HERITABILITY AND SELECTION ON CLUTCH SIZE IN DARWIN'S MEDIUM GROUND FINCHES (GEOSPIZA FORTIS)

I studied the causes of variation and selection on clutch size in a population of Darwin's Medium Ground Finches (Geospiza fortis) on Isla Daphne Major, using data collected over a nine-year period (1976–1984). Quantitative-genetic analyses were carried out using the first clutch laid by a female in a given year. I used both unadjusted clutch-size values and values adjusted for between-year differences in mean clutch size for repeatability and regression analyses. Repeatability of clutch size was small (≤8%) and nonsignificant in all cases. Sib-sib analyses and parent-offspring regressions gave no evidence of a significant additive genetic component to clutch-size variation. Slopes of mother-daughter regressions were actually negative, suggesting possible maternal effects of mother's clutch size on daughter's clutch size. There was a small positive relationship between female age and clutch size but no effect of male or female body size or of large-scale differences in habitat quality on clutch size. Selection on clutch size was generally directional and positive: in almost all years in which successful breeding occurred, large clutches tended to fledge more chicks and produce more young surviving to the following year, possibly because there was no trade-off between clutch size and the weights of individual chicks at fledging. Thus, sustained directional selection for large clutch size may have reduced additive genetic variation in clutch size to low levels in this population. The size of a female's clutch may be primarily determined by unidentified proximate environmental factors which vary from year to year, rather than by any long-term optimization of clutch size with respect to adult survival.     

Is there a trade-off between egg weight and clutch size in wild Lesser Snow Geese (Anser c. caerulescens)?

Previous studies of arctic nesting geese suggest that laying is limited by the size of a female's body reserves and that larger eggs contain more nutrients. These observations imply a life-history trade-off between egg size and clutch size which may give rise to a negative genetic correlation between the two characters. We estimated the genetic correlation between egg weight and clutch size using measurements from mothers and their daughters in a wild population of Lesser Snow Geese Anser caerulescens caerulescens. Between 65 and 80 % of the variance in egg weight is attributable to differences between individuals, and heritability of egg weight is about 60 %. In contrast, 10–20 % of the variance in clutch size is attributable to differences between individuals, and heritability of clutch size is about 15 %. The genetic correlation coefficient between egg weight and clutch size ranges from 0.09 to 0.32 and does not differ significantly from zero. We discuss the possible reasons for the lack of the expected negative genetic correlation.     

Bigger mothers are better mothers: disentangling size-related prenatal and postnatal maternal effects

Despite a vast literature on the factors controlling adult size, few studies have investigated how maternal size affects offspring size independent of direct genetic effects, thereby separating prenatal from postnatal influences. I used a novel experimental design that combined a cross-fostering approach with phenotypic manipulation of maternal body size that allowed me to disentangle prenatal and postnatal maternal effects. Using the burying beetle Nicrophorus vespilloides as model organism, I found that a mother''s body size affected egg size as well as the quality of postnatal maternal care, with larger mothers producing larger eggs and raising larger offspring than smaller females. However, with respect to the relative importance of prenatal and postnatal maternal effects on offspring growth, only the postnatal effects were important in determining offspring body size. Thus, prenatal effects can be offset by the quality of postnatal maternal care. This finding has implications for the coevolution of prenatal and postnatal maternal effects as they arise as a consequence of maternal body size. In general, my study provides evidence that there can be transgenerational phenotypic plasticity, with maternal size determining offspring size leading to a resemblance between mothers and their offspring above and beyond any direct genetic effects.     

GENETIC,ENVIRONMENTAL, AND CONDITION-DEPENDENT EFFECTS ON FEMALE AND MALE ORNAMENTATION IN THE BARN OWL TYTO ALBA

Secondary sexual characters are thought to indicate individual quality. Expression of sex-limited traits in an extravagant state may require both the underlying genes and the available nutrient resources. The assessment of the relative contribution of genes, environment, and body condition is relevant for understanding to that extent the extravagant trait may signal genotypic or phenotypic quality of the individual. In birds, usually only the males are ornamented. In the barn owl, Tyto alba, both females and males display sex-limited plumage traits. Males are commonly lighter colored and females spottier. In an experiment with combined cross-fostering and brood size manipulation we determined the relative contribution of genes, environment, and body condition to the variation in plumage coloration and plumage spottiness. The partial cross-fostering experiment tested the relative importance of shared genes and a shared environment for the resemblance of related birds. Siblings raised in different nests converged toward similar trait values, offspring resembled the true but not the foster parents, and plumage traits of unrelated nestlings sharing the same nest were not correlated. Results were not inflated by maternal effects detectable in the mother's phenotype, because middaughter to mother resemblance was not higher than midson to father resemblance. This suggests that plumage coloration and spottiness are largely genetically inherited traits, and that the rearing environment does not have a strong impact on the expression of these traits. To further investigate whether the two sex-limited traits are condition dependent, brood sizes were manipulated. Enlargement or reduction of broods by two nestlings resulted in lower and higher body mass of nestlings, respectively. However, nestlings raised in enlarged or reduced broods did not show either a significantly darker or lighter or a more or less spotted plumage. We did not detect any genotype-by-environment interaction. In conclusion, simultaneous cross-fostering and brood size manipulation demonstrate that additive genetic variance for plumage coloration and spottiness is maintained and that both the rearing environment and body condition do not account for a large proportion of the phenotypic variance in female and male ornamentations.     

Intra-specific variations of egg size,clutch size and larval survival relatedto maternal size in amphidromous <Emphasis Type="BoldItalic">Rhinogobius</Emphasis> goby

Synopsis Newly hatched amphidromous Rhinogobius sp. CB (cross band type) larvae drift downstream to the sea to grow and develop before returning upstream as juveniles. Since larger and older individuals usually inhabit the upper reaches of rivers, larvae from larger females are more likely to suffer higher risks of starvation or predation during their longer migration to the sea. We examined the relationship between reproductive parameters (egg volume and clutch size) and maternal length. We collected adult Rhinogobius sp. CB along the course of the Aizu River (Wakayama Prefecture, Japan) and spawned them under laboratory conditions. We measured egg volumes and clutch sizes, as well as larval starvation tolerance. Both egg volume and clutch size increased with standard length or age of the maternal fish, while egg density in these clusters did not correlate with standard length. Gonad-somatic index (GSI) also tended to increase with maternal standard length. There was a significant positive correlation between egg size and the 72-h survival rate of unfed hatchlings. Intra-specific variation of egg volumes and clutch sizes in this species seems to be an adaptation for enhancing offspring survivorship during migration to the sea. Some females spawned a second time. Second spawned egg sizes were smaller than first spawned egg sizes, although there was no difference in clutch size between the two. Egg size variation between subsequent spawns may be an adjustment to the changes in seasonal environmental conditions.     

Conflict between optimal clutch size for mothers and offspring in the leaf miner, Leucoptera sinuella

Abstract. 1. Clutch size in a leaf‐mining moth, Leucoptera sinuella (Reutti), was examined to determine whether the clutch size in natural populations meets the prediction of an optimal strategy, through comparisons between the optimal clutch sizes for offspring and for a mother. 2. A field experiment revealed that premature leaf abscission, egg dropping, and larval competition were important selective forces in determining the clutch size of this leaf miner on its host plant, Salix miyabeana. Then, optimal clutch size was predicted using the theoretical model of Weis et al. (1983 ), from the data obtained in the field experiment. 3. The model predicts that the clutch size that maximises offspring fitness is two, and that the clutch size that maximises reproductive success of the female varies from two to four, depending on the female's survival rate between oviposition events. The predicted clutch size (two) was identical to the clutch size observed most frequently in the field, assuming > 95% survival rate of females. Suitability of the model of Weis et al. (1983 ) was discussed based on these results.     

Parent–offspring conflict and selection on egg size in turtles

The trade‐off between offspring size and number can present a conflict between parents and their offspring. Because egg size is constrained by clutch size, the optimal egg size for offspring fitness may not always be equivalent to that which maximizes parental fitness. We evaluated selection on egg size in three turtle species (Apalone mutica, Chelydra serpentina and Chrysemys picta) to determine if optimal egg sizes differ between offspring and their mothers. Although hatching success was generally greater for larger eggs, the strength and form of selection varied. In most cases, the egg size that maximized offspring fitness was greater than that which maximized maternal fitness. Consistent with optimality theory, mean egg sizes in the populations were more similar to the egg sizes that maximized maternal fitness, rather than offspring fitness. These results provide evidence that selection has maximized maternal fitness to achieve an optimal balance between egg size and number.     

Repeatability and heritability of reproductive traits in free-ranging snakes

The underlying genetic basis of life-history traits in free-ranging animals is critical to the effects of selection on such traits, but logistical constraints mean that such data are rarely available. Our long-term ecological studies on free-ranging oviparous snakes (keelbacks, Tropidonophis mairii (Gray, 1841), Colubridae) on an Australian floodplain provide the first such data for any tropical reptile. All size-corrected reproductive traits (egg mass, clutch size, clutch mass and post-partum maternal mass) were moderately repeatable between pairs of clutches produced by 69 female snakes after intervals of 49-1152 days, perhaps because maternal body condition was similar between clutches. Parent-offspring regression of reproductive traits of 59 pairs of mothers and daughters revealed high heritability for egg mass (h2= 0.73, SE=0.24), whereas heritability for the other three traits was low (< 0.37). The estimated heritability of egg mass may be inflated by maternal effects such as differential allocation of yolk steroids to different-sized eggs. High heritability of egg size may be maintained (rather than eroded by stabilizing selection) because selection acts on a trait (hatchling size) that is determined by the interaction between egg size and incubation substrate rather than by egg size alone. Variation in clutch size was mainly because of environmental factors (h2=0.04), indicating that one component of the trade-off between egg size and clutch size is under much tighter genetic control than the other. Thus, the phenotypic trade-off between egg size and egg number in keelback snakes occurs because each female snake must allocate a finite amount of energy into eggs of a genetically determined size.     

Optimal offspring provisioning when egg size is "constrained": a case study with the painted turtle Chrysemys picta

Classic egg size theory predicts that, in a given environment, there is a level of maternal investment per offspring that will maximize maternal fitness. However, positive correlations among egg size and female body size are observed within populations in diverse animal taxa. A popular explanation for this phenomenon is that, in some populations, morphological constraints on egg size, such as ovipositor size (insects) or pelvic aperture width (lizards and turtles), limit egg size. Egg size may therefore increase with female body size due to body size‐specific constraints on investment per offspring, coupled with selection towards an optimal egg size. We use 17 years of data from a population of painted turtles Chrysemys picta to evaluate this hypothesis. In accordance with our predictions, we find that (1) morphological constraints on egg size are apparent only in relatively small females, similarly (2) egg mass exhibits a strong asymptotic relationship with female body size, suggesting egg mass is optimized only at large body sizes, (3) clutch size, not egg mass, varies with female condition, and (4) clutch size varies more than egg mass across years. Contrary to our predictions, we observe that (5) the egg mass‐clutch size tradeoff is not less pronounced at large body sizes. Our data do not fully support the traditional hypothesis, and recent models suggest that this hypothesis is indeed overly simplistic. When the selective environment of a female's offspring is influenced by her phenotype, optimal egg size may vary among maternal phenotypes. This concept can explain correlations among egg size and body size in many taxa, as well as the patterns observed in the present study. In this paradigm, a tight coupling of aperture width (or other ‘constraints’) and egg size may occur in small females, even when such morphological features are not causally related to variation in egg size. In this spirit, we question validity of invoking morphological constraints to explain covariation among egg size and female body size.     

WHY ARE CLUTCH SIZES MORE VARIABLE IN SOME SPECIES THAN IN OTHERS?

Animal species differ in the variability of their clutch sizes, as well as in mean clutch sizes. This phenomenon is particularly obvious in lizards, where virtually invariant clutch sizes have evolved independently in at least 23 lineages in seven families. Reduced variance in clutch size may arise either as an adaptation (because females with less variable clutch sizes have higher fitness) or as an indirect by-product of selection on other life-history characteristics. Comparative data on Australian scincid lizards indicate that variance in clutch sizes is lowest among species with low mean clutch sizes, small body sizes and a low variance in body sizes of adult females. Phylogenetic analysis shows that evolutionary decreases in the variance of clutch size have accompanied decreases in mean clutch sizes and decreases in the variance of adult female body sizes. Tropical lizards may also exhibit lower variance in clutch size. Most of these characteristics are correlated in occurrence, and may be allometrically tied to small body size. Hence, low variance in clutch size may be a consequence of allometric effects on a correlated suite of life-history characteristics. Exceptions to the general patterns noted above—especially, lizard species with invariant clutch sizes but large body sizes—may be due to loss of genetic variance for clutch sizes in lineages that have passed through a “bottleneck” of small body sizes and hence, low variance in clutch sizes.     

Domestic Calves (Bos taurus) Recognize their Own Mothers by Auditory Cues

The goal of this study was to determine if auditory cues are important in maternal recognition by domestic cattle calves, Bos taurus. Cows and their calves were separated and the vocalizations of the mothers were recorded. During experimental playbacks in a test enclosure, each calf (n = 9) was given a choice between a tape-recorded vocalization of its mother and that of a strange mother. Calves significantly preferred their own mother's vocalization as compared to the vocalization of the unfamiliar mother. Calves spent significantly more time near the speaker that played their own mother's call, and approached significantly closer to their mother's speaker. These results demonstrate that 3–5-wk-old calves can recognize their mothers by auditory cues alone. Visual inspection of audiospectrograms of the cows' vocalizations suggests that there are individual differences among cows.     

THE EVOLUTION OF CLUTCH SIZE. I. AN EQUATION FOR PREDICTING CLUTCH SIZE

We derive an equation for calculating the clutch sizes of birds and other long-lived animals from Murray's (1979) theory on the evolution of clutch size. For the Prairie Warbler (Dendroica discolor) in Indiana, this equation predicts an average clutch size of 3.49, less than half an egg smaller than the recorded average clutch size of 3.89. We attribute the discrepancy to sampling error and suggest that the equation satisfactorily identifies the important factors affecting the evolution of clutch size. The success of the equation in predicting clutch size of the Prairie Warbler provides additional support for Murray's theory on the evolution of clutch size.     

Reproductive variation and the egg size-clutch size trade-off within and among populations of painted turtles (Chrysemys picta bellii)

Interpopulation variation in egg size, clutch size and clutch mass was studied 3 years in four populations of painted turtles (Chrysemys picta bellii) from western Nebraska. Body size varied among all populations and was larger in two large (56–110 ha), sandhills lake populations than in two populations in smaller habitats (1.5–3.6 ha) of the Platte River floodplain. Reproductive parameters (egg mass, clutch mass, and clutch size) generally increased with maternal body size within populations. Clutch wet and dry mass varied among populations but largely as a function of maternal body size. Clutch size was largest in the sandhills lake populations, both absolutely and relative to maternal body size. Egg mass was smallest in the sandhills lakes and varied annually in one population. Over all populations, an egg sizeclutch size trade-off was detected (a negative correlation between egg mass and clutch size) after statistically removing maternal body size effects. Egg wet mass and clutch size were negatively correlated over all years within the sandhills populations and in some years in three populations. Although egg size varied within populations, egg size and clutch size covaried as expected by optimal offspring size models. Thus, patterns of egg size variation should be interpreted in the context of proximate or adaptive maternal body size and temporal effects. Comparisons among populations suggest that large egg size relative to maternal body size may occur when juvenile growth potential is poor and mean maternal body size is small.     

THE EVOLUTIONARY GENETICS OF AN ADAPTIVE MATERNAL EFFECT: EGG SIZE PLASTICITY IN A SEED BEETLE

In many organisms, a female's environment provides a reliable indicator of the environmental conditions that her progeny will encounter. In such cases, maternal effects may evolve as mechanisms for transgenerational phenotypic plasticity whereby, in response to a predictive environmental cue, a mother can change the type of eggs that she makes or can program a developmental switch in her offspring, which produces offspring prepared for the environmental conditions predicted by the cue. One potentially common mechanism by which females manipulate the phenotype of their progeny is egg size plasticity, in which females vary egg size in response to environmental cues. We describe an experiment in which we quantify genetic variation in egg size and egg size plasticity in a seed beetle, Stator limbatus, and measure the genetic constraints on the evolution of egg size plasticity, quantified as the genetic correlation between the size of eggs laid across host plants. We found that genetic variation is present within populations for the size of eggs laid on seeds of two host plants (Acacia greggii and Cercidium floridum; h² ranged between 0.217 and 0.908), and that the heritability of egg size differed between populations and hosts (higher on A. greggii than on C. floridum). We also found that the evolution of egg size plasticity (the maternal effect) is in part constrained by a high genetic correlation across host plants (r_G > 0.6). However, the cross-environment genetic correlation is less than 1.0, which indicates that the size of eggs laid on these two hosts can diverge in response to natural selection and that egg size plasticity is thus capable of evolving in response to natural selection.     

Influence of genetic and environmental factors on egg and clutch sizes among populations of Paratya australiensis Kemp (Decapoda: Atyidae) in␣upland rainforest streams, south-east Queensland

Headwater populations of the common shrimp Paratya australiensis were sampled to examine the factors influencing egg and clutch size. Much of the spatial variation in these reproductive traits was associated with differences in altitude, with upper sites having larger eggs and smaller clutches. Mean egg size at high-altitude sites was higher than that previously reported for this species. Temporal variation in egg and clutch size was also observed, with significant increases in egg size during the breeding season. At most sites, this increase was accompanied by a decrease in clutch size. The combined effect of the reciprocal patterns in egg volume and clutch size resulted in relatively little spatial and temporal variation in reproductive effort. In a field experiment, using a unique genetic marker, shrimps were translocated between two sites with significantly different egg sizes. After one generation (18 months), the mean egg size of translocated females was the same as that of females from the `source' population. In contrast, clutch size changed towards that of the resident females. This suggests that egg size is under strong genetic control, while clutch size is influenced by the environment. Received: 14 July 1997 / Accepted: 9 March 1998     

Paternal effects associated with melanism in Harmonia axyridis (Coleoptera: Coccinellidae): mating sequence asymmetries and interactions with age‐specific maternal effects

1. For this study a cohort of melanic Harmonia axyridis males homozygous for the spectabilis allele was produced. These were used to produce four kinds of twice‐mated females, comprising all four permutations of melanic and non‐melanic (succinic) males. A series of 12 larvae were then reared from the first and 10th clutches of each female to compare progeny developmental phenotypes. 2. There were no effects of mating treatment on overall female reproductive performance (preoviposition period, 20‐day fecundity, or egg fertility). 3. Age‐specific maternal effects were evident in progeny developmental phenotypes; larvae of 10th clutches developed more slowly, pupation was shorter, and adults emerged at heavier weights. 4. Paternal effects were superimposed on maternal effects and affected progeny independent of their paternity; melanic males induced slower larval development and slower pupation, but only in 10th clutches and only when they mated second. 5. There was a significant three‐way interaction between male mating treatment, clutch number and progeny phenotype, indicating that progeny developmental responses to (mixed) paternal effects varied depending on their own phenotype and the time elapsed since their mother's last mating. 6. Melanic males mating second obtained a P2 advantage over succinic males, which increased from first to 10th clutch, but the reverse was not true when succinic males mated second. Thus, polyandry in H. axyridis facilitates both genetic and epigenetic competition among males while simultaneously enabling the sharing of predominant paternal effects among the progeny of different fathers.     

Effects of carcass size and male presence on clutch size in Nicrophorus quadripunctatus (Coleoptera: Silphidae)

The effects of carcass size and male presence on clutch size in Nicrophorus quadripunctatus were examined. Male presence increased clutch size and improved the female's ability to produce replacement clutches. Clutch size was also related to carcass size. There was a negative correlation between number of clutches and clutch size for most carcass sizes. We conclude that N. quadripunctatus is potentially iteroparous and hypothesize that reproductive energy is reserved for brood failure.     

On the reproductive biology of the herbivorous spiny-tailed agamid Uromastyx philbyi in western Saudi Arabia

Abstract

The reproductive biology of the Spiny-tailed Agamid Uromastyx philbyi, a herbivorous desert lizard, was studied in western Saudi Arabia. Reproduction is seasonal, with mating in early spring (March), oviposition in late spring (May–June), and hatching in summer (July). The mean clutch size was 6.67 eggs. Eggs are large (mean mass = 7.48 g) and relative clutch mass (RCM) averaged 0.49. Clutch mass and RCM increased with increasing egg mass. Clutch size, egg size, clutch mass, and RCM were significantly correlated with maternal body size.     

Evolution of polyembryony: Consequences to the fitness of mother and offspring

Polyembryony, referring here to situations where a nucellar embryo is formed along with the zygotic embryo, has different consequences for the fitness of the maternal parent and offspring. We have developed genetic and inclusive fitness models to derive the conditions that permit the evolution of polyembryony under maternal and offspring control. We have also derived expressions for the optimal allocation (evolutionarily stable strategy, ESS) of resources between zygotic and nucellar embryos. It is seen that (i) Polyembryony can evolve more easily under maternal control than under that of either the offspring or the ‘selfish’ endosperm. Under maternal regulation, evolution of polyembryony can occur for any clutch size. Under offspring control polyembryony is more likely to evolve for high clutch sizes, and is unlikely for low clutch sizes (<3). This conflict between mother and offspring decreases with increase in clutch size and favours the evolution of polyembryony at high clutch sizes, (ii) Polyembryony can evolve for values of “x” (the power of the function relating fitness to seed resource) greater than 0.5758; the possibility of its occurrence increases with “x”, indicating that a more efficient conversion of resource into fitness favours polyembryony. (iii) Under both maternal parent and offspring control, the evolution of polyembryony becomes increasingly unlikely as the level of inbreeding increases, (iv) The proportion of resources allocated to the nucellar embryo at ESS is always higher than that which maximizes the rate of spread of the allele against a non-polyembryonic allele.     

CONDITION, GENOTYPE-BY-ENVIRONMENT INTERACTION, AND CORRELATIONAL SELECTION IN LIZARD LIFE-HISTORY MORPHS

Abstract We compared reproductive allocation and variation in condition and survivorship of two heritable female throat color morphs (orange and yellow) in a free‐living population of side‐blotched lizards (Uta stansburiana). Using path analysis and structural equation modeling, we investigated how variation in the social environment affected clutch size and egg mass and two condition traits (postlaying mass, immunological condition) and how these traits in turn affected female field survival. In the presence of many neighbors, both morphs increased their clutch sizes, although these effects were only significant in yellow females. In addition, yellow females increased their egg mass in the presence of many orange neighbors. Orange females surrounded by many orange neighbors showed sign of stress in the form of immunosuppression, whereas this effect was less pronounced in yellow females. The morphs also differed in the impact of variation in clutch size and egg mass on both condition traits. Finally, female morphotype and immune responsiveness affected fitness interactively, and hence these two traits showed signs of fitness epistasis: Selection gradients on this trait were opposite in sign in the two morphs. The correlational selection gradient (_{γthroatxantibody response}) between female throat color and antibody responsiveness was ‐0.365. Our data thus reveal important interactive effects such as genotype‐by‐environment interaction toward the social environment and morph‐specific trade‐offs as well as the occurrence of correlational selection. We discuss the use of naturally occurring and conspicuous genetic polymorphisms in field studies of selection and life‐history allocation.