Reduced Egg Size Research Articles

Early onset of reduced reproductive performance with age in the Treecreeper (Certhia familiaris)

Reductions in reproductive performance with age have been predicted to result from a general deterioration of performance, i.e. senescence. Variation among species in the onset and rate of this deterioration depends on the age-independent extrinsic mortality rate. If few individuals reach a specific age, the strength of selection for mechanisms that retard senescence will be reduced. The aim of this study was to investigate the age-dependent variation in two reproductive traits in a species, the Treecreeper (Certhia familiaris), with a low between-year survival rate. Clutch size did not vary with age, but egg size decreased from the first to the second breeding season. Compared with published age-dependent reductions in egg size, Treecreepers demonstrate the earliest onset of senescence, but they also have the highest total mortality rate, corroborating the predictions from the evolutionary theory of senescence. Production of eggs seems to be demanding for female Treecreepers, as egg size is also positively dependent on ambient temperature, further stressing the vulnerability of this trait for small reductions in female performance.

Influence of pea lectin expressed transgenically in oilseed rape (Brassica napus) on adult pollen beetle (Meligethes aeneus)

Abstract:Adult pollen beetles were exposed to three transgenic lines of Brassica napus L. expressing pea lectin up to 0.7% of total soluble protein in anthers. Two lines not expressing the pea lectin were used as controls. Adult responses were recorded in two experiments where beetles were fed either racemes or anthers. There was no effect of plant line on feeding rate, weight change or oviposition rate. A slightly reduced egg size was recorded when beetles were feeding on one of the plant lines with high concentration of pea lectin. Previous studies on the effect of pea lectin on pollen beetles have only focused on larval performance. The results of this study suggest that pea lectin has no adverse effect on the pest in its most damaging life stage.

Mothers adjust egg size to helper number in a cooperatively breeding cichlid

Mothers should adjust the size of propagules to the selective forces to which these offspring will be exposed. Usually, a larger propagule size is favored when young are exposed to high mortality risk or conspecific competition. Here we test 2 predictions on how egg size should vary with these selective agents. When offspring are cared for by parents and/or alloparents, protection may reduce the predation risk to young, which may allow mothers to invest less per single offspring. In the cooperatively breeding cichlid Neolamprologus pulcher, brood care helpers protect group offspring and reduce the latters' mortality rate. Therefore, females are expected to reduce their investment per egg when more helpers are present. In a first experiment, we tested this prediction by manipulating the helper number. In N. pulcher, helpers compete for dispersal opportunities with similar-sized individuals of neighboring groups. If the expected future competition pressure on young is high, females should increase their investment per offspring to give them a head start. In a second experiment, we tested whether females produce larger eggs when perceived neighbor density is high. Females indeed reduced egg size with increasing helper number. However, we did not detect an effect of local density on egg size, although females took longer to produce the next clutch when local density was high. We argue that females can use the energy saved by adjusting egg size to reduced predation risk to enhance future reproductive output. Adaptive adjustment of offspring size to helper number may be an important, as yet unrecognized, strategy of cooperative breeders.

How Does Metabolic Rate Scale With Egg Size? An Experimental Test With Sea Urchin Embryos

The consequences of changes in egg size for the development of marine invertebrates have been the subject of much theoretical and experimental work. Models that explore larval developmental modes in the context of maternal investment per offspring are often couched in an energetic framework, but the relationships between egg size and the energetics of larval development are poorly understood. We used blastomere separations to examine how experimental reductions in egg size affected (1) larval metabolic rate and (2) larval resistance to starvation. We found that separating blastomeres at the 2- and 4-cell stage resulted in average reductions of 50% and 75%, respectively, in larval metabolic rate. This suggests that, in an experimental context, mass-specific metabolic rate does not change with egg size. We also found that a 50% reduction in egg volume did not reduce the resistance of larvae to starvation when particulate food was withheld. This suggests that the material supplied to larvae in the egg is used primarily for construction of the larval body, rather than as a buffer against starvation or as a means of reducing reliance on exogenous fuel to sustain maintenance metabolism.

Experimental (Antiestrogen‐Mediated) Reduction in Egg Size Negatively Affects Offspring Growth and Survival

The relationship between egg size and offspring phenotype is critical to our understanding of the selective pressures acting on the key reproductive life-history traits of egg size and number. Yet there is surprisingly little empirical evidence to support a strong, positive relationship between egg size and offspring quality (i.e., offspring growth, condition, and survival) in birds, in part because of confounding effects of parental quality and the lack of experimental techniques for directly manipulating avian egg size independently of maternal condition. Previously, we showed that treatment of laying female zebra finches (Taeniopygia guttata) with the antiestrogen tamoxifen can decrease egg size by ca. 8% but that this reduction in egg size had few effects on offspring mass and size at fledging. Here, we extend the use of this technique to induce larger decreases in egg size (up to 50% in individual females) and show that a reduction in egg size of ca. 18% is associated with decreased embryo viability, increased hatchling mortality, and lower posthatching offspring survival. Furthermore, we show that although hatchlings from eggs reduced in size by ca. 9% can survive to fledging, these chicks show slower initial growth during the linear growth phase (5-10 d of age), fledge at lower masses than chicks from control eggs, and show postfledging compensatory growth. Our results provide empirical support for significant effects of egg size on offspring quality and further suggest that among individual females there is a minimum egg size required to maintain embryo viability and offspring quality.

Effects of egg size reduction and larval feeding on juvenile quality for a species with facultative-feeding development

In free-spawning marine invertebrates, larval development typically proceeds by one of two modes: planktotrophy (obligate larval feeding) from small eggs or lecithotrophy (obligate non-feeding) from relatively large eggs. In a rare third developmental mode, facultative planktotrophy, larvae can feed, but do not require particulate food to complete metamorphosis. Facultative planktotrophy is thought to be an intermediate condition that results from an evolutionary increase in energy content in the small eggs of a planktotrophic ancestor. We tested whether an experimental reduction in egg size is sufficient to restore obligate planktotrophy from facultative planktotrophy and whether the two sources of larval nutrition (feeding and energy in the egg) differentially influence larval survival and juvenile quality. We predicted, based on its large egg size, that a reduction in egg size in the echinoid echinoderm Clypeaster rosaceus would affect juvenile size but not time to metamorphosis. We reduced the effective size of whole (W) zygotes by separating blastomeres at the two- or four-cell stages to create half- (H) or quarter-size (Q) “zygotes” and reared larvae to metamorphosis, both with and without particulate food. Larvae metamorphosed at approximately the same time regardless of food or egg size treatment. In contrast, juveniles that developed from W zygotes were significantly larger, had higher organic content and had longer and more numerous spines than juveniles from H or Q zygotes. Larvae from W, H and Q zygotes were able to reach metamorphosis without feeding, suggesting that the evolution of facultative planktotrophy in C. rosaceus was accompanied by more than a simple increase in egg size. In addition, our results suggest that resources lost by halving egg size have a larger effect on larval survival and juvenile quality than those lost by withholding particulate food.

Genetic relationships between development of insecticide resistance and reduction of egg size as a negative effect on the fitness of the diamondback moth Plutella xylostella (Lepidoptera: Yponomeutidae)

Reciprocal crosses between susceptible (SS) and resistant (RR) lines to fenvalerate were conducted to determine whether or not there are any genetic relationships between insecticide resistance and reduction in egg size, as well to whether or not they affect the offspring fitness of the diamondback moth, Plutella xylostella. The dose-mortality regression lines of the F1 hybrid of susceptible females and resistant males (SR) and resistant females and susceptible males (RS) were roughly intermediate between those of parental lines, i.e., SS and RR. This result indicates that the mode of inheritance of fenvalerate resistance was neither complete dominance nor recessive, with no sex-linkage. The egg size of RR lines was significantly smaller than that of the SS lines. The RS and SR lines have an egg size close to that of their own matrilineage. We considered that the mode of inheritance of resistance and that of egg size were different from each other, although the selection experiment showed that there is a negatively correlated response between these parameters. The survival rate of the RR lines was significantly lower than that of the SS lines. However, the survival rate of the RS lines did not differ from that of the SR lines. Reproductive fitness is discussed with regard to different egg sizes among parental lines and reciprocal crosses.

PLACENTAL TRANSFER OF NUTRIENTS DURING GESTATION IN AN ANDEAN POPULATION OF THE HIGHLY MATROTROPHIC LIZARD GENUS MABUYA (SQUAMATA: SCINCIDAE)

Skinks of Mabuya genus exhibit the most specialized allantoplacenta among squamates (type IV), and the greatest degree of placentotrophy known in Reptilia. They ovulate microlecithal eggs (1-2 mm) that lack fatty yolk platelets; thus, it is suggested that virtually all of the nutrients for embryonic development are obtained by placental means. To test this inference, the net uptake of nutrients during gestation in an Andean population of Mabuya was quantified and compared with other oviparous and viviparous lizards, matrotrophic skinks, and eutherian mammals. Ionic, protein and lipid contents of recently ovulated eggs and neonates were measured. A significant net uptake of water, ions (calcium, potassium, sodium, magnesium), lipids, nitrogen (an index of protein), and dry matter was observed during development. Thus, in Mabuya the drastic reduction of egg size is related to the great reduction in the contribution of lecitotrophic nutrients to the embryo, an obligatory placentotrophy from early developmental stages, and the highest placental complexity known in Reptilia. All of these features converge with similar features found in eutherian mammals. Both clades evolved similar reproductive patterns and morphological features in their complex chorioallantoic placentae, which supplies all the nutrients for embryonic and fetal development.

Effects of metals on condition and reproductive output of the smooth toadfish in Sydney estuaries, south-eastern Australia

This study determined the condition and reproductive output of a common estuarine toadfish, Tetractenos glaber, in two metal contaminated and two reference estuaries near Sydney, Australia. Female toadfish from metal contaminated estuaries were smaller and younger than in reference estuaries; however, it could not be resolved whether these differences were due to direct effects of metal contamination or differences in nutritional value of prey. Lipid content in liver and gonad tissues was inversely related with levels of As, Pb, Cd and Co in sediment. In contrast, protein content in liver, gonad and muscle tissues was positively related to sediment levels of Ni and Co. Increased levels of Pb in gonads were associated with decreased oocyte diameter and density. This suggests a reduction in egg size and fecundity, which ultimately may lead to a decline in female reproductive output. Changes in fish health and reproduction caused by chemical pollutants may alter fish population and community structure.

The effect of dietary conjugated linoleic acid on egg yolk fatty acids and hatchability in Japanese quail

Conjugated linoleic acid (CLA) increased the ratio of saturated fatty acids to monounsaturated fatty acids in yolk and caused embryo mortality. Our preliminary studies showed that CLA had less of an effect on hatchability of quail than chickens. Hence, the objective was to determine the effects of dietary CLA on quail egg fatty acid content and hatchability. Eight male-female Japanese quail pairs per group were randomly assigned to diets containing 0 (canola oil; CO), 0.25, 0.5, 1, 2, or 3% CLA for 8 wk. Eggs were collected, held at 15 degrees C for 24 h, and then incubated. Three eggs from each group were collected for fatty acid analysis on the 45th day. At the end of the 8 wk, all quail were euthanized. Liver samples from female quail were obtained for fatty acid analysis. Diet containing 3, 2, or 1% CLA caused 100% embryo mortality after 6, 10, or 12 d of feeding, whereas overall hatchabilities in groups 0, 0.25, and 0.5 were 84, 86, and 64%, respectively. As the dietary CLA increased, egg and hepatic CLA increased, C16:0 increased and C16:1(n-7) and C18:1(n-9) decreased, whereas C18:0 remained unchanged. Diets containing 1, 2, or 3% CLA decreased the C20:4(n-6) levels in yolk (significantly) and liver (inconsistently) lipids. Yolk CLA levels from 0, 0.25, 0.5, 1, 2, and 3% CLA were 0.31, 0.90, 1.48, 2.44, 5.88, and 11.2%, respectively. The ratios of C16:0/C16:1(n-7) in yolks from groups fed 0, 0.25, 0.5, 1, 2, or 3% CLA were 8.2, 16.3, 20.4, 24.6, 26.1, and 28.6, respectively. The ratios of C18:0/C18:1(n-9) in yolks from hens fed 0, 0.25, 0.5, 1, 2, or 3% CLA were 0.28, 0.40, 0.48, 0.49, 0.69, and 0.83, respectively. Quail fed 0.25% CLA had increased egg size, whereas quail fed 2 or 3% had reduced egg size compared with those fed CO. Liver sizes (%) in all of the groups were increased, except for the group fed 0.25% CLA. These data suggest that CLA may affect hatchability possibly by changing the fatty acid composition of the yolk.

Timing of reproduction and egg quality covary with temperature in the insectivorous Barn Swallow,Hirundo rustica

SummaryEgg size and composition can be predicted to trade‐off against current condition of reproducing females. Adverse conditions for reproduction should thus give rise to a reduction in egg size and quality.Here we analyse timing of reproduction, egg mass and hatchability, and concentration of maternal carotenoids and components of the immune system in the eggs in relation to ambient temperature during the breeding season for the Barn Swallow (Hirundo rustica, Linnaeus).Relatively high temperatures during one day enhanced the probability that clutches were initiated the following days. Egg mass at laying, concentration of lutein, the main carotenoid in Barn Swallows, and lysozyme, a major maternal component of innate immunity in eggs, but not maternal antigen‐specific antibodies, covaried with temperature during the days preceding laying of individual eggs.Egg mass but not laying order or temperature during incubation predicted hatchability of the eggs.This study suggests that temperature affects not only egg mass, as previously shown for bird species, but also concentration of maternally derived substances, such as antioxidants and immune factors, that can affect egg hatchability and offspring antiparasite defence and viability.

Effects of reducing dietary protein, methionine, choline, folic acid, and vitamin B12 during the late stages of the egg production cycle on performance and eggshell quality

A series of four experiments was conducted to determine whether-shell quality during the late stages of egg production can be improved by using diets that are effective in reducing egg size. The experiments involved dietary manipulation of protein, methionine, choline, folic acid, and vitamin B12. In experiment 1, reducing dietary protein in combination of reducing the dietary methionine and choline or this diet without supplemental folic acid and vitamin B12 resulted in reduced egg weight and improved shell quality. However, egg production also was drastically reduced. In experiment 2, reducing the dietary level of methionine, without adding supplemental choline, folic acid, and vitamin B12 reduced egg size and improved shell quality, but egg production was reduced as well. In this experiment reducing the dietary methionine without supplemental folic acid and vitamin B12 reduced egg size and improved shell quality with no adverse effect on egg production. In experiment 3, reducing the dietary level of methionine and choline or reducing the dietary level of choline, folic acid, and vitamin B12 reduced egg size and improved shell quality without adverse effects on egg production. On the other hand, reducing dietary methionine, folic acid, vitamin B12, and supplemental choline reduced egg weight and improved shell quality but lowered egg production. In experiment 4, reducing dietary methionine together with reducing choline and vitamin B12 reduced egg size and improved shell quality with no adverse effect on egg production. The results of this series of experiments generally indicate that certain manipulations of the combination of methionine, choline, folic acid, and vitamin B12 have the potential to reduce egg weight and improve shell quality without affecting egg production during the latter stages of the egg production cycle.

Intraspecific competition and density dependence in an Ephestia kuehniella–Venturia canescens laboratory system

A model host‐parasitoid system of Ephestia kuehniella and Venturia canescens was used to examine the influence of host and parasitoid density on host and parasitoid life‐history parameters via a two‐way factorial experimental design (5 initial host densities×3 parasitoid densities). In the absence of parasitoids, E. kuehniella experienced scramble‐type competition with reduced growth, diminished adult size and a subsequent fecundity trade‐off for mortality. The mortality that did occur was confined to the late larval and pupal stages. In the presence of parasitoids attacking the late larval stage, competition changed from scramble for food to contest for enemy‐free space, with hosts escaping parasitism being small with low fecundity and reduced egg size, and with parasitoid adult size inversely dependent on host density. Total insect emergence (host+parasitoid), a measure of the influence of host resource competition on survivorship, exhibited a threshold effect as a function of initial host density; the threshold value was increased to a higher initial host density in the presence of parasitoids. Models of host self‐limitation were fitted to the data, with the generalized Beverton‐Holt model that incorporates a threshold effect providing the best fit, and the Ricker model with no threshold providing a very poor fit to the data.

Drinking water temperature effects on laying hens subjected to warm cyclic environments

Two experiments were conducted to evaluate the effects of drinking water temperature (Tw) on laying hens subjected to warm cyclic air temperature (Ta) conditions. Each experiment consisted of a 1-wk acclimation under thermoneutrality (TN) (Ta = Tw = 21 C), a 4-wk heat exposure or treatment period, and a 2-wk recovery under TN. Each experiment involved 24 individually caged hens at the initial age of 29 wk (Experiment 1) or 30 wk (Experiment 2). In Experiment 1, Tw of 18 or 27 C was provided to 12 birds per Tw regimen under a diurnal Ta of 27 to 35 C (daily mean of 31 C). In Experiment 2, Tw of 15, 19, 23, or 27 C was provided to six birds per Tw regimen under a diurnal Ta of 27 to 38 C (daily mean of 32.5 C). Experiment 1 showed that Tw of 18 C enhanced hourly and daily feed and water intake during the first 2 wk of heat exposure, as compared with Tw of 27 C. Experiment 2 showed that daily feed and water intake were greatest for hens in the 23 C Tw regimen and least for hens in the coolest or warmest Tw regimens. Reduction in daily feed intake with increase in daily mean Ta ranged from 2.0 to approximately 3.2 g/C-day (first week of heat exposure) to 1.1 to approximately 1.9 g/C-day (fourth week of heat exposure). Water to feed intake ratio was 1.8 to approximately 2.0 during acclimation and recovery, but increased to 3.0 to approximately 3.4 during heat exposure. Internal egg quality parameters were in general unaffected by Tw. The two warmer Tw regimens in Experiment 2 had less reduction in egg size than did the two cooler Tw. In both experiments, hens displayed anticipatory increase in feed and water intake 2 to 3 h prior to lights-off. However, the stimulus of lights-on did not elicit a strong return to feed and water consumption as typically seen in broilers. The results revealed the potential existence of an optimal Tw range (near 23 C) for heat-chal lenged laying hens. Larger-scale tests are warranted to further verify the findings.

Effects of Porcine Follicle-Stimulating Hormone on the Reproductive Performance of Female Zebra Finches (Taeniopygia guttata)

It has been suggested that follicle-stimulating hormone (FSH) may play a role in egg size/number trade-offs in oviparous vertebrates. We tested this hypothesis in an avian species by administering porcine FSH (pFSH) to intact, captive female zebra finches (Taeniopygia guttata) during egg formation. We predicted that (1) pFSH would increase the number of ovarian follicles recruited into rapid yolk development and so increase clutch size, (2) an increase in clutch size would lead to a reduction in egg size, and (3) doses of pFSH that were not sufficient to increase clutch size would increase yolk deposition and so increase egg mass. Although a range of pFSH doses decreased egg mass by ca. 10% in three separate experiments, the reduction in egg mass occurred in the absence of an increase in the number of eggs laid. Porcine FSH decreased mean clutch size significantly in one experiment and reduced median clutch size significantly in the other two experiments. The results of this study did not support the hypothesis that FSH mediates a trade-off between egg size and clutch size in birds.

The evolution of egg size in socially polyandrous shorebirds

In classical and multi‐clutch polyandry, females lay multiple clutches during a breeding season for more than one mate. The production of multiple clutches may be energetically demanding. We used comparative analyses to investigate three possible ways of reducing such egg‐laying costs in polyandrous shorebirds: (1) reduction in egg size, (2) reduction in clutch size, and (3) evolutionary increase in female size. Paired comparisons of polyandrous and non‐polyandrous taxa showed that females of polyandrous shorebirds lay smaller eggs than females of closely related monogamous and polygynous species. Directional analyses corroborated this result by indicating a significant decrease in egg size after phylogenetically independent origins of polyandry. The comparative analyses uniformly rejected the two alternatives, i.e. neither clutch size nor female size is related to social mating pattern. We also tested and rejected three alternative explanations for reduced egg size in polyandrous taxa. First, we found no evidence that polyandrous females have evolved smaller egg sizes in response to selection to match smaller size of males, which provide the parental care in these species. Second, reduction in egg size was not related to longer breeding seasons (and hence more opportunity for re‐nesting). Third, reduced egg sizes were also not related to rates of clutch predation (another potential correlate of multiple clutch production). Our results are thus consistent with the hypothesis that selection for reducing laying costs explains small egg size in socially polyandrous shorebirds.

Migratory costs and the evolution of egg size and number in introduced and indigenous salmon populations.

The trade-off between reproductive investment and migration should be an important factor shaping the evolution of life-history traits among populations following their radiation into habitats with different migratory costs and benefits. An experimentally induced difference in migratory rigor for families of chinook salmon (Oncorhynchus tshawytscha), of approximately 86 km and 413 m elevation, exacted a cost to somatic energy reserves (approximately 17% reduction in metabolizable mass) and ovarian investment (13.7% reduction in ovarian mass). This cost was associated with a reduction in egg size and paralleled the phenotypic pattern of divergence between two introduced New Zealand populations of common origin, presently breeding at sites with different migration distances. The genetic pattern of divergence of these same populations, detected under common rearing, was consistent with compensation for migratory costs (the population that migrates farther invested more in ovarian mass), but egg number more than egg size was associated with this evolution. These evolutionary patterns are consistent with what is known of the inheritance of these traits and with trade-offs and constraints favoring initial evolution in offspring number over offspring size. Analysis of egg number-size patterns of other Pacific salmon populations in their native range supported the hypothesis that migration strongly influences patterns of reproductive allocation, favoring a higher ratio of egg number to egg size with greater migration distance.

Effects of Light Intensity from Photostimulation in Four Strains of Commercial Egg Layers: 2. Egg Production Parameters

The effects of light intensity (LI) from photostimulation to 45 wk of age on egg production parameters and egg size characteristics were examined in four layer strains. Floor housed pullets were raised in a light-tight facility from 1 d of age until housing in individually illuminated cages at 17 wk of age. At 17 wk of age, two white egg strains, ISA-White (ISA-W) and Shaver 2000 (S2000), and two brown egg strains, ISA-Brown (ISA-B) and Shaver 579 (S579), were assigned to a processing group [Group 1 was killed at sexual maturity (first oviposition); Group 2 was kept to 45 wk] and were photostimulated at 18 wk of age using a LI of 1, 5, 50, or 500 lx (4 × 4 factorial design). One bird from Group 1 and one bird Group 2 were caged together in individually lit cages (one brown and one white egg layer). Cages were equipped with hardware to monitor egg laying time. Data of individual egg weight and time of lay were kept on Group 2 birds until 45 wk of age. Egg production data were analyzed for hen-day production, laying sequence length, egg and egg component weights, time of lay, and egg interval time as related to strain or LI.Hen-day production was greater in brown egg strains (ISA-B = 86.7%, S579 = 88.1%) than in white egg strains (ISA-W = 83.4%, S2000 = 82.3%) and was reduced in birds under 1 lx compared to 5 or 50 lx. A LI of 1 lx resulted in reduced egg production and laying sequence length compared to birds with a 50 or 500 lx. The effects of LI were strain dependant, however. Postpeak sequence length and egg production declined at more rapidly under 500 lx compared to other LI in brown egg strains, indicating possible development of a photorefractory condition. Mean settable egg weight was lower in 500 lx birds (56.1 g) compared to other groups (mean = 57.9 g), reducing total egg mass produced. Mean interval between successive eggs in a sequence was lengthened in 1 lx birds compared to other LI groups. Mean time of lay was earlier in brown egg strains than in white egg strains by 48 min. Mean time of lay was shifted to occur later by an increasing LI.Light intensity affected sexual maturation and egg production, as layers had differential responses to lighting. LI of 1 lx and 500 lx were found to be limiting to the egg production efficiency of layers. Whereas the birds receiving 1 lx had a reduced rate of egg production, those receiving 500 lx had reduced egg size later in the production period in combination with reduced shell quality, which indicated that inadequate feed intake under high LI conditions may be a factor affecting layer stocks. Exposure to high LI reduced egg size and total egg mass produced. Ultimately, the brown egg strains appeared to be more susceptible to the negative effects of low or high LI, indicating the importance of matching management practices to the particular hen genotype.

Macrogeographic clines in fecundity, reproductive allocation, and offspring size of the forest tent caterpillar Malacosoma disstria

Summary1. The fecundity of the forest tent caterpillar varies considerably across its geographic range. Field data indicate that populations in the southern United States (Gulf States) produce nearly twice as many eggs as females from Canada or the Lake States, with little or no difference in the size of adult females.2. In controlled rearing experiments, female forest tent caterpillar from the southern United States (Louisiana) had much larger clutch sizes than same sized females from northern populations in Michigan or Manitoba, Canada. Increased fecundity in Louisiana females was achieved through a significant reduction in egg size and a concomitant increase in the allocation of resources to egg production.3. Comparison of 10 forest tent caterpillar populations spanning a 27° latitudinal gradient, validated the results of detailed comparisons among the three populations above by confirming the strong negative correlation between latitude and clutch size.4. Neonate forest tent caterpillars from Manitoba were significantly larger than larvae from either Michigan or Louisiana. Michigan larvae were intermediate in size. It is postulated that large neonates are advantageous in thermally limiting environments. More than three times as many degree‐days are available to Louisiana neonates during the first 2 weeks after hatching. A consistently favourable climate during the vulnerable post‐hatching period may have allowed the evolution of larger clutches at the expense of neonate size in southern populations.

Does the concept of spawning per recruit make sense?

Density dependence means that exploited fish populations exhibit earlier maturity, a faster growth rate, increased fecundity and reduced egg size. Here, the consequences of these effects on population dynamics, the estimation of spawning biomass per recruit and associated biological reference points are examined by a simulation model. The model is a self-regenerating model in which the population parameters (age at maturity, growth, fecundity, egg size) vary according to three classes of population abundance. Early life stages are characterized by a size-dependent growth and mortality model. It is concluded that spawning per recruit is an ambiguous concept because, if density dependence occurs in the adult population, the spawning biomass of a cohort is not proportional to the number recruited. This leads to significant level of uncertainty in the estimates of spawning biomass per recruit and the associated biological reference points such as F-low, F-med, F-high and VSPR.