Insular shifts and trade-offs in life-history traits in pond frogs in the Zhoushan Archipelago, China

Island and mainland populations of animal species often differ strikingly in life-history traits such as clutch size, egg size, total reproductive effort and body size. However, despite widespread recognition of insular shifts in these life-history traits in birds, mammals and reptiles, there have been no reports of such life-history shifts in amphibians. Furthermore, most studies have focused on one specific life-history trait without explicit consideration of coordinated evolution among these intimately linked life-history traits, and thus the relationships among these traits are poorly studied. Here we provide the first evidence of insular shifts and trade-offs in a coordinated suite of life-history traits for an amphibian species, the pond frog Rana nigromaculata . Life-history data were collected from eight islands in the Zhoushan Archipelago and neighboring mainland China. We found consistent, significant shifts in all life-history traits between mainland and island populations. Island populations had smaller clutch sizes, larger egg sizes, larger female body size and invested less in total reproductive effort than mainland populations. Significant negative relationships were found between egg size and clutch size and between egg size and total reproductive effort among frog populations after controlling for the effects of body size. Therefore, decreased reproductive effort and clutch size, larger egg size and body size in pond frogs on islands were selected through trade-offs as an overall life-history strategy. Our findings contribute to the formation of a broad, repeatable ecological generality for insular shifts in life-history traits across a range of terrestrial vertebrate taxa.     

Covariation in Insular Life-history Traits of the Rice Frog(Fejervarya limnocharis)in Eastern China

Although shifts in life-history traits of insular vertebrates, as compared with mainland populations, have been observed in many taxa, few studies have examined the relationships among individual life-history traits on islands. Lifehistory theory also predicts that there is a trade-off between body size and reproductive effort, and between egg size and clutch size. We surveyed the rice frog, Fejervarya limnocharis, on 20 islands within the Zhoushan Archipelago and two nearby sites on the mainland of China to compare differences in life-history traits and to explore relationships among those traits. Rice frog females reached a greater body size on half of the smaller islands among the total 20 surveyed islands, and larger egg size, decreased clutch size and reduced reproductive effort on most of the islands when compared to the two mainland sites. Insular body size was negatively correlated with reproductive effort. There was a negative correlation between egg size and clutch size. Results suggest that life-history theory provides a good explanation for co-variation between body size and reproductive effort, and between egg size and clutch size in rice frogs on the islands.     

近期引入三刺鱼种群的骨板和体型的现时进化

近期引入到新环境中的种群给我们提供了一个推论种群过去微进化变化的难得的机会,而这些变化曾导致了种群在历史上对新栖息地拓殖的适应。自从1967年三刺鱼(GasterosteusaculeatusL.)被有意引入到不列颠哥伦比亚的Heisholt湖后,就隔离的淡水对其完全骨板化(CP)变体的相对适合度的影响已经做过多种多样的测定。CP变体的个体在早期的样本中比较常见(占20.3%-31.7%),而在现代的样本中比较稀少(占0%-5.0%)。后者样本中骨板弱化的变体占优势,这是绝大多数淡水种群的典型情形。我估测Heisholt湖的一个流域中三刺鱼体侧骨板数目从1974年到1997年的进化速率是-0.029海尔登,这比大多数对近期引入的或隔离的三刺鱼种群的现时进化的估测要低。最后,来自于Heisholt湖的CP个体比那些作为引入源溪流中的个体明显要小。总之,对应于已建群的自然淡水种群的文献资料,在引入种群中所观察到的个体大小和体侧骨板数目的变化,意味着三刺鱼对与淡水环境中生活相关的多种挑战的适应可以快速发生。     

Correlated contemporary evolution of life history traits in New Zealand Chinook salmon, Oncorhynchus tshawytscha

Size at age and age at maturity are important life history traits, affecting individual fitness and population demography. In salmon and other organisms, size and growth rate are commonly considered cues for maturation and thus age at maturity may or may not evolve independently of these features. Recent concerns surrounding the potential phenotypic and demographic responses of populations facing anthropogenic disturbances, such as climate change and harvest, place a premium on understanding the evolutionary genetic basis for evolution in size at age and age at maturity. In this study, we present the findings from a set of common-garden rearing experiments that empirically assess the heritable basis of phenotypic divergence in size at age and age at maturity in Chinook salmon (Oncorhynchus tshawytscha) populations introduced to New Zealand. We found consistent evidence of heritable differences among populations in both size at age and age at maturity, often corresponding to patterns observed in the wild. Populations diverged in size and growth profiles, even when accounting for eventual age at maturation. By contrast, most, but not all, cases of divergence in age at maturity were driven by the differences in size or growth rate rather than differences in the threshold relationship linking growth rate and probability of maturation. These findings help us understand how life histories may evolve through trait interactions in populations exposed to natural and anthropogenic disturbances, and how we might best detect such evolution.     

Age-dependent reproductive costs and the role of breeding skills in the Collared flycatcher

This study addressed whether there are any age‐related differences in reproductive costs. Of especial interest was whether young individuals increased their reproductive effort, and thereby their reproductive cost, as much as older birds when brood size was enlarged. To address these questions, a brood‐size manipulation experiment with reciprocal cross‐fostering of nestlings of young and middle‐aged female Collared flycatchers, Ficedula albicollis, was performed on the Swedish island of Gotland. Nestlings’ body mass, tarsus length and survival were recorded to estimate the parental ability and parental effort of the experimental female birds. Female survival and clutch size were recorded in the following years to estimate reproductive costs. We found that middle‐aged female flycatchers coped better with enlarged broods than younger females or invested more in reproduction. In the following year, young female birds that had raised enlarged broods laid smaller clutches than the females from all the other experimental groups. This result shows that the young female birds pay higher reproductive costs than the middle‐aged females. Both young and middle‐aged female flycatchers seemed to increase their reproductive effort when brood size was increased. However, such an increase resulted in higher reproductive costs for the young females. The difference in reproductive costs between birds of different ages is most likely a result of insufficient breeding skills of the young individuals.     

Genetic and morphological similarity of two California gull populations with different life history traits

California Gulls nesting at Mono Lake (ML) have a modal clutch size of two eggs, while at Great Salt Lake (GSL) the mode is three. We used starch gel electrophoresis to determine whether the populations are genically different. Average individual heterozygosity at ML averages 2.67% and at GSL 2.95%, both slightly below the average for other birds. The average number of alleles per locus is 1.2 for both sites. Variation between sites is insignificant, with a genetic distance of 0.001 and an F_ST of 0.0044. Therefore, the clutch size differences are without genic correlates. In fact, at the loci studied, the two samples are indistinguishable from two random samples drawn from a panmictic unit. Five morphological characteristics also exhibited little between-site difference, in agreement with the electrophoretic analysis. A marked historical reduction (bottleneck) in the Mono Lake population seems not to have affected levels of genetic variation. We conclude that the clutch size differences are either recently evolved, and determined by loci not surveyed by us, or they are environmentally induced. At the population level, these gulls are similar to passerines in terms of heterozygosity and among population differentiation.     

Life history evolution in cichlids 1: revisiting the evolution of life histories in relation to parental care

Empirical links between egg size and duration of parental care in fishes have generated a considerable amount of theory concerning life history evolution. However, to date, this link has not been investigated in relation to other important life-history traits such as clutch size and body size, or while controlling for shared ancestry between species. We provide the first phylogenetically based tests using a database with information on egg size, clutch size, body size and care duration in cichlid fishes (Cichlidae). Multiple regression analyses, based on independent contrasts on both the species and the genus level, showed that clutch size is the variable most closely related to duration of care. This pattern appeared to be driven by post-hatch care relationships. Our results show that, contrary to expectation, there is no positive link between egg size and care duration in Cichlidae. Instead, greater reproductive output through increased clutch size investment appears to have coevolved with greater care of offspring. We suggest that re-evaluation of the generality of current models of the evolution of egg size under parental care in fishes is needed.     

Phenotypic integration of life history and morphology: an example from three-spined stickleback, Gasterosteus aculeatus L.

Although typically life history and morphology are studied separately, they may evolve most often in concert. Therefore, a full understanding of the evolution of fish life-history patterns may require exploration of the evolutionary interplay between components of life history and other aspects of phenotype. One of the most promising approaches to understanding phenotypic integration is population comparison. This approach is particularly effective when ancestralderived relationships are understood and when multiple populations can be inferred to have evolved derived character states independently. Here we provide an example of this approach using five allopatric populations of freshwater three-spined stickleback ( Gasterosteus aculcatus L.) that have diverged in response to differences in selection regimes among the lakes they inhabit. We demonstrate a relationship between one aspect of reproductive life history, clutch volume and relative body shape. The differences are consistent with those predicted on the basis of differences in trophic habit and overall body form. Finally, we discuss the value of particular groups of fish for use in comparative studies and explore the kinds of evolutionary issues that can be addressed through population comparison.     

Habitat-specific clutch size and cost of incubation in common eiders, Somateria mollissima

Common eiders, Somateria mollissima, breed on two types of island in the Northern Baltic: open, sparsely vegetated islands and wooded islands with dense mixed forests. On open islands 79.8% of the nests were on open cliffs, exposed to wind and rain whereas on wooded islands 91.7% of the nests were sheltered inside dense spruce and pine thickets. We found that clutch size on open islands was significantly smaller than on wooded islands. Females started breeding simultaneously in the two habitats and they were similar in body size as measured by the length of the radio-ulna. During incubation females on open islands lost weight at a faster rate than females on wooded islands (34 g/day and 19 g/day, respectively). Heat loss is faster on open than wooded islands and therefore we suggest that the faster weight loss of females on open islands result from thermodynamically adverse incubation conditions. Because the eider is an extreme capital breeder, energy used for egg production cannot be used for incubation. To sustain a higher incubation cost on open islands, the optimal clutch size is therefore lower than on wooded islands. Received: 27 January 1997 / Accepted: 14 March 1997     

Rapid genetic divergence in postglacial populations of threespine stickleback (Gasterosteus aculeatus): the role of habitat type, drainage and geographical proximity

The goal of this study was to evaluate the role of common ancestry, and of geographical or reproductive isolation, in genetic divergence in populations of threespine sticklebacks (Gasterosteus aculeatus). Using seven DNA microsatellite loci we compared the effects of habitat type, drainage system and geographical proximity on genetic distance among 16 populations situated in an area in Schleswig-Holstein (Germany) that became deglaciated approximately 12 000 years ago. Stickleback population structure correlated only weakly with drainage system, whereas the primary divergence was among habitat types. Phylogenetic analysis revealed that lake (n = 7) and river (n = 5) populations formed two distinct clades (Cavalli-Sforza's and Edwards' chord distance, 82-100% bootstrap support) at approximately equal genetic distances to a third clade, comprising putative estuarine (n = 4) ancestors. Allele frequencies in lake and river populations represented different subsets of the genetically more diverse estuarine populations. In nested amovas approximately twice the genetic variance was distributed among lake vs. river vs. estuarine populations as compared with the combined effects of drainage system and geographical distance. Limited gene flow between habitat types must have been established after postglacial colonization, suggesting ecological hybrid inferiority or behavioural mating barriers between ecotypes. Within estuarine and lake populations, population differentiation followed an isolation-by-distance model. Given the high observed heterozygosities within the 16 study populations (HO = 0.65-0.87), the mean divergence between lake and river population pairs (FST = 0.18 +/- 0.007) would be reached after 300-6000 generations in a stepwise mutation model, depending on the size of N(e). This demonstrates both the utility of hypervariable microsatellites for detecting recent population divergences and the danger of operating at temporal or spatial scales which are beyond their resolution.     

Observations on some reproductive traits of Triops granarius (Lucas) (Crustacea: Notostraca)

Reproductive traits of Triops granarius females were examined. Fertilization by males was a prerequisite for oviposition. Clutch size ranged from 1 to 154, and clutch frequency (per 24 hours) from 1 to 9. Clutch size generally increased with increasing body size and increasing age. Total reproductive output was dependent upon life span, and estimated at around 200 eggs for a life span of 25 days. Clutch data indicate that T. granarius may have r-selected traits.     

Costs of reproduction in a tropical invariant-clutch producing lizard (Carlia rubrigularis)

Reproduction involves costs and benefits, whereby increased expenditure into current reproduction can reduce future reproductive success. Costs of reproduction, often assessed using the reduction in locomotor speed, have become well established in temperate-zone lizard species. However, substantial differences in biotic conditions and life-history traits between temperate- and tropical-zone lizards suggest that such costs may be different or of less importance for tropical species. This study examined the effects of reproduction on locomotor speed in the tropical invariant-clutch producing lizard Carlia rubrigularis . Counter to predictions and despite a low relative clutch mass (RCM), gravid and post-oviposition females experienced a reduction in locomotor speed with a physiological basis that was unrelated to the level of reproductive investment. In addition, gravid and postoviposition females exhibited locomotor speeds that were inversely related to the timing of oviposition and approached the speed of non-reproductive females after 3 weeks of oviposition. These results suggest that in addition to RCM, selection may act to reduce the period of recovery in species with extended reproductive seasons and which make numerous bouts of reproduction, such as C. rubrigularis .     

Seasonal variation of egg size and number in a Daphnia pulex population

Seasonal variation of egg size and number was examined in a Daphnia pulex population inhabiting a vernal pond. In this population, size at maturity declines at midseason, probably as an adaptive response to size-selective predation by larvae of the salamander Ambystoma. The larger early season individuals produce more and larger eggs than the smaller late season individuals. Age at maturity does not vary between seasons. Laboratory experiments indicate that temperature may affect egg size, egg number and size at maturity. However, field data suggest that temperature accounts for only a small fraction of the total variation in egg size and number. Indirect measures of nutrition indicate that food limitation does not cause the seasonal decline in egg size and number. The seasonal change in reproductive traits is well correlated with changes in invertebrate and vertebrate predation. Examination of predator feeding preferences and their impact on Daphnia mortality indicate that variation of reproductive traits is most likely a complex adaptation to changing predation regimes.     

Sexual cannibalism in Nephila plumipes as a consequence of female life history strategies

The evolution of sexual cannibalism has been modelled as both an adaptive and nonadaptive female strategy. Recent evidence from several species suggests a connection between female foraging and sexual cannibalism, but the precise benefits for females have remained obscure. Here, we investigate the difference between cannibalistic and noncannibalistic female Nephila plumipes by removing the potential nutritional benefit of cannibalism. Courting and mating males that were killed by a female were immediately removed so that the female could not consume them. Nevertheless, cannibalistic females gained more mass from maturation to oviposition and produced larger first clutches than noncannibalistic females, although cannibalistic females matured at a smaller size and mass than noncannibalistic females. In juvenile instars, mass gain was generally smaller in females that moulted in a good condition but intermoult intervals were shorter. However, the time from maturity to oviposition was not shorter in females that matured in a good condition. Male behaviour did not differ according to the risk of cannibalism. We suggest that sexual cannibalism in N. plumipes is a side‐effect of an increased foraging vigour of females that matured at a smaller size and body mass. Selection pressure on males to avoid cannibalism may be weak because of limited mating opportunities.     

Heuristic optimization of the general life history problem: A novel approach

Summary The general life history problem concerns the optimal allocation of resources to growth, survival and reproduction. We analysed this problem for a perennial model organism that decides once each year to switch from growth to reproduction. As a fitness measure we used the Malthusian parameterr, which we calculated from the Euler-Lotka equation. Trade-offs were incorporated by assuming that fecundity is size dependent, so that increased fecundity could only be gained by devoting more time to growth and less time to reproduction. To calculate numerically the optimalr for different growth dynamics and mortality regimes, we used a simplified version of the simulated annealing method. The major differences among optimal life histories resulted from different accumulation patterns of intrinsic mortalities resulting from reproductive costs. If these mortalities were accumulated throughout life, i.e. if they were senescent, a bangbang strategy was optimal, in which there was a single switch from growth to reproduction: after the age at maturity all resources were allocated to reproduction. If reproductive costs did not carry over from year to year, i.e. if they were not senescent, the optimal resource allocation resulted in a graded switch strategy and growth became indeterminate. Our numerical approach brings two major advantages for solving optimization problems in life history theory. First, its implementation is very simple, even for complex models that are analytically intractable. Such intractability emerged in our model when we introduced reproductive costs representing an intrinsic mortality. Second, it is not a backward algorithm. This means that lifespan does not have to be fixed at the begining of the computation. Instead, lifespan itself is a trait that can evolve. We suggest that heuristic algorithms are good tools for solving complex optimality problems in life history theory, in particular questions concerning the evolution of lifespan and senescence.