Individual Variation in Trophic Egg Production: Evidence for Maternal Manipulation in Response to Resource and Competition Levels

Females of the subsocial shield bug Parastrachia japonensis (Heteroptera: Parastrachiidae) incorporate trophic eggs (nutritive eggs) into their egg mass. Considerable variation occurs among females in trophic egg number and the proportion of an egg mass that is composed of trophic eggs. Because trophic eggs are essential to the development and survival of young, this variation could significantly impact female fitness. We tested the hypothesis that trophic egg abundance is induced by maternal phenotype (weight, body size) and resource exposure. We predicted that resource limitations would cause females to produce fewer fertile eggs and more trophic eggs and that larger and heavier females would produce more of each egg type. Females ovipositing early in the season are exposed to different resource conditions than those that oviposit late. Thus, we compared egg production patterns between these two groups and several other factors related to nesting. No correlation was seen between body size and trophic egg abundance, or, indeed, egg production, overall; however, heavier females produced heavier egg masses. Counter to our prediction, late females, which had greater access to food, produced significantly more total eggs, fewer fertile eggs, and more trophic eggs than early females. A binomial generalized linear model analysis indicated that the factors most correlated with the percentage of an egg mass destined to become trophic eggs were resource abundance, resulting from early or late oviposition, and distance of the nest from the host tree, with closer females producing more trophic eggs. The findings support our hypothesis that resource availability and, to a lesser extent, maternal phenotype affect trophic egg abundance.     

Does maternal care evolve through egg recognition or directed territoriality?

The mechanism that facilitates the evolution of maternal care is ambiguous in egg‐laying terrestrial vertebrates: does the ability of mothers to recognize their own eggs lead them under some circumstances to begin providing care or can maternal care evolve from simply being in close proximity to the eggs (e.g. through territorial behaviour)? This question is difficult to answer because in most species, parental care is either absent altogether or present; in only a few species we have the opportunity to observe intraspecific variation in the expression of parental care. We studied a population of long‐tailed skinks (Eutropis longicaudata) in which females have recently evolved maternal care from a noncaring state. Females on Orchid Island, Taiwan, remain with their eggs during incubation and when doing so, actively deter egg predation by egg‐eating snakes (Oligodon formosanus); in all other populations, females lack post‐ovipositional maternal care. Nest‐guarding females on Orchid Island (i) showed antipredator behaviours only in the original nest site in which they laid eggs, even after we removed all of the eggs or substituted them with those of a conspecific; (ii) protect any eggs present inside the original nest site (even when the eggs belong to a conspecific); and (iii) develop this behaviour while gravid (i.e. prior to laying eggs). This supports the hypothesis that long‐tailed skinks cannot recognize their own eggs, suggesting that maternal care is a directed form of territoriality only expressed towards egg‐eating snakes and only during reproduction. Nest guarding is among the most primitive forms of parental care, and the recent evolution of this behaviour in a single population provides insight into one of the mechanisms by which parental care can originate in terrestrial vertebrates.     

Environmental Effects on Male Reproductive Success and Parental Care in the Florida Flagfish Jordanella floridae

Models of optimal parental care predict that parental investment should depend on offspring value or the effect parental care has on offspring benefits. Few studies have examined the effect of external factors that influence offspring survival and the cost of care. In this study on the Florida flagfish (Jordanella floridae), a species with male parental care, we examined whether environmentally induced changes in care result from changes in egg requirements or in parental costs. We manipulated salinity and temperature, as these factors are known to affect the metabolic rate in both eggs and parents. We predicted that if the change in care behavior is determined by costs to the male then it should be paralleled by changes in non‐egg‐directed behavior. Conversely, if egg‐directed behavior changes independently of other behavior it would suggest that behavior is determined primarily by egg requirements. In addition we examined patterns of mating success under the assumption that if male care is affected by environmental factors then female preferences may change accordingly. Males decreased egg‐directed behavior (fanning and cleaning of eggs) at high salinity. Non‐egg‐directed behavior was unaffected by salinity. Temperature had no effects on behavior. Thus, we conclude that changes in egg demands are primarily responsible for the observed results. Successful males were bigger and more aggressive. This suggests that male dominance was an important determinant of male mating success. Unsuccessful males showed significantly more variation in number of red stripes with respect to salinity than successful males. Unsuccessful males may be less able to regulate color expression under varying environmental conditions, in which case color may be an indicator of male quality. We replicated the experiment early and late in the season. Males did not change their effort in care over the season. However, care (fanning) in the absence of eggs increased towards the end of the season. Since pre‐mating fanning was positively correlated to a male’s eventual mating success we conclude that males increased their effort to attract mates late in the season.     

The Adaptive Significance of Egg Attendance in a South‐East Asian Tree Frog

The arboreal frog, Chiromantis hansenae (Family: Rhacophoridae), is one of only a handful of South‐East Asian amphibian species reported with parental care. We present the first systematic observational and experimental study confirming offspring benefits as a result of this care, which has a number of unusual life‐history characteristics. Eggs are unusually small, breeding takes place in large pools, and females attend the eggs. Field observations and an adult removal experiment demonstrated a critical contribution of egg attendance to offspring survivorship. Harsh environmental conditions for offspring appeared to be the prime mover of parental care in this species, with desiccation as the main source of mortality when attending adults are absent. Results confirm females to be the caregivers, making C. hansenae a rare case of maternal egg attendance in a non‐directly developing anuran.     

Synchronization of laying by great spotted cuckoos and recognition ability of magpies

Brood parasites rely entirely on the parental care of host species to raise the parasitic nestlings until independence. The reproductive success of avian brood parasites depends on finding host nests at a suitable stage (i.e. during egg laying) for parasitism and weakly defensive (i.e. non‐ejector) hosts. Finding appropriate nests for parasitism may, however, vary depending on ecological conditions, including parasite abundance in the area, which also varies from one year to another and therefore may influence coevolutionary relationships between brood parasites and their hosts. In this scenario, we explored: 1) the degree of laying synchronization between great spotted cuckoos Clamator glandarius and magpies Pica pica during two breeding seasons, which varied in the level of selection pressure due to brood parasitism (i.e. parasitism rate); 2) magpie responses to natural parasitism in the pre‐laying period and successfulness of parasitic eggs laid at this stage; and 3) magpie responses to experimental parasitism performed at different breeding stages. We found that, during the year of higher parasitism rate, there was an increase in the percentage of parasitic eggs laid before magpies started laying. However, the synchronization of laying was poor both years regardless of the differences in the parasitism rate. The ejection rate was significantly higher during the pre‐egg‐laying and the post‐hatching stages than during the laying stage, and hatching success of parasitic eggs laid during the pre‐egg‐laying stage was zero. Thus, non‐synchronized parasitic eggs are wasted and therefore poor synchronization should be penalized by natural selection. We discuss four different hypotheses explaining poor synchronization.     

A new record of amphisexual care in an insect with exclusive paternal care: Rhynocoris tristis (Heteroptera: Reduviidae)

We describe the first record of amphisexual care in an insect with uniparental male care. Male Rhynocoris tristis are noted for aggressively guarding multiple egg masses. When a male is unable or unwilling to guard eggs, a female that has previously contributed eggs returns to the egg mass and defends it until the eggs hatch. We compared the frequency and intensity of parental aggression in both sexes and found no difference in either form of guarding behavior. Evidence of amphisexual care in R. tristis provides an opportunity to quantify the cost of care in both sexes within a single species.An erratum to this article can be found at     

Costly parenting: physiological condition over time and season in males of the giant waterbug Abedus dilatatus

Parental care patterns increase offspring fitness but may drive energetic costs to parents. The costs associated with parental care can change over time, decreasing the condition of parents that experience prolonged parental care. Thus, males can modulate parental effort based in the relative fitness cost/benefit pay‐offs under different stages and environmental conditions. The present study assesses the condition of parental males of Abedus dilatatus Say by measuring their lipid, glycogen and carbohydrate contents. We compare the condition in parental males that have experienced recent and prolonged care and that were also collected in the summer and winter. Waterbug males provide parental care via carrying and ventilating eggs on their back. Winter males are smaller and carry fewer eggs compared with summer males. Males with recent care and carrying more eggs present a more lipid content. However, at the end of care, males carrying more eggs present less lipids than males with smaller egg‐pads. Additionally, we find that males collected in the summer present more carbohydrates than males in the winter. Moreover, larger males with prolonged care present less carbohydrates than smaller males, in contrast to males with recent care where there is a positive relationship between size and carbohydrate content. Our results suggest that parental care in A. dilatatus may be a sexually‐selected trait, as has been found in related species, and further experiments could test this idea. This is the first study to provide evidence of physiological costs related to exclusive paternal care in arthropods.     

Attractive blue‐green egg coloration and cuckoo−host coevolution

Recent evidence suggests that blue‐green coloration of bird eggshells may be related to female and/or egg phenotypic quality, and that such colour may affect parental effort and therefore the nutritional environment of developing nestlings. Here we suggest that these relationships and the signal function of eggshell coloration would affect the outcome of coevolution between avian brood parasites and their hosts in at least three different non‐exclusive evolutionary pathways. First, by laying blue‐green coloured eggs, cuckoo females may exploit possible sensory biases of their hosts, constraining the evolution of parasitic egg recognition, and thus avoid rejection. Second, because of the relatively high costs of laying blue eggs, cuckoo females may be limited in their ability to mimic costly blue‐green eggs of their hosts because cuckoo females lay many more eggs than their hosts. Furthermore, costs associated with foreign egg recognition errors would be relatively higher for hosts laying blue eggs. Third, cuckoos may use coloration of host eggs for selecting individuals or specific hosts of appropriate phenotypic quality (i.e. parental abilities). We here explored some predictions emerging from the above scenarios and found partial support for two of them by studying egg coloration of European cuckoos (Cuculus canorus) and that of their 25 main hosts, as well as parasitism and rejection rate of hosts. Cuckoo hosts parasitized with more blue, green, and ultraviolet cuckoo eggs, or those laying more blue‐green eggs, were more prone to accept experimental parasitism with artificial cuckoo eggs. In addition, coloration of cuckoo eggs is more variable when parasitizing hosts laying bluer‐greener eggs, even after controlling for the effect of host egg coloration (i.e. degree of egg matching). Globally, our results are consistent with the proposed hypothesis that host egg traits that are related to phenotypic quality of hosts, such as egg coloration, may have important implications for the coevolutionary interaction between hosts and brood parasites. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 154–168.     

A limit on the extent to which increased egg size can compensate for a poor postnatal environment revealed experimentally in the burying beetle,Nicrophorus vespilloides

It is often assumed that there is a positive relationship between egg size and offspring fitness. However, recent studies have suggested that egg size has a greater effect on offspring fitness in low‐quality environments than in high‐quality environments. Such observations suggest that mothers may compensate for poor posthatching environments by increasing egg size. In this paper we test whether there is a limit on the extent to which increased egg size can compensate for the removal of posthatching parental care in the burying beetle, Nicrophorus vespilloides. Previous experiments with N. vespilloides suggest that an increased egg size can compensate for a relatively poor environment after hatching. Here, we phenotypically engineered female N. vespilloides to produce large or small eggs by varying the amount of time they were allowed to feed on the carcass as larvae. We then tested whether differences between these groups in egg size translated into differences in larval performance in a harsh postnatal environment that excluded parental care. We found that females engineered to produce large eggs did not have higher breeding success, and nor did they produce larger larvae than females engineered to produce small eggs. These results suggest that there is a limit on the extent to which increased maternal investment in egg size can compensate for a poor posthatching environment. We discuss the implication of our results for a recent study showing that experimental N. vespilloides populations can adapt rapidly to the absence of posthatching parental care.     

Maternal Vibration: An Important Cue for Embryo Hatching in a Subsocial Shield Bug

Hatching care has been reported for many taxonomic groups, from invertebrates to vertebrates. The sophisticated care that occurs around hatching time is expected to have an adaptive function supporting the feeble young. However, details of the characteristics of the adaptive function of hatching care remain unclear. This study investigated the hatching care of the subsocial shield bug, Parastrachia japonensis (Heteroptera: Parastrachiidae) to verify its function. Results show that the P. japonensis mothers vibrated the egg mass intermittently while maintaining an egg-guarding posture. Then embryos started to emerge from their shells synchronously. Unlike such behaviors of closely related species, this vibrating behavior was faint, but lasted more than 6 h. To investigate the effect of this behavior on hatching synchrony and hatching success, we observed the hatching pattern and the hatching rate in control, mother-removed, and two artificial vibration groups. Control broods experienced continuous guarding from the mother. Intermittent artificial vibration broods were exposed to vibrations that matched the temporal pattern of maternal vibration produced by a motor. They showed synchronous hatching patterns and high hatching rates. However, for mother-removed broods, which were isolated from the mother, and when we provided continuous artificial vibration that did not match the temporal pattern of the maternal vibration, embryo hatching was not only asynchronous: some embryos failed to emerge from their shells. These results lead us to infer that hatching care in P. japonensis has two functions: hatching regulation and hatching assistance. Nevertheless, several points of observational and circumstantial evidence clearly contraindicate hatching assistance. A reduction in the hatching rate might result from dependence on maternal hatching care as a strong cue in P. japonensis. We conclude that the hatching care of P. japonensis regulates the hatching pattern and serves as an important cue to induce embryo hatching.     

A review of the ecological parameters and implications of subsociality in Parastrachia japonensis (Hemiptera: Cydnidae), a semelparous species that specializes on a poor resource

Subsocial behavior or postovipositional parental care in insects has evolved in response to a variety of environmental stresses and ranges from briefly guarding eggs after oviposition to elaborate nidification and provisioning behaviors. Investment in parental care bears various costs, and should not continue beyond the point at which the costs to future reproductive success exceed the benefits to current reproductive effort. Progressive provisioning is a rare form of subsociality in insects. Females of the subsocial shield bug Parastrachia japonensis progressively provision their nymph-containing nests with drupes of the single resource, Schoepfia jasminodora, and this provisioning drastically enhances offspring survival. A female rears only one brood throughout her lifetime and continues provisioning the brood until about the third larval stadium, when the female dies. Thus, the female's entire reproductive effort is expressed in the success of that one brood, which suggests a reproductive strategy with enormous costs and risks. Why has such an extreme life history evolved in this species? Here, to answer that question and to contribute to our understanding of the evolutionary implications of subsocial behavior, in particular, progressive provisioning, we review what we have discovered about the ecological parameters of subsociality in this species during a long-term field study. We also discuss these parameters in P. japonensis in reference to other subsocial insects and related species and conclude with a suggestion that semelparity and progressive provisioning in this species are extreme adaptations to evolving complete dependency on an unreliable resource.     

Egg Discrimination and Sex-Specific Pecking Behaviour in Parasitic Cowbirds

We studied egg‐pecking behaviour in males and females of three cowbird species: the shiny cowbird (Molothrus bonariensis), a host generalist brood parasite, the screaming cowbird (M. rufoaxillaris), a host specialist brood parasite, and the bay‐winged cowbird (Agelaioides badius), a non‐parasitic species. We conducted three experiments in which we offered each bird an artificial nest with two plaster eggs and recorded whether egg pecking occurred and the number of pecks on each egg. In expt 1, we tested if there were species and sex differences in egg‐pecking behaviour by offering the birds two spotted eggs of similar pattern. Shiny and screaming cowbirds responded in 40.3% and 44% of the trials, respectively, with females and males presenting similar levels of response. In contrast, bay‐winged cowbirds did not show any response. In expt 2, we tested if shiny cowbirds responded differentially when they faced a choice between one host and one shiny cowbird egg, while in expt 3, we tested if screaming cowbirds responded differentially when they faced a choice between one shiny and one screaming cowbird egg. Shiny cowbirds pecked preferentially host eggs while screaming cowbirds pecked more frequently shiny cowbird eggs. Our results show that egg‐pecking behaviour is present in both sexes of parasitic cowbirds, but not in non‐parasitic birds, and that parasitic cowbirds can discriminate between eggs of their own species and the eggs of their hosts or other brood parasites.     

Postural Shifts During Egg‐Brooding and Their Impact on Egg Water Balance in Children’s Pythons (Antaresia childreni)

Parental care typically consists of distinct behavioral components that are balanced to address the multiple needs of offspring. Female pythons exhibit post‐oviposition parental care in which they coil around their parchment‐shelled eggs throughout incubation (40–80 d). Subtle postural shifts during egg‐brooding facilitate embryonic gas exchange but may entail hydric costs to the clutch. This study used a simple behavioral model to (1) further quantify the costs and benefits of specific parental behaviors to developing offspring and (2) determine the influence that developmental stage and relative clutch mass have on parental behavior. Although previous research has demonstrated that egg‐brooding as a whole reduces clutch water loss, we hypothesized that egg‐brooding female pythons specifically adopt a tightly coiled posture to conserve embryonic water, but must make postural adjustments to enhance gas exchange between the clutch and nest environments at the cost of increased clutch water loss. We measured rates of water loss in brooding Children’s pythons (Antaresia childreni) and their respective clutches (i.e., brooding units) and monitored changes in brooding posture. We conducted serial trials to elucidate the effect of developmental stage on postural adjustments and water loss. Results demonstrated that the proportion of time females spent in a tightly coiled posture was inversely related to mean water loss from the brooding unit. Analyses indicated that slight adjustments in posture led to bursts in brooding unit water loss. Indeed, brooding unit water loss during postural adjustments was significantly higher than during tight coiling. These findings imply that python egg‐brooding provides an adjustable diffusive barrier that leads to discontinuous gas exchange, which minimizes clutch water loss. Because females with larger relative clutch masses spent more time tightly coiled, egg‐brooding female pythons may use a ‘water first’ strategy in which they intentionally conserve clutch water at the cost of reduced embryonic respiratory gas exchange.     

Eat or be eaten: prevalence and impact of egg cannibalism on two-spot ladybirds, Adalia bipunctata

Ladybirds commonly engage in cannibalistic behaviour. Egg cannibalism by first instars is considered advantageous to the cannibal, because it not only results in direct metabolic gain but also a reduction in potential competitors. In this study, we quantified the effect of cannibalism on the development rate and survival of Adalia bipunctata (L.) (Coleoptera: Coccinellidae) larvae through development to the adult stage. We also assessed the synchrony of egg‐hatching in relation to laying order and compared the proportion of eggs cannibalized in egg batches laid as clusters or linearly. Larvae that had consumed a conspecific egg after hatching reached the adult stage 1.65 days earlier than those larvae that had not. Larval and pupal mortality was lower for cannibals compared to non‐cannibals; only 46% of non‐cannibalistic individuals reached the adult stage whereas 81% of cannibals pupated successfully. Egg cannibalism is undoubtedly advantageous to A. bipunctata larvae both in terms of faster development and increased survival. There is a positive correlation between laying and hatching order for eggs laid linearly or in a cluster. There was no significant difference in the proportion of eggs hatching in clusters or in a line (80 and 77%, respectively). The remaining eggs were either cannibalized or did not hatch. The ecological implications of these results are discussed with particular reference to trophic egg plasticity.     

Parental care decreases the survival of adults in the waterbug Abedus ovatus

Among animals, especially insects, care of offspring exclusively performed by males is rare. Paternal care increases survival of their offspring but may be costly for males. Our objective was to evaluate the potential survival costs of parental care in the waterbug Abedus ovatus where males only care their eggs. We conducted a capture-mark-recapture study and estimated apparent survival probabilities of parental males, non-parental males and females in a Mexican waterbug population. In addition, we also estimated recapture rates and the transition rates between non-parental males and parental males. Our results suggest that paternal care negatively affects the survival of A. ovatus since survival probability of parental males is lower than non-parental males. The recapture probability was higher in non-parental males than in parental males. The decreased probability of survival in parental males may be due to different factors. As males carry the eggs on their backs this could probably affect their swimming performance and therefore affect their ability to capture prey. Additionally, the physiological wear derived from the intense muscular activity when performing subaquatic parental care behaviors such as brood pumping, could deteriorate their condition and subsequently, their survival. Furthermore, parental males are more conspicuous to predators, especially in the later stages of egg development, which may increase mortality rates. Our results support the trade-off between current and future reproduction and provide evidence of parental care survival costs in a model system where only males care the progeny.     

Potential for controlling codling moth Cydia pomonella (Linnaeus) (Lepidoptera: Tortricidae) in Argentina using the sterile insect technique and egg parasitoids

Codling moth is the main pest affecting apples and pears worldwide. Most pest control strategies used against this insect have relied on the use of broad‐spectrum insecticides which have led to non‐desirable effects like pesticide resistance, residues in the environment, human health concerns and the reduction of access to international markets. Therefore, alternative pest control strategies that would result in sustainable fruit production systems while taking care of the environment are strongly promoted. The use of the sterile insect technique has proven to be a valuable pest control tactic within area‐wide integrated pest management strategies, and its synergistic effect for Lepidoptera pests when combined with other biological control tactics such as parasitoids has been documented. The purposes of this research were to evaluate the response of an Argentinean codling moth strain to a sub‐sterilizing radiation dose of 100 Gy and to assess the acceptability and suitability of sterile codling moth eggs by the egg parasitoids, Trichogramma cacoeciae (Marchal) and Trichogramma nerudai (Pintureau and Gerding). Irradiated female moths survived better than irradiated male moths and non‐irradiated male and female moths. Also, the fecundity of irradiated female moths was reduced by more than 30% as compared to non‐irradiated ones whereas their fertility was close to zero. The F₁ generation was male biased with a lower fertility (inherited sterility) than the parental generation. Trichogramma cacoeciae and T. nerudai parasitized both fertile and sterile eggs. However, there was a significant reduction in acceptability for sterile eggs. Trichogramma nerudai parasitized more eggs than T. cacoeciae, but egg acceptability for this species was proportionally lower than for T. cacoeciae especially on eggs oviposited by irradiated females. Development to adult of both parasitoids species was not substantially affected by the origin of the eggs and the wasps had acceptable levels of adult emergence, survival and fecundity. These results provided useful information on the potential for controlling the codling moth using egg parasitoids and the sterile insect technique in Argentina.     

Influence of Shape on Egg Discrimination in American Robins and Gray Catbirds

The eggs of some obligate brood parasites are more spherical than the eggs of their non‐parasitic relatives and hosts, which contributes to the increased strength of their shells. We examined whether egg shape, including the more spherical shape of brown‐headed cowbird eggs (Molothrus ater), influenced egg discrimination in American robins (Turdus migratorius) and gray catbirds (Dumetella carolinensis). We added a series of artificial objects to robin and catbird nests that varied in shape from a control host egg, a rounded, cowbird‐like egg, to odd‐shaped objects. Real cowbird eggs were significantly more spherical than catbird and robin eggs, which confirmed a potential cue for egg recognition. Object shape significantly influenced the probability of rejection and time to rejection in both robins and catbirds. However, rounded eggs and spheres were rejected infrequently and at frequencies similar to control eggs. Therefore, the shape of a brood parasite's egg does not appear to influence egg discrimination in these two rejecters. Robins and catbirds rejected significantly more odd‐shaped objects than egg‐shaped objects and odd‐shaped objects were rejected significantly sooner than egg‐shaped objects. The rejection of odd‐shaped objects likely represents an expression of nest‐sanitation behaviour where debris is removed from the nest. By comparison with other studies of accepters of cowbird eggs, robins and catbirds appear to reject higher proportions of odd‐shaped objects, which suggests they may have more refined abilities to discriminate against foreign objects in their nests.     

Exclusive Male Egg Care and Determinants of Brooding Success in a Millipede

The evolution of exclusive male parental care is a major and controversial issue in behavioural ecology. Although arthropods practicing paternal care are thought to be key taxa for investigating this issue, few studies have attempted to clarify the selection factors associated with male behaviour and fitness consequences in arthropods. In the millipede Brachycybe nodulosa, males curl their bodies around egg masses on the undersides of decaying logs. Male‐removal experiments in the laboratory strongly suggest that males defend the eggs against fungal infection. Orphaned eggs were soon covered by hyphae and no eggs hatched, whereas almost all eggs brooded by males successfully hatched. The egg‐brooding males showed no aggressive responses when disturbed. Only some mature males bred in the field. Furthermore, the number of eggs brooded varied greatly among the males. Selected generalized linear models revealed that males with a wide seventh body segment, which possesses gonopods (genital legs), tended to succeed in brooding; and males with a wider body also obtained more eggs. Colony attributes had no significant effects on male brooding. We discuss the possible sexual selection mechanisms that could accomplish this pattern of brooding success among male B. nodulosa.     

Adaptive egg size plasticity for larval competition and its limits in the seed beetle Callosobruchus chinensis

Life‐history theory predicts that females who experienced stressful conditions, such as larval competition or malnutrition, should increase their investment in individual offspring to increase offspring fitness (the adaptive parental hypothesis). In contrast, it has been shown that when females were reared under stressful conditions, they become smaller, which consequently decreases egg size (the parental stress hypothesis). To test whether females adjust their egg volume depending on larval competition, independent of maternal body mass constraint, we used a pest species of stored adzuki beans, Callosobruchus chinensis (L.) (Coleoptera: Chrysomelidae: Bruchinae). The eggs of females reared with competitors were smaller than those of females reared alone, supporting the parental stress hypothesis; however, correcting for female body size, females reared with competitors produced larger eggs than those reared in the absence of competition, supporting the adaptive parental hypothesis, as predicted. The phenotypic plasticity in females' investment in each offspring in stressful environments counteracts the constraint of body size on egg size.     

Factors promoting maternal trophic egg provisioning in non-eusocial animals

The 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.