The Expensive Brain: A framework for explaining evolutionary changes in brain size

To explain variation in relative brain size among homoiothermic vertebrates, we propose the Expensive Brain hypothesis as a unifying explanatory framework. It claims that the costs of a relatively large brain must be met by any combination of increased total energy turnover or reduced energy allocation to another expensive function such as digestion, locomotion, or production (growth and reproduction). Focusing on the energetic costs of brain enlargement, a comparative analysis of the largest mammalian sample assembled to date shows that an increase in brain size leads to larger neonates among all mammals and a longer period of immaturity among monotokous precocial species, but not among the polytokous altricial ones, who instead reduce their litter size. Relatively large brained mammals, altricial and precocial, also show reduced annual fertility rates as compared to their smaller brained relatives, but allomaternal energy inputs allow some cooperatively breeding altricial carnivores to produce even more offspring in a shorter time despite having a relatively large brain. Thus, the Expensive Brain framework explains why brain size is linked to life history pace in some, but not all mammalian lineages. This framework encompasses other hypotheses of energetic constraints on brain size variation and is also compatible with the Brain Malnutrition Risk hypothesis, but the absence of a mammal-wide correlation between brain size and immature period argues against the Needing-to-Learn explanation for slower development among large brained mammals.     

Energy beyond the pupal stage: larval nutrition and its long-time consequences for male mating performance in a scorpionfly

The basic requirement for selection to take effect is variation in fitness relevant traits among individuals of a population. This study is concerned with the question whether environmental conditions met during an early phase of life history that is dominated by the natural component of selection will affect traits and behaviour in a sexual selection context after metamorphosis in a holometabolous insect species. We examined the effects of nutrition as a proximate factor responsible for intrasexual phenotypic variation in the mating performance of male Panorpa vulgaris (Mecoptera: Panorpidae). For this purpose, we manipulated food availability during larval development as well as during adulthood. To obtain matings and to increase their reproductive success males must secrete salivary masses which are then consumed by the females during copulation. The results of the present study are consistent with those of previous studies demonstrating a strong effect of nutrition during adulthood on various fitness relevant traits (salivary gland development, saliva investment in copulations, etc.). But moreover, we could show that food availability during larval development affected male body weight and that there was an interaction between larval and adult diet affecting salivary gland weight relative to body weight. Therefore, food availability during the larval stage can become an important and limiting factor for salivary gland development (and mating success) depending on food availability during adulthood. Several other variables (number of salivary masses, copulation duration, salivary mass weight and saliva investment) seemed not to be associated with larval nutrition.     

Adaptive variation in energy acquisition and allocation among latitudinal populations of the Atlantic silverside

Understanding the evolution of growth rate requires knowledge of the physiology of growth. This study explored the physiological basis of countergradient variation (CnGV) in somatic growth across latitudinal populations of the Atlantic silverside, Menidia menidia. Energetics of northern (Nova Scotia, Canada) and southern (South Carolina, USA) genotypes were compared across resource levels, temperatures, and fish sizes to identify trade-offs to rapid growth. Offered unlimited resources, genotypes differed in both energy acquisition and allocation. Food consumption, growth, and efficiency of northern genotypes were consistently higher than in southern genotypes, across temperatures and body sizes. Feeding metabolism (specific dynamic action; SDA) was proportional to meal size, differing between genotypes to the extent that food consumption differed. Given limited resources, northern and southern genotypes displayed similar growth, efficiency, routine activity, and SDA across temperatures and fish sizes. Routine metabolism was equal at 17°C and 22°C, yet was significantly higher in northern fish at 28°C. Growth rates in M. menidia do not appear to trade off across environments or body sizes, i.e., at no temperature, ration, or size do southern fish outgrow northern conspecifics. Nor does submaximal growth result from increased costs of maintenance, tissue synthesis, or routine activity. Based on our findings, we propose that CnGV consumption and growth in M. menidia likely result from trade-offs with other energetic components, namely sustained and burst swimming. Received: 26 January 1999 / Accepted: 14 September 1999     

Within-trophic group interactions of bacterivorous nematode species and their effects on the bacterial community and nitrogen mineralization

Knowledge of the interactions between organisms within trophic groups is important for an understanding of the role of biodiversity in ecosystem functioning. We hypothesised that interactions between bacterivorous nematodes of different life history strategies would affect nematode population development, bacterial community composition and activity, resulting in increased N mineralization. A microcosm experiment was conducted using three nematode species (Bursilla monhystera, Acrobeloides nanus and Plectus parvus). All the nematode species interacted with each other, but the nature and effects of these interactions depended on the specific species combination. The interaction between B. monhystera and A. nanus was asymmetrically competitive (0,–), whereas that between B. monhystera and P. parvus, and also A. nanus and P. parvus was contramensal (+, –). The interaction that affected microcosm properties the most was the interaction between B. monhystera and P. parvus. This interaction affected the bacterial community composition, increased the bacterial biomass and increased soil N mineralization. B. monhystera and P. parvus have the most different life history strategies, whereas A. nanus has a life history strategy intermediate to those of B. monhystera and P. parvus. We suggest that the difference in life history strategies between species of the same trophic group is of importance for their communal effect on soil ecosystem processes. Our results support the idiosyncrasy hypothesis on the role of biodiversity in ecosystem functioning.     

Meeting the energetic demands of insect diapause: nutrient storage and utilization

Insects in diapause characteristically feed very little or not at all, thus they are largely or totally dependent on energy reserves sequestered prior to the entry into diapause. Fats are the dominant reserve used during this period, but non-fat reserves are also important for some species, especially during certain phases of diapause. Metabolic depression, coupled with the low temperatures of winter, facilitates the economic utilization of reserves during the many months typical of most diapauses. Though many insects store additional lipid prior to the entry into diapause, our review of the literature indicates that this is not always the case. We provide evidence that interactions between nutrient storage and metabolism can influence the decision to enter diapause and determine how long to remain in diapause. In addition, the energy reserves expended during diapause have a profound effect on post-diapause fitness. Though the physiological and biochemical mechanisms that regulate nutrient homeostasis prior to and during diapause remain poorly known, we propose several mechanisms that have the potential to contribute to diapause-associated nutrient homeostasis. Potential players include insulin signaling, neuropeptide F, cGMP-kinase, AMP-activated protein kinase, and adipokinetic hormone.     

Life history patterns of river invertebrates

The initiation of invertebrate distribution patterns in rivers occurs by choice of oviposition sites and is influenced by the evolved reproductive strategies of the individual species. Subsequent redistribution by migration or drifting establishes patterns which are then modified by environmental influences on growth and mortality. Continuity of life cycles is sustained by variations on a number of defined life history strategies combined with evolved behavioural responses.     

An evolutionary life-history framework for understanding sex differences in human mortality rates

Sex differences in mortality rates stem from genetic, physiological, behavioral, and social causes that are best understood when integrated in an evolutionary life history framework. This paper investigates the Male-to-Female Mortality Ratio (M:F MR) from external and internal causes and across contexts to illustrate how sex differences shaped by sexual selection interact with the environment to yield a pattern with some consistency, but also with expected variations due to socioeconomic and other factors. Daniel J. Kruger earned his Ph.D. in Social Psychology at Loyola University Chicago and is currently an Assistant Research Scientist at the University of Michigan. His evolutionary research interests include altruism, cooperation, competition, risk, life history, mortality patterns, mating strategies, and applications for social and ecological sustainability. Randolph M. Nesse is Professor of Psychiatry, Professor of Psychology, and Research Professor at the Institute for Social Research at the University of Michigan, where he directs the Evolution and Human Adaptation Program. His research interests are focused on Darwinian medicine, especially the origins of depression and other forms of psychopathology.     

Circularity in linear programming models of optimal diet

The linear programming model of optimal diet for herbivores has been criticized for being biologically unrealistic, for being too successful given statistical realities, and for being circular. I try to clarify the issue of circularity. Circularity arises if constraint lines are estimated from average values for governing parameters, when the assumed constraints are not effective. This may occur (1) under benign season conditions when consumers do not maximize their food intake, because of costs associated with food processing and storage, or (2) when an unidentified constraint limits intake. To evaluate hypotheses about the factors controlling diet composition, it must be shown that (1) consumers respond appropriately to variation in the parameters controlling constraint settings, and (2) the assumed constraints are close to their maximum (or minimum) settings.     

黑线仓鼠的食物与食量

作者从1983年至1989年,在河北省三个不同的气候区,对农田中的黑线仓鼠(Cricetulus,bara-bensis)的食物与食量进行了调查。结果表明,黑线仓鼠主要取食农作物种子。同时取食草籽和根、茎、叶、花、果实及动物性食物。不同地区、不同作物地、不同季节和年度、不同性别鼠的食物差异显著。食物差异与鼠的种群繁值及数量动态有关。月夹捕率与颊囊中食物的总检出率和种子检出率正相关非常显著。直线回归方程式分别为y=38.388+2.866x n=60 r=0.343>r_(0.01)=0.325和y=29.734+2.669x r=0.394>r_(0.01360)=0.325。黑线仓鼠对食物的选择,主要受到食物质量的影响,同时也受到食物频度、作物的物候变化、动物的群落结构、种群密度、种内与种间竞争等多种因素的综合影响。在不同的时间和栖息地影响它食物选择的主要因素不同。日食量4-5g。     

Dietary restriction in two rotifer species: the effect of the length of food deprivation on life span and reproduction

According to resource allocation theory, animals face a trade off between the allocation of resources into reproduction and into individual growth/maintenance. This trade off is reinforced when food conditions decline. It is well established in biological research that many animals increase their life span when food is in suboptimal supply for growth and/or reproduction. Such a situation of reduced food availability is called dietary restriction. An increase in life span under dietary restricted conditions is seen as a strategy to tolerate periods of food shortage so that the animals can start reproduction again when food is in greater supply. In this study, the effect of dietary restriction on life span and reproduction in two rotifer species, Cephalodella sp. and Elosa worallii, was investigated using life table experiments. The food concentration under dietary restricted conditions was below the threshold for population growth. It was (1) tested whether the rotifers start reproduction again after food replenishment, and (2) estimated whether the time scale of dietary restricted conditions is relevant for the persistence of a population in the field. Only E. worallii responded to dietary restriction with an increase in life span at the expense of reproduction. After replenishment of food, E. worallii started to reproduce again within 1 day. With an increase in the duration of dietary restricted conditions of up to 15 days, which is longer than the median life span of E. worallii under food saturation, the life span increased and the life time reproduction decreased. These results suggest that in a temporally (or spatially) variable environment, some rotifer populations can persist even during long periods of severe food deprivation.     

Food limitation in a wild cyclopoid copepod population: direct and indirect life history responses

We studied the effects of food limitation on the population dynamics of the freshwater cyclopoid copepod Diacyclops thomasi in Oneida Lake, New York. In the field population, maximum juvenile abundance coincided seasonally with high phytoflagellate concentration. During the clear-water phase (a seasonal period of low algal density), D. thomasi disappeared from the water column, but fourth-instar copepodids (CIV) were found encysted in developmental arrest in the sediment. Laboratory assays of the effect of the density of two types of food on copepod life history parameters showed that temporal variation in the concentration of relatively small phytoflagellates significantly affected stage-specific development times. This food limitation was most pronounced during the clear-water phase, and supplementation of the diet with a laboratory-cultured phytoflagellate, Chlamydomonas, prevented food limitation. Although developmental arrest appears to be controlled primarily by photoperiod, availability of the larger, more mobile food, Euglena, also influenced the percentage of individuals entering developmental arrest in the laboratory. An investigation of the spatial and temporal emergence pattern in the field revealed that CIV copepodids started to emerge in late autumn and that emergence rates were significantly greater at deep-water locations (9–12 m water depth) compared with shallow-water locations (5–7 m). The clear-water phase in Oneida Lake is an annual event, probably produced by intense grazing by Daphnia pulicaria and Daphnia galeata. Food limitation is thus very likely a recurrent phenomenon for D. thomasi. This apparent seasonal competitive impact of Daphnia on Diacyclops affects both nauplii and immature copepodids. Diacyclops shows two types of responses to the food limitation: (1) the physiological response of slowed active development, and (2) the adaptive response of developmental arrest. Received: 3 November 1997 / Accepted: 1 March 1998     

温度和食物浓度对三品系萼花臂尾轮虫实验种群动态的影响

应用个体培养方法,研究了温度(20、2和30 ℃)和藻类食物浓度(1.5×10₆、3.0×10₆、6.0×10₆和9.0×10₆ cells·ml^-1)对青岛、芜湖、广州三品系萼花臂尾轮虫种群动态的影响.结果表明,温度仅对轮虫的世代时间和种群内禀增长率有显著影响,而品系对所有生命表参数均无显著影响.轮虫种群的内禀增长率随培养温度的升高而增大,世代时间则随培养温度的升高而缩短.食物浓度仅对轮虫的生命期望值和平均寿命有显著影响,品系对轮虫的净生殖率、世代时间、生命期望值和平均寿命也有显著影响.三品系间,以广州品系轮虫的净生殖率、世代时间、生命期望值和平均寿命最大,芜湖品系最短.当食物浓度为3.0×10₆ cells·ml^-1时,轮虫的生命期望值和平均寿命最长,9.0×10₆ cells·ml^-1时最短.各品系轮虫的净生殖率、世代时间、总生殖率、生命期望值和平均寿命均随培养温度的升高而减小,广州品系的净生殖率除外.轮虫种群的内禀增长率和广州品系轮虫的总生殖率则随培养温度的升高而增大.青岛和广州品系轮虫的各生命表参数,均与食物浓度呈曲线相关,但芜湖品系仅世代时间、平均寿命和生命期望值随食物浓度的增大而缩短.     

Phenotypic plasticity and interpopulation differences in life history traits of<Emphasis Type="Italic"> Armadillidium vulgare</Emphasis> (Isopoda:Oniscidae)

The hypothesis that the balance of trade-offs between survivorship, growth and reproductive allocation in the terrestrial isopod Armadillidium vulgare will change when resource input is increased has been investigated experimentally. When the quality of food available was increased, by adding a mixture of litter from herbaceous dicotyledonous plants to a background low-quality food of dead grasses, survivorship was found to be the most phenotypically plastic trait, increasing by 168%. Growth rates increased by 99% but reproductive allocation by only 21%. In the field, members of a population from a site with more high-quality food grew more than twice as fast as those from a site where less high-quality food was available. The population from the site with higher food availability, contrary to predictions from the laboratory study, did not survive as well as that from the site with less available high-quality food. This may be because the site that is more favourable for growth has a more stressful physical environment due to much bigger temperature fluctuations, which are known to be an important cause of mortality in this species. When individuals from both populations were reared under controlled laboratory conditions, both the parental and F1 generations from the poor growth environment survived better than those from the good growth habitat. However, even when given an excess of high-quality food those from the poor growth environment continued to grow more slowly and had a lower reproductive allocation than those from the site with higher food availability. We conclude that microevolutionary changes may have occurred in the balance of resource allocation between survivorship, growth and reproductive allocation, to favour higher survivorship during the longer prereproductive period at the site where growth to the threshold size for reproduction takes longer.     

Dietary effects on life history traits in a terrestrial isopod: the importance of evaluating maternal effects and trade-offs

Studies of life history aim to explain patterns in the evolution of reproductive investment, growth, and survival. Trade-offs between traits are a fundamental component of life history theory. In herbivorous arthropods life history traits are often responsive to variation in numerous environmental factors, especially diet quality. Using three artificial diets under controlled laboratory conditions, we examined changes in life history traits (i.e. growth rate, offspring number, offspring size, incubation period), trade-offs between traits, and maternal effect on the growth rate of offspring, in the common woodlouse (terrestrial isopod), Porcellio laevis. The high protein diet had significant impacts on offspring production, triggering a smaller-sized offspring, and demonstrating a trade-off between these last two traits. The high carbohydrate diet seldom exerted a significant effect on incubation period. The quality of dietary items evidently has important consequences on the life history of the mother and, thus, on offspring growth; the directions of these effects, however, were opposite. Mothers fed diets with high protein concentrations presented significant maternal effects, measured as offspring growth rate during later ontogeny. Our results support the notion that protein, rather than carbohydrate, concentrations in the diet limit herbivorous arthropods, and have significant consequences on life history traits, as was seen for P. laevis. Clearly, the change in phenotypic correlations between incubation period and offspring number from negative to positive is an empirical demonstration of the context dependence of life history trait trade-offs.     

The force of selection on the human life cycle

In this article, I present evidence for a robust and quite general force of selection on the human life cycle. The force of selection acts in remarkably invariant ways on human life histories, despite a great abundance of demographic diversity. Human life histories are highly structured, with mortality and fertility changing substantially through the life cycle. This structure necessitates the use of structured population models to understand human life history evolution. Using such structured models, I find that the vital rates to which fitness is most sensitive are prereproductive survival probabilities, particularly the survival of children ages 0 to 4 years. The fact that the preponderance of selection falls on transitions related to recruitment combined with the late age at first reproduction characteristic of the human life cycle creates a fitness bottleneck out of recruitment. Because of this, antagonistic pleiotropy with any trait that detracts from the constituent transitions to recruitment is expected. I explore the predictors of variation in the force of selection on early survival. High fertility increases the selective premium placed on early survivorship, whereas high life expectancy at birth decreases it.     

Sexual size dimorphism in caecilian amphibians: analysis, review and directions for future research

Sexual dimorphism, widespread in the animal kingdom, describes differences between the sexes in size, shape and many other traits. Sexual size dimorphism (SSD) plays a significant role in understanding life history evolution and mating systems. The snakelike morphology of limbless caecilian amphibians lacking obvious secondary sexual characters (in contrast to frogs and salamanders) impedes accurate intrasexual comparisons. In this study, sexual size dimorphism in the oviparous caecilian Ichthyophis cf. kohtaoensis, a phylogenetically basal caecilian, was analysed. Females were larger in all body and head characters tested. However, when adjusted to body size (total length), females differed only in their cloacal shape. Clutch volume was positively correlated to female body size, thus female fecundity increased with body size supporting the hypothesis of a fecundity-selected SSD in the oviparous Ichthyophis cf. kohtaoensis. A review of the present SSD data for caecilians shows that many species are monomorphic for body size but show dimorphism in head size, while other species demonstrate female-biased SSD. Male-biased SSD has not been reported for caecilians. To understand life history evolution in caecilians, further studies on the reproductive biology of other taxa are urgently needed, in particular for rhinatrematids and uraeotyphlids. New data will allow phylogenetically controlled comparative analyses to fully explore the pattern of SSD among caecilian lineages.     

北草蜥种群间生活史变异的成因分析:热环境、食物可利用性和体温的岛屿间差异

比较浙江温州北麂岛与洞头岛的热环境、食物可利用性以及动物体温,以辨析北草蜥岛屿种群间生活史特征差异中环境因子的作用。通过测定岛屿上北草蜥栖息地植被和环境温度,比较岛屿热环境的差异。野外测定活动蜥蜴的体温、环境温度和活动规律,实验室温梯板中测定动物喜好体温。用陷阱法测定无脊椎动物多样性和丰度,以比较岛屿蜥蜴种群的食物可利用性。岛屿植被存在差异导致热环境的差异。洞头岛植被高于北麂岛,地表层光线透入率则低于北麂岛。因而,洞头岛郁闭地表的平均温度和最高温度显著低于北麂岛,但两岛裸露地面的温度无显著差异。热环境的岛屿间差异进而影响北草蜥的野外体温。在春季,洞头岛的野外有效温度和基底温度显著大于北麂岛,而两岛北草蜥的体温无显著差异;在夏季,洞头岛的北草蜥体温、有效温度和基质温度均显著高于北麂岛;到秋季,北麂岛蜥蜴体温和环境温度高于洞头岛。地面无脊椎动物多样性和丰度的岛屿间差异表明北麂岛食物可利用性大于洞头岛。岛屿间北草蜥日活动规律和喜好体温无显著差异。本研究表明:(1)温度和食物可利用性存在岛屿间差异,岛屿种群间生活史特征差异可能与之有关;(2)两岛屿北草蜥主要采取行为调节对策来适应自然界的热环境变化,尚未发现热生理学特征的进化性漂移[动物学报51(5):797-805,2005]。     

温度和食物浓度对三品系萼花臂尾轮虫实验种群动态的影响

应用个体培养方法,研究了温度(20、25和30℃)和藻类食物浓度(1．5×10^6、3．0×10^6、6．0×10^6和9．0×10^6cells·ml^-1)对青岛、芜湖、广州三品系萼花臂尾轮虫种群动态的影响．结果表明,温度仅对轮虫的世代时间和种群内禀增长率有显著影响,而品系对所有生命表参数均无显著影响．轮虫种群的内禀增长率随培养温度的升高而增大,世代时间则随培养温度的升高而缩短．食物浓度仅对轮虫的生命期望值和平均寿命有显著影响,品系对轮虫的净生殖率、世代时间、生命期望值和平均寿命也有显著影响．三品系间,以广州品系轮虫的净生殖率、世代时间、生命期望值和平均寿命最大,芜湖品系最短．当食物浓度为3．0×10^6cells·ml^-1时,轮虫的生命期望值和平均寿命最长,9．0×10^6cells·ml^-1时最短．各品系轮虫的净生殖率、世代时间、总生殖率、生命期望值和平均寿命均随培养温度的升高而减小,广州品系的净生殖率除外．轮虫种群的内禀增长率和广州品系轮虫的总生殖率则随培养温度的升高而增大．青岛和广州品系轮虫的各生命表参数,均与食物浓度呈曲线相关,但芜湖品系仅世代时间、平均寿命和生命期望值随食物浓度的增大而缩短．