Decreased Egg Size Research Articles

The molecular basis of socially induced egg-size plasticity in honey bees.

Reproduction involves the investment of resources into offspring. Although variation in reproductive effort often affects the number of offspring, adjustments of propagule size are also found in numerous species, including the Western honey bee, Apis mellifera. However, the proximate causes of these adjustments are insufficiently understood, especially in oviparous species with complex social organization in which adaptive evolution is shaped by kin selection. Here, we show in a series of experiments that queens predictably and reversibly increase egg size in small colonies and decrease egg size in large colonies, while their ovary size changes in the opposite direction. Additional results suggest that these effects cannot be solely explained by egg-laying rate and are due to the queens' perception of colony size. Egg-size plasticity is associated with quantitative changes of 290 ovarian proteins, most of which relate to energy metabolism, protein transport, and cytoskeleton. Based on functional and network analyses, we further study the small GTPase Rho1 as a candidate regulator of egg size. Spatio-temporal expression analysis via RNAscope and qPCR supports an important role of Rho1 in egg-size determination, and subsequent RNAi-mediated gene knockdown confirmed that Rho1 has a major effect on egg size in honey bees. These results elucidate how the social environment of the honey bee colony may be translated into a specific cellular process to adjust maternal investment into eggs. It remains to be studied how widespread this mechanism is and whether it has consequences for population dynamics and epigenetic influences on offspring phenotype in honey bees and other species.

Terrestrial reproduction and parental care drive rapid evolution in the trade-off between offspring size and number across amphibians

The trade-off between offspring size and number is central to life history strategies. Both the evolutionary gain of parental care or more favorable habitats for offspring development are predicted to result in fewer, larger offspring. However, despite much research, it remains unclear whether and how different forms of care and habitats drive the evolution of the trade-off. Using data for over 800 amphibian species, we demonstrate that, after controlling for allometry, amphibians with direct development and those that lay eggs in terrestrial environments have larger eggs and smaller clutches, while different care behaviors and adaptations vary in their effects on the trade-off. Specifically, among the 11 care forms we considered at the egg, tadpole and juvenile stage, egg brooding, male egg attendance, and female egg attendance increase egg size; female tadpole attendance and tadpole feeding decrease egg size, while egg brooding, tadpole feeding, male tadpole attendance, and male tadpole transport decrease clutch size. Unlike egg size that shows exceptionally high rates of phenotypic change in just 19 branches of the amphibian phylogeny, clutch size has evolved at exceptionally high rates in 135 branches, indicating episodes of strong selection; egg and tadpole environment, direct development, egg brooding, tadpole feeding, male tadpole attendance, and tadpole transport explain 80% of these events. By explicitly considering diversity in parental care and offspring habitat by stage of offspring development, this study demonstrates that more favorable conditions for offspring development promote the evolution of larger offspring in smaller broods and reveals that the diversity of parental care forms influences the trade-off in more nuanced ways than previously appreciated.

Survival and spawning success of American shad (Alosa sapidissima) in varying temperatures and levels of glochidia infection.

Temperature fluctuations and climate change impacts may substantially affect spawning success of fish, especially migratory species with a limited spawning window. Factors affecting American shad (Alosa sapidissima) spawning success and survival were investigated at different temperatures and periods (peak- and late-spawning periods) during the Connecticut River, USA, spawning migration in 2017. Wild caught American shad were exposed to constant temperatures regimes of 15, 18, 21, 24 and 27°C for 2weeks. During the peak-spawning period, an increase in temperature (15-24°C) was shown to increase spawning success factors, including spawning probability, number of eggs, and fertilization success, but decreased egg size. Temperatures between 18 and 27°C did not affect these factors during the late-spawning period. Glochidia infection by the alewife floater (Anodonta implicata) was much higher in the late-spawning period and significantly decreased the survival of American shad. Further research should investigate the parasite-host relationship between the alewife floater and American shad to determine annual variability of glochidia infections and how they affect American shad from physiological and passage perspectives. Higher temperatures were shown to increase spawning success of American shad during the peak-spawning period, but temperature had no effect during the late-spawning period. However, any effect during the late-spawning period may have been masked by a high level of glochidia infection.

Egg size and composition in an ageing capital breeder – consequences for offspring performance

1. Egg size is often used as a proxy of egg quality although size and composition may vary, e.g. in insects egg size usually decreases as female ages. Whether this decrease in size reflects reduced concentrations of essential nutrients such as lipids and proteins of eggs laid by ageing females, or does reduced size per se explain often observed lower fitness of later laid eggs is poorly explored.2. Egg properties were compared with fitness parameters of offspring laid on the first and fourth night during the oviposition period of a capital breeding moth, Cleorodes lichenaria (Hufnagel). The study aim was to explore whether decreased egg size is caused by decreased provisioning into later laid eggs measured as egg protein and lipid concentration, and whether it results in lower fitness of later laid offspring.3. The fresh and dry weight of eggs decreased over the oviposition period, but the protein and lipid concentration remained constant. Survival of larvae was lower among the fourth night laid offspring on a low quality host Parmelia sulcata Taylor compared to a high quality host Ramalina fraxinea (L.) Ach. No differences were observed in egg fertility or hatchability, neonate survival without food and pupal mass between the offspring produced on different nights.4. Decreased survival of offspring produced later was rather attributable to absolute provisioning (i.e. lower weight of eggs) than relative provisioning (i.e. decreased concentrations of nutrients in eggs). It is argued that lower survival of later laid smaller eggs on low quality diet is likely attributable to physical and chemical characteristics of host lichens and/or physical properties of tiny neonate larvae.

Helpers at the nest compensate for reduced maternal investment in egg size in carrion crows

Life history theory predicts that mothers should trade off current and future reproductive attempts to maximize lifetime fitness. When breeding conditions are favourable, mothers may either increase investment in the eggs to improve the quality of the offspring or save resources for future reproduction as the good raising environment is likely to compensate for a 'bad start'. In cooperatively breeding birds, the presence of helpers improves breeding conditions so that mothers may vary the number, size and quality of the eggs in response to the composition of the group. Here, we show that in cooperatively breeding carrion crows Corvus corone corone, where nonbreeding males are more philopatric and more helpful at the nest than females, breeding females decreased egg size as the number of subordinate males in the group increased. However, despite the smaller investment in egg size, fledglings' weight increased in groups with more male subordinates, improving post-fledging survival and indicating that helpers fully compensated for the initial 'bad start'. These results highlight a 'hidden effect' of helpers that bears profound implications for understanding the ultimate function of helping.

Population response of pale chub (Zacco platypus) exposed to wastewater effluents in Gap Stream

The effects of effluent from a municipal wastewater treatment plant (MWTP) on population level responses of the pale chub (Zacco platypus), as sentinel species, were evaluated at four sites of the Gap Stream (GS) in June 2007. At GS 7.2 and GS 2.7, downstream of the municipal wastewater outfall, the response patterns of the pale chub population in the age at maturation were not changed; mean age, fecundity, and condition factor were increased; age distribution was shifted to older; growth rate was increased/or not changed; and the egg size and population size were decreased compared with reference site (GS 26.6). These observed responses of the pale chub population matched well with the characteristics expected from a fish population experiencing chronic recruitment failure, except for the decreased egg size due to the elevation in nutrients and pollutants by treated sewage discharge. The observed response pattern at the downstream sites might be caused by the deterioration of the spawning or nursery habitat, or by the contaminant-induced chronic spawning failures. Thus, these results suggest that the effluent of the MWTP might impact on the fish population structure and health status of pale chub population.

Nonlinear continuum of egg size-number trade-offs in a snake: is egg-size variation fitness related?

The relationship between offspring size and offspring number is crucial to life history evolution. To examine how these two life history variables are coupled and whether an altered balance between them will result in changes in maternal fitness, we manipulated clutch size of the Chinese cobra (Naja atra) by using the techniques of hormonal manipulation and follicle ablation. Females receiving exogenous follicle-stimulating hormone produced more but smaller eggs, and females undergoing follicle ablation produced fewer but larger eggs. Neither body size (body mass and snout-vent length) at hatching nor egg mass at oviposition had a role in determining hatchling survival and growth. Female hatchlings were more likely to die in early post-hatching days and grew more slowly than male hatchlings. Our data show that: (1) there is a nonlinear continuum of egg size-number trade-offs in N. atra within which there is a single inflexion where the rate at which egg size decreases with increasing clutch size, or clutch size increases with decreasing egg size, is maximized; (2) there is a fixed upper limit to egg size for a given-sized female, and the limit is not determined by her body volume; (3) egg size has no role in determining hatchling survival and growth; and (4) the extent to which females may enjoy reproductive benefits in a given reproductive episode depends on how well egg size and egg number are balanced.

Assessing effects of a mining and municipal sewage effluent mixture on fathead minnow ( Pimephales promelas) reproduction using a novel, field-based trophic-transfer artificial stream

The Junction Creek watershed, located in Sudbury, ON, Canada receives effluent from three metal mine wastewater treatment plants, as well as a municipal wastewater (MWW) discharge. Effects on fish have been documented within the creek (decreased egg size and increased metal body burdens). It has been difficult to identify the cause of the effects observed due to the confounded nature of the creek. The objectives of this investigation were to assess the: (1) effects of a mine effluent and municipal wastewater (CCMWW) mixture on fathead minnow (FHM; Pimephales promelas) reproduction in an on-site artificial stream and (2) importance of food ( Chironomus tentans) as a source of exposure using a trophic-transfer system. Exposures to CCMWW through the water significantly decreased egg production and spawning events. Exposure through food and water using the trophic-transfer system significantly increased egg production and spawning events. Embryos produced in the trophic-transfer system showed similar hatching success but increased incidence and severity of deformities after CCMWW exposure. We concluded that effects of CCMWW on FHM were more apparent when exposed through the water. Exposure through food and water may have reduced effluent toxicity, possibly due to increased nutrients and organic matter, which may have reduced metal bioavailability. More detailed examination of metal concentrations in the sediment, water column, prey ( C. tentans) and FHM tissues is recommended to better understand the toxicokinetics of potential causative compounds within the different aquatic compartments when conducting exposures through different pathways.

Experimental (Antiestrogen‐Mediated) Reduction in Egg Size Negatively Affects Offspring Growth and Survival

The relationship between egg size and offspring phenotype is critical to our understanding of the selective pressures acting on the key reproductive life-history traits of egg size and number. Yet there is surprisingly little empirical evidence to support a strong, positive relationship between egg size and offspring quality (i.e., offspring growth, condition, and survival) in birds, in part because of confounding effects of parental quality and the lack of experimental techniques for directly manipulating avian egg size independently of maternal condition. Previously, we showed that treatment of laying female zebra finches (Taeniopygia guttata) with the antiestrogen tamoxifen can decrease egg size by ca. 8% but that this reduction in egg size had few effects on offspring mass and size at fledging. Here, we extend the use of this technique to induce larger decreases in egg size (up to 50% in individual females) and show that a reduction in egg size of ca. 18% is associated with decreased embryo viability, increased hatchling mortality, and lower posthatching offspring survival. Furthermore, we show that although hatchlings from eggs reduced in size by ca. 9% can survive to fledging, these chicks show slower initial growth during the linear growth phase (5-10 d of age), fledge at lower masses than chicks from control eggs, and show postfledging compensatory growth. Our results provide empirical support for significant effects of egg size on offspring quality and further suggest that among individual females there is a minimum egg size required to maintain embryo viability and offspring quality.

Cottonseed Meal in Poultry Diets: A Review

The residue of cottonseed oil extraction industry popularly known as cottonseed meal (CSM) is a fairly good source of protein (222.0 to 560.2gkg-1) and metabolizable energy (7.4 to 11.99MJkg-1). The use of CSM in poultry diet is limited due to the presence of gossypol, cyclopropenoid fatty acids (CPFA), high fibre and poor protein quality. The most critical amino acid in CSM is lysine. During the oil extraction from cotton- seeds, a portion of free gossypol (FG) binds with the epsilon amino group of lysine, thereby reducing the availability of lysine to the chicken. The concentration of nutrients, FG, CPFA in CSM depends on the variety of cultivars, methods of oil extraction, proportion of kernel to husk, lint and seed coat. The concentration of FG in various varieties of CSM ranges from 200 and 5300mgkg-1. In glandless variety, the FG concentration is less (>130mgkg-1). Gossypol inhibits the activity of pepsin and trypsin in gastro-intestinal tract, thereby reducing the digestibility of protein. The reported toxic effects of gossypol are growth depression, lameness, decreased egg size and hatchability. Gossypol and CPFA also deteriorates the internal egg quality by causing yolk mottling and pinkish discoloration of albumen, respectively. The tolerance levels of FG in birds vary widely depending on the age and strain of birds, protein content and quality, duration of feeding, presence of minerals especially the iron content in the diet. The tolerance of FG in chicks varies widely (90 to 1000mgkg-1 diets), however performance of layer chicks could be affected even at a lower FG intake of 2.02mg day-1 bird-1. Though, growth was not affected in broilers at FG concentration in diet below 150mgkg-1, feed efficiency was reduced at lower levels i.e. 60-100mgkg-1. The depression in egg production in layers fed diets containing more than 100mgkg-1 was mainly due to lowered feed intake. The egg weight and egg quality was not affected at FG levels upto 200mgkg-1 diet. Iron treatment (4 : 1; Fe : FG) proved to be effective in alleviating the negative effects on egg quality, but could not alleviate the depression in laying performance. Lysine supplementation to CSM containing diets is also effective in counteracting the ill effects of FG in chicken, especially chicks and broilers. Other processing methods like solid substrate fermentation, production of glandless cotton seed and use of organic solvents were though effective in improving the nutritive value of CSM and reducing the FG content in CSM, these methods are of little practical relevance and uneconomical. Production of glandless variety of cottonseed through genetic modification (Bacillus thuringiensis cotton) is one of the recent trends of reducing the FG content. Some other methods like pelleting, extrusion, cooking and Ca (OH)2 treatment of CSM could be tried for improving the feeding value of CSM in poultry.

Low energy reserves and energy allocation decisions affect reproduction by Mountain Pine Beetles,Dendroctonus ponderosae

SummaryLow internal energy reserves at the beginning of the breeding season may impose physiological constraints on an animal's reproductive investment and may alter the optimal trade‐off between investment in reproduction and somatic condition.Here we examine how the energetic condition of female Mountain Pine Beetles (Dendroctonus ponderosae) affects their reproductive investment. We starved beetles to simulate the decrease in energy that accompanies dispersal and tested whether starved beetles had decreased egg number and decreased egg size, or both. We further distinguished whether changes are due to physiological constraints or shifts in allocation between reproduction and somatic condition.We found that starved beetles produced smaller eggs than non‐starved beetles, but females were able to partially offset the energetic deficit by feeding at their breeding habitat. Starvation did not decrease the number of eggs beetles produced.The number and size of eggs produced depended on whether females allocated energy to reproduction or to somatic condition. However, this life‐history allocation decision was independent of the amount of energy beetles had at the beginning of reproduction.Our results demonstrate the importance of assessing reproductive investment in the context of other life‐history trade‐offs. Specifically, since egg size in Mountain Pine Beetles was highly dependent on both the amount of energy remaining after dispersal and whether energy was allocated to reproduction or somatic maintenance, we expect both of these trade‐offs to be under strong selection.

Proximate Mechanisms Influencing Egg Size Plasticity in the Seed Beetle <I>Stator limbatus</I> (Coleoptera: Bruchidae)

The seed beetle Stator limbatus adjusts egg size in response to the quality of the oviposition host, laying small eggs on the high quality host, Acacia greggii seeds, and larger eggs on the poor quality host, Cercidium floridum seeds. We examined various proximate mechanisms of this egg-size plasticity. Once exposed to a new host, it takes 24 to 48 h for females to change the size eggs they lay. Beetles that came into direct contact with seeds adjusted egg size to a much greater extent than beetles that only came in contact with the fruits or were physically separated from the seeds. Egg size was not affected by size of the seed. Both the host seed in which the larvae were reared as well as the host to which adults were subsequently exposed influenced egg size. The default egg size in the absence of a suitable host is slightly larger than the size egg laid on A. greggii. Thus, beetles increase egg size in response to C. floridum seeds and slightly decrease egg size in response to A. greggii seeds.

The evolution of cuckoo parasitism: a comparative analysis

Cuckoos (family Cuculidae) show the highest diversity of breeding strategies within one bird family (parental care, facultative and obligate brood parasites). We used independent contrasts from two phylogenies to examine how this variation was related to 13 ecological and life-history variables. The ancestral state was probably tropical, resident, forest cuckoos with parental care. The evolution of brood parasitism was correlated with a shift to more open habitats, a change in diet, increases in species breeding-range size and migration, and a decrease in egg size. Once parasitism had evolved, more elaborate parasitic strategies (more harmful to host fitness) were correlated with decreased egg size, a change in diet, increased breeding-range size and migration, a shortened breeding season and a decrease in local abundance. Establishing the most probable evolutionary pathways, using the method of Pagel, shows that changes in ecological variables (such as migration, range size and diet type) preceded the evolution of brood parasitism, which is likely to be a later adaptation to reduce the cost of reproduction. By contrast, brood parasitism evolved before changes in egg size occurred, indicating that egg size is an adaptive trait in host--parasite coevolution. Our results suggest that the evolution of cuckoo brood parasitism reflects selection from both ecological pressures and host defences.

Changes in reproductive strategy in the ruffe during a period of establishment in a new habitat

The pattern of maturation, body size and fecundity was examined in a population of ruffe (Gymnocephalus cernuus L.) three times during a period of rapid growth, and eventual stabilization, following its introduction to a new habitat. When the ruffe were less common, maturing ruffe were relatively large and immature ruffe relatively small, compared with when the ruffe were abundant. Intermediate ruffe population size showed a maturation pattern intermediate between these two extremes. It is suggested that this pattern of maturation is a response of the ruffe population to changing growth opportunity induced by changing intraspecific competition. This fluctuating maturation pattern is interpreted in terms of a threshold‐dependent maturation trigger, operating on the rate of accumulation of energy and a trade‐off between somatic growth and gonad development. When the ruffe population was large, high intraspecific competition resulted in low opportunity for growth; only fish with the highest rate of food acquisition were able to mature in a given year–the investment in gonadal tissue reducing somatic growth. When the ruffe population was low, the high rate of energy acquisition in the population resulted in the triggering of maturation, even at small size, only the very smallest fish remaining immature. High growth opportunity allowed maturing fish to develop gonad and maintain somatic growth. The pattern of size related fecundity also changed over the three periods. When growth opportunity was low, size related fecundity was greater than when opportunity for growth was high. This suggests that maturing females faced with poor growth conditions compensated by increasing egg number for a given body size either by decreasing egg size or by increasing total investment in ovarian tissue.

Egg size and hybrid syndrome-dependent embryo mortality in Chironomus hybrids (Diptera: Chironomidae)

Female hybrids of the cross Chironomus t. thummi female × Ch. t. piger male which are largely affected by the sterility inducing Rud syndrome were backcrossed with males of both parental strains. The aim of the study was to provide information about those egg volumes that are insufficient for a normal embryogenesis and to ascertain whether in the hybrids the lethally small egg size represents a new abnormal trait of the Rud syndrome. The egg masses obtained contain eggs of very different sizes with volumes ranging from 0.5 nl to 3.49 nl. Embryo mortality is unusually frequent in those eggs of the backcrosses and of the parental strains that have volumes smaller than 1.5 nl. An egg volume of 1.5 nl represents in Ch. thummi the lower limit for those volumes that are sufficient for a normal embryogenesis. Mortality increases with decreasing egg size, reaching 100% in backcross eggs with volumes of 0.99 nl and smaller. Small egg size is a new trait of the Rud syndrome affected thummi female × piger male hybrids. This trait is part of a postzygotic reproductive isolation barrier between thummi and piger and manifests first in the backcrosses. Most backcross eggs show volumes between 1.5 nl and 2.99 nl. Within this volume range the amount of mortality does not depend upon egg volume. Here, embryo death is great in the backcrosses but normal in the parental strains. The high frequency of embryo death in the backcrosses must be predominantly due to the action of the Rud syndrome and a second hybrid syndrome, called HLE syndrome. Since further characteristic traits of these syndromes could be detected in surviving backcross individuals, the study demonstrates the occurrence of the syndromes in this generation also. Therefore, the postzygotic reproductive isolation mechanism of both hybrid syndromes is effective in the hybrids and in their progeny as well.

Clutch Size in Spiders: Is More Better?

1. After controlling for phylogenetic correlation, clutch size in spiders (suborder Araneomorphae) increases with body mass. 2. Clutch mass scales isometrically with adult female body mass while individual egg mass scales with negative allometry. Clutch size therefore is a result of scaling constraints on egg size and total clutch mass. 3. There is no empirical evidence for a size-number trade-off in allocation of resources within clutches. Theory predicts that spiders are selected to maximize clutch size, with no cost of decreased egg size. Spiders are constrained from achieving an optimal clutch size

Biology of the rainbowfish,Glossolepis multisquamatus (Melanotaeniidae), from the Sepik River floodplains, Papua New Guinea

G. multisquamatus is abundant in the Sepik River, preferring floodplain lakes and marginal areas of floodplain with moderate turbidity. It is the only rainbowfish documented to inhabit river floodplains. Migrations onto and off the floodplain occur in response to changes in flood conditions. Seasonality is evident in reproduction, condition factor and fat deposit and stomach fullness indices; all increasing in the flood season. Fecundity is high compared with stream dwelling rainbowfish species mainly due to decreased egg size. The reproductive strategy ofG. multisquamatus is interpreted as a response to increased adult mortalities encountered on floodplains in comparison to more stable stream habitats occupied by related species.G. multisquamatus is carnivorous, its diet being similar to other rainbowfishes, consisting of a variety of small invertebrates taken from mid-water or the water surface. The ecology and behaviour ofG. multisquamatus is explained as a partial colonisation of newly developed floodplains in this river.

Effects of natural exposure to copper and zinc on egg size and larval copper tolerance in white sucker ( Catostomus commersoni)

Sucker eggs were collected from a metal-contaminated site where previous collections had documented a maternal yolk factor associated with an increased larval copper tolerance during yolk nutrition. In this study, white sucker eggs were divided into groups which were water hardened and/or incubated at the contaminated site or a reference (control) site. Incubation at the contaminated site was associated with a decreased egg size and an increased deformity rate, but had no effect on fertilization rate or larval size. Incubation in contaminated streams increased the larval whole-body metal content, and decreased the tolerance and resistance of larvae exposed to copper in laboratory bioassays. An increase in water temperature accelerated the developmental rate of the larvae, but had no detectable effect on copper tolerance. Although metals did enter the eggs during water hardening, there was no difference in egg metallothionein content, and the lack of obvious hepatic activity until several days after hatching may prevent the formation of protective proteins. The authors hypothesize that the transfer of metals to white sucker eggs in association with yolk precursors, and the subsequent mobilization of these metals during yolk utilization, can account for the observed differences in larval tolerance and growth.

Egg and brood sizes of oceanic pelagic crustaceans

The eggs and broods of crustaceans that carry their embryos attached to their appendages or within a marsupium are examined. Larger crustaceans within an order or suborder have larger eggs than smaller crustaceans but the egg size relative to body size decreases except in 2 groups: the relative volume of the eggs of calanoid copepods and deep-living mysids is maintained at about 1.0 O/O and 0.4 % of body volume respectively across the ranges of size of the females examined. Brood volume relative to body volume tends to remain constant as body size increases and represents about 10 to 15 % of body volume. The constraints on brood volume probably devolve from the possession of a ngid exoskeleton but some variation independent of body volume is possible. Egg size and number are dependent variables. Mysids, euphausiids and decapods divide into species that produce large and those that produce small eggs. There may be a minimum viable size of egg. Consequently, small crustaceans may increase fecundity through production of successive broods whereas larger crustaceans can increase the number of eggs by decreasing egg size. The effect of increased lipid content of large eggs relahve to an increase in egg size and incubation time is discussed.

Adaptive Versus Constraint Explanations for Egg-to-Body Size Relationships in Two Butterfly Families

We have used a comparative approach in order to understand what determines egg size by analyzing relationships of egg to female body size in 11 pierid and 17 satyrid butterflies. Adult female weight varies between 30 mg and 300 mg, and since scaling of morphological character traits is usually observed, we would expect a positive correlation between egg and female size as a result of nonadaptive allometry (in the sense of Huxley 1932) in the absence of selective pressures on egg size. However, two such pressures can readily be recognized, both of which should cause egg size to depart from the baseline allometry. First, a positive correlation between egg size and offspring fitness may select for bigger eggs. Second, since there is a negative relationship between egg size and fecundity, selection on females to maximize fecundity confers a selective pressure to decrease egg size. Since these selective pressures signify a parent-offspring conflict, we follow Parker and Begon's assumption that "selection acts to maximize the reproductive success of the female parent" (1986, p. 574). Then, assuming that the critical minimum value for egg viability is independent of female body size, we believe that the strong selection pressure on females to maximize their fecundity will reduce egg size to this minimum viable size, irrespective of female body size. Only if female ability to achieve high fecundity is somehow constrained would we expect scaling of egg to body size as a result of either nonadaptive allometry (in the sense of Huxley 1932) or adaptive adjustment of egg size. Among the 11 pierids tested, egg weight does not increase with female weight. Therefore, we must conclude that pierids are selected to maximize fecundity and that the strength of this selection pressure overrides the effects of any positive correlation between egg size and offspring fitness. By contrast, egg size does increase with body size among the 14 satyrids that are adapted to low-temperature environments but not among the 3 species from sunny habitats. We tentatively conclude that female ability to achieve high fecundity is somehow constrained among the 14 species and that the constraints are related to their adaptation to low temperatures (by butterfly standards). This would be true if there is an upper limit to the rate at which eggs are laid each day and if egg laying can be extended over a limited number of days. Thus, the observed scaling of egg to body size among these satyrids can be due to an unraveling either of a positive correlation between egg size and offspring fitness or of a nonadaptive "baseline" scaling expected from some common law of growth (see Huxley 1932). Two lines of evidence suggest that the latter explanation agrees better with the data. First, the adaptive explanation rests on the assumption that egg size and offspring fitness correlate positively. Empirical tests for a correlation between egg weight and seven offspring-fitness parameters in two of the satyrid butterflies, Pararge aegeria and Lasiommata megera, have failed to reveal any such positive relationship (Wiklund and Persson 1983; Karlsson and Wiklund 1984), suggesting a low correlation between egg size and offspring fitness. If so, the adaptive explanation becomes more or less identical with the nonadaptive-allometry explanation. Second, the slope of the log-log correlation between egg and body weight for the 14 satyrids adapted to low-temperature habitats was 0.677, which is in general agreement with the correlation for the 17 orders of birds that have been tested, all of which appear to have the same slope, 0.675 (Rahn et al. 1975). This may suggest that some common kind of nonadaptive (in the sense of Williams 1966) growth mechanism is responsible for the observed scaling. The inference that temperature constrains the realization of high fecundity in the 14 satyrids above is corroborated by the demonstration that the 3 satyrids adapted to sun-exposed habitats effectively behave like pierids; that is, they lay a higher number of eggs per day compared with the species adapted to low temperatures, and they also lay eggs smaller than expected for their body size.