Reproductive Allocation Patterns Research Articles

The spatial arrangement of sexes is related to reproductive allocation in mosses: a comparative study of reproductive allocation in three different monoicous sexual systems.

We examined the relationship between reproductive allocation and vegetative growth in three monoicous sexual systems of bryophytes. The sexual systems show a gradient of increasing distance between the sexes, from gonioautoicous to cladautoicous to rhizautoicous. Here, we investigated the following two hypotheses: (1) reproductive allocation differs between sexes and sexual systems, and male reproductive allocation increases with increasing distance between male and female gametangia; and (2) reproductive allocation is negatively related to vegetative growth. We sampled the three sexual systems, represented by three moss species of the genus Fissidens in the Atlantic Forest of Southeastern Brazil. Ramets were washed in the laboratory; the reproductive structures were detached from the vegetative ramets and sorted regarding sex and individual, dried at 70 °C for 72 h, and weighed in an ultramicrobalance. We calculated the mean reproductive and vegetative mass and reproductive allocation and used generalized linear models to test our predictions. Reproductive allocation differed between species and sexes. It was higher in the rhizautoicous than in the cladautoicous and gonioautoicous species. Mean reproductive allocation was greater in males than in females of the rhizautoicous species, greater in females than males of the cladautoicous species, and did not differ between the sexes in the gonioautoicous species. Estimates of reproductive and vegetative mass were positively related in females of the rhizautoicous species. Vegetative mass was not related to reproductive allocation in the gonioautoicous species, but negatively related to reproductive allocation in the male and female branchlets of the cladautoicous species and in the female ramets of the rhizautoicous species. The reproductive allocation patterns differ between the rhizautoicous species and the 'truly' monoicous species, with shorter intersexual distances, which implies that our hypotheses were supported only in part. We suggest that the hypotheses should be reformulated and tested further by comparing 'truly' monoicous species with dioicous species and by including other genera.

Bigger Is Better, Sometimes: The Interaction between Body Size and Carcass Size Determines Fitness, Reproductive Strategies, and Senescence in Two Species of Burying Beetles

The cost of reproduction hypothesis suggests that allocation to current reproduction constrains future reproduction. How organisms accrue reproductive costs and allocate energy across their lifetime may differ among species adapted to different resource types. We test this by comparing lifetime reproductive output, patterns of reproductive allocation, and senescence between two species of burying beetles, Nicrophorus marginatus and N. guttula, that differ in body size, across a range of carcass sizes. These two species of burying beetles maximized lifetime reproductive output on somewhat different–sized resources. The larger N. marginatus did better on large and medium carcasses while the smaller N. guttula did best on small and medium carcasses. For both species, reproduction is costly and reproduction on larger carcasses reduced lifespan more than reproduction on smaller carcasses. Carcass size also affected lifetime reproductive strategies. Each species’ parental investment patterns were consistent with terminal investment on carcasses on which they performed best (optimal carcass sizes). However, they exhibited reproductive restraint on carcass sizes on which they did not perform as well. Reproductive senescence occurred largely in response to carcass size. For both species, reproduction on larger carcasses resulted in more rapid senescence. These data suggest that whether organisms exhibit terminal investment or reproductive restraint may depend on type and amount of resources for reproduction.

Intraspecific variation of reproductive traits between migratory and resident populations of the rice plant skipper Parnara guttata guttata

Much empirical data obtained across taxa is necessary to evaluate how variation in migratory costs and benefits can play a role in shaping the phenotypic and genetic patterns of reproductive investment between populations. Reproductive resources are allocated to both the size and number of eggs, and females may vary their allocation to each of these traits in response to increased movement capacity. In central regions of Japan, the rice plant skipper Parnara guttata guttata migrates in a southwesterly direction every year during late summer and early autumn (‘migratory’ population), but this migration is not observed in populations in southern or parts of western Japan (‘resident’ population). If migratory costs and benefits vary between these populations, they may cause their reproductive strategies to differ. Moreover, migration combined with a shift of reproductive allocation that evolved in one generation may influence the flight ability and patterns of reproductive allocation in the other generation in which migration has not evolved. In this study, a common-garden experiment was conducted to compare reproductive traits (egg size and number of eggs), flight ability and body size between these populations under two daylength regimes, at which different forms of adult P. g. guttata emerge. Under both daylength regimes the female adults of migratory populations had greater flight ability and tended to lay a greater number of eggs, which were on average smaller than the eggs laid by resident population females. A population-level difference in reaction norm elevation for flight ability and number of eggs was detected, but no interactions between population and daylength. The effects of the energetic cost of flight, adaptive constraints (e.g., genetic correlations and trade-offs), and the optimal strategy may explain the difference between populations of this species in the patterns of reproductive allocation to egg size and number of eggs.

Soil biocrusts reduce seed germination and contribute to the decline in Artemisia ordosica Krasch. shrub populations in the Mu Us Sandy Land of North China

The sand-stabilising, xeric shrub Artemisia ordosica Krasch. has experienced widespread decline decades after being widely distributed through seeding in the Mu Us Sandy Land in China. The role of biocrusts in the decline of A. ordosica populations has received attention but has not been directly confirmed. To clarify the effects of biocrusts on shrub decline, a field study was conducted with different biocrust treatments (bare sand, lichen biocrust, moss biocrust, biocrust removed) in August 2019 in the Mu Us Sandy Land in North China. In each biocrust treatment, the age structure, dynamic indices, and reproductive allocation patterns of the A. ordosica population were determined, in addition to soil characteristics. A separate laboratory study was conducted to determine the effects of soil moisture and nutrients on seed germination. With the development of biocrust, the age structure of A. ordosica populations began to decline, but when the biocrust was removed, A. ordosica seedlings re-established. Soil moisture and nutrient contents were higher under lichen biocrust (earlier stage of biocrust development) and biocrust removal than under moss biocrust (later stage of biocrust development). In response to the adverse conditions under biocrust, A. ordosica increased reproductive output while reducing vegetative growth, thereby increasing the possibility for offspring dispersal and colonisation of new habitats. By contrast, with biocrust removal, A. ordosica reduced reproductive allocation. In addition, with the development of biocrust, soil moisture was reduced, and seed germination was limited. Thus, biocrusts are important in the decline of A. ordosica in sandy lands because of reductions in seed germination, which further suggests that moderate removal of biocrusts to improve seed germination can be key in the sustainable development of sandy land vegetation.

Effects of Water Addition on Reproductive Allocation of Dominant Plant Species in Inner Mongolia Steppe.

Extreme events such as extreme drought and precipitation are expected to increase in intensity and/or duration in the face of climate change. Such changes significantly affect plant productivity and the biomass allocation between reproductive and vegetation organs (i.e., reproductive allocation). Our aims are to test the effects of water addition on the trade-offs in allocation of plant biomass and whether such effects are modified by species. A manipulative experiment was conducted from May 2000 to October 2001, where four dominant plant species (i.e., Leymus chinensis, Stipa grandis, Artemisia frigida, and Potentilla acaulis) in the Inner Mongolia steppe in China were treated with 8 levels of water addition. Results demonstrated that water addition significantly affected the reproductive allocation of plants, and such effects were modified by species. Specifically, with increasing water availability, L. chinensis was not impacted, while A. frigida allocated more biomass to reproductive organs than to vegetative organs, while such allocation in S. grandis and P. acaulis first decreased, and then increased after reaching a peak. Our results indicated that plant species can adjust their reproductive allocation patterns to deal with water availability gradients. Climatic factors such as rainfall and temperature usually co-appearing, thus future research should explore the joint effects of several climate change factors on grasslands in order to maintain the health and sustainability of grasslands.

Soil temperature drives elevational patterns of reproductive allometry in a biodiversity hotspot

Understanding the geographic patterns of reproductive allocation helps in clarifying the selective forces that shape the reproductive strategies of plants. However, studies on the elevational patterns of reproductive allocation remain limited. Moreover, although soil attributes have long been suspected to drive elevational patterns of reproductive allocation, few studies have explored this relationship. Delaying reproduction and allocating a high proportion of biomass to vegetative organs may be risky for plants living under high-elevation habitats, as these two processes can potentially lead to plant reproductive failure due to the low temperatures and short growing seasons at high elevations. Thus, we hypothesize that reproductive effort will increase with elevation and the elevational pattern of reproductive allocation will be largely driven by soil attributes, given their covariation with elevation. To test these hypotheses, we determined the vegetative and reproductive biomass of individual Impatiens arguta (Balsaminaceae) plants across 12 populations in the Gaoligong Mountains (China), and collected data on soil temperature, nutrients, moisture, and pH for each population. Based on standard major axis regression and linear regression models, we found that (1) both vegetative and reproductive biomass decreased with elevation; (2) all populations demonstrated significant allometric slopes (i.e., linear coefficients of log[reproductive biomass] − log[vegetative biomass] regressions) > 1; (3) allometric slopes decreased with elevation; and (4) soil temperature was a better predictor of the allometric slope than elevation, i.e., the allometric slope decreased with soil temperature. These results suggest that plant species growing at high elevation invest proportionately more resources to reproduction as an adaptation to low-temperature environments, and reproductive output is heavily dependent on vegetative growth. This study provides the first evidence of soil temperature driving reproductive allocation patterns, which suggests that plant species will favor allocation to growth under increasing soil temperatures with climate warming.

Testes mass in the livebearing fish Brachyrhaphis rhabdophora (Poeciliidae) varies hypoallometrically with body size but not between predation environments.

In this study, we considered potential causes of variation in testis size in the livebearing fish Brachyrhaphis rhabdophora. We evaluated variation in testes mass among individual males and among populations that occupy different selective environments. First, we predicted that small males should allocate more to testes mass than large males (i.e., hypoallometric pattern) based on a sperm competition argument. Second, based on life history theory and associated differences in mortality rates between populations that coexist with many fish predators and those with few predators, we predicted that males in high‐predation environments should allocate more to testes mass than males in habitats with few predators. Our results showed that small males allocated proportionally more to testes mass than larger males (slope of testes mass to body mass was hypoallometric). However, there was no effect of predator environment on testes mass independent of body size differences. In this system, size‐specific patterns of reproductive allocation in males (hypoallometry) differ from that seen in females (hyperallometry). Allocation to testes mass may respond to differences in mortality rate through selection on body size.

Reproductive effort and success of males in scramble-competition polygyny: Evidence for trade-offs between foraging and mate search.

Patterns of male reproductive allocation provide insight into life-history characteristics. The trade-offs associated with resource and female group defence are well-defined. However, less is understood about trade-offs in species that practise scramble-competition polygyny, where successful strategies may favour competitive mate-searching rather than contest competition and fighting. White-tailed deer (Odocoileus virginianus) practise scramble-competition polygyny where solitary males search for and assess receptivity of females scattered across the landscape. Physically mature males are expected to do most of the breeding because of the high energetic costs of reproduction and high social status. However, young males may collectively sire one-third of offspring. To gain a better understanding of trade-offs associated with scramble-competition polygyny, we quantified metrics associated with reproductive effort and success. We quantified changes in body mass of harvested males, energetic costs of locomotion based on movements of GPS radiocollared males and timing of reproduction via temporal genetic parentage assignments. Young males (1.5 and 2.5years old) sired offspring, but their mating success was mainly limited to peak rut, when most females were in oestrus. Furthermore, multiple paternity was common, indicating opportunistic reproduction. Reproductive effort, indexed by body mass loss, was highest in prime-age males (5.5-6.5years old). Surprisingly, young and postprime males also exhibited significant body mass loss, indicative of investment in reproductive effort. Movement rates increased twofold to fourfold during rut as a function of mate search activities, but cost of locomotion would cause only about one-third of observed body mass loss. Because males are capital breeders, we infer most of body mass loss is due to reduced foraging. In scramble-competition polygyny, the repeated location of potential mates and assessment of their oestrous status appear to be important constituents of male mating strategies. Therefore, mating success may be influenced by time management and spatial memory, and not based solely on social dominance. Thus, reproductive effort should be greater for individuals capable of reducing time foraging. For those that cannot, opportunistic mating opportunities may arise when operative adult sex ratios are low. Our analyses reveal valuable insight into the trade-offs associated with scramble-competition polygyny.

Influence of prey size on reproduction among populations of Diamond-backed Watersnakes (Nerodia rhombifer)

Mean prey size often varies across landscapes, resulting in predator populations having differing access to energetic resources. Shifts in resource quality are likely to cause differences in energy allocation of reproduction. Thus, additional energy intake may lead to (i) increased offspring size, (ii) increased numbers of offspring, (iii) increased relative proportion of energy allocated to reproduction, (iv) increased absolute amount of energy allocated to reproduction or (v) increased energy allocated to growth of some other nonreproductive function, or (vi) some combination of the above outcomes. To test this hypothesis, we evaluated the reproductive allocation patterns of four populations of Diamond-backed Watersnakes (Nerodia rhombifer (Hallowell, 1852)) that differ in their mean prey size. Snakes at large prey sites produced longer, heavier babies compared with snakes from small prey sites. Statistical interactions among sites confounded our ability to compare differences in litter size, litter mass, and relative clutch mass. We suggest that increased prey size results in populations shifting reproductive allocation to increase offspring size. Effects of prey size on litter size, litter mass, and relative clutch mass remain unclear.

Male parental effort predicts reproductive contribution in the joint-nesting, Smooth-billed Ani (Crotophaga ani)

Co-operative breeders provide parental care to non-filial offspring—a behaviour known as ‘alloparental care’. While inclusive fitness benefits are a widely accepted driver of alloparental care in kin-based social groups, such indirect benefits are lost in non-kin societies. Among such societies, theory predicts that the degree of parental and alloparental effort should therefore be proportional to an individual’s genetic contribution to mixed broods—depending upon reproductive options. Using genotyping data across five to 12 microsatellite loci for individuals from 20 social groups (67 adults and 153 nestlings), we assessed whether kinship or proportional reproductive success explained trends in parental and alloparental effort in the Smooth-billed Ani (Crotophaga ani), a joint-nesting cuckoo species. Nocturnal incubation in this species appears to be performed almost exclusively by a single male. We first report significantly higher degrees of relatedness between adults within social groups (\(\bar{r}\) = 0.208, n = 114 dyads), than between social groups (\(\bar{r}\) = 0.120, n = 893 dyads), suggesting that inclusive fitness benefits may in part explain uneven allocation of parental effort. Second, we show that nocturnal incubation status is a significant predictor of reproductive success in males, as nocturnal incubators sire a greater proportion of nestlings in mixed-parentage broods. While patterns of reproductive skew appear high at 80% paternal confidence (\(\bar{B}\) = 0.052, p = 0.061), we report no significant deviation from an egalitarian breeding framework. Our results revealed similar patterns of reproductive allocation to closely related Groove-billed Anis (Crotophaga sulcirostris); however, differences in male reproductive skew and within-group relatedness across crotophagids are highlighted and offer insight into social evolution among anis.

Variation in the reproductive strategy of a lichenized fungus along a climatic gradient.

Onset of reproduction and reproductive allocation patterns are key components of plant reproductive strategies. Life history theory predicts that plants in adverse environments for juvenile performance start reproduction at smaller sizes and exhibit higher reproductive allocation compared to their counterparts in favourable environments. Life history theory will gain in generality if its predictions are shown to apply to a broad range of organisms and modes of reproduction. This study tested whether the asexual reproductive strategy of a lichenized fungus changed along a climatic gradient. The variation in threshold size for asexual reproduction and asexual reproductive allocation of the lichen Lobarina scrobiculata was assessed in 18 populations (9665 individuals) along a climatic gradient spanning 800 km in latitude in Southern Europe. Using generalized linear models and standardized major axis regressions, the allometric relationships and the associated variation in climatic factors according to the changes in the threshold size for reproduction and reproductive allocation patterns were assessed. The onset of reproduction was size-dependent and the reproductive allocation increased with individual size. Both the threshold size for reproduction and the reproductive allocation varied along the rainfall gradient. A lower threshold size for reproduction and higher reproductive allocation in drier, adverse locations were found. Therefore, populations in drier locations fitted the predictions of life history theory for sexually reproducing organisms in adverse environments for juvenile performance. This study highlights the applicability of the life history theory to fungi and to modes of reproduction other than sexual reproduction. Based on the intraspecific variation in the asexual reproductive strategy of a fungal organism with climatic factors, these findings expand the scope of life history theory predictions and increase our understanding of life history diversity and reproductive strategies across environments.

Reproductive allocation and fruit-set pattern in the euhalophyte Suaeda salsa in controlled and field conditions

Reproductive allocation and fruit-set pattern are important characteristics of halophytes, and these characteristics were investigated in euhalophyte Suaeda salsa (S. salsa). The plants comprised of those treated with 1 mM (control) or 500 mM (high salinity) of NaCl for 120 days in a glasshouse and those collected in the field. In controlled conditions, high salinity increased reproductive allocation and the number of fruits and seeds in each node of the branches. In field conditions, reproductive allocation and the number of fruits and brown seeds in S. salsa at the upper tide line were higher than at the lower tide line. There were more fruits and seeds distributed in the central part than those in the base or top of branches in both controlled and field conditions. Soil organic matter, total nitrogen and total phosphorus content at the upper tide line were lower than at the lower tide line. An opposite trend could be observed in the total soluble salt, Na+ and Cl− content in soil. In conclusion, the fruit-set pattern of S. salsa may be affected by both resource constraints and resource allocation. Reproductive allocation and fruit-set pattern may ensure the establishment of S. salsa in high saline and low nutrient environments.

Participant perceptions of Range Rider Programs used to mitigate wolf-livestock conflicts in the western United States

Reproduction in mammals is costly, with implications for trade-offs between current reproduction and survival. Life history theory suggests the amount of energy allocated to reproduction should change as a function of increasing maternal age, and empirical work supports both within-individual decreases (consistent with senescence) and increases (consistent with constraint and restraint). The Weddell seal (Leptonychotes weddellii) is an ideal organism with which to study patterns of maternal energy allocation due to the life history. Here, we used a long-term mark-recapture database of individually marked mothers and pups in Erebus Bay, Antarctica, to develop a stratified sample of masses from pups and known-age females across maternal ages in 11 different years. Hierarchical modeling of potential sources of variation in: 1) pup masses at parturition, 2) pup mass gains from parturition to mid-lactation (~20 days), and 3) pup mass gains from mid-lactation to late-lactation (~20-days to ~35-days) was used to 1) evaluate the relative support for increases/decreases in reproductive allocation as a function of maternal age, 2) assess how patterns of reproductive allocation may reflect pre-parturition resource acquisition and/or post-parturition resource allocation, and 3) estimate the magnitude of individual heterogeneity in maternal effects after accounting for maternal and offspring characteristics. Our results provide strong evidence that reproductive investment at parturition and pup mass gains from parturition through late-lactation vary with maternal age and breeding history and result in important differences in late-lactation pup masses. Such variation may have consequences for the early-life success of offspring, and thus implications at the population level.

Differences in Patterns of Reproductive Allocation between the Sexes in Nicrophorus orbicollis.

Organisms are selected to maximize lifetime reproductive success by balancing the costs of current reproduction with costs to future survival and fecundity. Males and females typically face different reproductive costs, which makes comparisons of their reproductive strategies difficult. Burying beetles provide a unique system that allows us to compare the costs of reproduction between the sexes because males and females are capable of raising offspring together or alone and carcass preparation and offspring care represent the majority of reproductive costs for both sexes. Because both sexes perform the same functions of carcass preparation and offspring care, we predict that they would experience similar costs and have similar life history patterns. In this study we assess the cost of reproduction in male Nicrophorus orbicollis and compare to patterns observed in females. We compare the reproductive strategies of single males and females that provided pre- and post-hatching parental care. There is a cost to reproduction for both males and females, but the sexes respond to these costs differently. Females match brood size with carcass size, and thus maximize the lifetime number of offspring on a given size carcass. Males cull proportionately more offspring on all carcass sizes, and thus have a lower lifetime number of offspring compared to females. Females exhibit an adaptive reproductive strategy based on resource availability, but male reproductive strategies are not adaptive in relation to resource availability.

How Diverse Detrital Environments Influence Nutrient Stoichiometry between Males and Females of the Co-Occurring Container Mosquitoes Aedes albopictus, Ae. aegypti, and Culex quinquefasciatus.

Allocation patterns of carbon and nitrogen in animals are influenced by food quality and quantity, as well as by inherent metabolic and physiological constraints within organisms. Whole body stoichiometry also may vary between the sexes who differ in development rates and reproductive allocation patterns. In aquatic containers, such as tree holes and tires, detrital inputs, which vary in amounts of carbon and nitrogen, form the basis of the mosquito-dominated food web. Differences in development times and mass between male and female mosquitoes may be the result of different reproductive constraints, which could also influence patterns of nutrient allocation. We examined development time, survival, and adult mass for males and females of three co-occurring species, Aedes albopictus, Ae. aegypti, and Culex quinquefasciatus, across environments with different ratios of animal and leaf detritus. We quantified the contribution of detritus to biomass using stable isotope analysis and measured tissue carbon and nitrogen concentrations among species and between the sexes. Development times were shorter and adults were heavier for Aedes in animal versus leaf-only environments, whereas Culex development times were invariant across detritus types. Aedes displayed similar survival across detritus types whereas C. quinquefasciatus showed decreased survival with increasing leaf detritus. All species had lower values of 15N and 13C in leaf-only detritus compared to animal, however, Aedes generally had lower tissue nitrogen compared to C. quinquefasciatus. There were no differences in the C:N ratio between male and female Aedes, however, Aedes were different than C. quinquefasciatus adults, with male C. quinquefasciatus significantly higher than females. Culex quinquefasciatus was homeostatic across detrital environments. These results allow us to hypothesize an underlying stoichiometric explanation for the variation in performance of different container species under similar detrital environments, and if supported may assist in explaining the production of vector populations in nature.

Reproductive allocation between the sexes, across natural and novel habitats, and its impact on genetic diversity

Allocation to reproductive mode (sexual and asexual) can vary depending on environmental conditions but is often examined at the population level, whereas selection acts upon the individual. We examined individual variation in reproductive mode to identify how the interaction of sex and the environment affect population genetic diversity. Using the plant Marchantia inflexa, we tested whether reproductive allocation pattern varies consistently between males and females and among plants collected from different environments, and determined if morphological responses were the result of individual plasticity or genetic differences. We then quantified genetic variability between the different environments and between the sexes. Male and female plants were collected from two strikingly different habitats within the same region: along natural sites (rivers) and along novel human-modified sites (roadsides). Using a common garden approach, we found a strong sex by habitat interaction: male and female responses differed significantly by their source habitat. For females, relative to river-collected, road-collected plants had higher growth and asexual reproduction, while the pattern was reversed, although not significant, for males. Genetic differentiation was significant between the two habitats with no evidence of individual differences in plasticity for growth, but there was a genotype effect for asexual propagule production. Males and females did not differ genetically; but river-collected plants with lower sexual potential were more diverse than roadside-collected plants, possibly the result of founder events. These results show that individual variation in reproduction is controlled by the interaction of both the environment and genetics. Due to different selection pressures between natural and novel habitats, there are observable differences in life history traits with an associated evolutionary response to the novel habitat.

Maternal allocation in bison: co-occurrence of senescence, cost of reproduction, and individual quality

Parental allocation strategies are of profound interest in life history because they directly impact offspring fitness and therefore are highly valuable for understanding population dynamics and informing management decisions. Yet, numerous questions about reproductive allocation patterns for wild populations of large mammals remain unanswered because of the challenges for measuring allocation in the wild. Using a nine-year longitudinal data set on life-history traits of mother–calf bison pairs, we identified sources of variation in relative maternal allocation (calf mass ratio on mother mass) and assessed the occurrence of reproductive costs associated with differential maternal allocation. We found that heavy mothers provided a lower allocation but still produced heavier calves than light mothers. Older females produced lighter calves and tended to decrease allocation as they aged, supporting the occurrence of reproductive senescence. Mothers that had produced a calf the previous year produced lighter calves and allocated less than mothers that did not lactate the previous year, revealing reproductive costs. However, greater maternal allocation did not reduce the probability of breeding in successive years, and the amount of allocation provided by a mother was positively correlated among the offspring she produced, illustrating individual heterogeneity. Although life-history studies are usually classified as either supporting costs of reproduction or individual quality, our study demonstrates that these contrasting evolutionary forces can shape variation within a single trait. Our work illustrates that many processes can coevolve within a population, emphasizing the need to integrate multiple concepts to better understand the evolution of life-history traits. With regard to management of bison herds, if the goal of culling programs is to select for animals with the best performance, this research suggests that managers should account for the condition and previous reproductive status of mothers when taking culling decisions on juvenile bison.

Maternal allocation in bison: co-occurrence of senescence, cost of reproduction, and individual quality

Parental allocation strategies are of profound interest in life history because they directly impact offspring fitness and therefore are highly valuable for understanding population dynamics and informing management decisions. Yet, numerous questions about reproductive allocation patterns for wild populations of large mammals remain unanswered because of the challenges for measuring allocation in the wild. Using a nine-year longitudinal data set on life-history traits of mother-calf bison pairs, we identified sources of variation in relative maternal allocation (calf mass ratio on mother mass) and assessed the occurrence of reproductive costs associated with differential maternal allocation. We found that heavy mothers provided a lower allocation but still produced heavier calves than light mothers. Older females produced lighter calves and tended to decrease allocation as they aged, supporting the occurrence of reproductive senescence. Mothers that had produced a calf the previous year produced lighter calves and allocated less than mothers that did not lactate the previous year, revealing reproductive costs. However, greater maternal allocation did not reduce the probability of breeding in successive years, and the amount of allocation provided by a mother was positively correlated among the offspring she produced, illustrating individual heterogeneity. Although life-history studies are usually classified as either supporting costs of reproduction or individual quality, our study demonstrates that these contrasting evolutionary forces can shape variation within a single trait. Our work illustrates that many processes can coevolve within a population, emphasizing the need to integrate multiple concepts to better understand the evolution of life-history traits. With regard to management of bison herds, if the goal of culling programs is to select for animals with the best performance, this research suggests that managers should account for the condition and previous reproductive status of mothers when taking culling decisions on juvenile bison.

Contrasting Life Histories in Neighbouring Populations of a Large Mammal

BackgroundA fundamental life history question is how individuals should allocate resources to reproduction optimally over time (reproductive allocation). The reproductive restraint hypothesis predicts that reproductive effort (RE; the allocation of resources to current reproduction) should peak at prime-age, whilst the terminal investment hypothesis predicts that individuals should continue to invest more resources in reproduction throughout life, owing to an ever-decreasing residual reproductive value. There is evidence supporting both hypotheses in the scientific literature.Methodology/Principal FindingsWe used an uncommonly large, 38 year dataset on Alpine chamois (Rupicapra rupicapra) shot at various times during the rutting period to test these two hypotheses. We assumed that body mass loss in rutting males was strongly related to RE and, using a process-based approach, modelled how male relative mass loss rates varied with age. For different regions of our study area, we provide evidence consistent with different hypotheses for reproductive allocation. In sites where RE declined in older age, this appears to be strongly linked to declining body condition in old males. In this species, terminal investment may only occur in areas with lower rates of body mass senescence.Conclusions/SignificanceOur results show that patterns of reproductive allocation may be more plastic than previously thought. It appears that there is a continuum from downturns in RE at old age to terminal investment that can be manifest, even across adjacent populations. Our work identifies uncertainty in the relationship between reproductive restraint and a lack of competitive ability in older life (driven by body mass senescence); both could explain a decline in RE in old age and may be hard to disentangle in empirical data. We discuss a number of environmental and anthropogenic factors which could influence reproductive life histories, underlining that life history patterns should not be generalised across different populations.

Life History of Brachyrhaphis parismina: Variation Within and Among Populations

Poeciliids provide a model system for comparative studies of life-history variation because of the relatively large number of species and the diversity of environments they occupy. Brachyrhaphis parismina is a narrow-bodied poeciliid that occupies rivers and streams in the eastern lowlands of Costa Rica. Detailed life-history information on species in the narrow-bodied clade of the genus Brachyrhaphis is lacking compared to the many studies on the round-bodied species of the genus. We test for variation in life history among five populations of Brachyrhaphis parismina, and for patterns of allometry associated with the reproductive value hypothesis among individuals. Life-history traits exhibited little variation among populations in contrast to variation observed among populations of round-bodied congeners. Furthermore, within locations females exhibit isometric patterns of reproductive allocation with female body size, thus life history varies little over a female's lifetime. This pattern is inconsistent with that expected from the reproductive value hypothesis. Variation among and within populations in this species may be constrained by the high flow environment they occupy.