habitats greatly differing in seawater temperature and light regimes. It remains largely unknown, however, how the demography of corals differs across this gradient of environmental conditions. Variation in coral growth is especially important to coral populations, because aspects of coral demography are dependent on colony size, with both fecundity and survivorship increasing with larger colonies. Here we tested for latitudinal variation in annual growth rate and survival of juvenile corals, using 11 study locations extending from 17° S to 33° N in the West and South Pacific. Regression analyses revealed a significant decline in annual growth rates with increasing latitude, whereas no significant latitudinal pattern was detected in annual survival. Seawater temperature showed a significant and positive association with annual growth rates. Growth rates varied among the four common genera, allowing them to be ranked Acropora > Pocillopora > Porites > Dipsastraea. Acropora and Pocillopora showed more variation in growth rates across latitudes than Porites and Dipsastraea. Although the present data have limitations with regard to difference in depths, survey periods, and replication among locations, they provide evidence that a higher capacity for growth of individual colonies may facilitate population growth, and hence population recovery following disturbances, at lower latitudes. These trends are likely to be best developed in Acropora and Pocillopora, which have high rates of colony growth.

     

Latitudinal variation in genetic divergence of populations and the potential for future speciation

The increase in biological diversity with decreasing latitude is widely appreciated but the cause of the pattern is unknown. This pattern reflects latitudinal variation in both the origin of new species (cladogenesis) and the number of species that coexist. Here we address latitudinal variation in species origination, by examining population genetic processes that influence speciation. Previous data suggest a greater number of speciation events at lower latitudes. If speciation events occur more frequently at lower latitudes, we predicted that genetic divergence among populations within species, an important component of cladogenesis, should be greater among lower latitude populations. We tested this prediction using within-species patterns of mtDNA variation across 60 vertebrate species that collectively spanned six continents, two oceans, and 119 degrees latitude. We found greater genetic divergence of populations, controlling for geographic distance, at lower latitudes within species. This pattern remained statistically significant after removing populations that occur in localities previously covered by continental glaciers during the last glaciation. Results suggest that lower latitude populations within species exhibit greater evolutionary independence, increasing the likelihood that mutation, recombination, selection, and/or drift will lead to divergence of traits important for reproductive isolation and speciation. Results are consistent with a greater influence of seasonality, reduced energy, and/or glacial (Milankovitch) cycles acting on higher latitude populations, and represent one of the few tests of predictions of latitudinal variation in speciation rates using population genetic data.