The developmental and physiological basis of body size evolution in an insect

The evolution of body size is a dominant feature of animal evolution. However, little is known about how the underlying developmental mechanisms that determine size change as body size evolves. Here we report on a case of body size evolution in the tobacco hornworm Manduca sexta that occurred over a period of nearly 30 years. We take advantage of an extensive series of physiological studies performed in the early 1970s that established the parameters that regulate body size in this species and compare their values with those of modern individuals that are descendants of the same colony. We show that three of the five processes that determine adult body size changed during this period, while two remained constant. Changes in these three developmental processes completely account for the observed evolutionary change in body size.     

Seasonal changes in body size,sexual size dimorphism and sex ratio in relation to mating system in an adult odonate community

Seasonal environments impose developmental time constraints on insects which can be reflected in body size and sex ratio. By tracking these two aspects in recently emerged adults of 10 species of an odonate community in a number of lakes, we investigated whether (a) body size in both sexes decreased as the flight season progressed and whether this led to seasonal changes in sexual size dimorphism (SSD); (b) SSD patterns were related to mating systems; (c) biases in sex ratio could be explained by mortality rates associated with the largest sex (e.g. in species with male-biased SSD, a female-biased sex ratio; in species with female-biased SSD, a male-biased sex ratio). Our results indicated that adults in most species, but not all, tend to reach a smaller body size as the season progressed. However, the opposite pattern was found in a few species. Predictions about the relation between SSD and mating systems were confirmed: a female-biased SSD in nonterritorial species and monomorphism for territorial species. However, predictions of biases in sex ratio according to SSD were not met in all species. Interestingly, changes in body size and SSD along the season were lake-specific in two species in which these patterns could be examined. These results, although partially supportive of environmental and sexual selection patterns acting on size and sex ratio as documented in other odonate species, indicate that we are still far from understanding seasonal constrains in these animals.     

Small population size limits reproduction in an invasive grass through both demography and genetics

Small populations of founding individuals or survivors of incomplete management programs often represent critical transitions in biological invasions. Theory predicts that population size affects reproduction and, consequently, a population’s expansion, but there are few empirical tests, and fewer that account for the reduced genetic diversity that often accompanies small population size. We created experimental small populations of invasive ryegrass (Lolium multiflorum) with population size varying independently from genetic diversity. Treatment independence was achieved by cloning plants to increase population size without changing diversity. Plant fitness was measured as the proportion of florets producing a seed. We analyzed the effects of population size, genetic diversity, and their interaction using ANCOVAs, one of which accounted for variation in individual plant growth. As predicted, smaller populations produced significantly lower proportion seed set. Low genetic diversity also reduced seed set, but this was best interpreted as part of a significant interaction with population size. Specifically, the effect of population size on the proportion seed set was over five times larger for populations in the medium genetic diversity treatment than the highest diversity treatment, and 6.7 times larger for populations with the lowest level of diversity. Population size variation had biologically meaningful consequences, as the rate of seed set within the low diversity treatment increased by 80 % with increasing population size. The results indicate that both the demographics and genetics of populations can influence reproduction and invasive potential, and must be considered when assessing risk and designing management plans for invasive plants.     

Insular shifts in body size of rice frogs in the Zhoushan Archipelago, China

1. Differences in body size between mainland and island populations have been reported for reptiles, birds and mammals. Despite widespread recognition of insular shifts in body size in these taxa, there have been no reports of such body size shifts in amphibians. 2. We provide the first evidence of an insular shift in body size for an amphibian species, the rice frog Rana limnocharis. We found significant increases in body size of rice frogs on most sampled islands in the Zhoushan archipelago when compared with neighbouring mainland China. 3. Large body size in rice frogs on islands was significantly related to increased population density, in both breeding and non-breeding seasons. Increases in rice frog density were significantly related to higher resource availability on islands. Increased resource availability on islands has led to higher carrying capacities, which has subsequently facilitated higher densities and individual growth rates, resulting in larger body size in rice frogs. We also suggest that large body size has evolved on islands, as larger individuals are competitively superior under conditions of harsh intraspecific competition at high densities. 4. Increases in body size in rice frogs were not related to several factors that have been implicated previously in insular shifts in body size in other taxa. We found no significant relationships between body size of rice frogs and prey size, number of larger or smaller frog species, island area or distance of islands from the mainland. 5. Our findings contribute to the formation of a broad, repeatable ecological generality for insular shifts in body size across a range of terrestrial vertebrate taxa, and provide support for recent theoretical work concerning the importance of resource availability for insular shifts in body size.     

The relationship between genome size, development rate, and body size in copepods

Freshwater cyclopoid copepods exhibit at least a fivefold range in somatic genome size and a mechanism, chromatin diminution, which could account for much of this interspecific variation. These attributes suggest that copepods are well suited to studies of genome size evolution. We tested the nucleotypic hypothesis of genome size evolution, which poses that variation in genome size is adaptive due to the bulk effects of both coding and noncoding DNA on cell size and division rates, and their correlates. We found a significant inverse correlation between genome size and developmental (growth) rate in five freshwater cyclopoid species at three temperatures. That is, species with smaller genomes developed faster. Species with smaller genomes had significantly smaller bodies at 22 °C, but not at cooler and warmer temperatures. Species with smaller genomes developed faster at all three temperatures, but had smaller bodies only at 22 °C. We propose a model of life history evolution that adds genome size and cell cycle dynamics to the suite of characters on which selection may act to mold life histories and to influence the distribution of traits among different habitats.     

Burrow defense behaviors in a sand-bubbler crab, Scopimera globosa, in relation to body size and prior residence

The burrow defense behaviors in a sand-bubbler crab, Scopimera globosa, living on a tidal flat, were experimentally examined. Body size and prior residence influenced the results of struggles for the burrows, and large individuals or the burrow owners won in most cases when the intruders were not significantly larger than the owners. Most large owners defended their burrows by directly fighting their opponents. On the other hand, small owners defended their burrows in three different ways. (1) Owners fought directly against same-sized or smaller intruders. For larger intruders, (2) most owners returned to their burrows when the owner was nearer to the burrow than the intruder (returning behavior), and (3) owners sat motionless when the intruder was nearer to the burrow than the owner (sitting behavior). Success ratios of the three types of burrow defense were 38.2%, 88.5%, and 100%, respectively. It was considered that sitting behavior of the cryptically colored S. globosa has evolved because intruders cannot see motionless owners and consequently cannot detect the owner's burrow. Received: October 6, 2000 / Accepted: January 22, 2001     

Polyphagy and adult body size in geometrid moths

We compared the average body size (wing span) of Finnish geometrid moth species in relation to their degree of polyphagy and quality of food. The first hypothesis, originally constructed for mammals and birds, states that smaller species should more often be specialists than large species, because of the different relationships between body size and home range size, and body size and daily energy requirements. According to the second hypothesis, smaller species should feed more often on herbs than do larger species, because of the different defence mechanisms of herbs and woody plants. The results support both of these hypotheses. Specialist species are smaller than oligophagous or polyphagous species, and small species concentrate on herbs. We conclude that quality and quantity of food resources may explain the pattern.     

The effects of food shortage during larval development on adult body size, body mass, physiology and developmental time in a tropical damselfly

Few studies have looked jointly at the effects of larval stressors on life history and physiology across metamorphosis, especially in tropical insects. Here we investigated how the variation of food availability during the larval stage of the tropical and territorial American rubyspot damselfly (Hetaerina americana) affects adult body size and body mass, and two physiological indicators of condition--phenoloxidase activity (an indicator of immune ability) and protein concentration. We also investigated whether larval developmental time is prolonged when food is scarce, an expected situation for tropical species whose larval time is less constrained, compared to temperate species. Second instar larvae were collected from their natural environments and reared in one of two diet regimes: (i) "rich" provided with five Artemia salina prey every day, and (ii) "poor" provided with two A. salina prey every day. In order to compare how distinct our treatments were from natural conditions, a second set of last-instar larvae were also collected and allowed to emerge. Only body size and phenoloxidase increased in the rich regime, possibly to prioritize investment on sexually selected traits (which increase mating opportunities), and immune ability, given pathogen pressure. The sexes did not differ in body size in relation to food regimes but they did differ in body mass and protein concentration; this can be explained on the basis of the energetically demanding territorial activities by males (for the case of body mass), and female allocation to egg production (for the case of protein). Finally, animals delayed larval development when food was scarce, which is coherent for tropical environments. These findings provide key insights in the role of food availability in a tropical species.     

Growth in early life and relative body size among adult polar bears (Ursus maritimus)

Data on the body length (cm) of polar bear ( Ursus maritimus ), collected as part of a long-term ecological study in the Hudson Bay region of Canada, were examined to test the hypothesis that growth early in life influences adult body size. Data for 42 females and 30 males that were captured as two-year-olds (2.6-2.9 years), after the period of maternal care, and later recaptured as adults were analysed. On average, females increased in length by 11.2% and males by 23.1 % between captures. Relative adult body length was significantly correlated with relative length as a two-year-old in females but not in males. Thus, body length attained by two-year-olds is a weaker determinant of adult body length in males than in females. We suggest that, in comparison with females, prolonged growth beyond the period of maternal care may predispose the growth and eventual adult body size of male offspring to a greater degree of environmentally mediated variation. Furthermore, while data on maternal investment strategies in polar bears are lacking, variation in the allocation of maternal resources to cubs, within and among litters, may have a limited impact on the eventual adult body size of any surviving male offspring.     

A phylogenetic analysis of egg size,clutch size,spawning mode,adult body size,and latitude in reef fishes

Theoretical treatments of egg size in fishes suggest that constraints on reproductive output should create trade-offs between the size and number of eggs produced per spawn. For marine reef fishes, the observation of distinct reproductive care strategies (demersal guarding, egg scattering, and pelagic spawning) has additionally prompted speculation that these strategies reflect alternative fitness optima with selection on egg size differing by reproductive mode and perhaps latitude. Here, we aggregate data from 278 reef fish species and test whether clutch size, reproductive care, adult body size, and latitudinal bands (i.e., tropical, subtropical, and temperate) predict egg size, using a statistically unified framework that accounts for phylogenetic correlations among traits. We find no inverse relationship between species egg size and clutch size, but rather that egg size differs by reproductive mode (mean volume for demersal eggs = 1.22 mm³, scattered eggs = 0.18 mm³, pelagic eggs = 0.52 mm³) and that clutch size is strongly correlated with adult body size. Larger eggs were found in temperate species compared with tropical species in both demersal guarders and pelagic spawners, but this difference was not strong when accounting for phylogenetic correlations, suggesting that differences in species composition underlies regional differences in egg size. In summary, demersal guarders are generally small fishes with small clutch sizes that produce large eggs. Pelagic spawners and egg scatterers are variable in adult and clutch size. Although pelagic spawned eggs are variable in size, those of scatterers are consistently small.     

Poor growth prior to early childhood: decreased health and life-span in the adult

Previous studies in animal populations have shown that stunted neural and thymolymphatic growth early in development may result in permanently impaired neural and immune function, decreased body growth, vertebral wedging, and decreased life-span. In the human adult, small vertebral neural canal (VNC) diameters may reflect early stunted neural and immune development and impaired function that leads to decreased health (inferred by greater vertebral wedging) and life-span in the adult. VNC, which complete their growth by early childhood (age 4), are markers of early development in adults. On the other hand, features following general body growth, such as height, weight (represented here by vertebral body height) continues to grow until young adulthood. They are less reliable, because they readily experience catch-up growth (even in chronically stressed populations) and, unlike VNC, may mask poor early growth. To test associations between early growth and adult health and life-span in humans, we measured 2,060 VNC, vertebral heights, vertebral wedging, nerve-root tunnel lengths, severity of vertebral osteophytosis, and ages at death in 90 adult (aged 15-55 years) prehistoric skeletons (950-1300 A.D.). Tibial lengths were also measured in a subsample (n = 30). Multivariate, bivariate, and nonparametric analyses showed that small VNC are significantly associated with greater vertebral wedging and decreased life-span (P less than 0.05-0.00001). VNC are independent of vertebral body heights and tibial lengths (general body growth). VNC, but not statural components, are useful in predicting adult health, presumably because they reflect neural and immune development and do not readily experience catch-up growth. Thus, longitudinal retrospective measures of early growth and adult health were systematically linked within individuals regardless of confounding factors operating over the 350-year time period. Since this research was completed, this model has repeatedly been independently confirmed in four living urban industrial populations. Longitudinal retrospective analysis was employed together with direct measures of VNC, neural and immune function. Together these results suggested that it may be essential to improve growth prior to early childhood in order to maximize adult health and life-span.     

Production,reproduction and size in mammals

It is argued that birth rate, turnover rate and production/biomass ratios (P/B) are equivalent in stable populations. It is then shown from field data on 21 mammal species that production scales as a 2/3 power of body mass, suggesting that size rather than life history characteristics explain most interspecific variation. The relationship simolifies calculations of annual production in populations and energy flow through communities.     

Effect of four chlorophytes on larval survival, development and adult body size of the mosquito Aedes aegypti

The effect of four microalgal chlorophytes, Chlorella vulgaris,Scenedesmus quadricauda, Chlorococcum sp. and Ankistrodesmus convolutus, on the survival, larval development and adultbody size of the mosquito Aedes aegypti was investigated. The percentage mortality of larvae fed with C. vulgaris, Chlorococcum sp.or S. quadricauda after six days was 100%, 84% and 88%,respectively. Delayed pupation and body size reduction of the mosquitoesfed with C. vulgaris, Chlorococcum sp. and S. quadricaudawere observed. In contrast, larvae fed with A. convolutus werebigger than those fed with normal insectory feed. The study showed thatC. vulgaris, S. quadricauda and Chlorococcum sp. have potentialto be used as larvicidal agents.     

Worker reproduction and policing in insect societies: an ESS analysis

Insect societies are vulnerable to exploitation by workers who reproduce selfishly rather than help to rear the queen's offspring. In most species, however, only a small proportion of the workers reproduce. Here, we develop an evolutionarily stable strategy (ESS) model to investigate factors that could explain these observed low levels of reproductive exploitation. Two key factors are identified: relatedness and policing. Relatedness affects the ESS proportion of reproductive workers because laying workers generally work less, leading to greater inclusive fitness costs when within-colony relatedness is higher. The second key factor is policing. In many species, worker-laid eggs are selectively removed or 'policed' by other workers or the queen. We show that policing not only prevents the rearing of worker-laid eggs but can also make it unprofitable for workers to lay eggs in the first place. This can explain why almost no workers reproduce in species with efficient policing, such as honeybees, Apis, and the common wasp, Vespula vulgaris, despite relatively low relatedness caused by multiple mating of the mother queen. Although our analyses focus on social insects, the conclusion that both relatedness and policing can reduce the incentive for cheating applies to other biological systems as well.     

Transgenerational effects of parental larval diet on offspring development time, adult body size and pathogen resistance in Drosophila melanogaster

Environmental conditions experienced by parents are increasingly recognized to affect offspring performance. We set out to investigate the effect of parental larval diet on offspring development time, adult body size and adult resistance to the bacterium Serratia marcescens in Drosophila melanogaster. Flies for the parental generation were raised on either poor or standard diet and then mated in the four possible sex-by-parental diet crosses. Females that were raised on poor food produced larger offspring than females that were raised on standard food. Furthermore, male progeny sired by fathers that were raised on poor food were larger than male progeny sired by males raised on standard food. Development times were shortest for offspring whose one parent (mother or the father) was raised on standard and the other parent on poor food and longest for offspring whose parents both were raised on poor food. No evidence for transgenerational effects of parental diet on offspring disease resistance was found. Although paternal effects have been previously demonstrated in D. melanogaster, no earlier studies have investigated male-mediated transgenerational effects of diet in this species. The results highlight the importance of not only considering the relative contribution each parental sex has on progeny performance but also the combined effects that the two sexes may have on offspring performance.