Evolution of clutch size in insects. II. A test of static optimality models using the beetle Callosobruchus maculatus (Coleoptera: Bruchidae)

Wilson and Lessells (1993) analysed the effect of constraint assumptions on the predictions of static optimality models for insect clutch size. They concluded that the models could be reliably distinguished between (and hence the main constraints identified) only after precise quantitative predictions had been examined. The present paper describes a series of laboratory experiments, using the bruchid beetle Callosobruchus maculatus, that allow these quantitative predictions to be made and tested. Experiments in which female encounter rate with hosts was altered gave qualitative support for 3 out of 6 basic (single oviposition) models, but the quantitative fit of them all was poor. However, when the (a priori) condition was included in these models that several other females would oviposit on the same hosts (the multiple oviposition models), the time limiting multiple oviposition model alone produced quantitative predictions that were supported by observations. In other words, the results suggest that the main constraints on bruchid oviposition behaviour are the amount of time available for laying eggs and the number of other females ovipositing. However, additional qualitative predictions indicate that the number of eggs available to the female may also constrain clutch size evolutionarily. The usefulness of static optimality models for examining clutch size decisions in insects is discussed in the context of these results.     

A comparative study of egg mass and clutch size in the Anseriformes

The factors explaining interspecific differences in clutch investment in precocial birds are poorly understood. We investigated how variations in clutch characteristics are related to environmental factors in a comparative study of 151 extant species of ducks, geese and swans (Anseriformes). Egg mass was negatively related to clutch size in a phylogenetic regression, a relationship that was much stronger when controlling for female mass. Nest placement was related to both egg size and clutch size, with cavity-nesting species laying more but smaller eggs. Egg size was positively correlated with incubation period and with female mass, and also with sexual size dimorphism (i.e. male mass relative to that of the female). Clutch size was not related to female mass. Species with long term pair bonds laid smaller clutches and larger eggs. The size of the breeding range was strongly positively correlated with clutch size and clutch mass, and its inclusion in multivariate models made other biogeographical variables (hemisphere, breeding latitude or insularity) non-significant. The small clutches in insular species appear to be a product of small range size rather than insularity per se. Our results suggest there is an evolutionary trade-off between clutch and egg size, and lend support to Lack’s resource-limitation hypothesis for the waterfowl.     

Intra-seasonal decline of clutch size in Lesser Snow Geese

Summary Different mechanisms proposed to explain the intra-seasonal decline in clutch size of Lesser Snow Geese (Anser caerulescens caerulescens) were tested at the La Perouse Bay colony, Manitoba, Canada. Ovary examination of females collected after laying revealed that the actual number of eggs produced per female decreased over the laying period. This finding eliminates nest-parasitism, partial clutch predation and renesting attempts as sufficient explanations for seasonal clutch size decline. Follicular atresia induced by reserve depletion was also rejected since its occurrence was similar among early and late nesters. The decline in clutch size was observed within age-classes, and therefore age effects on clutch size and laying date per se do not account for the observed relationship. Clutch size and laying date were respectively positively and negatively correlated with the amount of nutrient reserves in females at the onset of laying, and also covaried within individuals observed breeding in several seasons. Laying date repeatability was estimated at 0.22. It is postulated that the seasonal decline in clutch size results mainly from a positive feed-back of female's nutrient reserves on the hormones controlling ovary development. Hatching synchrony or shortness of the breeding season may be the ultimate factors responsible for the intra-seasonal clutch size decline in Lesser Snow Geese.     

On the evolution of clutch size and nest size in passerine birds

Summary I examined the hypothesis that the clutch size of some altricial birds may be limited by over-crowding of the nestlings in the nest, by comparing data on different species of European passerines. Large-sized birds build, relative to the body, larger nests than small-sized birds, both as regards the inner and the outer nest widths and as regards edge breadth; only inner nestcup depth did not change relatively to body size. Nest size also varied in relation to nesting place. Birds with open nests built off the ground had a rather narrow nestcup, whereas those with a domed nest, or which nest in a cavity, had a wide nestcup. When only open-nesters were compared, birds nesting on, or close to, the ground tended to have a wider nestcup than birds nesting above the ground. Inner nestcup width was correlated with the amount of mosses and lichens used in building the nest; the more of such materials the narrower the nestcup. The three variables: standardised body size, nesting place, and type of nesting material used accounted for 92% of the overall variation observed in inner nestcup width. When controlling for adult body size, clutch size was positively correlated with the size of the nestcup. A multiple regression analysis showed that relative nestcup depth, nest site, and type of nesting materials used, accounted for 64% of the overall variation in clutch size.     

Population density, body size, and phenotypic plasticity of brood size in a burying beetle

Theory predicts that organisms living in heterogeneous environmentswill exhibit phenotypic plasticity. One trait that may be particularlyimportant in this context is the clutch or brood size becauseit is simultaneously a maternal and offspring characteristic.In this paper, I test the hypothesis that the burying beetle,Nicrophorus orbicollis, adjusts brood size, in part, in anticipationof the reproductive environment of its adult offspring. N. orbicollisuse a small vertebrate carcass as a food resource for theiryoung. Both parents provide parental care and actively regulatebrood size through filial cannibalism. The result is a positivecorrelation between brood size and carcass size. Adult bodysize is an important determinant of reproductive success forboth sexes, but only at higher population densities. I testthree predictions generated by the hypothesis that beetles adjustbrood size in response to population density. First, averageadult body size should vary positively with population density.Second, brood size on a given-sized carcass should be larger(producing more but smaller young) in low-density populationsthan in high-density populations. Third, females should respondadaptively to changes in local population density by producinglarger broods when population density is low and small broodswhen population density is high. All three predictions weresupported using a combination of field and laboratory experiments.These results (1) show that brood size is a phenotypically plastictrait and (2) support the idea that brood size decisions arean intergenerational phenomenon that varies with the anticipatedcompetitive environment of the offspring.     

Experimentally induced clutch size enlargements affect reproductive success in the Pied Flycatcher

We performed a food provisioning experiment in a population of Pied Flycatchers Ficedula hypoleuca breeding at high altitude in central Spain to test if food availability before and during laying determines clutch size. Food was provided to one of two pairs with the same date of initiation of nest-building (15 dyads of subsequently reproducing pairs were thus created). Food provisioning began on the day of initiation of nest-building and ended on the day after the last egg was laid. Although laying date was unaffected by the experiment, clutch size in the experimental treatment was significantly larger. This result could indicate that food availability at laying (1) proximately constrained clutch size or (2) that females evaluated future conditions for incubating eggs and feeding nestlings based on food availability at laying. Reproductive success (proportion of eggs that resulted in fledged young) was significantly reduced in the experimental treatment. This effect suggest that supplemented females were tricked by the experiment into laying more eggs than the number of eggs they were able to incubate with success and the number of nestlings they were able to feed, a source of error in clutch size adjustment which could be common in non-experimental situations.     

Effect of marking pheromone on clutch size in the Mediterranean fruit fly

Abstract Using acridine orange to selectively stain eggs, we showed that wild-collected female Mediterranean fruit flies (Ceratitis capitata Wiedemann) laid fewer eggs per clutch in fruit previously infested with eggs than in uninfested fruit. This effect is apparently attributable to marking pheromone deposited by females after oviposition: clutch size on fruit infested with eggs but free of marking pheromone was not statistically different from that on uninfested fruit. Clutch size on uninfested fruit on which marking pheromone was artificially transferred was significantly lower than that on uninfested and untreated fruit. Marking pheromone had a comparable though not statistically significant effect on the clutch size of females originating from a strain maintained in the laboratory for several hundred generations.     

Influence of fine-scale habitat structure on nest-site occupancy,laying date and clutch size in Blue Tits Cyanistes caeruleus

Most birds have specific habitat requirements for breeding. The vegetation structure surrounding nest-sites is an important component of habitat quality, and can have large effects on avian breeding performance. We studied 13 years of Blue Tit Cyanistes caeruleus population data to determine whether characteristics of vegetation structure predict site occupancy, laying date and number of eggs laid. Measurements of vegetation structure included the density of English Oak Quercus robur, European Beech Fagus sylvatica, and other deciduous, coniferous and non-coniferous evergreen trees, within a 20-m radius of nest-boxes used for breeding. Trees were further sub-divided into specific classes of trunk circumferences to determine the densities for different maturity levels. Based on Principal Component Analysis (PCA), we reduced the total number of 17 measured vegetation variables to 7 main categories, which we used for further analyses. We found that the occupancy rate of sites and the number of eggs laid correlated positively with the proportion of deciduous trees and negatively with the density of coniferous trees. Laying of the first egg was advanced with a greater proportion of deciduous trees. Among deciduous trees, the English Oak appeared to be most important, as a higher density of more mature English Oak trees was associated with more frequent nest-box occupancy, a larger number of eggs laid, and an earlier laying start. Furthermore, laying started earlier and more eggs were laid in nest-boxes with higher occupancy rates. Together, these findings highlight the role of deciduous trees, particularly more mature English Oak, as important predictors of high-quality preferred habitat. These results aid in defining habitat quality and will facilitate future studies on the importance of environmental quality for breeding performance.     

The evolution of reaction norms: simple models for age and size at maturity

This paper presents a simple model for the evolution of reaction norms for age and size at maturity that predicts reaction norms with a variety of shapes. Using realistic parameter values the model predicts reaction norms close to those observed in Drosophila. The major assumptions of the model are: 1) that net reproductive rate is maximized, 2) that growth is determinate, and 3) that mortality rates are independent of age and size at maturity. If, additionally, juvenile mortality is uncorrelated with a growth coefficient, k, the model predicts that selection favors maturation later at a smaller size when k is reduced by environmental factors and that decreased juvenile mortality leads to delayed maturity. These two predictions conform with those found by previous models using other measures of fitness. Correlations between k and juvenile mortality can change the shape of the predicted reaction norm. Depending on the precise form of the correlation, the model can predict done- or bowl-shaped reaction norms and can predict delayed or earlier maturity as k decreases. These shapes are qualitatively different from those predicted by previous models that used different fitness measures. Systematic estimates of the parameter values for this and for related models are required to determine the appropriate fitness measure for models of reaction norms.     

The genetics of phenotypic plasticity I. Heritability

Methods for estimating the genetic component of phenotypic plasticity are presented. In the general case of clonal replicates or full-sibs raised in several environments, the heritability of plasticity can be measured as the ratio of the genotype-environment interaction variance to the total phenotypic variance. In the special case of only two environments plasticity also can be measured as the difference among environments in genotype or family means. In that case, the heritability of plasticity can be measured as either a ratio of variance components or as the slope of a parent-offspring regression. The general measure suffers because no least-square standard errors have been developed, although they can be calculated by maximum-likelihood or bootstrapping techniques. For the other two methods least-square standard errors can be calculated but require very large experiments for statistical significance to be achieved. The heritability measures are compared using data on plasticity of thorax size in response to temperature in Drosophila melanogaster. The heritability estimates are all in close agreement. Models of the evolution of phenotypic plasticity have treated it as a trait in its own right and as a cross-environment genetic correlation. Although the first approach is the one used here, neither one is preferred.     

Effect of food abundance on laying date and clutch size in the White Stork Ciconia ciconia

CapsuleFood independently affects both laying date and clutch size, suggesting that seasonal decline in clutch size is related to a decrease in food availability.

Aim To test the effect of food abundance on laying date and clutch size of the White Stork and identify the cause of seasonal decline in the number of eggs laid.

Methods During 1991 and 1996 we recorded clutch size and laying date of pairs breeding next to rubbish dumps (food abundant and constant throughout the breeding season) and birds breeding far from rubbish dumps (using natural food sources).

Results In 1991 there was no difference in mean laying date between pairs nesting at rubbish dumps and control pairs. Clutch size was significantly larger at rubbish dump nests. In contrast, mean laying date was earlier in control pairs in 1996 and there was no significant differences in clutch sizes, even when controlling for laying date effect.

Conclusion The results support the hypothesis that food availability independently affects both laying date and clutch size. The seasonal decline in clutch size close to rubbish dumps was negligible (1991) or much smaller than in the control zone (1996) suggesting that a progressive deterioration of natural food sources is the most probable reason for a decline in clutch size as the season advances.     

Effects of predation on clutch size and egg dispersion in the codling moth Laspeyresia pomonella

ABSTRACT.

• 1 The effects of predation on clutch size and egg dispersion in the codling moth Laspeyresia pomonella (L.) were experimentally studied in an orchard by recording survival of eggs glued onto different substrates.
• 2 Egg mortality due to sucking insects was readily distinguishable from that due to chewing insects.
• 3 In both the field and laboratory, females preferred to deposit eggs singly on the uppersides of leaves.
• 4 Eggs on the uppersides of leaves and on fruit survived significantly better than did those on the undersides of leaves or on twigs.
• 5 Egg losses were highest during the early part of the flight season.
• 6 The smallest (one egg) and largest (nine eggs) clutches, associated with one egg-bearing leaf per twig, both suffered the lowest rates of predation. Factors selecting against oviposition on fruits and production of large clutch sizes are presented and discussed.

     

Sex-specific plasticity of growth and maturation size in a spider: implications for sexual size dimorphism

Sex-specific plasticity in body size has been recently proposed to cause intraspecific patterns of variation in sexual size dimorphism (SSD). We reared juvenile male and female Mediterranean tarantulas (Lycosa tarantula) under two feeding regimes and monitored their growth until maturation. Selection gradients calculated across studies show how maturation size is under net stabilizing selection in females and under directional selection in males. This pattern was used to predict that body size should be more canalized in females than in males. As expected, feeding affected male but not female maturation size. The sex-specific response of maturation size was related to a dramatic divergence between subadult male and female growth pathways. These results demonstrate the existence of sex-specific canalization and resource allocation to maturation size in this species, which causes variation in SSD depending on developmental conditions consistent with the differential-plasticity hypothesis explaining Rensch's Rule.     

Reproductive biology of the fire-bellied toads, Bombina bombina and B. variegata (Anura: Discoglossidae): egg size, clutch size and larval period length differences

Clutch size, egg size, larval size and duration of larval period were estimated for samples of Bombina bombina and B. variegata from several populations. Bombina variegata lays fewer eggs per batch (mean 17.4) than B. bombina (mean 32.5). The volume of B. variegata eggs is 2.4 times greater and the dry weight 2.3 times greater than that of B. bombina. As a consequence, the volume of B. variegata hatchlings and their dry weight are respectively 1.9 times and 2.5 times greater than that of B. bombina. Analysis of variance showed that variation of egg size in B. variegata is significantly greater than in B. bombina. When reared under laboratory conditions freshly metamorphosed individuals of both species showed no difference in body size or weight. Although growth rates are equal, tadpoles of B. variegata reached metamorphosis 14 days earlier than B. bombina tadpoles because of differences in size of hatchlings. Differences in reproductive parameters found for the two European Bombina species can be interpreted as adaptations to contrasting environments in which they breed.     

A comparative study of the relationship between host size and brood size in Apanteles spp. (Hymenoptera: Braconidae)

ABSTRACT. 1. Current models of insect oviposition predict that clutch size in parasitoids should correlate with host size, with a continuum from solitary species at one end to large gregarious broods at the other. This prediction is tested for the genus Apanteles (sensu lato).
2. The distribution of brood sizes in Apanteles is bimodal, with peaks at one (solitary species) and at about twenty (gregarious species).
3. Brood size of gregarious species correlates with host size, but when a measure of the total volume of a parasitoid brood is plotted against host size, solitary species do not lie on the same regression slope as gregarious species.
4. There is a relative shortage of gregarious species on small hosts, and a relative excess of solitary species on large hosts. Solitary species on large hosts do not fully consume the host resource.
5. The possible role of evolutionary constraints to adaptive progeny allocation in Apanteles is discussed.     

A comparative analysis of predator-induced plasticity in larval Triturus newts

Species that occupy similar habitats are expected to show convergent phenotypes. If habitats are defined by the presence of predators, then traits that modify vulnerability to predation, including predator-induced phenotypic plasticity, should be similar within habitats. We tested this idea using larvae of six syntopic newt species belonging to the two Triturus clades. Behavioural plasticity induced by odonate predators was strongly dissimilar between the two main clades but similar within them. Morphological plasticity was variable among species, even between one pair of closely related species. A predation experiment tested whether differences between clades could be caused by differences in body size. Size-specific vulnerability differed between newts in the small-bodied and large-bodied clades, indicating that similar predators may affect the two clades differently. The results showed both similarity and dissimilarity in predator-induced phenotypic plasticity in syntopic larval newts although theory suggests that divergence is unlikely in such ecologically similar species.     

Macroevolutionary pattern of sexual size dimorphism in geckos corresponds to intraspecific temperature‐induced variation

Many animal lineages exhibit allometry in sexual size dimorphism (SSD), known as ‘Rensch’s rule’. When applied to the interspecific level, this rule states that males are more evolutionary plastic in body size than females and that male‐biased SSD increases with body size. One of the explanations for the occurrence of Rensch’s rule is the differential‐plasticity hypothesis assuming that higher evolutionary plasticity in males is a consequence of larger sensitivity of male growth to environmental cues. We have confirmed the pattern consistent with Rensch’s rule among species of the gecko genus Paroedura and followed the ontogeny of SSD at three constant temperatures in a male‐larger species (Paroedura picta). In this species, males exhibited larger temperature‐induced phenotypic plasticity in final body size than females, and body size and SSD correlated across temperatures. This result supports the differential‐plasticity hypothesis and points to the role phenotypic plasticity plays in generating of evolutionary novelties.