Maternal nutritional condition and genetic differentiation affect brood size and offspring body size in Nicrophorus

In most animal species, brood size and body size exhibit some variation within and between populations. This is also true for burying beetles (genus Nicrophorus), a group in which the body size of offspring depends critically on the number of offspring competing for food due to the discrete nature of resource used for larval nutrition (vertebrate carcasses). In one species, brood size and body size are correlated with population density, and appear to be phenotypically plastic. We investigated potential proximate causes of between-population variation in brood size and body size in two species, Nicrophorus vespilloides and Nicrophorus defodiens. Our first experiment supported the notion that brood size is phenotypically plastic, because it was affected by environmental variation in adult nutritional condition. We found that the pre-breeding nutritional status of female N. vespilloides affected the number of eggs they laid, the number of surviving larvae in their broods, and the body size of their offspring. We do not know whether this plasticity is adaptive because greater offspring body size confers an advantage in contests over breeding resources, or whether starved females are constrained to produce smaller clutches because they cannot fully compensate for their poor pre-breeding nutritional status by feeding from the carcass. Our second experiment documents that brood size, specifically the infanticidal brood-size adjustment behavior, has undergone genetic differentiation between two populations of N. defodiens. Even under identical breeding conditions with identical numbers of first-instar larvae, females descended from the two populations produced broods of different size with corresponding differences in offspring body size.     

A sex-specific trade-off between mating preferences for genetic compatibility and body size in a cichlid fish with mutual mate choice

Mating preferences for genetic compatibility strictly depend on the interplay of the genotypes of potential partners and are therein fundamentally different from directional preferences for ornamental secondary sexual traits. Thus, the most compatible partner is on average not the one with most pronounced ornaments and vice versa. Hence, mating preferences may often conflict. Here, we present a solution to this problem while investigating the interplay of mating preferences for relatedness (a compatibility criterion) and large body size (an ornamental or quality trait). In previous experiments, both sexes of Pelvicachromis taeniatus, a cichlid fish with mutual mate choice, showed preferences for kin and large partners when these criteria were tested separately. In the present study, test fish were given a conflicting choice between two potential mating partners differing in relatedness as well as in body size in such a way that preferences for both criteria could not simultaneously be satisfied. We show that a sex-specific trade-off occurs between mating preferences for body size and relatedness. For females, relatedness gained greater importance than body size, whereas the opposite was true for males. We discuss the potential role of the interplay between mating preferences for relatedness and body size for the evolution of inbreeding preference.     

The trade-off between number and size of offspring in humans and other primates

Life-history theory posits a fundamental trade-off between number and size of offspring that structures the variability in parental investment across and within species. We investigate this 'quantity-quality' trade-off across primates and present evidence that a similar trade-off is also found across natural-fertility human societies. Restating the classic Smith-Fretwell model in terms of allometric scaling of resource supply and offspring investment predicts an inverse scaling relation between birth rate and offspring size and a (-1/4) power scaling between birth rate and body size. We show that these theoretically predicted relationships, in particular the inverse scaling between number and size of offspring, tend to hold across increasingly finer scales of analyses (i.e. from mammals to primates to apes to humans). The advantage of this approach is that the quantity-quality trade-off in humans is placed into a general framework of parental investment that follows directly from first principles of energetic allocation.     

Maternal effects, maternal body size and offspring energetics: a study in the common woodlouse Porcellio laevis

What are the consequences of the natural variation in maternal body mass on offspring energetic performance? How are performance traits related to thermal physiology and energetics phenotypically integrated on offspring? To answer these questions, fifty breeding pairs of the common terrestrial isopod Porcellio laevis were set up in the lab. Physiological performance, thermal tolerance and thermal sensitivity were measured in F1 adults. Maternal effects were estimated as: the direct influence of maternal body mass and the variation associated with mothers. Phenotypic integration was evaluated using path analysis. Our results show that: (1) maternal body size affects positively offspring long-term metabolism, (2) maternal variation was significant in many of the physiological traits and (3) there is an intricate set of relationships among traits and importantly, that offspring exhibited compensational strategies among metabolism, thermal sensitivity and thermal tolerance traits. Even if we cannot clearly state whether these maternal influences were because of the genes or the environment that mothers provided and thus no predictions can be done regarding their evolutionary consequences, it seems clear that the role of maternal effects on physiology can no longer be ignored. In this sense, there's a lot to be gained by incorporating explicit experimental protocols to test for maternal effects.     

Influence of brood size and offspring size on parental investment in a biparental cichlid fish,Neolamprologus moorii

Both males and females ofNeolamprologus moorii, a substrate-spawning cichlid fish in Lake Tanganyika, tend their eggs and fry. In this study, the influence of brood size and fry size on parental investment of this species was examined under natural conditions. Breeding parents intensively attacked fry-eating fishes that approached their broods. The attack rate by the parents against the approaching fry-eaters was positively correlated with brood size and negatively with fry size, but was much more strongly related to brood size. This suggests that the parental decision for brood defense appeared to be primarily determined by brood size. The reason for fry size being less important in the parental decision may be that even large fry have a low survival ability on their own under the high predation pressure in the study sites.     

Relationships between body size and abundance in ecology

Body size is perhaps the most fundamental property of an organism and is related to many biological traits, including abundance. The relationship between abundance and body size has been extensively studied in an attempt to quantify the form of the relationship and to understand the processes that generate it. However, progress has been impeded by the under appreciated fact that there are four distinct, but interrelated, relationships between size and abundance that are often confused in the literature. Here, we review and distinguish between these four patterns, and discuss the linkages between them. We argue that a synthetic understanding of size-abundance relationships will result from more detailed analyses of individual patterns and from careful consideration of how and why the patterns are related.     

Mothers influence offspring body size through post-oviposition maternal effects in the redbacked salamander, Plethodon cinereus

In the terrestrial salamander (Plethodon cinereus), previous work has shown that mothers body size is positively correlated to offspring size at the time of hatching even after controlling for the effects of egg size. This study was designed to determine whether maternal body size affects offspring size via pre-oviposition factors (e.g., yolk quality, jelly coat composition, or maternal genes) or post-oviposition factors (e.g., parental care behaviors, parental modification of environment). Gravid females were captured and induced to lay eggs in experimental chambers in which the environment was standardized. Fifteen clutches were exchanged, or cross-fostered, between female pairs differing in body size. Ten females whose eggs were taken away and then returned served as controls for the crossing treatment. Foster mothers did not significantly differ from control mothers in the time spent with eggs, body position, or number of egg movements during brooding. Average egg mass measured midway through development was not significantly correlated to the body size of either the genetic or foster mother, but was correlated to pre-oviposition oocyte size. At hatching, offspring body length was positively correlated to egg size and the foster mothers body size. This correlation suggests that in P. cinereus post-oviposition maternal effects have a greater impact on offspring size than other maternal factors incorporated into the egg prior to oviposition. While our study showed that larger mothers moved their eggs less often and tended to spend more time in contact with their eggs, further work needs to be done to identify the specific mechanisms through which larger mothers influence the body size of their offspring. This is the first experimental demonstration of post-oviposition maternal effects for any amphibian with parental care.     

Relationships between body size and biomass of aquatic insects

SUMMARY. Predictive equations were developed to estimate dry weight from body length measurements for forty-three taxa of aquatic insects. The inter-relationships between dry weight, body length and head capsule width of individuals grouped according to the eight major orders of aquatic insects were also examined. Regression analysis indicated that the relationship between biomass and body size was best expressed by a power equation, Y =_aX^b, rather than by linear or exponential equations. Changes in body length versus head capsule width were best expressed by linear equations, with three distinct relationships being observed. Body length estimated biomass better than head capsule width. Populations often species of insects collected from two different rivers generally did not differ significantly in their dry weight to body length relationships.     

Intergenerational effects of maternal birth season on offspring size in rural Gambia

Environmental conditions experienced in early life can influence an individual's growth and long-term health, and potentially also that of their offspring. However, such developmental effects on intergenerational outcomes have rarely been studied. Here we investigate intergenerational effects of early environment in humans using survey- and clinic-based data from rural Gambia, a population experiencing substantial seasonal stress that influences foetal growth and has long-term effects on first-generation survival. Using Fourier regression to model seasonality, we test whether (i) parental birth season has intergenerational consequences for offspring in utero growth (1982 neonates, born 1976-2009) and (ii) whether such effects have been reduced by improvements to population health in recent decades. Contrary to our predictions, we show effects of maternal birth season on offspring birth weight and head circumference only in recent maternal cohorts born after 1975. Offspring birth weight varied according to maternal birth season from 2.85 to 3.03 kg among women born during 1975-1984 and from 2.84 to 3.41 kg among those born after 1984, but the seasonality effect reversed between these cohorts. These results were not mediated by differences in maternal age or parity. Equivalent patterns were observed for offspring head circumference (statistically significant) and length (not significant), but not for ponderal index. No relationships were found between paternal birth season and offspring neonatal anthropometrics. Our results indicate that even in rural populations living under conditions of relative affluence, brief variation in environmental conditions during maternal early life may exert long-term intergenerational effects on offspring.     

Flavobacterium psychrophilum in Baltic salmon Salmo salar brood fish and their offspring.

Baltic salmon brood fish were investigated for the presence of Flavobacterium psychrophilum in the kidney, spleen, brain and sexual products (ovarian fluid, unfertilised eggs and milt). Samples for bacteriology were taken at capture, when the fish were ascending their native river to spawn, and after a period of captivity in indoor pools, at stripping. During captivity, abnormal wiggling behaviour was recorded in some of the fish. Bacterial samples were taken to determine if F. psychrophilum had any role in the aetiology of the condition. Furthermore, the presence of F. psychrophilum on egg surfaces during incubation was investigated. F. psychrophilum was isolated from internal organs and/or sexual products in 7 out of 50 (14.0%) fish sampled at capture and 63 out of 272 (23.2%) fish sampled at stripping. The bacteria was isolated from either spleen or gonads in 2 out of 19 (10.5%) fish with abnormal wiggling behaviour but no bacteria was isolated from the brain. No F. psychrophilum was isolated from eggs at the eyed stage. Just before hatching, the bacterium was isolated from 5 out of 15 (33.3%) family groups. The present study shows that Baltic salmon brood fish are carriers of F. psychrophilum during their spawning migration. The presence of the bacteria in sexual products from both females and males indicates that transmission from the brood fish to the offspring should be considered an important route of infection.     

Direct genetic and postnatal maternal genetic effects on body composition in mice selected for body weight.

Line crossfostering techniques were used to study differences among selected and control lines of mice in direct genetic and postnatal maternal genetic influences on preweaning (day 12) body weight and composition. The lines were selected for high (H6) and low (L6) 6-week body weight and the control line (C2) was maintained by random selection. There were positive correlated responses to selection in both direct genetic and postnatal maternal genetic effects on body weight and weights of all body components (P less than 0.01) except for water and ash weight in H6. The correlated responses in postnatal maternal genetic effects were of the same order of magnitude as those in direct genetic effects. Correlated responses were greater in L6 than in H6. Correlated responses in direct genetic effects were positive (P less than 0.01) for water percent in H6 and ether extract percent in L6, and negative (P less than 0.01) for water percent and lean percent in L6. Correlated responses in postnatal maternal genetic effects were positive for ether extract percent and negative for water percent (P less than 0.01). Correlated responses were far greater in L6 than in H6 and were greater for postnatal maternal genetic effects than for direct genetic effects. Analyses of covariance results indicated line differences in the relative growth rates of the body components.     

Disentangling prenatal and postnatal maternal genetic effects reveals persistent prenatal effects on offspring growth in mice

Mothers are often the most important determinant of traits expressed by their offspring. These "maternal effects" (MEs) are especially crucial in early development, but can also persist into adulthood. They have been shown to play a role in a diversity of evolutionary and ecological processes, especially when genetically based. Although the importance of MEs is becoming widely appreciated, we know little about their underlying genetic basis. We address the dearth of genetic data by providing a simple approach, using combined genotype information from parents and offspring, to identify "maternal genetic effects" (MGEs) contributing to natural variation in complex traits. Combined with experimental cross-fostering, our approach also allows for the separation of pre- and postnatal MGEs, providing rare insights into prenatal effects. Applying this approach to an experimental mouse population, we identified 13 ME loci affecting body weight, most of which (12/13) exhibited prenatal effects, and nearly half (6/13) exhibiting postnatal effects. MGEs contributed more to variation in body weight than the direct effects of the offsprings' own genotypes until mice reached adulthood, but continued to represent a major component of variation through adulthood. Prenatal effects always contributed more variation than postnatal effects, especially for those effects that persisted into adulthood. These results suggest that MGEs may be an important component of genetic architecture that is generally overlooked in studies focused on direct mapping from genotype to phenotype. Our approach can be used in both experimental and natural populations, providing a widely practicable means of expanding our understanding of MGEs.     

Relationships between brood size and parental provisioning performance in chinstrap penguins during the chick guard phase

 Chinstrap penguins (Pygoscelis antarctica) normally lay two eggs, but brood size is often reduced by mortality during incubation or after hatching. We hypothesized that this variation in brood size would affect the parents’ foraging behavior and their chick provisioning performance. We studied patterns of adult foraging trip duration and frequency, food load delivery, and chick growth rates in relation to brood size during the guard phase in four breeding seasons (1991–1994) on Seal Island, Antarctica. Within a given year, parents with two chicks made more frequent foraging trips to sea and may have transported larger food loads to the nest; however, the duration of foraging trips was unrelated to brood size. Overall, parents with two chicks spent ∼15% more time at sea than parents with only one chick. Both the frequency and duration of foraging trips varied between years. Foraging trip duration may partly reflect the birds’ foraging radius, which probably varies with time in response to shifts in krill distribution. Chick growth rate varied betwen years, but was related to brood size only in 1992, when chicks from two-chick broods grew significantly more slowly than chicks from one-chick broods. Food loads transported to chicks, as well as chick growth rates, were highest in 1994, when concurrent hydroacoustic studies indicated that regional krill biomass was severely depressed. This apparent anomaly suggests that the spatial scale of the krill survey may have been too coarse to detect some high-density krill aggregations within the penguins’ foraging range. Received: 26 September 1995 / Accepted: 12 May 1996     

Relationships of latitude,altitude, and body size to litter size and mean annual production of offspring inPeromyscus

Litter size was positively correlated with latitude and altitude but not with production of offspring or with body size in Peromyscus. Increased litter size in northern populations probably reflects shortening the breeding season by climate and not a greater mortality rate at northern latitudes compared to southern latitudes. Production of offspring was negatively correlated with body size but not with latitude, altitude, or litter size. This is probably due to larger species living longer and taking longer to mature.     

A model for optimal offspring size in fish, including live-bearing and parental effects

Since Smith and Fretwell's seminal article in 1974 on the optimal offspring size, most theory has assumed a trade-off between offspring number and offspring fitness, where larger offspring have better survival or fitness, but with diminishing returns. In this article, we use two ubiquitous biological mechanisms to derive the shape of this trade-off: the offspring's growth rate combined with its size-dependent mortality (predation). For a large parameter region, we obtain the same sigmoid relationship between offspring size and offspring survival as Smith and Fretwell, but we also identify parameter regions where the optimal offspring size is as small or as large as possible. With increasing growth rate, the optimal offspring size is smaller. We then integrate our model with strategies of parental care. Egg guarding that reduces egg mortality favors smaller or larger offspring, depending on how mortality scales with size. For live-bearers, the survival of offspring to birth is a function of maternal survival; if the mother's survival increases with her size, then the model predicts that larger mothers should produce larger offspring. When using parameters for Trinidadian guppies Poecilia reticulata, differences in both growth and size-dependent predation are required to predict observed differences in offspring size between wild populations from high- and low-predation environments.     

Size-number trade-off and optimal offspring size for offspring produced sequentially using a fixed amount of reserves

We analysed the nature of size-number trade-off of offspring when multiple cohorts of such offspring are produced sequentially using a fixed amount of reserves. In the model, we incorporated sink-limitation in the resource absorption rate of offspring from the mother tissue and the loss of resources by maintenance respiration. We found that the later the initiation of a cohort, the greater the cost of producing a cohort with the same size and number of offspring. This is due to the loss of resources by maintenance respiration during the period from the beginning of reproduction to the initiation of the cohort. Also, the extra cost increases with an increase in the specific maintenance respiration rate. Thus, resources lost to respiration over time reduces the fitness value of producing late cohorts. Hence, it is advantageous to produce all offspring simultaneously unless there are fitness advantages of producing offspring sequential which overcome this cost or constraints preventing simultaneous production. Sequentially offspring production evolves if there is a constraint on the number of offspring of each cohort. With this constraint, the optimal offspring size decreases with the production sequence of cohorts.     

Growing through predation windows: effects on body size development in young fish

The degree to which growth in early life stages of animals is regulated via density‐dependent feedbacks through prey resources is much debated. Here we have studied the influence of size‐ and density‐dependent mechanisms as well as size‐selective predation pressure by cannibalistic perch Perca fluviatilis on growth patterns of young‐of‐the‐year (YOY) perch covering several lakes and years. We found no influence of initial size or temperature on early body size development of perch. In contrast, there was a negative relationship between reproductive output and the length of YOY perch at five weeks of age. However, rather than an effect of density‐dependent growth mediated via depressed resources the relationship was driven by positive size‐selective cannibalism removing large individuals. Hence, given a positive correlation between the density of victims and predation pressure by cannibals, size‐dependent interactions between cannibals and their victims may wrongly be interpreted as patterns of density‐dependent growth in the victim cohort. Overall, our results support the view that density‐dependent resource‐limitation in early life stages is rare. Still, patterns of density‐dependent growth may emerge, but from variation in size‐selective predation pressure rather than density as such. This illustrates the importance of taking overall population demography and predatory interactions into account when studying growth patterns among recruiting individuals.     

Bigger is better: changes in body size explain a maternal effect of food on offspring disease resistance

Maternal effects triggered by changes in the environment (e.g., nutrition or crowding) can influence the outcome of offspring–parasite interactions, with fitness consequences for the host and parasite. Outside of the classic example of antibody transfer in vertebrates, proximate mechanisms have been little studied, and thus, the adaptive significance of maternal effects on infection is not well resolved. We sought to determine why food‐stressed mothers give birth to offspring that show a low rate of infection when the crustacean Daphnia magna is exposed to an orally infective bacterial pathogen. These more‐resistant offspring are also larger at birth and feed at a lower rate. Thus, reduced disease resistance could result from slow‐feeding offspring ingesting fewer bacterial spores or because their larger size allows for greater immune investment. To distinguish between these theories, we performed an experiment in which we measured body size, feeding rate, and susceptibility, and were able to show that body size is the primary mechanism causing altered susceptibility: Larger Daphnia were less likely to become infected. Contrary to our predictions, there was also a trend that fast‐feeding Daphnia were less likely to become infected. Thus, our results explain how a maternal environmental effect can alter offspring disease resistance (though body size), and highlight the potential complexity of relationship between feeding rate and susceptibility in a host that encounters a parasite whilst feeding.