The influence of a positive correlation between clutch size and offspring fitness on the optimal offspring size

Summary The effect is modeled of a positive relationship between clutch size and offspring fitness on the optimal investment in offspring. In species which meet the assumptions of the model, the model predicts a positive correlation between maternal resource level and offspring size. If larger mothers are able to allocate more resources to offspring, then the model would also predict a positive correlation between maternal size and offspring size when the assumptions of the model are met. Thus, this model may help explain both among and within individual variation in offspring size. When offspring are produced in groups and the number of offspring killed per clutch is limited by predator satiation, offspring in larger clutches may experience a higher probability of survival. Such a life style may be found in animals such as sea turtles. Offspring size is positively correlated with maternal size in some members of this group.     

Optimal offspring sizes in small litters

Summary Numerous evolutionary models explore the trade-off between offspring size and offspring number. However, such models often fail when the number of offspring is small because optimal litter size (or litter size at optimal offspring size) may fall between the necessarily integer values for real litters. This paper extends a classic model for optimal investment per offspring to the case of small litters and predicts that range in offspring size and the largest (smallest) offspring size should decline (increase) with increased litter size. Application of the model to egg size data from a poeciliid fish,Gambusia hubbsi, reveals a surprisingly close approximation to the largest offspring size and variation in offspring size at small litter sizes.     

Hatching asynchrony can have long‐term consequences for offspring fitness in zebra finches under captive conditions

Hatching asynchrony can have profound short‐term consequences for offspring, although the long‐term consequences are less well understood. The purpose of this study was to examine the long‐term consequences of hatching asynchrony for offspring fitness in birds. Specifically, we aimed to test the hypothesis that hatching asynchrony increases the sexual attractiveness and fecundity, respectively, of early‐hatched male and female zebra finch, Taeniopygia guttata (Vieillot, 1817) offspring. Mate‐choice trials comparing male nestlings with the same parents, but that were reared in asynchronous or experimentally synchronous broods, revealed no female preference in relation to hatching regime. We did however find strong evidence that, as adults, late‐hatched males were more attractive to females than siblings that had hatched earlier. Meanwhile, we found a weak trend towards early‐hatched females depositing more carotenoids and retinol in the egg yolk than late‐hatched or synchronously hatched females, although there were no differences in terms of clutch characteristics or the deposition of α‐tocopherol or γ‐tocopherol in the egg yolk. Therefore, we found that the beneficial long‐term consequences of hatching asynchrony were sex specific, being accrued by late‐hatched male nestlings and by early‐hatched female nestlings. Consequently, we conclude that the long‐term consequences of hatching asynchrony are more complex than previously realised. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 430–438.     

DO EMBRYOS INFLUENCE MATERNAL INVESTMENT? EVALUATING MATERNAL‐FETAL COADAPTATION AND THE POTENTIAL FOR PARENT‐OFFSPRING CONFLICT IN A PLACENTAL FISH

The evolution of matrotrophy introduces the potential for genomic conflicts between mothers and embryos. These conflicts are hypothesized to accelerate the evolution of reproductive isolation and to influence the evolution of life-history traits, reproductive structures, and genomic imprinting. These hypotheses assume offspring can influence the amount of maternal investment they receive and that there is a trade-off between maternal investment into individual offspring and maternal survival or fecundity. We used field data and laboratory crosses to test whether these assumptions are met in the matrotrophic poeciliid fish Heterandria formosa . Comparisons of life histories between two natural populations demonstrated a trade-off between the level of maternal investment into individual embryos and maternal fecundity. Laboratory crosses between individuals from these populations revealed that offspring genotype exerts an influence on the level of maternal investment and affects maternal fecundity through higher rates of embryo abortion and lower numbers of full-term offspring. Our results show that the prerequisites for parent–offspring conflict to be a potent evolutionary force in poeciliid fish are present in H. formosa. However, determining whether this conflict has shaped maternal investment in nature will require disentangling any effects of conflict from those of several ecological factors that are themselves correlated with the expected intensity of conflict.     

HERITABILITY OF NESTLING BEGGING INTENSITY IN THE HOUSE SPARROW (PASSER DOMESTICUS)

Evolutionary theory of parent–offspring conflict assumes that offspring food solicitation behavior, known as begging, and parental response to begging are subjected to selection and coevolution. This assumption implies that begging intensity should be heritable, at least to some degree. Although some studies have suggested that begging is heritable, the evidence for this is rare and mostly indirect. To assess the heritability of begging we used artificial selection, sibling analysis, and the monitoring of begging intensity in four generations of cross-fostered captive house sparrow nestlings. We also contrasted the heritability of begging with that of morphological traits, known to be heritable in this species. Our results show that adult wing length and body mass were heritable as expected. The heritability estimates of the visual and vocal components of nestling begging (standardized for food deprivation and body mass) were low to moderate, as expected for behavioral traits in general, and lower than previously reported for passerine birds. Our sibling analysis shows that common environment had much greater effect on begging than genetic origin, suggesting that begging evolution may be strongly influenced by gene–environment interaction, probably through the mechanisms that adjust begging response to environmental and social conditions.     

接种量对花绒寄甲繁殖的影响

释放花绒寄甲防治天牛已成为重要手段,而繁殖花绒寄甲时,接种花绒寄甲幼虫到大麦虫蛹体时,控制合适的接种量是提高花绒寄甲繁殖数量和质量的关键技术。本研究通过在替代寄主上人工接种不同数量的花绒寄甲幼虫,观察其发育情况及其子代数量、质量等指标,明确最佳接种量。结果表明：随着接种量的增加,花绒寄甲幼虫历期和蛹历期明显缩短,其中接种4头/个和6头/个,幼虫历期为13 d,蛹历期为33 d,而接种14头/个和16头/个时,幼虫历期短于12 d,蛹历期明显缩短为28~29 d。花绒寄甲结茧数随接种量增加而增加,接种数为16头/个时,结茧数最多,接近7个。花绒寄甲结茧率随着接种量增多而降低,4头/个时,结茧率最高为72.3%。接种量对花绒寄甲子代个体数量和大小均有显著影响,其中接种8头/个时,羽化数平均为4.3头,显著高于接种4头/个（羽化数平均为2.8头）,明显低于接种16头/个(羽化数平均为6.9头)。接种量为4头/个和6头/个时,子代成虫个体最大,单头重平均每头可达0.035 g,接种量为8头/个时,成虫单头重平均每头0.032 g左右,接种量达到16头/个时,单头重最轻,为0.023 g。对花绒寄甲羽化率无显著影响,7个处理下子代羽化率均较高,平均在94.4%~100%。接种量越少,更利于花绒寄甲的生长发育,当接种量为4头/个时,花绒寄甲成虫发育最好,其子代个体最大,但子代数较少。因此,利用大麦虫蛹繁育花绒寄甲种虫时,最佳接种量为4头/个,而需要规模化繁育花绒寄甲作为天敌使用时,综合考虑子代数量和质量以及经济成本,最佳接种量为8头/个。     

Parent-offspring conflict and co-adaptation: behavioural ecology meets quantitative genetics

The evolution of the complex and dynamic behavioural interactions between caring parents and their dependent offspring is a major area of research in behavioural ecology and quantitative genetics. While behavioural ecologists examine the evolution of interactions between parents and offspring in the light of parent-offspring conflict and its resolution, quantitative geneticists explore the evolution of such interactions in the light of parent-offspring co-adaptation due to combined effects of parental and offspring behaviours on fitness. To date, there is little interaction or integration between these two fields. Here, we first review the merits and limitations of each of these two approaches and show that they provide important complementary insights into the evolution of strategies for offspring begging and parental resource provisioning. We then outline how central ideas from behavioural ecology and quantitative genetics can be combined within a framework based on the concept of behavioural reaction norms, which provides a common basis for behavioural ecologists and quantitative geneticists to study the evolution of parent-offspring interactions. Finally, we discuss how the behavioural reaction norm approach can be used to advance our understanding of parent-offspring conflict by combining information about the genetic basis of traits from quantitative genetics with key insights regarding the adaptive function and dynamic nature of parental and offspring behaviours from behavioural ecology.     

Reducing losses to offspring mortality by redistributing resources

1. It is shown that reallocation of resources from dying offspring to their surviving siblings leads to significant reductions of fitness losses due to early developmental errors.
2. The reason resource reallocation can improve offspring fitness is because mothers do not provide offspring with the optimal amount of resources from the offspring's point of view. Rather, mothers trade their investment per offspring off against the number of offspring. Hence, surviving offspring can use reallocated resources fruitfully.
3. Animals suffering high offspring mortality can reduce this cost by producing large packages of resources shared by offspring. This allows for better reallocation of resources. Furthermore, by overstocking their resource packages with eggs they can anticipate embryo mortality and obtain offspring that will on average be more optimal in size.
4. In accordance with our prediction, parthenogenetic flatworms studied here produce larger cocoons than sexuals and they overstock smaller cocoons with eggs. However, higher embryo survival in large cocoons may also explain both these phenomena.     

DECLINE IN OFFSPRING VIABILITY AS A MANIFESTATION OF AGING IN DROSOPHILA MELANOGASTER

Abstract The evolutionary explanation of senescence proposes that selection against alleles with deleterious effects manifested only late in life is weak because most individuals die earlier for extrinsic reasons. This argument also applies to alleles whose deleterious effects are nongenetically transmitted from mother to progeny, that is, that affect the performance of progeny produced at late ages rather than of the aging individuals themselves. We studied the effect of maternal age on offspring viability (egg hatching success and larva-to-adult survival) in two sets of Drosophila melanogaster lines (HAM/LAM and YOUNG/OLD), originating from two long-term selection experiments. In each set, some lines (HAM and YOUNG, respectively) have been selected for early reproduction, whereas later reproduction was favored in their counterparts (LAM and OLD). In the HAM and LAM lines, both egg hatching success and larval viability declined with mother's age and did so with accelerating rates. The hatching success declined significantly faster with maternal age in HAM than in LAM lines, according to one of two statistical approaches used. Egg hatching success also declined with maternal age in YOUNG and OLD lines, with no difference between the selection regimes. However, the relationship between mother's age and offspring larva-to-adult viability differed significantly between these two selection regimes: a decline of larval viability with maternal age occurred in YOUNG lines but not in OLD lines. This suggests that the rate with which offspring viability declines with mother's age responded to selection for early versus late reproduction. We suggest broadening the evolutionary concept of senescence to include intrinsically caused declines in offspring quality with maternal age.     

Defending the Weak: Parental Defense Peaks When Chick Vulnerability is Greatest in the Herring Gull (Larus argentatus)

Successful reproduction often depends upon parents providing offspring with resources and protection. In birds, reproductive success can often be enhanced by parents engaging in antipredator behaviors, but these behaviors can be costly. Theoretically, individuals should temporally modify the intensity of nest defense behavior to balance the costs and benefits of current and future reproductive success. More specifically, nest defense should vary throughout a nesting attempt to maximize fitness of the adults. Here, we consider the relationship between nest defense behavior and chick vulnerability in the herring gull (Larus argentatus), where chicks are under high predation risk. We estimated chick vulnerability by quantifying survival probabilities at different periods of the nestling stage. Simultaneously, we quantified changes in parental aggression throughout the nesting cycle by simulating predation attempts using a human predator model. We found that chick survival probabilities were lowest (i.e., vulnerability was highest) and parental aggression in nest defense was greatest during the first 10 days after hatching. Thus, we show that parents are most defensive when chicks are most vulnerable and that adults optimize nest defense behaviors in a way that maximizes their fitness.     

Parental age difference, educationally assortative mating and offspring count: evidence from a contemporary population in Taiwan

Using contemporary population data from Taiwan, we examine the relationships between parental age difference, educationally assortative mating, income and offspring count. Controlling for women's reproductive value (measured by age at first birth), we find that an older husband is associated with fewer offspring, whereas a husband with similar or higher education is associated with more offspring. Concerning resources, we find that women's income is negatively associated with fertility and husband's income is positively associated with fertility among highly educated women. These results are consistent with the view that women compensate for trade-offs between education, income generation and childbearing by seeking mates with a higher status.     

Immune Effects of Nonylphenol on Offspring of Rats Exposed During Pregnancy

Effects of nonylphenol on immune system of male rats were examined. Dams were treated orally with nonylphenol at doses of 0, 20, 40, 80, or 200 mg/kg, respectively, from pregnant days 14 to 19. The offspring rats were investigated at postnatal day 60. Compared with the control groups, the doses of 80 and 200 mg nonylphenol/kg induced an obvious decrease in the absolute and relative weight of spleen and thymus. In the 200 mg/kg nonylphenol-treated group, the proliferative responses of murine spleen lymphocytes cultured in vitro were suppressed, Cytokine productions of interferon-gamma and interleukin-6 in serum were markedly lower than those in the control group. Histologically, the boundary between splenic red pulp and white pulp was unclear, expansion and congestion appeared in splenic sinus, lymphocytes in spleen and thymus dramatically reduced, and lots of focal necrosis cells were present. The results of this study show that nonylphenol can cross the placenta barrier, and that in utero exposure to 200 mg/kg/day nonylphenol can inhibit immune function in male offspring rats.     

Delusions of grandeur: Seed count is not a good fitness proxy under individual variation in phenology

The concept of fitness is central to evolutionary biology, yet it is difficult to define and to measure. In plant biology, fitness is often measured as seed count. However, under an array of circumstances, seed count may be a biased proxy of fitness, for example when individuals vary in allocation to sexual versus asexual reproduction. A more subtle example, but also likely to be important in natural populations, is when interindividual variation in conditions during development results in variation in offspring quality among seed parents. In monocarpic (semelparous) plants, this is expected to result from variation in effective season length experienced among individuals that reach reproductive maturity at different times. Here, we manipulate growing season length to ask whether seed count is an accurate representation of parental fitness in the monocarpic herb Lobelia inflata. Simple seed count suggests a paradoxical fitness advantage under constrained‐season length. However, we find that the apparent fitness advantage of a constrained‐season length is overridden by low relative per‐seed fitness. Furthermore, the fitness deficit in the constrained environment is associated primarily with an accelerating decrease in viability and seedling survival in seeds derived from fruits produced progressively later in the season. In this study, the overall fitness value of a seed under a constrained season is 0.774 of that observed under a long season.     

Giving offspring a head start in life: field and experimental evidence for selection on maternal basking behaviour in lizards

The timing of birth is often correlated with offspring fitness in animals, but experimental studies that disentangle direct effects of parturition date and indirect effects mediated via variation in female traits are rare. In viviparous ectotherms, parturition date is largely driven by female thermal conditions, particularly maternal basking strategies. Our field and laboratory studies of a viviparous lizard (Niveoscincus ocellatus) show that earlier‐born offspring are more likely to survive through their first winter and are larger following that winter, than are later‐born conspecifics. Thus, the association between parturition date and offspring fitness is causal, rather than reflecting an underlying correlation between parturition date and maternal attributes. Survival selection on offspring confers a significant advantage for increased maternal basking in this species, mediated through fitness advantages of earlier parturition. We discuss the roles of environmentally imposed constraints and parent–offspring conflict in the evolution of maternal effects on parturition date.     

Transgenic offspring by transcaryotic implantation of transgenic ovaries into normal mice

Female transgenic mice may be unable to reproduce successfully if the product encoded by the transgene results in pathological changes or affects the fertility of the mouse. To approach this problem, we have produced chimaeras by transferring the ovaries of transgenic mice into normal mice of the same strain. Such chimaeras will be an ideal tool for investigating the interactions between transgenic ovaries and normal mice or vice versa. Here we show that, using this method, we were able to get large numbers of transgenic offspring even from founder transgenic female mice that were themselves infertile as a result of the overexpression of growth hormone genes. Although none of the ovary recipients were given immunosuppressant treatment, 60% of the recipients had biologically active ovaries over a mean period of about 100 days.     

Cannibalism as a selective force on offspring size in fish

Cannibalism may cause considerable mortality on juvenile fish and it has been hypothesised that it may exercise selection on offspring size in that larger offspring may enjoy a size refuge. For this to be evolutionarily advantageous the survival of individual offspring must compensate for the reduced fecundity implied by larger offspring size. We develop a model which combines standard assumptions of size‐dependent mortality with adult cannibalism to investigate the potential for cannibalism to act as selective force on offspring size. We find that for this potential to be realised, the mortality due to cannibalism must exceed a threshold value that is a decreasing function of non‐cannibalistic predation intensity, cannibalized size range width and the average cannibalized size. If cannibalism exceeds this threshold, the model predicts evolution of offspring size towards refuges above or below cannibalized size range depending on initial offspring size. Cannibalistic mortality cannot be so great that the population is non‐viable, however, the range of parameter values describing cannibalistic intensity allowed within these boundaries is wide. On this basis, we suggest that cannibalism is a potential mechanism for offspring size selection.     

Persistent directional selection on body size and a resolution to the paradox of stasis

Directional selection on size is common but often fails to result in microevolution in the wild. Similarly, macroevolutionary rates in size are low relative to the observed strength of selection in nature. We show that many estimates of selection on size have been measured on juveniles, not adults. Further, parents influence juvenile size by adjusting investment per offspring. In light of these observations, we help resolve this paradox by suggesting that the observed upward selection on size is balanced by selection against investment per offspring, resulting in little or no net selection gradient on size. We find that trade‐offs between fecundity and juvenile size are common, consistent with the notion of selection against investment per offspring. We also find that median directional selection on size is positive for juveniles but no net directional selection exists for adult size. This is expected because parent–offspring conflict exists over size, and juvenile size is more strongly affected by investment per offspring than adult size. These findings provide qualitative support for the hypothesis that upward selection on size is balanced by selection against investment per offspring, where parent–offspring conflict over size is embodied in the opposing signs of the two selection gradients.     

Hatching Asynchrony Decreases the Magnitude of Parental Care in Domesticated Zebra Finches: Empirical Support for the Peak Load Reduction Hypothesis

Parent–offspring conflict over the supply of parental care results in offspring attempting to exert control using begging behaviours and parents attempting to exert control by manipulating brood sizes and hatching patterns. The peak load reduction hypothesis proposes that parents can exert control via hatching asynchrony, as the level of competition amongst siblings is determined by their age differences and not by their growth rates. Theoretically, this benefits the parents by reducing both the peak load of the offspring's demand and their overall demand for food and benefits the offspring by reducing the amplification of their competition. However, the peak load reduction hypothesis has only received mixed support. Here, we describe an experiment where we manipulated the hatching patterns of domesticated zebra finch Taeniopygia guttata broods and quantified patterns of nestling begging and parental feeding effort. There was no difference in the begging intensity of nestlings raised in asynchronous or experimentally synchronous broods, yet parental feeding effort was lower when provisioning asynchronous broods and particularly so when levels of nestling begging were low. Further, both parents acted in unison, as there was no evidence of parentally biased favouritism in relation to hatching pattern. Therefore, our study provided empirical support for the prediction that hatching asynchrony reduces the feeding effort of parents, thereby providing empirical support for the peak load reduction hypothesis.