Trophic-egg production in a subsocial bug: adaptive plasticity in response to resource conditions

Adomerus triguttulus (Heteroptera: Cydnidae) females provision host Lamium spp. seeds to their nymphs. Females also produce trophic eggs, which are inviable and usually function as a food supply for hatched nymphs. Here we report experimental evidence of the potentially adaptive maternal effects of this unusual resource investment. To investigate the effects of food-resource environments on trophic-egg production, we reared females under different resource conditions prior to oviposition and then compared the subsequent allocations of trophic eggs. Females that had been supplied with less-developed seeds produced fewer viable eggs than those supplied with well-developed seeds. However, there was no difference in the number of trophic eggs produced between the two treatments, and thus the trophic-egg ratio, i.e. the number of trophic eggs per viable egg, was higher in females supplied with less-developed seeds. Trophic-egg feeding by hatched nymphs enhances their growth or survival, and a higher trophic-egg ratio would be adaptive particularly under limited resource conditions. In A . triguttulus , adults and offspring depend on the same food resource, i.e. host seeds, and the resource conditions experienced by females prior to oviposition should be well correlated with those experienced later by the offspring. This may enable females to adjust the trophic-egg ratio in response to varying resource environments prior to oviposition.     

Trophic eggs compensate for poor offspring feeding capacity in a subsocial burrower bug

Various animals produce inviable eggs or egg-like structures called trophic eggs, which are presumed to be an extended maternal investment for the offspring. However, there is little knowledge about the ecological or physiological constraints associated with their evolutionary origin. Trophic eggs of the seminivorous subsocial burrower bug (Canthophorus niveimarginatus) have some unique characteristics. Trophic eggs are obligate for nymphal survival, and first-instar nymphs die without them. To identify the cause of nymphal death, we hypothesized that first-instar nymphs starve to death because they cannot feed on anything but trophic eggs. Although first-instar nymphs fed on artificially exposed endosperm did survive, nymphs that were provided with intact seed were not able to penetrate the seed vessel and starved to death. Another hypothesis that trophic eggs play a role in transferring the midgut symbiont, essential for survival in heteropteran bugs, from mother to offspring was rejected because almost all nymphs had retained the symbiont without feeding on trophic eggs. These results suggest that poor feeding capacity of the offspring is the cause of nymphal death, and the important constraint that promotes the evolution of the curious trophic egg system in C. niveimarginatus.     

Intraclutch egg‐size variation in acanthosomatid bugs: adaptive allocation of maternal investment?

If there are differences in predation risk among the offspring within a clutch, parents may allocate less resources to the offspring facing higher risk. I examined parental investment in terms of egg size within clutches in five species of stink bugs (Heteroptera, Acanthosomatidae). In subsocial Elasmucha and Sastragala species, the female guards her eggs and first-instar nymphs against invertebrate predators by covering her clutch with her body. Large differences in survival from predation between offspring at the centre and offspring at the periphery of the clutch have been reported in such subsocial insects. I found that Elasmucha and Sastragala females laid significantly smaller eggs in the peripheral (and thus more vulnerable) part of the clutch. Phenotypic trade-offs between egg size and clutch size were detected in these subsocial species. Egg size was positively correlated with hatched first-instar nymph size: smaller nymphs hatched from smaller peripheral eggs. In asocial Elasmostethus humeralis , however, no significant difference in size was detected between the eggs at the centre of and those at the periphery of the clutch. Thus, in subsocial acanthosomatid bugs, females seem to allocate their resources according to the different predation risks faced by the offspring within the clutch.     

Maternal Care in the Burrower Bug <Emphasis Type="Italic">Adomerus triguttulus</Emphasis>: Defensive Behavior

Some burrower bugs (Heteroptera: Cydnidae) show complex patterns of maternal care, including defense against predators and the provisioning of food to nymphs. Recently, the subsocial cydnid bugs have attracted the interest of researchers as model systems to study the behavioral ecology of parental investment. However, there have been few attempts to quantify the fitness benefits of maternal behavior other than provisioning. Here, we examined the maternal behavior of Adomerus triguttulus and its adaptive significance in terms of offspring survival in the field. A. triguttulus young depend on fallen nutlets of myrmecophorous mints, Lamium spp. Under field conditions, females attend offspring, from eggs to second instar nymphs, in nests on the ground under the litter. When disturbed, the females showed aggressive responses against the source of disturbance. The females often carried spherical clutches of eggs away from the nest when heavily disturbed. Female-removal experiments in the field demonstrated a defensive function of the female behavior; predators, such as ants, attacked egg clutches without females and the clutches often disappeared during the experiment. Egg clutches without females sometimes also suffered from fungal infection. Selective factors on maternal defensive behavior in A. triguttulus are discussed in terms of habitat properties possibly emerging from insect–plant associations.     

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.     

Brood conspicuousness and clutch viability in male‐caring assassin bugs (Rhinocoris tristis)

Abstract 1. Conspicuousness to mates can bring benefits to both males (increased mating success) and females (reduced search costs), but also brings costs (e.g. increased predation and parasitism). Assassin bugs, Rhinocoris tristis, lay egg clutches either on exposed stems or hidden under leaves. Males guard eggs against parasitoids. Guarding males are attractive to females who add subsequent clutches to the brood. This is an excellent opportunity to study the effects of conspicuousness on the fitness of males and females. 2. Using viable eggs in a multi‐clutch brood as a correlate of fitness, the present study examined whether laying eggs on stems affected (1) female fitness, through exposure to parasitism and cannibalism, and (2) male fitness, through attracting further females. 3. Stem broods were more parasitised. However, males on stems accumulated more mates and more eggs, a net benefit even accounting for parasitism. The eggs gained from being on a stem were cannibalised. By contrast, higher mortality on stems suggests that females should gain by ovipositing on leaves. To the extent that egg viability represents fitness, male and female interests may therefore differ. This suggests a potential for sexual conflict that may affect other species with male care. 4. Despite higher costs, females actually initiated more broods, and subsequently added bigger clutches to broods, on stems than under leaves. This suggests either that viable eggs do not reflect fitness, or that females laid in unfavourable locations. The key is now to address lifetime fitness, since unmeasured factors may affect offspring viability post‐hatching, and to investigate who controls the location of oviposition in R. tristis.     

Differences in the effects of parental age on offspring life history between tropical and temperate populations of milkweed bugs (Oncopeltus spp.)

Summary We compare the effects of parental age on several offspring life history traits in two milkweed bug populations: a typically univoltine population ofOncopeltus fasciatus from the Sacramento Valley of California and a typically multivoltine population ofO. cingulifer from Monteverde, Costa Rica. Reared under identical conditions (27°C, 12 h light: 12 h dark photoperiod), these bugs exhibit significant differences in the effects of parental age on offspring life history. As they age,O. fasciatus females from Sacramento lay clutches of eggs of decreasing average weight and a decreasing proportion of their eggs produce offspring that survive to adulthood. Those offspring which do survive have a significantly faster developmental rate and females have a larger body size at adult eclosion. AsO. cingulifer from Monteverde age, they also produce lighter eggs, but there is no significant change in the offspring developmental rate, survival or female adult body size. We suggest that these results are largely explicable as the consequence of different parental investment strategies associated with the predictable relationship between parental age and time of season inO. fasciatus but not inO. cingulifer. AsO. fasciatus from Sacramento age, they may be increasing their investment per developing offspring so as to increase the probability that nymphs hatching late in the season will reach a prereproductive adult diapause before the first killing frost.     

Egg and larval load assessment and its influence on oviposition behaviour of the leaf beetle Galerucella nymphaeae

The oviposition behaviour of the water-lily beetle Galerucella nymphaeae was examined. This species is a specialist herbivore on the floating leaves of nymphaeids Nymphaeaceae and especially on the yellow water-lily, Nuphar lutea. Females lay their eggs in clutches on the leaves, and after hatching, the larvae feed on the leaves. The quality of the leaves decreases quickly after the larvae hatch, and eventually the leaves will sink below the water surface, whereupon the eggs, 1st-instar larvae and pupae are killed by drowning. The influence of conspecific eggs, larvae and feeding tracks on the oviposition preferences of the beetles was tested. Females were allowed to choose between fresh leaves and leaves with conspecific eggs and larvae as well as between leaves with larvae and leaves with feeding tracks but no larvae. An attempt was also made to determine whether eggs and larvae affect the oviposition rate of females when they are not given the opportunity to oviposit on untouched leaves. The results indicate that females tended to avoid leaves with conspecific larvae or to exhibit a decreased oviposition rate on such leaves. Females also avoided conspecific eggs, although the oviposition rate was not influenced by the presence of conspecific eggs. When females were allowed to choose between leaves with larvae and leaves with feeding tracks, possible discrimination against leaves with larvae just fails to reach the 5% level.     

Natural selection in the tropical treehopper <Emphasis Type="Italic">Alchisme grossa</Emphasis> (Hemiptera: Membracidae) on two sympatric host-plants

The pronotum is the most distinctive and representative structure in treehoppers. Although several functions have been proposed for this structure, its involvement in fitness has not been formally evaluated. Given the high degree of maternal investment exhibited by the subsocial membracid Alchisme grossa (Hoplophorionini), the pronotum has been suggested as a shield for offspring protection. We performed selection gradient analyses on A. grossa in order to evaluate how natural selection is acting upon two traits associated with this structure considering three different fitness proxies: (1) number of eggs, (2) the ratio between the number of first-stage nymphs and the number of eggs (early survival), and (3) the number of third-stage nymphs (late survival). Since A. grossa feeds, oviposits and mates on two alternative host-plants (Brugmansia suaveolens and Solanum ursinum, both Solanaceae), we also evaluated and compared selection gradients between these host-plants. We found positive linear selection acting upon pronotum length considering the number of eggs and the late survival fitness proxies and positive linear selection acting on the distance between the suprahumeral horns considering the early survival proxy on females ovipositing on both host-plants. These results highlight the importance of the pronotum in treehoppers females’ fitness and suggest that maternal care appears to have a greater importance in the way that natural selection is operating than the host-plant where oviposition occurs.     

Does host plant quality affect the oviposition decisions of an omnivore?

Optimal oviposition theory predicts a positive relationship between female preference for oviposition hosts and offspring performance. Interspecies effects on oviposition preference have been widely investigated, especially for herbivores. However, intraspecies variation, such as nitrogen content, might also influence female preference for oviposition hosts and subsequent offspring performance. To evaluate this possibility, we investigated the oviposition preference of a zoophytophagous omnivore and the development and survival of its nymphs on a single species of host plant that varied in nitrogen content. In choice and no‐choice experiments without prey, female omnivores were allowed to oviposit on plants that had been fertilized using 4 rates of nitrogen fertilizer (39, 78, 156, and 311 mg/L nitrogen) for 72 h. After 72 h, the most females were found on tomato plants receiving high concentrations of nitrogen fertilizer and more eggs were laid on those plants. First instar nymphs developed more rapidly on high‐nitrogen plants and third instar nymphs developed faster on low‐nitrogen plants. Plant nitrogen did not affect nymph survival to the adult stage, or the probability of survival over time. Although female omnivores did discriminate between potential oviposition hosts based on plant nitrogen, their choices did not significantly impact nymph development or survival. This is the first study to show that intraspecies variation in nitrogen content between plants affects the oviposition preference of female omnivores, but not offspring performance.     

Trophic Egg Production in a Subsocial Shield Bug, Parastrachia japonensis Scott (Heteroptera: Parastrachiidae), and its Functional Value

Females of the gregarious shield bug, Parastrachia japonensis Scott (Heteroptera: Parastrachiidae) engage in extensive parental care behaviors that include egg and nymph guarding and progressive provisioning of drupes of the solitary host tree, Schoepfia jasminodora (Olacaceae: Rosidae: Santales). We noted that some eggs in every egg mass failed to turn pink and develop eye‐spots indicative of developing embryos, suggesting that they are infertile, and therefore non‐viable. We also observed newly hatched nymphs probing, and presumably feeding, on the egg mass remains. In the present report, through field observations and experiments involving removal of these non‐viable eggs in the laboratory, we demonstrate that their presence is correlated with significant increases in nymphal weight, developmental rate and survival in the absence of other food. Thus, we conclude that an additional manifestation of the parental care behaviors that P. japonensis females use to increase their reproductive success is the production of trophic eggs. Some physical traits of the trophic eggs and their functional role in this system are discussed in the context of our current theoretical understanding of extended parental care.     

Prenatal sex ratios influence sexual dimorphism in a reptile

The prenatal environment influences offspring traits in a variety of ways and in a wide range of taxa. For example, maternal allocation of steroids to the eggs influences offspring traits in birds, and in some mammals the intrauterine position influences morphological, behavioural, and physiological traits due to sex-related steroid transfer between sibling fetuses. We show that similar phenomena occur in the common lizard (Lacerta vivipara), a viviparous reptile. Females developing in male-biased clutches had a more masculine allometry (relatively larger heads) at parturition than females developing in female-biased clutches. Males were correspondingly feminized in female-biased clutches. The effects could either be due to diffusion of steroids produced by the offspring or by a general tendency for females to allocate steroids according to the sex ratio of her clutch. Subsequent to parturition, the sexes differed in their growth trajectories depending on sex ratio environment. In males, the difference in allometry between sex ratio environments remained over time, whereas in females the corresponding effect disappeared.     

Influence of egg size differences within egg clutches on larval parameters in nine libellulid species (Odonata)

Abstract.  1. In libellulids, egg size differs between species and populations. There are also size differences within egg clutches of individual females.
2. Past experiments suggest that there are two different types of egg clutches in libellulids. Egg size decreases significantly during oviposition in species that perform non-contact guarding during oviposition. In contrast, in species ovipositing in tandem, egg size is randomly distributed.
3. This study deals with the possible consequences of egg size variation within the different egg clutch types. The study examined whether there is a correlation between egg development time, offspring sex or larval size and egg size.
4. The current experiments were conducted in Namibia and Germany. Five non-contact guarding and four tandem guarding libellulid species were used.
5. In some species larger eggs needed more time to develop, in some species no correlation between egg size and egg development time could be found, whereas in other species larger eggs developed faster.
6. The sex ratio was biased towards females in Leucorrhinia dubia and in Sympetrum striolatum and egg size was not associated with gender.
7. In both egg clutch types larger eggs resulted in larger larvae. In this study, evidence was found that the effects of egg size diminished with progressing larval development under good conditions. However, it is possible that the effects may have a greater influence under harsh circumstances.     

Oviposition site preference and developmental performance of a gall‐inducing leafhopper on galled and non‐galled host plants

Oviposition preferences of herbivorous insects affect offspring performance. Both positive and negative links between oviposition preference and offspring performance have been reported for many species. A gall‐inducing leafhopper, Cicadulina bipunctata Melichar (Hemiptera: Cicadellidae), feeds on various Poaceae plants and induces galls of enhanced nutritional value for their offspring. Although gall induction by C. bipunctata improves nymphal performance, the oviposition preference of females between galled and non‐galled host plants is still unclear. In this paper, the nymphal performance and oviposition and feeding‐site preference of C. bipunctata were investigated using galled wheat, Triticum aestivum L., and non‐galled barley, Hordeum vulgare L., as host plants. The survival rate of C. bipunctata on wheat was significantly higher than on barley. In the choice test, significantly more eggs were laid into barley, whereas the number of eggs deposited on both hosts was not significantly different in the no‐choice test. The number of settling individuals per leaf area was not significantly different between wheat and barley, suggesting no clear preference for oviposition between these plants. Experience as a nymph with a growing host did not affect oviposition preference as adult female. The inconsistent correspondence between offspring performance and oviposition preference of C. bipunctata may reflect the high mobility of nymphs and/or differences in leaf area between host plants. The results indicate that the previous finding that oviposition preference and offspring performance are not always positively correlated in herbivorous insects is applicable to gall‐inducing insects.     

Unequal maternal investment in offspring quality in relation to predation risk

Equal investment within broods does not always maximize parental reproductive value if the reproductive value of some of the young is low. We examined maternal investment in terms of offspring size in relation to the prospects of survival from predation within broods of the shield bug Elasmucha ferrugata Fabr. (Heteroptera; Acanthosomatidae). Shield bug females guard eggs and first instar nymphs against invertebrate predators by covering the clutch with their body and by behaving aggressively towards their enemies. Survival of eggs was not possible without maternal care. When females were allowed to guard their brood, eggs at the periphery were more vulnerable to predators than eggs at the centre. We found that females laid significantly larger eggs in the safest, central part of the clutch. There seems to be an advantage of large nymph size, since when nymphs were reared separately with low food resources, the larger ones were more likely to survive. Larger nymphs were also more likely to push themselves to the safest, central part of the clutch. Females seem to allocate their resources more to the offspring with the highest probability of avoiding predation. Thus our study supports unequal maternal investment within broods of E.ferrugata.     

Females of the weevil Zabrotes subfasciatus manipulate the size and number of eggs according to the host seed availability

Abstract. In some insects, the finding of oviposition substrate triggers the uptake into oocytes of yolk proteins that are stored in the fat body during post‐embryonic development. The main host of the bean weevil Zabrotes subfasciatus (Coleoptera; Chrysomelidae; Bruchinae; Amblycerini), in which larval resources are the sole source for future egg maturation, is Phaseolus vulgaris. Despite not feeding as adults, females of this species are able to lay eggs after encountering host seeds but it is not known how females react to changes in the availability of bean seeds. In the present study, the behaviour of Z. subfasciatus facing two very different environments for oviposition is investigated, as well as how this influences offspring fitness. The results obtained show that females of Z. subfasciatus react to variations in the availability of seeds belonging to the same host species by adjusting egg size and number. Females on low bean seed density lay larger and fewer eggs than those on high bean seed density, demonstrating a trade‐off between these reproductive traits. Moreover, females can adjust egg size to changing levels of host availability during the first 4 days of their oviposition period. Although no difference in offspring weight is found, those from small eggs (low competition environment) result in larger adults. No response to selection on these traits after rearing beetles on the same host for 40 generations is observed. This unresponsiveness may indicate that beetle populations behave according to their reaction norm that already allows rapid adaptation to a varying amount of host‐seed availability and better exploitation of the environments of this widespread stored‐seed pest.     

The seasonal timing of oviposition in sand lizards (Lacerta agilis): why early clutches are better

We studied a population of sand lizards (Lacerta agilis) near the northern edge of the species' range in coastal Sweden. We captured, marked, released and recaptured 98 adult female lizards over 5 years. Hatchlings from 146 laboratory-incubated clutches (1279 eggs) from field-caught gravid females were measured, weighed, marked and released at the study site. Female sand lizards usually laid only a single clutch of 4 to 15 eggs each year, but varied considerably in the time of year at which they laid their eggs. Oviposition dates shifted between years depending on weather (basking opportunities), but the relative timing of oviposition was consistent within a given female from year-to-year. The first females to oviposit each year were large animals in good physical condition, that had grown rapidly in previous years. “Early” clutches were larger than “later” clutches, had higher hatching success, and tended to have higher post-hatching survival rates. Offspring from early clutches were larger than “later” hatchlings, and differed in body proportions (probably because seasonal changes in maternal temperatures directly modified offspring phenotypes). Overall, our study documents several strong correlates of the timing of oviposition, and suggests that variation in this trait among females has strong fitness consequences, perhaps related to maternal “quality”. The correlations we observed between oviposition date and other traits that have been invoked as determinants of hatchling survival in reptiles (e.g., hatchling size, body shape, opportunities for multiple mating by the mother) suggest that hypotheses advocating simple causal connections between these traits and hatchling success should be viewed with caution.     

Interfemale variation in egg yolk androgen allocation in the European starling: do high-quality females invest more?

Androgenic hormones occur naturally in bird egg yolk and are known to enhance growth in canary and gull chicks. Gil et al. (1999, Science, 286, 126-128) have recently proposed that female allocation of androgens to eggs represents a form of costly maternal investment. This hypothesis predicts that females of high quality or high reproductive potential should invest more yolk androgens in their eggs than females of low quality or low reproductive potential. We tested these predictions by examining interfemale variation in allocation of the androgenic steroids testosterone and androstenedione to eggs in a wild population of European starlings, Sturnus vulgaris. We collected 30 full clutches of eggs and captured 22 of the females that laid these clutches for phenotypic measurements. In agreement with the hypothesis, we found that there was significant interfemale variation in yolk androgen concentrations. Furthermore, older females deposited more androstenedione and testosterone in eggs than 1-year-old females, and females laying early or large clutches deposited more testosterone in eggs than females laying late or small clutches. However, females in good body condition did not deposit more androgens in eggs than females in poor body condition. Large-yolked eggs had a higher total yolk androgen content than small-yolked eggs, but larger eggs did not have higher concentrations of yolk androgens. The results in general support the investment hypothesis of yolk androgen allocation. However, further data are needed regarding the costs of yolk androgen allocation and the effects of yolk androgens on offspring to substantiate the assumptions of the hypothesis. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Clutch size manipulation, hatching success and offspring phenotype in the ball python (Python regius)

In a diverse array of avian and mammalian species, experimental manipulations of clutch size have tested the hypothesis that natural selection should adjust numbers of neonates produced so as to maximize the number of viable offspring at the end of the period of parental care. Reptiles have not been studied in this respect, probably because they rarely display parental care. However, females of all python species brood their eggs until hatching, but they do not care for their neonates. This feature provides a straightforward way to experimentally increase or reduce clutch size to see whether the mean clutch size observed in nature does indeed maximize hatching success and/or optimize offspring phenotypes. Eggs were removed or added to newly laid clutches of Ball Pythons ( Python regius ) in tropical Africa (nine control clutches, eight with 50% more eggs added, six with 42% of eggs removed). All clutches were brooded by females throughout the 2-month incubation period. Experimental manipulation of clutch-size did not significantly affect the phenotypes (morphology, locomotor ability) of hatchlings, but eggs in 'enlarged' clutches hatched later, and embryos were more likely to die before hatching. This mortality was due to desiccation of the eggs, with females being unable to cover 'enlarged' clutches sufficiently to retard water loss. Our results support the notion of an optimal clutch size, driven by limitations on parental ability to care for the offspring. However, the proximate mechanisms that generate this optimum value differ from those previously described in other kinds of animals. © 2003 The Linnean Society of London . Biological Journal of the Linnean Society 2003, 78 , 263–272.     

Access to multiple mates increases fecundity but does not affect per‐offspring maternal investment in a marine gastropod

Females of many organisms mate more than once and with more than one male, suggesting that polyandry confers some advantage to the female or her offspring. However, variation in maternal investment in response to mate choice and mate number can confound efforts to determine if there are benefits of polyandry. Access to multiple mates could increase maternal investment in offspring via a number of different mechanisms. Few studies have determined if investment is influenced by mate choice and number, and data are particularly lacking for marine invertebrates. This study was designed to determine if maternal investment and offspring size increase with access to increasing numbers of mates in the protandrous intertidal slipper snail Crepidula cf. marginalis. Virgin female slipper limpets were exposed to one, three, or five potential mates and their fecundity, egg size, and hatchling size were measured for multiple clutches. Treatment had a significant effect on fecundity, with fecundity increasing with the number of potential mates. Treatment did not have an effect on the size of eggs or hatchlings, on the variation in egg size or hatchling size within broods, or on the frequency of oviposition. Treatment did alter the variation in average offspring size among females, but not in the way predicted by theory. The main result, that access to multiple mates does not have an effect on per offspring maternal investment, makes C. cf. marginalis an ideal candidate to study the effects of polyandry on offspring fitness without having to take into account confounding effects of variation in maternal investment.