Relative Egg Size Research Articles

Morpho-anatomical traits explain the effects of bacterial-feeding nematodes on soil bacterial community composition and plant growth and nutrition

Bacterial-feeding nematodes are abundant soil animals regulating microbial communities and enhancing plant nutrition and growth. However, the biological metrics driving the variable within-trophic group effects of these organisms on soil and plant functions are not yet identified. In this study, we determined the effects of eight bacterial-feeding nematode genera (Acrobeloides butschlii, Brevibbuca sp., Cephalobus sp., Diploscapter sp., Oscheius akosreti, Panagrellus redivivus, Pristionchus pacificus, Zeldia sp.) on soil bacterial communities and Pinus pinaster growth and nutrition from organic N (glutamate) and P (phytate) supplied in a high P-adsorbing soil. Because nematode species harbored contrasting morpho-anatomical characteristics, we hypothesized that traits, in comparison to species identity or life strategy groups (cp classes), could explain the variable effects of the nematode species. After 50 days of growth, the inoculation of bacterial-feeding nematodes caused variable but never negative effect on plant growth and net plant N and P uptake depending on nematode species. We also observed significant shifts in root traits and bacterial community composition. For both soil bacterial composition and plant growth and nutrition, a selected panel of traits (e.g. body size, mouth width, procorpus width, demanian ratios b and c, relative egg size) explained soil and plant function more than did species identity or life strategy groups. For instance, traits explained 77 % of plant N and P amounts while cp classes and species-identity explained 33 % and 46 %, respectively, according to PLS models. The identity of traits varied according to the functions. Structural equation modeling with latent variables emphasized the direct effects of nematodes on plant nutrition through excretion in comparison to indirect effects through shifts in bacterial composition and improved root ramification. We discuss the relevance of bacterial-feeding nematode traits as promising biological metrics to explain the effect of these worms within the soil microbial loop framework.

How do brood-parasitic cuckoos reconcile conflicting environmental and host selection pressures on egg size investment?

Brood-parasitic cuckoos lay their eggs in the nests of other birds and thereafter abandon their young to the care of the host. Thus, all maternal investment is restricted to investment in the egg. Optimal investment at this stage is likely to have a large impact on maternal reproductive success. Many bird species optimize the size of their eggs to suit both the prevailing environmental conditions and the number of individuals that will provide care to the chicks. However, relatively few cues are available to avian brood parasites to facilitate optimal investment in their eggs. Moreover, optimization of egg size to suit environmental conditions or the social structure of the host group may be constrained by stronger selection for egg mimicry, which reduces the likelihood of detection and rejection of foreign eggs by the host. We aimed to test how the conflicting selection pressures of (1) selection for large eggs in harsh environmental conditions versus (2) rejection of large eggs by hosts interact to influence the size and shape of Horsfield's bronze-cuckoo, Chalcites basalis , eggs. Using a sliding window approach to investigate periods of climate sensitivity, we show that climatic variables did not predict egg size. Nor did cuckoos modify their egg size or shape in relation to the group size of the host (superb fairy-wren, Malurus cyaneus ). Conversely, host defences did appear to have influenced cuckoo egg morphology: eggs that were relatively short and round had a higher probability of being abandoned by the host than eggs that were long and narrow. This suggests that host defences are the overriding selection pressure on cuckoo egg morphology, and, unlike their hosts, cuckoos do not adapt their eggs to the prevailing climate or host group size. • Female brood-parasitic cuckoos invest only in the egg. • Selection pressures of host rejection and environment on egg size conflict. • Sliding window approach was used to investigate periods of climate sensitivity. • Host defences are the overriding selection pressure on cuckoo egg morphology. • Hosts use adult cuckoo presence and cuckoo egg shape to detect parasitism.

Age, Growth, Reproduction and Fecundity of Roach Rutilus rutilus from Volvi Lake, Northern Greece

The roach, Rutilus rutilus, is a Cyprinid which is widely distributed throughout freshwater systems in Europe. It is one of the most abundant and important commercial fish species in Greece. Age, growth, reproduction and fecundity of roach were studied in Lake Volvi from 1997-1998 females had higher growth in length (L∞=277.18 mm, k=0.097) than adult males (L∞=185.60 mm, k=0.148). All males older than one year and larger than 70 mm, and all females older than two years and larger than 80 mm were sexually mature. The breeding period spanned from the end of March till mid April. Fecundity ranged from 2,036 to 31,653 eggs (mean 8,102 eggs) and relative fecundity from 76.06 to 333.94 eggs/g (mean 186.78 eggs/g). Diameter of eggs were 1.3 ± 0.07 mm and weighted 0.0011 ± 0.0002 g . The weight of eggs were independent of length and age of individuals (P>0.05), however, there was a significant relationship between weight and egg diameter (P<0.05). Absolute fecundity depended on length, weight and age of the females. The relationship between absolute fecundity and length were less obvious in the case of relative fecundity and egg size. Estimated growth was at the lower end when compared with other studies conducted elsewhere.

The comparative energetics of petrels and penguins

Petrels and penguins form the clade Austrodyptornithes. The Add-my-Pet collection of referenced data and Dynamic Energy Budget (DEB) parameters presently has 64 members of this clade. Adaptations to their wandering life style could now be studied; I here try to characterise this clade from an energy perspective. It turned out that petrels are in several respects extreme among birds, showing adaptations to life at low mean food levels. Penguins show the same tendency, but less extreme. Almost all petrel species show a weight-reduction well prior to fledging till the adult weight. This permanent weight reduction suggests that the mean adult feeding level is about 80% of the maximum one. This pattern is well-captured by the standard DEB model, in which (energy) reserve plays an important role. They also have relatively large eggs, and long incubation times and low reproduction rates. I argue that large eggs minimize the age at fledging, given that a particular maturity threshold needs to be reached for fledging. Austrodyptornithes have a high age at puberty, which, in combination with low reproduction rates, causes very low maximum population growth rates. The energy allocation to reproduction for petrels is typical among birds, so the low reproduction rates are the results of the large eggs. Another adaptation to low mean food levels is the large stomach/gut capacity, which appeared to be coupled to a high reserve capacity. This points to be an adaptation to a patchy occurrence of their food, probably in combination with spells of poor weather conditions for feeding. Petrels have a high supply stress and altriciality coefficient, compared to birds, while penguins are a very small altriciality coefficient. This reflects a very high maturity level at puberty for both taxa, in combination with high maturity level at birth for petrels and a typical one for penguins. They have a relatively long life span, compared to other birds, but not extreme. The relative egg size of penguins is typical for birds, to smaller than typical. Penguins combine small eggs with long foraging trips, while small young need frequent feeding. They evolved two adaptation for this: crop milk feeding and collecting lipid-rich food for their offspring. The energetic differences that have been found between petrels and penguins may directly relate to feeding in surface waters by petrels and in deeper waters by penguins; seasonal fluctuations in food density are more pronounced in surface waters. The higher clades Aequornithes and Aequorlitornithes are in many respect intermediate between Austrodyptornithes and other birds, demonstrating a clear taxonomic coherence in energy parameters.

Egg-size plasticity in Apis mellifera: Honey bee queens alter egg size in response to both genetic and environmental factors.

Social evolution has led to distinct life-history patterns in social insects, but many colony-level and individual traits, such as egg size, are not sufficiently understood. Thus, a series of experiments was performed to study the effects of genotypes, colony size and colony nutrition on variation in egg size produced by honey bee (Apis mellifera) queens. Queens from different genetic stocks produced significantly different egg sizes under similar environmental conditions, indicating standing genetic variation for egg size that allows for adaptive evolutionary change. Further investigations revealed that eggs produced by queens in large colonies were consistently smaller than eggs produced in small colonies, and queens dynamically adjusted egg size in relation to colony size. Similarly, queens increased egg size in response to food deprivation. These results could not be solely explained by different numbers of eggs produced in the different circumstances but instead seem to reflect an active adjustment of resource allocation by the queen in response to colony conditions. As a result, larger eggs experienced higher subsequent survival than smaller eggs, suggesting that honey bee queens might increase egg size under unfavourable conditions to enhance brood survival and to minimize costly brood care of eggs that fail to successfully develop, and thus conserve energy at the colony level. The extensive plasticity and genetic variation of egg size in honey bees has important implications for understanding life-history evolution in a social context and implies this neglected life-history stage in honey bees may have trans-generational effects.

The combined effects of hypoxia and fish kairomones on several physiological and life history traits of Daphnia

Abstract Numerous studies have tested the combined effect of the threat of predation by fish and low oxygen concentrations on the phenotypic plasticity of Daphnia. These studies assessed the trade‐off between minimising predation risk and the negative effects of oxygen deficiencies in the context of depth selection behaviour. We tested whether this trade‐off also affects physiological and life history traits. We expected an interactive effect between the threat of fish predation and low oxygen concentrations, such, that the net effect of both stressors would be antagonistic (lower than the sum of each of the stressors acting separately), rather than additive (or synergistic) on the majority of traits investigated, but we predicted synergistic effects on heat shock proteins (HSPs). To test this, we performed life table experiments in different oxygen concentrations (normoxia and hypoxia) and levels of predation threat (the presence and absence of fish kairomones) on HSP70 and putative HSP110, haemoglobin concentration and life history traits with small‐bodied Daphnia galeata and large‐bodied Daphnia pulex originating from waterbodies where there were different risks of fish predation. As predicted, the net effect of both stressors was antagonistic for most of the physiological and ecological variables studied. The presence of kairomones resulted in decreased body size of adults, egg size, egg size in relation to brood chamber volume, and in increased clutch size in relation to body size. These effects were weaker in hypoxia than in normoxia, which may suggest an existence of adaptive responses caused by a lower perceived risk in hypoxia than in normoxia, as the foraging abilities of fish are limited by oxygen deficiencies. The presence of kairomones hampered the production of haemoglobin in hypoxia for the clones of larger‐bodied species, which suggests the existence of a trade‐off between reduced visibility under positive‐size selective predation risk and increased efficiency of oxygen transport to body tissues. The presence of kairomones and hypoxia resulted in an increased level of putative HSP110, and the effect of kairomones was stronger in hypoxia than in normoxia. More complex results were obtained for the effect of both stressors on the level of HSP70. The findings of our study provide a new insight on the interactions between planktivorous fish and zooplankton in aquatic food webs. More specifically, these findings suggest the existence of unexplored size‐dependent eco‐physiological trade‐offs between minimising predation risk and mitigating the negative effects of oxygen deficiencies.

Effects of Maternal Reproductive Investment on Sex-Specific Pollutant Accumulation in Seabirds: A Meta-Analysis.

Avian egg production demands resources such as lipids and proteins. Relative egg size and mass varies across species, reflecting differences in maternal investment. This variability may affect the maternal transfer of anthropogenic pollutants including lipophilic polychlorinated biphenyls (PCBs) and protein-associated per- and polyfluoroalkyl substances (PFASs) and mercury (Hg). We conducted a meta-analysis on seabirds and investigated whether interspecies variation in maternal investment contributes toward skewed pollutant concentration ratios between males and females, as Cmale/Cfemale (80 studies). Overall concentrations of PCBs and perfluorooctanesulfonic acid (PFOS) were 1.6 and 1.3 times higher, respectively, in males than females, whereas mercury was similar between sexes. Few studies compared females and eggs ( n = 6), highlighting a knowledge gap. We found that an increasing maternal investment as a clutch-to-female mass ratio resulted in lower PCB concentrations in females than in males during the incubation period, but no sex-specific differences were observed for mercury and PFOS. Egg production is both a lipid dominated and protein-limited process. Females transfer lipophilic pollutants more easily to eggs, and to a higher degree with increasing maternal investment, but feeding ecology may be more important. Interspecies variation in maternal pollutant transfer may lead to negative effects scaling from an offspring to population level.

Laboratory rearing of walleye pollock Gadus chalcogrammus from egg to first-time spawning season, and egg production and egg size in relation to ration level

Laboratory rearing of walleye pollock <i>Gadus chalcogrammus</i> from egg to first-time spawning season, and egg production and egg size in relation to ration level

Quantification of 27 yolk steroid hormones in seven shrubland bird species: interspecific patterns of hormone deposition and links to life history, development, and predation risk

Steroid hormones play critical organizational and activational roles during vertebrate development, impacting everything from sexual differentiation to metabolic activity. For oviparous species such as birds, these hormones are transferred from female to egg during follicle maturation, and differences in relative and absolute concentrations of the steroid hormones may reflect differences in life history, developmental, and ecological conditions. Prior work on yolk steroid hormones has focused on a handful of candidate hormones (e.g., testosterone, androstenedione, and corticosterone), but we used high-performance liquid chromatography with tandem mass spectroscopy (LC–MS–MS) to quantify 27 yolk steroids from the eggs of seven shrubland bird species (American Robin, Turdus migratorius Linnaeus, 1766; Brown-headed Cowbird, Molothrus ater (Boddaert, 1783); Brown Thrasher, Toxostoma rufum (Linnaeus, 1758); Eastern Towhee, Pipilo erythrophthalmus (Linnaeus, 1758); Field Sparrow, Spizella pusilla (A. Wilson, 1810); Gray Catbird, Dumetella carolinensis (Linnaeus, 1766); Northern Cardinal, Cardinalis cardinalis (Linnaeus, 1758)). In addition to comparing steroid profiles across species, we conducted exploratory analyses to determine how the hormones clustered using a principal component (PC) approach and if PCs were correlated with aspects of egg resources (relative egg size, proportion yolk), life-history traits (embryonic and nestling development speed), and nest-predation risk (daily survival rate (DSR)). We documented substantial interspecific variation in both absolute and proportional endocrine profiles. PCAs indicated that glucocorticoids generally clustered together (PC1), but other classes of steroids did not. PC2 and PC3 strongly covaried with egg resources, DSR, and development speed, suggesting that they reflect adaptive patterns of maternal hormone deposition.

Rates of parasitism, but not allocation of egg resources, vary among and within hosts of a generalist avian brood parasite.

Brown-headed cowbirds (Molothrus ater) deposit their eggs into the nests of other birds, which then raise the cowbird chick. Female cowbirds thus have limited options for impacting their offspring's development via maternal effects compared to most other passerines. Cowbirds can impact their offspring's phenotype by choosing among potential host nests, and by adjusting egg resources based on host characteristics. To examine whether cowbirds exhibit either or both of these strategies, we investigated rates of cowbird parasitism and egg investment (egg size, yolk-to-albumen ratio, and yolk testosterone and androstenedione) among and within host species in a shrubland bird community. We found that the probability of being parasitized by cowbirds, controlling for host status as a cowbird egg accepter or rejecter and ordinal date, varied significantly among host species, indicating an apparent preference for some hosts. Parasitism rates did not differ with host size, however, and despite variation in cowbird egg size among host species, this variation was not related to host size or cowbird preference. Among host species with eggs that are larger than those of the cowbird, cowbirds were significantly more likely to parasitize nests with relatively smaller eggs, whereas parasitism rates did not vary with relative egg size in host species with smaller eggs. There was no evidence for variation in cowbird egg components among or within host species. Our data indicate that cowbirds discriminate among host nests, but do not appear to adjust the composition of their eggs based on inter- or intraspecific host variation.

Egg size decreases with increasing female tail fork depth in family Hirundinidae

Female ornamentation evolves as a nonfunctional copy of functional male ornamentation or through direct selection on female ornamentation. Because females invest many resources in both reproduction and ornamentation, an evolutionary tradeoff between female ornamentation and maternal investment can be predicted, but it has rarely been demonstrated. Using phylogenetic comparative analysis, we studied egg size in relation to female fork depth in the family Hirundinidae, which has a wide range of fork depth (from 0 to >50 mm). Because deeply forked tails are aerodynamically costly, we predicted that species in which females have deeply forked tails suffer reduced egg size due to inefficient foraging during the egg formation period. Egg length was significantly related to nest type but not to female or male tail fork depth, whereas we found a significant negative relationship between egg breadth and female, but not male, tail fork depth. As a consequence, a significant negative relationship was found between egg volume and female, but not male, tail fork depth. Because female and male fork depths were not significantly related to clutch size, clutch size would not compensate the relationship between egg size and fork depth. The current finding supports the hypothesis that female ornamentation trades off with maternal investment.

Bathymetric gradients of fecundity and egg size in fishes: A Mediterranean case study

There is a general hypothesis that species inhabiting deep-sea waters have lower fecundity and larger eggs than shallower species. However, there are few comparative studies which explore this trend because of the complexity of sampling in deep waters, especially in fishes. We present here the first analysis of fecundity and egg size with depth along an isothermal environment. We calculate the relative fecundity and egg size of 11 species of demersal deep-sea fish from the western Mediterranean and included in our analyses published data for an additional 14 species from the same geographic area. The results show that the relative fecundity (eggs per g of individual) of the analyzed fishes slightly decreased along the bathymetric gradient, whereas egg size increased with depth. When the analysis was conducted including only species from the order Gadiformes, the most speciose group in the region and with the widest depth range of distribution (50–2000m), there was no relationship between relative fecundity and depth, while the deepest species had larger egg sizes than shallower ones. The finding of similar relative fecundities but larger egg sizes suggests that these deep-sea species are investing a higher amount of energy in the production of offspring than shallower water counterparts. The results are discussed in relation to the isothermal characteristics of the deep Mediterranean Sea and ecological adaptations for reproductive success.

Fish egg size: a characteristic determined by parental care

Fish egg size: a characteristic determined by parental care

Effects of intraclutch variation in egg size and hatching asynchrony on nestling development and survival in semi-precocial Herring Gulls

Intraclutch egg size variation may non-adaptively result from nutritional/energetic constraints acting on laying females or may reflect adaptive differential investment in offspring in relation to laying/hatching order. This variation may contribute to size hierarchies among siblings already established due to hatching asynchrony, and resultant competitive asymmetries often lead to starvation of the weakest nestling within a brood. The costs in terms of chick mortality can be high. However, the extent to which this mortality is egg size-mediated remains unclear, especially in relation to hatching asynchrony which may operate concomitantly. I assessed effects of egg size and hatching asynchrony on nestling development and survival of Herring Gulls (Larus argentatus), where the smaller size and later hatching of c-eggs may represent a brood-reduction strategy. To analyze variation in egg size, I recorded the laying order and laying date of 870 eggs in 290 three-egg clutches over a 3-yr period (2010-2012). I measured hatchlings and monitored growth and survival of 130 chicks from enclosed nests in 2011 and 2012. The negative effect of laying date ( =− 0.18 ± SE 0.06, P = 0.002) on c-egg size possibly reflected the fact that late breeders were either low quality or inexperienced females. The mass, size, and condition of hatchling Herring Gulls were positively related to egg size (all P < 0.0001). C-chicks suffered from increased mortality risk during the first 12 d, identified as the brood-reduction period in my study population. Although intraclutch variation in egg size was not directly related to patterns of chick mortality, I found that smaller relative egg size interactively increased differences in relative body condition of nestlings, primarily brought about by the degree of hatching asynchrony during this brood-reduction period. Thus, the value of relatively small c-eggs in Herring Gulls may lie in reinforcing brood reduction through effects on nestling body condition. A reproductive strategy Herring Gulls might have adopted to maintain a three-egg clutch, but that also enables them to adjust the number of chicks they rear relative to the prevailing environmental conditions and to their own condition during the nestling stage.

Reproductive biology of the Amazon Lava Lizard (Tropidurustorquatus) from the Wet Chaco of Corrientes (Argentina): congeneric comparisons of ecotypic and interspecific variations

Herein we describe the reproductive biology of a population of the Amazon Lava Lizard (Tropidurus torquatus (Wied-Neuwied, 1820)) from Corrientes, northeastern Argentina (Wet Chaco). We describe the male and female reproductive cycles, minimum body size for adults, reproductive output, mean relative clutch mass, fat body cycles, and sexual dimorphism. Our results were compared with data on the reproductive biology of Brazilian populations of T. torquatus and congeneric species. In Corrientes, males of T. torquatus exhibited a continuous reproductive cycle, but with annual variation of testicular parameters associated with spermatogenic activity. In contrast, females were reproductive only from winter to summer (July–February), laying at least two clutches each of six eggs, on average, per reproductive season. The relative clutch mass and egg size values in Corrientes were the highest reported for the species. The annual cycle of energy storage (as fat bodies) was inversely correlated with reproductive activity in both sexes. Males differed from females in snout–vent length, head size, interlimb length, and tail length. We observed interpopulational differences in relative clutch mass, egg volume and mass, incubation period and hatching time, and the minimum body size for sexual maturity probably as a result of phenotypic plasticity or adaptation to local environmental conditions and likely both.

Effect of Dietary Protein and Lipid Levels on the Reproductive Performance and Body Composition of Angelfish, Pterophyllum scalare (Schultze, 1823)

A study to determine the effect of increasing levels of dietary protein and lipid on Pterophyllum scalare was carried out using twelve semi-purified diets varying in dietary protein (P) and lipid (L) levels (48P8L, 48P12L, 48P16L, 50P8L, 50P12L, 50P16L, 52P8L, 52P12L, 52P16L, 54P8L, 54P12L and 54P16L). Broodstock performance was evaluated based on relative fecundity, fertilization rate, hatching rate, egg size, spawning interval, egg and body composition of P. scalare broodstock. Egg production was highest from fish fed with 50P12L, 52P12L and 52P16L diets. Fish fed the 50P12L and 52P12L diets tended to spawn at shorter intervals. Relative fecundity, fertilization rate, hatching rate, egg size and relative hatchling number, were all significantly higher on diets containing 52% protein than at lower levels. There was no significant difference in hatchling length but hatchling weight differed significantly (P &lt; 0.05) with dietary protein and lipid levels. The protein content of fish and eggs was also influenced (P &lt; 0.05) by dietary protein and lipid levels. It is therefore suggested that in P. scalare broodstock fed with 50-52% protein and 12% lipid, reproductive performance, egg and larval quality improved compared with other diets.

Revisiting Sabath's "Larger Avian Eggs" from the Gobi Cretaceous

In 1991, Sabath described “larger avian eggs” from the Upper Cretaceous Barun Goyot and Djadokhta Formations of Mongolia. These were later included in the ootaxon Gobioolithus major. Here we recognize the larger avian eggs of Sabath as a distinct ootaxon, Styloolithus sabathi, oogen. et oosp. nov. These eggs differ from those of Gobioolithus in being larger (70 by 32 mm) and more elongate. Microscopically, the shell bears a third layer (possible external zone) thicker than the mammillary layer and nearly as thick as the second layer (possible squamatic zone); the continuous layer (including layers two and three) to mammillary layer thickness ratio is 3.1:1. Within the clutch, the tightly spaced eggs stand with their long axes steeply inclined. Adult remains are associated with two clutches, suggesting an incubation mode similar to that of troodontid maniraptorans, where adults sat atop largely buried eggs. S. sabathi provides evidence that relative egg size in Mesozoic non-ornithuromorph birds had increased markedly from the non-avian theropod condition in oviraptorids and troodontids, but had not yet reached the modern egg-adult proportions of Neornithes. Sediment-bound upright eggs appear common to Enantiornithes and more basal avians, suggesting that like non-avian theropods, these birds lacked chalazae, the chords of albumen allowing egg rotation in modern birds. Absence of this simple structure may have restricted these basal birds to ground nesting in areas with appropriate substrates and not permitted the type of nesting diversity found in Neornithes. Neornithes are the only Mesozoic clade of Dinosauria to nest completely free of sediment; this may have played a crucial role in their surviving the K-Pg mass extinction event.

Maternal Investment in Diamond FiretailsStagonopleura guttata: Female Spot Numbers Predict Egg Volume and Yolk Lutein Content

It is widely known that parents invest into reproduction to increase offspring survival and thereby increase reproductive success. In particular, female birds allocate maternal resources including lipids, hormones, and nutrients into the egg yolk. Carotenoid pigments, which are responsible for the characteristic yellow, orange, and red colours, are particularly important as antioxidant for the developing embryo. The Investment Hypothesis addresses variable female allocation of maternal resources into egg yolk and predicts that high quality females and females paired to high-quality males will invest more in their eggs. We test predictions of the Investment Hypothesis in the Diamond Firetail Stagonopleura guttata. In captive females and their pair males, we compare yolk carotenoid (lutein and zeaxanthin) allocation and egg size in relation to traits indicative of individual quality (body condition, flank spot number, bill and rump colour). Females with more white flank spots laid larger eggs that contained more lutein, the major carotenoid found in the Diamond Firetail eggs. Total carotenoid concentration in egg yolk was not related to any other female characteristic. Females laid larger eggs when paired with a high quality male (dark red rump).

Clutch Investment in the Chilean Swallow (Tachycineta meyeni) Shifts with Time of Breeding and Position in the Sequence of Laying

Within a population, the sizes of eggs and clutches vary, and the combination of both determines a female's reproductive investment. We investigated females' investment in clutches of the Chilean Swallow (Tachycineta meyeni) at the southern limit of its range, where it breeds in an extreme climate. We analyzed variation in clutch, egg and yolk size in relation to the female's condition, date of laying, and position in the sequence of laying. As predicted by energy/nutrient constraints, clutch size and yolk size decreased over the breeding season, though egg mass increased. Females' investment strategy differed with clutch size: their investment in eggs increased with each successive egg in clutches of 4 and 5 eggs but decreased with each successive egg in clutches of 3 eggs. Eggs that were heavier for their size were more likely to hatch than eggs light for their size. Females may make a strategic decision, with high investment in eggs laid early resulting in a short sequence (small clutch) and delayed investment resulting in a long sequence (large clutch). Alternatively, the shift in clutch investment may simply be a consequence of physiology. Better estimates of female body condition are needed for these options to be distinguished.

Factors promoting maternal trophic egg provisioning in non‐eusocial animals

AbstractThe adaptive function of trophic egg‐laying is generally regarded as extended parental investment to the offspring. However, the evolutionary factors promoting trophic egg‐laying are still unclear, because the amount of maternal investment per offspring should be ideally equal between smaller offspring with trophic eggs and larger offspring without any additional investment. Several authors have suggested that trophic egg‐laying should evolve only when egg size is constrained, but this hypothesis has not been evaluated. We investigated the evolutionary mechanisms of trophic egg‐laying by two different approaches. First, we evaluated morphological constraints on egg size in two sibling ladybird species, Harmonia axyridis, which is known to produce trophic eggs, and H. yedoensis. Second, we theoretically predicted the optimal proportion of trophic eggs to total eggs and egg size in relation to environmental heterogeneity, predictability of environmental quality, and investment efficiency of trophic egg consumption. The intra‐ and interspecific morphological comparisons suggest that morphological constraints on the evolutionary determination of egg size are weak at best in the two ladybird species. Moreover, we theoretically showed that small egg size and trophic egg‐laying are favoured in heterogeneous environments when mothers cannot adjust egg size plastically. We also showed that even a small reduction in investment efficiency makes a trophic egg strategy unlikely, despite relatively high environmental predictability. We conclude that trophic egg provisioning may be a flexible maternal adaptation to a highly heterogeneous environment rather than a response to a morphological constraint on egg size.