Density-dependent growth and sexual maturity of silver hake in the north-west Atlantic

Stock abundance was the most important predictor of sexual maturation of age 2 and 3 silver hake in the north-west Atlantic between 1973–1990. Growth at ages 1 and 2 in the southern stock was significantly and negatively correlated with stock abundance (r²>0.56, P <0.01). Sexual maturity may be mediated through competition and growth during the first and second years of life. In the northern stock, growth and stock abundance was not significantly correlated ( P >0.05). Here, increased sexual maturity at ages 2 and 3 during 1973–1990 may have occurred without increased somatic growth, already near its maximum, due to the relatively cooler water temperatures of the Gulf of Maine. Logistic regression models indicated that maturation is dependent on age, stock density and an age-density interaction.     

Lifetime reproductive effort in humans

Lifetime reproductive effort (LRE) measures the total amount of metabolized energy diverted to reproduction during the lifespan. LRE captures key components of the life history and is particularly useful for describing and comparing the life histories of different organisms. Given a simple energetic production constraint, LRE is predicted to be similar in value for very different life histories. However, humans have some unique ecological characteristics that may alter LRE, such as the long post-reproductive lifespan, lengthy juvenile period and the cooperative nature of human foraging and reproduction. We calculate LRE for natural fertility human populations, compare the findings to other mammals and discuss the implications for human life-history evolution. We find that human life-history traits combine to yield the theoretically predicted value (approx. 1.4). Thus, even with the subsidized energy budget and uniqueness of the adult lifespan, human reproductive strategies converge on the same optimal value of LRE. This suggests that the fundamental demographic variables contained in LRE trade-off against one another in a predictable and highly constrained manner.     

The influence of male age and simulated pathogenic infection on producing a dishonest sexual signal

In recent years, studies have shown that reproductive effort decelerates in response to pathogenic infection. If infection substantially reduces a host''s residual reproductive value (RRV), however, then an acceleration of effort may instead occur (e.g. terminal investment). Reproductive acceleration would theoretically allow hosts to maintain or exaggerate their sexual signal upon infection. This would create a deceptive message from the perspective of the chooser, who may unwittingly copulate with an infected mate to their detriment. Using the cricket Allonemobius socius, we assessed the potential for reduced RRV to accelerate male reproductive effort and create a dishonest signal. RRV was manipulated through male age and simulated pathogenic insult. Reproductive effort was measured as calling song energetics, mating success, latency to mate and nuptial gift size. We show that males adopted either an accelerated or decelerated reproductive strategy upon infection, and that this decision was probably mediated by RRV. Moreover, males who accelerated their effort produced a dishonest signal by increasing their song energetics while providing fewer paternal resources (i.e. smaller gifts). Our study is one of the few to document the existence of dishonest signals and relate dishonesty to a potential reduction in female fitness, underscoring the conflict inherent in sexual reproduction.     

The cost of floral longevity in Clarkia tembloriensis: An experimental investigation

The hypothesis that flower maintenance requires resources that would be used to support other plant functions (i.e. a cost of floral maintenance) was tested by experimentally manipulating floral longevity. Plants of Clarkia tembloriensis, a species with pollination-induced flower senescence, received either early or late pollinations (long and short longevities, respectively). We examined the effect of this manipulation on (1) per-flower allocation to nectar production and (2) flower, fruit and seed production per plant under two levels of resource availability. The direct costs of floral longevity measured in terms of nectar sugar were high: flowers that were maintained 35% longer invested proportionately more in nectar sugar (30%). At the whole-plant level, a cost of floral longevity was manifested as reduced seed production, but the magnitude of this cost varied with resource level. While plants with longer-lived flowers showed a 12% reduction in seed production, those that experienced reduced resource levels via partial defoliation, showed a decrement in seed production that was almost three times larger (34%). These differences were not brought about by changes in the number of flowers and fruits, but by significant alterations in their sizes. A model that expresses the cost of flower maintenance as a trade-off between floral longevity and seed production shows that an optimal flower longevity is determined by both the rate of fitness accrual and the cost of floral maintenance.     

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.     

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.     

青藏高原东缘块茎堇菜鳞茎分配的个体大小依赖性

块茎堇菜(Viola tuberifera)为青藏高原特有两型闭锁花植物,属多年生草本,具独特的混和交配系统,既可通过早春开放花异花受精和夏季地上地下闭锁花自花受精有性繁殖,还可通过秋季新鳞茎无性繁殖产生后代。高山环境下,异花受精常因花粉限制而无法正常进行,自花受精和克隆繁殖成为保障植物种群正常繁衍的不二之选,而克隆繁殖更能在植株资源消耗最小的情况下保障子代的存活。该文以青藏高原东缘高寒草甸的混合繁育植物块茎堇菜为研究对象,探索其生长期内鳞茎分配的个体大小依赖性,以及植株如何权衡鳞茎的资源分配以适应个体大小的变化。结果表明：块茎堇菜生活史阶段的鳞茎分配具有个体大小依赖性,鳞茎分配与个体大小呈极显著负幂指数相关关系(P<0.01),个体越大,鳞茎分配越小;反之,个体越小,鳞茎分配越高。即块茎堇菜对鳞茎的资源投入受个体大小的制约,通过鳞茎分配比例的高低响应植株自身资源状况的变化,保障在高寒环境下植物种群的生存和繁衍。该研究结果为高山植物克隆繁殖的生活史进化提供了依据。     

草地植物繁殖性状权衡关系研究进展

植物繁殖性状间的权衡关系反映了植物在产生子代过程中资源获取与分配的生态策略;这影响植物子代萌发、生长及竞争能力。草地植物具有重要的生产和生态功能。草地植物繁殖性状的生态策略研究发现;营养生长和繁殖、繁殖方式以及植物花性状间存在普遍的权衡关系;而缺乏对植物自身繁殖性状间权衡关系的关注。本文综述了草地植物繁殖特征权衡关系的研究进展;从有性与无性繁殖、花部性状、种子大小和数量;到地下芽库与土壤种子库之间的多层面权衡;发现种群内个体和花序水平性分配方面鲜有关于权衡关系研究;揭示了繁殖体权衡如何塑造植物的繁殖策略和生存适应性的机制。通过深入分析权衡机制在植物生长、种群动态和生态系统多样性中的关键作用;阐述了其在理解全球变化背景下植物生长、繁殖及胁迫响应方面的理论价值;旨在为未来草地植物繁殖性状权衡关系研究提供不同角度的参考。     

Physiological costs of growing fast: does accelerated growth reduce pay-off in adult fitness?

Accumulating evidence suggests that, in contrast to earlier assumptions, juvenile growth rates are optimised by means of natural and sexual selection rather than maximised to be as fast as possible. Owing to the generally accepted advantage of growing fast to adulthood, such adaptive variation strongly implies the existence of costs attached to rapid growth. By using four populations of protandrous copper butterflies with known differences in intrinsic growth rates within and across populations, we investigate a potential trade-off between rapid growth and the proportionate weight loss at metamorphosis. While controlling for effects of pupal time and mass, we demonstrate that (1) protandrous males, exhibiting higher growth rates, suffer a higher weight loss than females throughout, that (2) population differences in weight loss generally follow known differences in growth rates, and that (3) males have by 6 higher metabolic rates than females during pupal development. These results support the notion that a higher weight loss during the development to adulthood may comprise a physiological cost of rapid development, with the pay-off of accelerated growth being reduced by a disproportionally smaller adult sizeCo-ordinating editor: Leimer