Geographical variation in egg size of the Great Tit Parus major: a new perspective

A recent study on geographical variation in egg size of Great Tits Parus major concluded that: (1) mean egg size tended to increase with increasing latitude; and (2) mean egg size was positively correlated with mean clutch size. Including new data on both egg and clutch size, we reanalysed the relationships between egg size, clutch size and latitude, and investigated the possible effects of habitat type, female body size and egg shape on these relationships. We found that (1) egg volume showed minimum values around 51°N, increasing both north and southwards; (2) female body size increased linearly with increasing latitude; (3) female body size was positively correlated with egg breadth, but not with egg length or egg volume; (4) the sphericity index of the eggs (breadth to length ratio) was largest at medium latitudes, and eggs were more elongated towards the north and the south; (5) the relationship between clutch size and latitude was curvilinear, with the largest clutch sizes at intermediate latitudes; (6) egg size was not correlated with clutch size when the complete latitudinal range was considered, but egg size was negatively correlated with clutch size between 40 and 51°N; and (7) egg size did not differ among habitat types. We suggest that female body size (which probably limits egg breadth), and the pressure for producing large eggs (which in turn increases the reproductive success) are the main determinants of geographical variation in egg size and shape. Populations of small-bodied Great Tits seem to escape from the limits of their size, producing relatively elongated eggs, so that from a certain latitude southwards, egg volume does not decrease in spite of a decrease in female body size. Moreover, the negative relationship between egg and clutch size at low latitudes suggests that energetic trade-offs may also contribute to determine egg size in the south.     

Variation in offspring size within a population of the web-building spiderAgelena limbata

Within a population of the web-building spiderAgelena limbata, the weight of the first instar nymphs ranged from 1.187 to 6.559 mg. Both intraclutch and interclutch variation were recorded. The mean weights were different among clutches and the coefficients of variation within a clutch ranged from 3.3 to 29.2%. Variation in the nymphal weight was certainly derived from variation in the egg weight because there was a high correlation between the two weights. Factors affecting interclutch variation in nymphal weight were examined by multiple regression analysis. Nymphal weight was positively correlated with the body size and food conditions of female parents, and negatively correlated with the clutch size. Among these three factors, the food conditions of female parents had the largest apparent effect on the interclutch variation. The results suggest that females with larger body size and more food produce larger offspring, and that there is a trade-off between offspring size and clutch size. Heavier nymphs had larger body size (carapace width) and may have larger energy reserves. Heavier nymphs survived experimental starvation for a significantly longer period.     

Widespread reproductive variation in North American turtles: temperature, egg size and optimality

Theory predicts the existence of an optimal offspring size that balances the trade-off between offspring fitness and offspring number. However, in wild populations of many species, egg size can still vary from year to year for unknown reasons. Here, we hypothesize that among-year variation in population mean egg size of freshwater turtles is partly a consequence of their gonadal sensitivity to seasonal temperatures, a physiological mechanism which principally functions to synchronize reproduction with a favorable time of year. As part of this process, among-year variation in seasonal temperatures modifies the extent of egg follicle development, and this may translate into variation in mean egg size among years (both at the individual and population level). To test this hypothesis, we applied an information-theoretic approach to model relationships between mean egg mass and the temperatures experienced during discrete periods of follicular development in wild populations of three turtle species (Chrysemys picta, Chelydra serpentina, Glyptemys insculpta) over 12 consecutive years. Because follicular development occurs in the fall for C. serpentina and G. insculpta, whereas it occurs both in the fall and spring for C. picta, we expected only fall temperatures would explain egg size variation in C. serpentina and G. insculpta, whereas both fall and spring temperatures would correlate with egg size variation in C. picta. These predictions were upheld. We then compared among-year variation in within-female egg and clutch sizes of each species in order to evaluate whether such variation might still be consistent with some tenets of optimal egg size theory. In all three species, we found that clutch sizes vary more than egg sizes in spite of temperature-induced egg size variation, and this pattern of relatively high clutch-size variation matches theoretical predictions. Future work should explore the roles of direct and indirect (i.e., nutritional) influences of temperature on egg size in natural settings.     

Effects of temperature and season on egg size, hatchling size and adult size in Notiophilus biguttatus

1. Temperature-induced egg size variation and its effects on successive life stages in the carabid beetle Notiophilus biguttatus were examined.
2. In the laboratory, across temperature regimes, egg size and number were inversely related; number of eggs and total egg mass were higher, but egg size was smaller at high temperatures.
3. Food intake rate was shown not to be involved in the temperature effect on egg size.
4. Within the higher temperature regimes, among females, egg size was negatively correlated with number of eggs and with total egg mass.
5. Data on egg mortality and egg development time did not explain why at low temperature eggs were larger than at high temperature.
6. Larvae hatching from eggs produced at a low temperature were heavier than larvae from eggs produced at a high temperature, irrespective of temperature during development.
7. In a prolonged outdoor experiment (January – July), encompassing the main breeding period of N. biguttatus , egg size decreased and both egg production rate and total egg mass increased in the course of the experiment.
8. In the field, an effect of seasonal change in temperature on adult body size was found; teneral beetles that had their juvenile period early in the season were larger than those that had their juvenile period later in the season.
9. The results of the study suggest a mechanistic explanation in which the egg size response to temperature follows from a difference in temperature sensitivity between two processes in oogenesis; adaptiveness of the temperature response was not clarified.     

High variability in egg size and energetic content among intertidal mussels

Maternal investment is a fundamentally important parameter in life-history theory and models, yet the scales at which it varies (among individuals vs. among populations) is rarely reported. In this study, variability in attributes of eggs and early larvae of Mytilus californianus was examined from four sites spanning Point Conception, California, in June and September 2001. The effects of female, site, and month were examined for the following variables: egg volume (microl), egg energy content (microg carbon per egg), and initial larval size (microm). The only significant effect on both egg traits was that of female. Females differed by up to 57% in mean egg volume and 116% in mean egg energetic content. Although there were significant effects of rearing environment, female, site, and month on initial larval size, variability in larval length was small compared to the egg traits. Mean larval length was maximally 11% different among females. Neither female body weight nor length was correlated to mean offspring traits, and there were also no significant relationships between egg traits and initial larval size. The primary source of variation in maternal investment in this system appears to be among individual females rather than over space or time.     

Controlling resource acquisition to reveal a life history trade-off: egg mass and clutch size in an iteroparous seed predator, Prostephanus truncatus

1. A trade-off constraining egg production was investigated and the consequences of egg mass for the offspring life history in Prostephanus truncatus investigated. Fresh egg mass was found to vary between 0.065 and 0.109 mg (about 20% of fresh body mass at emergence) and was correlated both with dry egg mass ( r = 0.8) and with the mass of first-instar larvae ( r = 0.8).
2. There was a negative correlation between egg mass and clutch size: doubling clutch size from eight to sixteen eggs resulted in a reduction in egg mass from 0.09 to 0.07 mg. Resource allocation per clutch was not constant but increased with clutch size. After allowing for the relationship between egg mass and clutch size, heavier females were found to oviposit both heavier eggs and more eggs in a clutch.
3. By placing eggs in prepared seeds that minimized variation in resource acquisition, it was discovered that development period of females, but not of males, was negatively correlated with egg mass. Egg size was not correlated with body size at emergence in either sex.
4. The results are interpreted as revealing a resource-allocation trade-off in egg production.     

Inter-clutch egg size variation in kestrels Falco tinnunculus: seasonal decline under fluctuating food conditions

We examined inter-clutch egg size variation of Eurasian kestrels Falco tinnunculus in western Finland over a period of 12 years, during which their main prey, Microtus voles, fluctuated in three-year population cycles. Females that bred twice in the area had highly repeatable egg size, and the main part of the observed variation was likely to be due to among-female differences. Laying date also explained some variation in egg size, but compared to the variation between individuals its effect was small and varied among the phases of the vole cycle. During decrease phases and years with low vole abundance mean egg size of clutches declined with laying date, whereas during increase phases mean egg size remained stable within the season. Although egg size was not related to fledging success, we found that egg size may have value for the development of eggs as the probability of total hatching success of a clutch increased with increasing mean egg size.     

Larval host plant affects fitness consequences of egg size variation in the seed beetle Stator limbatus

Egg size variation often has large effects on the fitness of progeny in insects. However, many studies have been unable to detect an advantage of developing from large eggs, suggesting that egg size variation has implications for offspring performance only under adverse conditions, such as during larval competition, periods of starvation, desiccation, or when larvae feed on low-quality resources. We test this hypothesis by examining the consequences of egg size variation for survivorship and development of a seed-feeding insect, Stator limbatus, on both a low-quality (Cercidium floridum) and a high-quality (Acacia greggii) host plant. Our results are consistent with the hypothesis. S. limbatus larval performance was affected by egg size only when developing on the poor-quality host (C. floridum); larvae from large eggs survived better on C. floridum than those from small eggs, while there was no evidence of an effect of egg size on progeny development time, body weight, or survivorship when larvae developed on A. greggii. These results indicate intense selection for large eggs within C. floridum-associated populations, but not in A. greggii-associated populations, so that egg size is predicted to vary among populations associated with different hosts. Our results also support this hypothesis; females from a C. floridum-associated population (Scottsdale) laid larger eggs than females from an A. greggii-associated population (Black Canyon City).     

Egg size and clutch size in the reproductive investment of American Kestrels

We studied causes and consequences of egg-size variation among clutches of American kestrels ( Falco sparverius ). Egg. from 275 clutches were measured 1990 to 1992. To test the hypothesis that the size of eggs was contrained by food availability in the pre-laying period, we censused small mammal populations in the three years and performed a food supplementation experiment in 1990 and 1991. Kestrels did not advance the date they laid their first egg but did lay significantly larger eggs in response to extra food. The size of eggs was correlated with small mammal abundance on the territoty, and females in good body condition tended to lay large eggs. Body size did not affect egg size, and there were no relationship between agg size and laying date except in 1900, the poorest food year. Clutches with a large mean egg volume had better hatching success than clutches containing small eggs. We argue that there is a phonetypic component to egg size in kestrels, and that kestrels use egg size to fine-tune reproductive investiment to available resources.     

Egg size, first instar behaviour and the ecology of Lepidoptera

There is striking variation in egg size among Lepidoptera. Part of the explanation could be a link between egg size and larval feeding ecology.
The relationship between absolute egg size and aspects of feeding ecology for different Lepidoptera families from different temperate regions is examined. Species that overwinter in the egg stage have larger eggs. There are significant differences in egg size with respect to feeding specificity but different families show different patterns. Woody plant feeders have larger eggs than herb feeders. There is little effect of proximity of the egg to the plant part that is eaten.
Patterns in the behaviour and survival of newly hatched larvae of 42 spp. of British Lepidoptera and their relationship to egg and larval size and to food plant characteristics are examined. Patterns in egg size with respect to feeding ecology are similar to those described above. There is a strong correlation between egg size and the size of newly hatched larvae. Newly hatched larvae survived for a mean of 1–20 days without food. Survival is not correlated with larval weight. Grass feeders survive longer than herb and woody plant feeders; the species surviving the longest feeds on lichens. Newly hatched larvae moved at a mean speed of 0.7-267.8 cm h^-1. Speed is not correlated with larval weight or survival time. Grass feeders move faster than woody plant feeders which, in turn, move faster than herb feeders. Woody plant feeders tend to move upwards, grass feeders downwards and herb feeders both upwards and downwards. The proportion of larvae silking is negatively correlated with larval weight.
The strong links between egg size and larval feeding ecology and between feeding ecology and larval behaviour are discussed. It is surprising that larval body size does not appear to constrain the speed of movement, nor tolerance to starvation.     

Reproductive variation and the egg size-clutch size trade-off within and among populations of painted turtles (Chrysemys picta bellii)

Interpopulation variation in egg size, clutch size and clutch mass was studied 3 years in four populations of painted turtles (Chrysemys picta bellii) from western Nebraska. Body size varied among all populations and was larger in two large (56–110 ha), sandhills lake populations than in two populations in smaller habitats (1.5–3.6 ha) of the Platte River floodplain. Reproductive parameters (egg mass, clutch mass, and clutch size) generally increased with maternal body size within populations. Clutch wet and dry mass varied among populations but largely as a function of maternal body size. Clutch size was largest in the sandhills lake populations, both absolutely and relative to maternal body size. Egg mass was smallest in the sandhills lakes and varied annually in one population. Over all populations, an egg sizeclutch size trade-off was detected (a negative correlation between egg mass and clutch size) after statistically removing maternal body size effects. Egg wet mass and clutch size were negatively correlated over all years within the sandhills populations and in some years in three populations. Although egg size varied within populations, egg size and clutch size covaried as expected by optimal offspring size models. Thus, patterns of egg size variation should be interpreted in the context of proximate or adaptive maternal body size and temporal effects. Comparisons among populations suggest that large egg size relative to maternal body size may occur when juvenile growth potential is poor and mean maternal body size is small.     

Avian egg size: variation within species and inflexibility within individuals

Egg size is a widely-studied trait and yet the causes and consequences of variation in this trait remain poorly understood. Egg size varies greatly within many avian species, with the largest egg in a population generally being at least 50% bigger, and sometimes twice as large, as the smallest. Generally, approximately 70% of the variation in egg mass is due to variation between rather than within clutches, although there are some cases of extreme intra-clutch egg-size variation. Despite the large amount of variation in egg size between females, this trait is highly consistent within individuals between breeding attempts; the repeatability of egg size is generally above 0.6 and tends to be higher than that of clutch size or laying date. Heritability estimates also tend to be much higher for egg size (> 0.5) than for clutch size or laying date (< 0.5). As expected, given the high repeatability and heritability of egg size, supplemental food had no statistically significant effect on this trait in 18 out of 28 (64%) studies. Where dietary supplements do increase egg size, the effect is never more than 13% of the control values and is generally much less. Similarly, ambient temperature during egg formation generally explains less than 15% of the variation in egg size. In short, egg size appears to be a characteristic of individual females, and yet the traits of a female that determine egg size are not clear. Although egg size often increases with female age (17 out of 37 studies), the change in egg size is generally less than 10%. Female mass and size rarely explain more than 20% of the variation in egg size within species. A female's egg size is not consistently related to other aspects of reproductive performance such as clutch size, laying date, or the pair's ability to rear young. Physiological characteristics of the female (e.g. endogenous protein stores, oviduct mass, rate of protein uptake by ovarian follicles) show more promise as potential determinants of egg size. With regards to the consequences of egg-size variation for offspring fitness, egg size is often correlated with offspring mass and size within the first week after hatching, but the evidence for more long-lasting effects on chick growth and survival is equivocal. In other oviparous vertebrates, the magnitude of egg-size variation within populations is often as great or greater than that observed within avian populations. Although there are much fewer estimates of the repeatability of egg size in other taxa, the available evidence suggests that egg size may be more flexible within individuals. Furthermore, in non-avian species (particularly fish and turtles), it is more common for female mass or size to explain a substantial proportion of the variation in egg size. Further research into the physiological basis of egg-size variation is needed to shed light on both the proximate and ultimate causes of intraspecific variation in this trait in birds.     

Seasonal differences in egg size in three species of crabs from a tropical upwelling zone

Egg size and offspring size are fundamentally important aspects of the life histories of all animals. However the impact of environmental conditions on intraspecific variation in egg size of marine invertebrates is poorly documented. Here we followed three species of intertidal crabs Xanthodius sternberghii, Petrolisthes armatus and Clibanarius albidigitus to understand how seasonal environmental variation in temperature and salinity associated with seasonal upwelling impacts egg size. Ovigerous females of both P. armatus and C. albidigitus were found year round, while X. sternberghii has a limited reproductive season, with ovigerous females found only between November and February. In all three species more than half of the variation in egg size was attributable to variation among broods from different females. Eggs collected during the dry, upwelling season were significantly larger than those collected during the wet, non-upwelling season. Multiple regression analysis showed that average egg size from each brood was significantly negatively correlated with temperature for all three species. Egg size was also negatively correlated with salinity in P. armatus when we controlled for temperature. Overall these results support the idea that changes in environmental temperature caused by seasonal upwelling play a significant role in generating seasonal differences in egg size.     

Egg size variation in European Satyrine butterflies (Nymphalidae, Satyrinae)

The interspecific relationships between egg size and female size, sexual size dimorphism, latitude, preoviposition and egg-laying behaviour, was investigated in European Satyrine butterflies by controlling for taxonomic relatedness and by simple cross-species comparisons. Egg size scales positively to female size, although the relationship is not significant (but nearly so) when taxonomic effects are controlled for. Egg size is correlated with latitude. When the effects of latitude are removed, the relationship between egg and female size becomes more evident. Sexual size dimorphism is negatively correlated with egg size, but the effect disappears when adult size is controlled for. Small egg size is related to long preoviposition. No relationship between egg laying mode and egg size appears. The results are interpreted as evidence for positive scaling of egg to female size, with superimposed effects of egg size radiation. The correlation of egg size with latitude suggests that temperature, or season length, is likely to have made an important contribution to egg size selection within this geographic area.     

Tooth size and arcadal length correlates in man

Intra-arcadal mesiodistal and buccolingual tooth size correlations were evaluated in a sample of 125 caucasoids with ideal occlusion. Dental dimensions were corrected for arcade mength (as a measure of jaw size) by a series of regression analyses of each mesiodistal dimension on the sum of the mesiodistal dimensions within each arcade. Regression coefficients of tooth dimension on arcade length were calculated to gain an insight into the dimensional sensitivity of individual teeth to arcade length variation. The data presented here suggest a strong association between arcadal length (jaw size) dependence, and the dimensional stability of individual teeth. When corrected for arcade length, a definite pattern of tooth size correlation emerges: postcanine maxillary and mandibular teeth are negatively correlated to the anterior teeth and are positively correlated to one another. The hypothesis is developed that anterior and postcanine teeth should be viewed as two separate and negatively size-correlated units, beyond the boundaries of the four morphological tooth classes. Recognition of this basic dichotomous size arrangement within each jaw allows for a reassessment of some of the problems associated with hominid dental evolution.     

Habitat-specific clutch size and cost of incubation in eiders reconsidered

The energetic incubation constraint hypothesis (EICH) for clutch size states that birds breeding in poor habitat may free up resources for future reproduction by laying a smaller clutch. The eider (Somateria mollissima) is considered a candidate for supporting this hypothesis. Clutch size is smaller in exposed nests, presumably because of faster heat loss and higher incubation cost, and, hence, smaller optimal clutch size. However, an alternative explanation is partial predation: the first egg(s) are left unattended and vulnerable to predation, which may disproportionately affect exposed nests, so clutch size may be underestimated. We experimentally investigated whether predation on first-laid eggs in eiders depends on nest cover. We then re-evaluated how nesting habitat affects clutch size and incubation costs based on long-term data, accounting for confounding effects between habitat and individual quality. We also experimentally assessed adult survival costs of nesting in sheltered nests. The risk of egg predation in experimental nests decreased with cover. Confounding between individual and habitat quality is unlikely, as clutch size was also smaller in open nests within individuals, and early and late breeders had similar nest cover characteristics. A trade-off between clutch and female safety may explain nest cover variation, as the risk of female capture by us, mimicking predation on adults, increased with nest cover. Nest habitat had no effect on female hatching weight or weight loss, while lower temperature during incubation had an unanticipated positive relationship with hatching weight. There were no indications of elevated costs of incubating larger clutches, while clutch size and colony size were positively correlated, a pattern not predicted by the ‘energetic incubation constraint’ hypothesis. Differential partial clutch predation thus offers the more parsimonious explanation for clutch size variation among habitats in eiders, highlighting the need for caution when analysing fecundity and associated life-history parameters when habitat-specific rates of clutch predation occur.     

Seasonal variation of egg size and number in a Daphnia pulex population

Seasonal variation of egg size and number was examined in a Daphnia pulex population inhabiting a vernal pond. In this population, size at maturity declines at midseason, probably as an adaptive response to size-selective predation by larvae of the salamander Ambystoma. The larger early season individuals produce more and larger eggs than the smaller late season individuals. Age at maturity does not vary between seasons. Laboratory experiments indicate that temperature may affect egg size, egg number and size at maturity. However, field data suggest that temperature accounts for only a small fraction of the total variation in egg size and number. Indirect measures of nutrition indicate that food limitation does not cause the seasonal decline in egg size and number. The seasonal change in reproductive traits is well correlated with changes in invertebrate and vertebrate predation. Examination of predator feeding preferences and their impact on Daphnia mortality indicate that variation of reproductive traits is most likely a complex adaptation to changing predation regimes.     

Variation in sexual size dimorphism among populations: testing the differential‐plasticity hypothesis

Sexual size dimorphism (SSD) is a common phenomenon in animals and varies widely among species and among populations within species. Much of this variation is likely due to variance in selection on females vs. males. However, environmental variables could have different effects on females vs. males, causing variation in dimorphism. In this study, we test the differential‐plasticity hypothesis, stating that sex‐differential plasticity to environmental variables generates among‐population variation in the degree of sexual dimorphism. We examined the effect of temperature (22, 25, 28, and 31 °C) on sexual dimorphism in four populations of the cockroach Eupolyphaga sinensis Walker (Blattaria: Polyphagidae), collected at various latitudes. We found that females were larger than males at all temperatures and the degree of this dimorphism was largest at the highest temperature (31 °C) and smallest at the lowest temperature (22 °C). There is variation in the degree of SSD among populations (sex*population interaction), but differences between the sexes in their plastic responses (sex*temperature interaction) were not observed for body size. Our results indicated that sex‐differential plasticity to temperature was not the cause of differences among populations in the degree of sexual dimorphism in body size.     

Relationships between egg size, chick size at hatching, and chick survival in the Whimbrel Numenius phaeopus

The relationships between egg size, chick size at hatching and chick survival in Whimbrels Numenius phaeopus were studied over a three-year period in the Shetland Isles. Three measurements of chick size at hatching were all positively correlated with egg volume, though the relationship was strongest with hatchling body-weight. In two of the three years the proportion of chicks from a brood which survived to fledging increased significantly with the mean hatching weight of chicks in the brood. Within broods, a significant effect of hatching weight on survival was detectable only up to 7 days after hatching. Between years the egg volumes and hatchling weights of individual female Whimbrels showed relatively little variability, indicating that these attributes could be controlled to a large extent by inheritance.     

Testing ecological and developmental hypotheses of mean and variation in adult size in nephilid orb-weaving spiders

Fecundity selection has been hypothesized to drive the evolution of female gigantism in the orb-weaving family Nephilidae. Several species of these spiders also exhibit large amounts of variation in size at maturity in one or both sexes. In this article, we attempt to detect correlations of mean and variation in adult size at a phylogenetic scale between the sexes and with latitude. We tested six predictions derived from three broad developmental, ecological, and age structure hypotheses, using independent contrasts and a recent species-level nephilid phylogeny as well as least squares and other conventional statistics: 1. In both sexes, species with larger mean size will have greater variation in size; 2. Males and females will show correlated changes in mean size and of variation in size; 3. In both sexes, mean size will be negatively correlated with the midpoint of the latitudinal range; 4. In both sexes, tropical species will be more variable; 5. In both sexes, more widespread species will be more variable; 6. Variation in male size will be positively correlated with mean female size. In no cases were male and female development correlated, suggesting that in this lineage male and female body size evolve independently. The only significant trend detected was a positive phylogenetic correlation between variation in female size and latitude, the opposite of prediction 4. Power tests showed that in all tests of the ecological hypothesis, sample sizes were more than adequate to detect significant trends, if present. Our results suggest that evolutionary trends in juvenile development among species are too weak to be detectable in such data sets.