Contrasting patterns of body size for Daphnia species that segregate by habitat

Summary We investigate how body size of two coexisting Daphnia species varies among 7 lakes that represent a gradient of predation risk. The two species segregate vertically in stratified lakes; D. galeata mendotae is typically smaller and more eplimnetic than D. pulicaria. The extent of vertical habitat partitioning, however, varies seasonally within and among lakes in apparent response to predation intensity by epilimnetic planktivorous fishes. Daphnia pulicaria uses the epilimnion at low levels of fish predation but is restricted to the hypolimnion under high fish predation, whereas D. galaeta mendotae always utilizes the epilimnion. The species display contrasting patterns of genetic variation in neonate size and size at maturity. D. pulicaria is larger in lakes with higher fish and Chaoborus densities whereas D. galeata mendotae is smaller. This contrast in body size in lakes with high predation is associated with greater habitat segregation in those lakes. In lakes with low predation risk, the two species are similar in body size at birth and maturity.Authorship order alphabetical     

Consequences and causes of geographic variation in the body size of a keystone predator,Notophthalmus viridescens

Two subspecies of the predatory aquatic salamanderNotophthalmus, N. viridescens viridescens andN. v. dorsalis, differ in adult body size and geographic distribution. We tested whether experimental populations of the two predator subspecies differed in their effects on prey populations ofB. americanus, and whether observed differences in predator body size were genetic and/or environmentally induced. We compared the effects of predation by bothNotophthalmus subspecies on larvalBufo americanus by experimentally manipulating the densities (0, 2, or 4 newts/m³) and subspecies ofNotophthalmus (N. v. viridescens orN. v. dorsalis) added to artificial ponds. BothNotophthalmus subspecies significantly reducedB. americanus survival, but differed significantly in this effect. FewerBufo survived with the larger subspecies,N. v. viridescens, than with the smallerNotophthalmus subspecies,N. v. dorsalis. TheNotophthalmus subspecies differed in their patterns of adult and larval growth. Adults of the smaller subspecies,N. v. dorsalis, had a significantly higher growth rate than the larger subspecies,N. v. viridescens, under common environmental conditions, suggesting that differences in predator size were partly genetic, rather than entirely environmentally induced. LarvalN. v. dorsalis metamorphosed significantly later in the season than larvae ofN. v. viridescens, suggesting that larvalN. v. dorsalis had a lower growth rate than larvalN. v. viridescens. Differences in adult and larval growth, together with differences in the minimum adult size observed in natural populations, suggest that differences in the rate or duration of pre-adult growth may contribute substantially to observed differences in size.     

Environmental influences on the sexual dimorphism in body size of western bobcats

Sexual size dimorphism might be influenced by environmental constraints on sexual selection or by intraspecific competition between males and females. We studied bobcats (Lynx rufus) in collections of museum specimens from western North America to examine these hypotheses. Structural body size was estimated from several measurements of the skull, ln-transformed and indexed through principal components analysis. Sexual dimorphism in body size was estimated from the difference in size index of males and females, and compared to geographic and climatic variables associated with biotic provinces (ecoregions). Of several climatic variables that were associated with bobcat body size, only seasonality of climate was associated with sexual dimorphism. Sexual size dimorphism, longitude, elevation, and seasonality were intercorrelated. As longitude decreased (moving inland from west-coastal ecoregions), sexual dimorphism decreased with the increased elevation and seasonality of continental climates of the Rocky Mountains. We suggest that increased seasonality and the need for fasting endurance by females may place constraints on the degree of sexual dimorphism in bobcats. Sexual dimorphism of body size and sexual size dimorphism of trophic structures (teeth) exhibited a strong positive association over geography, thus indirectly supporting the hypothesis that intrasexual competition for prey could account for the geographic variation in sexual size dimorphism. Thus, both environmental constraints on sexual selection of body size and intersexual competition were supported as possible explanations of the degree of sexual size dimorphism that occurs in populations of bobcats.     

Evaluation of the sample size used for the rapid bioassessment of rivers using macroinvertebrates

Experiments were designed to investigate selective predation by medium (40–55 mm carapace width: CW) and large (55–70 mm CW) Carcinus maenas when feeding on four bivalves of contrasting shell morphology. Size-selection was examined by presenting individual crabs with a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas and Cerastoderma edule. Medium-sized crabs preferred mussels 5–15 mm shell length (maximum shell dimension: SL) and cockles 5–10 mm SL, whereas large crabs preferred mussels 15–25 mm and cockles 10–20 mm SL. Crabs generally showed no preference for any particular size of either oyster species. Species-selection was examined by presenting individual crabs with paired combinations of the four bivalves in various proportions. When offered mussels and oysters simultaneously, both size categories of crabs consistently selected mussels, and food choice was independent of prey relative abundance. By contrast, C. maenas selected mussels and cockles as expected by the frequency in which each size category of crab encountered the preferred size ranges of prey. Crab preference clearly paralleled the rank order of prey profitability, which in turn was mainly determined by prey biomass, suggesting that active selection takes place at some point of the predation cycle. Experiments with epoxy resin models showed that initial reluctance of crabs to attack oysters was not associated with the ultimate energy reward. Moreover, they suggest that foraging decisions are partly based on evaluations of overall prey shape and volume, and that the minimum dimension of the shell constitutes an important feature which crabs recognise and associate with prey value.     

Conflicting processes in the evolution of body size and development time

Body size and development time of Manduca sexta are both determined by the same set of three developmental–physiological factors. These define a parameter space within which it is possible to analyse and explain how phenotypic change is associated with changes in the underlying factors. Body size and development time are determined by the identical set of underlying factors, so they are not independent, but because the mechanisms by which these factors produce each phenotype are different, the two phenotypes are only weakly correlated, and the correlation is context dependent. We use a mathematical model of this mechanism to explore the association between body size and development time and show that the correlation between these two life-history traits can be positive, zero or negative, depending entirely on where in parameter space a population is located, and on which of the underlying factors has a greater variation. The gradient within this parameter space predicts the unconstrained evolutionary trajectory under directional selection on each trait. Calculations of the gradients for body size and development time revealed that these are nearly orthogonal through much of the parameter space. Therefore, simultaneous directional selection on body size and development time can be neither synergistic nor antagonistic but leads to conflicting selection on the underlying developmental parameters.     

Niche differentiation depends on body size in a cichlid fish: a model system of a community structured according to size regularities

1. Communities of different species are often structured according to niche differentiation associated with competitive interactions. We show that similar principles may apply on an ecological time-scale when individuals of a species having a wide size variation compete for resources, using the Lake Tanganyika cichlid Lobochilotes labiatus (5-30 cm). This species has a mouth especially adapted to suck up invertebrates from rock crevices. 2. Individuals defended feeding territories against similar-sized conspecifics, but not against different-sized ones. Thus, territories of similar-sized fish rarely overlapped, but up to a total of seven individuals (of seven size-ranks) had broadly overlapping territories with dissimilar-sized individuals. Comparison with expectation from the null model demonstrated clearly that observed size ratios between adjacent size rank were determined non-randomly regardless of sexual combinations. 3. Larger individuals took larger prey types of larger average size, but more importantly used wider rock crevices from which to suck food than smaller individuals. We calculated pairwise values of Schoener's index of diet overlap C(d) and the values of Levin's index of diet breadth B(d) (prey type and prey size) and the same for the width of the rock crevices used for foraging (C(r) and B(r)). C(d) remained high among all combinations of the seven ranks. In contrast, C(r) declined strongly in combinations of adjacent ranks (to 0.27), and was low or zero among further different size ranks. This shows that fish with overlapping territories divided the food resources largely through foraging site partitioning. Accordingly, B(d) did not depend on the size difference to the nearest two coinhabiting fish, whereas B(r) did. 4. We conclude that this L. labiatus community is structured non-randomly: body size-dependent effects on foraging site usage result in competition with, and territorial exclusion of, similar-sized individuals, but not of dissimilar-sized individuals that were accepted as coinhabitants. Accordingly, mean body size ratios (large/small) between two adjacent ranks were consistently approximately 1.28 [standard deviation (SD) = 0.07, n = 104], while approximately 1.34 from the null model (SD = 0.34, n = 10 400 simulations). We discuss our results as an example of Hutchinson's rule, applied originally to size ratios of different species.     

On the importance of body size in the colonisation of ephemeral resource patches by vagile consumers

A study on the colonisation of leaf detritus patches by vagile macroinvertebrates in a brackish lagoon is presented in the framework of a conceptual model where a body size-related constraint on patch use behaviour is explicitly considered. Abundance patterns of dominant macroinvertebrate taxa were characterised by short-term, non-random fluctuations, showing significant site-dependent variations. Yet, a site-independent covariation was observed between patterns’ fractal dimension and the average body mass of each taxon, indicating that, while the temporal scales characterising the colonisation patterns may be highly species-specific, cross-species generalisations are possible based on body size. The generality of these results was supported by literature data on temporal patterns of carcass colonisation by bathyal fish. The importance of size-related mechanisms in regulating the aggregation of vagile consumers on resource patches and, ultimately, their coexistence at both an inter- and intra-specific level, is discussed.     

Allometric allocation in fruit and seed packaging conditions the conflict among selective pressures on seed size

Selective pressures on seed size could vary among the different stages of plant life cycles, so no simple relation could explain a priori its evolution. Here, we determined the relationships between seed size and two fitness components—seed dispersal and survival from predation—in a bird-dispersed tree, Crataegus monogyna. We interpret these relationships in relation to the patterns of mass allocation to fruit and seed components. Selection patterns were assessed at two levels (1) selection pressures on the parent tree; comparing seed dispersal efficiency among individual plants and (2) selection pressures at the individual seed level; comparing seed size variation (i) before and after dispersal, and (ii) before and after postdispersal seed predation. Dispersal efficiency (percentage of seed crop dispersed) was positively correlated with fruit mass and fruit width. Differences in crop size did not offset this effect, and larger seeds were overrepresented in the seed rain relative to the seed pool before dispersal. However, the advantage of larger seeds during the dispersal stage was cancelled later by an opposite selection pressure exerted by seed predators. As a result, smaller seeds had a higher probability of surviving postdispersal seed predation, establishing an evolutionary conflict imposed by the need for dispersal and the danger of being predated. Birds and rodents preferentially selected highly profitable fruits and seeds in terms of the relative proportion of their components. Larger fruits had a higher pulp to seed proportion than smaller ones, and all seeds had the same proportion of coat relative to the embryo-plus-endosperm fraction. Hence, although predator pressures were stronger than disperser ones, larger seeds invested proportionally less in structural defense than in dispersal.     

Benefits and costs to pollinating,seed-eating insects: the effect of flower size and fruit abortion on larval performance

Plant–pollinator interactions are well-known examples of mutualism, but are not free of antagonism. Antagonistic interactions and defenses or counter-defenses are expected particularly in nursery pollination. In these systems, adult insects, while pollinating, lay their eggs in flowers, and juveniles consume the seeds from one or several fruits, thereby substantially reducing plant fitness. The outcome of such interactions will depend, for the plant, on the balance between pollination versus seed predation and for the larvae on the balance between the food and shelter provided versus the costs imposed by plant defenses, e.g., through abortion of infested fruits. Here, we examine the costs and benefits to the larvae in the nursery-pollination system Silene latifolia/Hadena bicruris. Using selection lines that varied in flower size (large- vs. small-flowered plants), we investigated the effects of variation in flower and fruit size and of a potential defense, fruit abortion, on larval performance. In this system, infested fruits are significantly more likely to be aborted than non-infested fruits; however, it is unclear whether fruit abortion is effective as a defense. Larger flowers gave rise to larger fruits with more seeds, and larvae that were heavier at emergence. Fruit abortion was frequently observed (ca. 40% of the infested fruits). From aborted fruits, larvae emerged earlier and were substantially lighter than larvae emerging from non-aborted fruits. The lower mass at emergence of larvae from aborted fruits indicates that abortion is a resistance mechanism. Assuming that lower larval mass implies fewer resources invested in the frugivore, these results also suggest that abortion is likely to benefit the plant as a defense mechanism, by limiting both resources invested in attacked fruits, as well as the risk of secondary attack. This suggests that selective fruit abortion may contribute to the stability of mutualism also in this non-obligate system.     

Fish predation on Eudiaptomus gracilis in relation to clutch size,body size,and sex: a field experiment

An enclosure experiment was performed to test for direct predationeffects on fecundity and adult body size of the copepod Eudiaptomus gracilis in the field. By introducing a high densityof fish (15 underyearling roach, Rutilus rutilus, per 350litre enclosure) and documenting the short-term effects on traitsin a rapidly decreasing prey population, responses to changes inthe phytoplankton community were minimized. After 68 hours ofpredation, clutch size and frequency of females carrying eggs weresignificantly lower in fish enclosures. Female density was moreaffected than male density. Predation selected against large bodysize in both sexes but less so in females, leading to an increasedsexual size dimorphism. The results agree with predictions based onprey selectivity in fish. Predation risk should increase withclutch size and body size since these traits increase theconspicuousness of prey. The size of the highly visible egg-clutchmay be more important than body size. Female body size wasuncorrelated to clutch size, which may explain the weaker sizeeffect among females and the changed sexual sizedimorphism.     

The phylogenetic relationships and generic limits of finches (Fringillidae)

Phylogenetic relationships among the true finches (Fringillidae) have been confounded by the recurrence of similar plumage patterns and use of similar feeding niches. Using a dense taxon sampling and a combination of nuclear and mitochondrial sequences we reconstructed a well resolved and strongly supported phylogenetic hypothesis for this family. We identified three well supported, subfamily level clades: the Holoarctic genus Fringilla (subfamly Fringillinae), the Neotropical Euphonia and Chlorophonia (subfamily Euphoniinae), and the more widespread subfamily Carduelinae for the remaining taxa. Although usually separated in a different family-group taxon (Drepanidinae), the Hawaiian honeycreepers are deeply nested within the Carduelinae and sister to a group of Asian Carpodacus. Other new relationships recovered by this analysis include the placement of the extinct Chaunoproctus ferreorostris as sister to some Asian Carpodacus, a clade combining greenfinches (Carduelis chloris and allies), Rhodospiza and Rhynchostruthus, and a well-supported clade with the aberrant Callacanthis and Pyrrhoplectes together with Carpodacus rubescens. Although part of the large Carduelis-Serinus complex, the poorly known Serinus estherae forms a distinct lineage without close relatives. The traditionally delimited genera Carduelis, Serinus, Carpodacus, Pinicola and Euphonia are polyphyletic or paraphyletic. Based on our results we propose a revised generic classification of finches and describe a new monotypic genus for Carpodacus rubescens.     

Interactive effects of sublethal predation and body size on siphon production of the bivalve Nuttallia olivacea

This paper is intended to reveal effects of siphon cropping on siphon production of a tellinacean bivalve Nuttallia olivacea, an important prey for juvenile stone flounder. We carried out a two-way field experiment in which bivalves of three treatments (3-times siphon removal, 1-time removal, no removal) and four size classes (9-50 mm shell length) were caged and placed in the field for 3 mo. Growth of repeatedly-removed bivalves was inhibited, indicating reduced siphon growth (natural increase of siphon size according to somatic growth). However, siphon production of removed bivalves was larger than non-removed bivalves, possibly because of siphon regeneration. Juveniles (N. olivacea < 20 mm shell length) showed high growth performance. Their siphon growth was greater than their siphon regeneration. In all bivalves except juveniles, siphon regeneration was greater than siphon growth and engendered high siphon production. Siphon growth was dependent on bivalve size and was only slightly reduced by siphon loss, but siphon regeneration seemed to be dependent mostly on the extent of siphon loss. Greater siphon removal enhanced larger siphon production. These results indicate that intensive siphon cropping by juvenile stone flounder induces high siphon production without serious impact on N. olivacea.     

Population cycles and changes in body size of the lynx in Alaska

The lynx Lynx canadensis is a common predator in the boreal forests of North America. Its population fluctuates during a 9- to 11-year cycle in synchrony with the population size of its main prey, the snowshoe hare Lepus americanus. Using adult museum specimens, we studied changes in skull (and hence body) size of the lynx in Alaska during the second half of the 20th century. The population cycle in Alaska averaged 9 years, similar to that reported in the neighbouring Yukon. Using harvest data of lynx as an estimate of population size, we found that skull size was negatively related to population size. This relationship was strongest not for the population density in the year of death (X), but for year X-3, a carry-over effect from the first year (or years) of life, indicating that conditions during the fast-growth years are determining body size. We suggest that the density-dependent effect is probably due to changes in food supply, either resulting from the adverse effects of competition or a possible diminished availability of food. Two skull parameters decreased significantly during the second half of the 20th century. We do not know the cause for the year effect and suggest that it might be due to a long-term change in the availability of prey. Canine size did not change during the study period, probably an indication that snowshoe hares maintained their status as the main prey of the lynx throughout the study period. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Brain size at birth throughout human evolution: a new method for estimating neonatal brain size in hominins

An increase in brain size is a hallmark of human evolution. Questions regarding the evolution of brain development and obstetric constraints in the human lineage can be addressed with accurate estimates of the size of the brain at birth in hominins. Previous estimates of brain size at birth in fossil hominins have been calculated from regressions of neonatal body or brain mass to adult body mass, but this approach is problematic for two reasons: modern humans are outliers for these regressions, and hominin adult body masses are difficult to estimate. To accurately estimate the brain size at birth in extinct human ancestors, an equation is needed for which modern humans fit the anthropoid regression and one in which the hominin variable entered into the regression equation has limited error. Using phylogenetically sensitive statistics, a resampling approach, and brain-mass data from the literature and from National Primate Research Centers on 362 neonates and 2802 adults from eight different anthropoid species, we found that the size of the adult brain can strongly predict the size of the neonatal brain (r² = 0.97). This regression predicts human brain size, indicating that humans have precisely the brain size expected as an adult given the size of the brain at birth. We estimated the size of the neonatal brain in fossil hominins from a reduced major axis regression equation using published cranial capacities of 89 adult fossil crania. We suggest that australopiths gave birth to infants with cranial capacities that were on average 180 cc (95% CI: 158–205 cc), slightly larger than the average neonatal brain size of chimpanzees. Neonatal brain size increased in early Homo to 225 cc (95% CI: 198–257 cc) and in Homo erectus to approximately 270 cc (95% CI: 237–310 cc). These results have implications for interpreting the evolution of the birth process and brain development in all hominins from the australopiths and early Homo, through H. erectus, to Homo sapiens.     

Fitness of juvenile lizards depends on seasonal timing of hatching,not offspring body size

To understand how selection shapes life-history traits, we need information on the manner in which offspring phenotypes influence fitness. Life-history allocation models typically assume that “bigger offspring are better”, but field data paint a more complex picture: larger offspring size sometimes enhances fitness, and sometimes not. Additionally, higher survival and faster growth of larger offspring might be due to indirect maternal effects (e.g., mothers allocate hormones or nutrients differently to different-sized eggs), and not to offspring size per se. Alternative factors, such as seasonal timing of hatching, may be more important. We examined these issues using 419 eggs from captive jacky dragon lizards (Amphibolurus muricatus). The mothers were maintained under standardized conditions to minimize variance in thermal and nutritional history, and the eggs were incubated under controlled conditions to minimize variance in offspring phenotypes due to incubation temperature and moisture. We reduced the size of half the eggs (and, thus, the size of the resultant hatchlings) from each clutch by yolk extraction. The hatchlings were marked and released at a field site over a 3-month period, with regular recapture surveys to measure growth and survival under natural conditions. Growth rates and survival were strongly enhanced by early-season hatching, but were not affected by hatchling body size.     

Colony size affects collective decision-making in the ant Temnothorax albipennis

Social insects are well-known for their ability to achieve robust collective behaviours even when individuals have limited information. It is often assumed that such behaviours rely on very large group sizes, but many insect colonies start out with only a few workers. Here we investigate the influence of colony size on collective decision-making in the house-hunting of the ant Temnothorax albipennis. In experiments where colony size was manipulated by splitting colonies, we show that worker number has an influence on the speed with which colonies discover new nest sites, but not on the time needed to make a decision (achieve a quorum threshold) or total emigration time. This occurred because split colonies adopted a lower quorum threshold, in fact they adopted the same threshold in proportion to their size as full-size colonies. This indicates that ants may be measuring relative quorum, i.e. population in the new nest relative to that of the old nest, rather than the absolute number. Experimentally reduced colonies also seemed to gain more from experience through repeated emigrations, as they could then reduce nest discovery times to those of larger colonies. In colonies of different sizes collected from the field, total emigration time was also not correlated with colony size. However, quorum threshold was not correlated with colony size, meaning that individuals in larger colonies adopted relatively lower quorum thresholds. Since this is a different result to that from size-manipulated colonies, it strongly suggests that the differences between natural small and large colonies were not caused by worker number alone. Individual ants may have adjusted their behaviour to their colony’s size, or other factors may correlate with colony size in the field. Our study thus shows the importance of experimentally manipulating colony size if the effect of worker number on the emergence of collective behaviour is to be studied. Received 13 December 2005; revised 9 May 2006; accepted 15 May 2006.     

Independent evolution of the same set of characters in fig flies (Lissocephala,Drosophilidae)

Summary Comparison of species of African fig flies (Lissocephala, Drosophilidae) shows that an unusual phenotype has evolved independently when a particular environment prevails. Each time the adaptation is observed, associated morphological and behavioural characters also appear. Such convergence suggests that selection operates on sets of predetermined morphogenetic pathways.     

Maternal body size as a morphological constraint on egg size and fecundity in butterflies

It is a widespread notion that in arthropods female reproductive output is strongly affected by female size. In butterflies egg size scales positively with female size across species, suggesting a constraint imposed by maternal size. However, in intraspecific comparisons body size often explains only a minor part of the variation in progeny size. We here include representatives of various butterfly families to test the generality of this phenomenon across butterflies. Phenotypic correlations between egg and maternal body size were inconsistent across species: correlations were non-significant for Pararge aegeria and Lycaena tityrus, significantly positive for Papilio machaon, significantly negative for Araschnia levana, and contradictory for Pieris napi. Thus, there was no general pattern linking egg size to maternal size, e.g., caused by an allometric relationship. Consequently, there was at best limited evidence for maternal size acting as a morphological constraint on egg size within butterfly species. Realized fecundity depended on maternal size in P. napi and A. levana, but not in P. aegeria, suggesting that maternal size may affect egg number more strongly than egg size. Yet, variation in fecundity was primarily explained by variation in longevity as is expected for income breeders. Heritability estimates across species were rather similar for pupal mass (ranging between 0.14 and 0.19), but more variable for egg size (0.17–0.31).     

crm-1 facilitates BMP signaling to control body size in Caenorhabditis elegans

We have identified in Caenorhabditis elegans a homologue of the vertebrate Crim1, crm-1, which encodes a putative transmembrane protein with multiple cysteine-rich (CR) domains known to have bone morphogenetic proteins (BMPs) binding activity. Using the body morphology of C. elegans as an indicator, we showed that attenuation of crm-1 activity leads to a small body phenotype reminiscent of that of BMP pathway mutants. We showed that the crm-1 loss-of-function phenotype can be rescued by constitutive supply of sma-4 activity. crm-1 can enhance BMP signaling and this activity is dependent on the presence of the DBL-1 ligand and its receptors. crm-1 is expressed in neurons at the ventral nerve cord, where the DBL-1 ligand is produced. However, ectopic expression experiments reveal that crm-1 gene products act outside the DBL-1 producing cells and function non-autonomously to facilitate dbl/sma pathway signaling to control body size.