Critical weight in the development of insect body size

Body size is one of the most important life history characters of organisms, yet little is known of the physiological mechanisms that regulate either body size or variation in body size. Here, we examined one of these mechanisms, the critical weight, which is defined as the minimal mass at which further growth is not necessary for a normal time course to pupation. The critical weight occurred at 55% of peak larval mass in laboratory-reared larvae of the tobacco hornworm Manduca sexta. We examined the effects of genetic and environmental variation in the critical weight on body size. As in many other insects, Manduca larvae reared on poor diets were smaller and those reared at lower temperatures were larger than control animals. We demonstrated that the critical weight was lower on low quality diets but did not change with temperature. There was significant genetic variation for body size, for plasticity of body size, and for critical weight, but not for plasticity of critical weight. Variation in the critical weight accounted for 73% of between-family variance in peak larval size, whereas plasticity of critical weight was not significantly correlated with plasticity of body size. Our results suggest that although critical weight is an important factor in determining body size and enabling the evolution of body size, it may, at the same time, act as a constraint on the evolution of plasticity of body size. Thus, the determinants of body size and the determinants of plasticity of body size do not need to be identical.     

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.     

An integrated analysis of phenotypic selection on insect body size and development time

Most studies of phenotypic selection do not estimate selection or fitness surfaces for multiple components of fitness within a unified statistical framework. This makes it difficult or impossible to assess how selection operates on traits through variation in multiple components of fitness. We describe a new generation of aster models that can evaluate phenotypic selection by accounting for timing of life‐history transitions and their effect on population growth rate, in addition to survival and reproductive output. We use this approach to estimate selection on body size and development time for a field population of the herbivorous insect, Manduca sexta (Lepidoptera: Sphingidae). Estimated fitness surfaces revealed strong and significant directional selection favoring both larger adult size (via effects on egg counts) and more rapid rates of early larval development (via effects on larval survival). Incorporating the timing of reproduction and its influence on population growth rate into the analysis resulted in larger values for size in early larval development at which fitness is maximized, and weaker selection on size in early larval development. These results illustrate how the interplay of different components of fitness can influence selection on size and development time. This integrated modeling framework can be readily applied to studies of phenotypic selection via multiple fitness components in other systems.     

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.     

Monogenean body size,but not reproduction,increases with infracommunity density

Body size reveals a plethora of life-history, ecological, and evolutionary information about a species. It plays a critical role in success or failure during competitive, reproductive, or predator–prey interactions. Typically, there is a negative relationship between body size and population density in natural populations and communities. I analysed this relationship within and among multiple populations of two prominent monogenean parasites (>90% prevalence) on Lepomis macrochirus in three lakes in New Jersey (USA), using multiple regression models. To elucidate the causes and benefits of this relationship, I also measured host body condition via a regression index, and reproductive output of the parasite community by measuring parasite eggs shed from the host. The relationship between body size and density of infrapopulations (parasites of a single species on a single host) was positive, and the strength of this relationship for both species depended on which lake they occupied, indicating the potential for Allee effects. This relationship persists at the infracommunity level, where there was a similar positive relationship between a community weighted mean body size and density. However, this relationship did not result in greater reproductive success as measured by infracommunity egg production per individual per 24 h or egg size. The cause of this relationship also remains elusive; it was not explained by host condition or age. The results suggest that there is either no reproductive advantage to this increase in body size or the advantage conferred was not related to these measured fitness components. These findings indicate that researchers should be cautious using body size as a proxy for fitness or reproduction, while also raising further questions about the nature of the relationship between parasites on a host and that between those parasites and the host.     

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.     

Host body size as a factor determining the egg complement of Strepsiptera,an insect parasite

The egg complement (total number of eggs produced by a single female) differs greatly among the species of Strepsiptera. The maximum is found in Stichotrema dallatorreanum (750,000 eggs), and the minimum in Triozocera minor (984 eggs). Based on the egg complement of 31 species in 11 genera, the following conclusions were drawn: (1) The egg complement is generally smaller in those species whose hosts gregariously cohabit in a very limited area, or are distinct flower-visitors, compared with those whose hosts display the above two traits weakly; (2) The egg complement is determined by the size of the maternal body. The size of female strepsipterans is reduced when they parasitize smaller host such as males and workers, as compared with those that parasitize larger host such as females and queens; likewise, the size of the strepsipterans becomes larger on increase in size of hosts, showing that their egg complements are principally determined by the size of host species; (3) The increase in the egg complement is compensated for by the reduction in egg size. The relative egg size (length of the firstinstar larva/length of maternal body) is conspicuously reduced according to an increase in the size of the female strepsipterans.     

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.     

How body size and development biased the direction of evolution in early amphibians

Evolutionary change does not proceed in every direction with equal probability. Evolutionary biases or constraints are limitations on the mode, direction and tempo of evolution. Early tetrapods provide interesting examples, especially Paleozoic and Mesozoic amphibians. (1) Body size had a strong impact on morphology and development in early amphibians, resulting in manifold convergences imposed by design limitations. Miniaturisation had similar effects in a wide range of Paleozoic tetrapods, which are consistent with observations on extant salamanders. Gigantism was a common feature of Triassic temnospondyls, correlating with slow developmental rates similar to those of gigantic salamanders and the convergent evolution of bone density. (2) Ontogeny imposes constraints on evolution by canalised (buffered) developmental sequences. In Paleozoic temnospondyls, ontogenetic trajectories evolved by several different modes (truncation of the trajectory, shifting of events or condensation of events). Metamorphosis is an extreme example of a condensed developmental sequence, which first evolved in Paleozoic temnospondyls, increased in salamanders and culminated in anurans. It imposes strong biases that may be broken by three conceivable modes: (1) loss of the adult period (neoteny), (2) loss of the larval period (direct development) and (3) ‘unpacking’ of metamorphosis by re-evolving the plesiomorphic trajectory.     

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     

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.     

Eco‐energetic consequences of evolutionary shifts in body size

Size imposes physiological and ecological constraints upon all organisms. Theory abounds on how energy flux covaries with body size, yet causal links are often elusive. As a more direct way to assess the role of size, we used artificial selection to evolve the phytoplankton species Dunaliella tertiolecta towards smaller and larger body sizes. Within 100 generations (c. 1 year), we generated a fourfold difference in cell volume among selected lineages. Large‐selected populations produced four times the energy than small‐selected populations of equivalent total biovolume, but at the cost of much higher volume‐specific respiration. These differences in energy utilisation between large (more productive) and small (more energy‐efficient) individuals were used to successfully predict ecological performance (r and K) across novel resource regimes. We show that body size determines the performance of a species by mediating its net energy flux, with worrying implications for current trends in size reduction and for global carbon cycles.     

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.     

Latitudinal shifts in body size of Enallagma cyathigerum (Odonata)

Aim Survey of the latitudinal body size pattern for populations of Enallagma cyathigerum (Odonata) across a south‐north transect. Location A transect covering the whole distribution range from south to north across Europe was sampled. Methods Newly emerged adults were collected from five major sites across Europe and one to four localities were sampled within each site. In total 253 adults were collected from fourteen localities. Body size was measured using thorax length, length of right front wing and length of right hind tibia. These body size estimates were thereafter related to latitude and mean temperature in January and July. Results Body size showed a U‐shaped pattern with latitude, being large at low and high latitudes and small at intermediate latitudes. The same U‐shaped pattern was found for mean January and July temperature, with large animals at low and high temperatures. Conclusion The U‐shaped relationship between body size and latitude is suggested to be a combination of two effects: (1) the length of the season favourable for growth and development, and (2) variation in life cycle length with latitude.     

Freezing tolerance in relation to cooling rate in an adult insect

In the adult tenebrionid beetle Upis ceramboides unusually low cooling rates are required to demonstrate maximum freezing tolerance, and a very slight change in rate can reduce survival from 100 to 0%. Freezing to ?50 °C results in 100% mortality at rates above 0.35 °C/min, but no injury is apparent if the cooling rate is 0.28 °C/min. The lower lethal temperature, determined with a cooling rate of 0.17 °C/min, is about ?60 °C. The maximum cooling rate which allows full survival is nearly identical to optimal cooling rates previously found for mouse embryos and some lymphocytes, but the striking sensitivity to very slight changes in rate is unique to Upis. Most studies dealing with insect freezing tolerance have utilized rates of 1 °C/min or faster, and the failure of some of these laboratory studies to observe freezing survival may be due to the use of lethal cooling rates.     

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.