Age‐specific maternal effects interact with larval food supply to modulate life history in Coleomegilla maculata

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Reproductive senescence and dynamic oviposition behaviour in insects

We explore the relationship between clutch size behaviour and the likelihood of egg limitation in insects with parasitoid-like life histories. We compare the incidence of egg limitation in insects that produce clutch sizes consistent with maximizing fitness per host (i.e. the single host maximum, or Lack clutch size) with insects exhibiting two types of dynamic behaviour. In the first case, insects produce clutch sizes according to a rule that specifies a negative relationship between egg complement and clutch size; in the second case, clutch size is determined by both egg complement and the host encounter rate using a dynamic state-variable model. The analyses are an extension of models presented by Rosenheim (1996), which predicted a strong positive relationship between the risk of egg limitation and the host encounter rate in the absence of dynamic oviposition behaviour. Our results are consistent with Rosenheim's, but they show that dynamic behaviour can greatly reduce th e risk of egg limitation and weaken the relationship between the host encounter rate and egg limitation.     

Morphological correlates of life‐history variation: is lizard clutch size related to the number of germinal beds in the ovary?

Clutch size varies widely in reptiles, both intraspecifically and interspecifically. The mechanisms that generate this variation have attracted detailed study, focusing primarily on ecological factors (e.g. food availability), trade‐offs with other traits (e.g. offspring size), and physical constraints (e.g. maternal body shape). Does ovarian morphology, specifically the number of germinal beds from which ova are produced, also correlate with clutch size? Our review of published data on 58 lizard species reveals that clutch size is correlated with the number of germinal beds per ovary (more fecund species have more germinal beds), and that phylogenetic changes in germinal beds have been consistently associated with concurrent phylogenetic changes in fecundity. These correlations imply a causal connection: either clutch size is constrained by ovarian morphology, and/or ovarian morphology evolves to allow adaptive shifts in clutch size. The latter hypothesis is more consistent with available data. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94 , 81–88.     

Progressive changes in brain size and musculo-skeletal traits in seven hominoid populations

Neurological complexity has increased over evolutionary time for invertebrates and vertebrates alike, with the hominid brain tripling in size over the last 3 million years. Since magnetic resonance imaging (MRI) studies among humans indicate a significant correlation (meanr>0.40) between individual differences in brain size and general cognitive ability, it is reasonable to hypothesize that increasing brain size confers greater intelligence. However, larger brains have associated costs, taking longer to build and requiring more energy to run. Sufficient advantages must have accrued for them to override these trade-offs. The present paper documents that in hominoids, as brain size increased from 380 to 1364 cm³ over seven hominoid groups (chimpanzees to australopithecines toHomo habilis toHomo erectus to differences amongHomo sapiens), it was accompanied by changes in 74 musculo-skeletal traits (rs=0.90). These occurred on both cranial traits (temporalis fossae, post-orbital constrictions, mandibles, dentition, nuchal muscle attachments) and on post-cranial traits (pelvic widths, femoral heads, tibial plateaus). It is concluded that in the evolutionary competition to find and fill new niches, there was “room at the top” for greater behavioral complexity and larger brain size, leading to cascading effects on other traits.     

Global warming and positive fitness response in mountain populations of common lizards Lacerta vivipara

Recent global warming threatens many species and has already caused population‐ and species‐level extinctions. In particular, high risks of extinction are expected for isolated populations of species with low dispersal abilities. These predictions rely on widely used ‘climatic envelope’ models, while individual responses, the ultimate driver of a species response to climate change, have been most often neglected. Here, we report on some changes in life‐history traits of a dispersal‐limited reptile species (a poorly studied taxa) living in isolated populations. Using long‐term data on common lizards collected in southern France, we show that individual body size dramatically increased in all the four populations studied over the past 18 years. This increase in body size in all age classes appeared related to a concomitant increase in temperature experienced during the first month of life (August). Daily maximum temperature in August increased by 2.2°C and yearling snout‐vent‐length increased by about 28%. As a result, adult female body size increased markedly, and, as fecundity is strongly dependent on female body size, clutch size and total reproductive output also increased. For one population where capture–recapture data were available, adult survival was positively related to May temperature. All fitness components investigated therefore responded positively to the increase in temperature, such that it might be concluded that the common lizard has been advantaged by the shift in temperature. We contrast these short‐term results with the long‐term habitat‐based prediction that these populations located close to mountain tops on the southern margin of the species range should be unable to cope with the alteration of their habitat. To achieve a better prediction of a species persistence, one will probably need to combine both habitat and individual‐based approaches.     

The song of the old mother: Reproductive senescence in female drosophila

Among animals with multiple reproductive episodes, changes in adult condition over time can have profound effects on lifetime reproductive fitness and offspring performance. The changes in condition associated with senescence can be particularly acute for females who support reproductive processes from oogenesis through fertilization. The pomace fly Drosophila melanogaster is a well-established model system for exploring the physiology of reproduction and senescence. In this review, we describe how increasing maternal age in Drosophila affects reproductive fitness and offspring performance as well as the genetic foundation of these effects. Describing the processes underlying female reproductive senescence helps us understand diverse phenomena including population demographics, condition-dependent selection, sexual conflict, and transgenerational effects of maternal condition on offspring fitness. Understanding the genetic basis of reproductive senescence clarifies the nature of life-history trade-offs as well as potential ways to augment and/or limit female fertility in a variety of organisms.     

Prolonged offspring dependence and cooperative breeding in birds

It has been suggested that the evolution of cooperative breedingin birds is associated with unusually long periods of offspringdependence ; this appears paradoxical because cooperative breedersoften produce more broods than their noncooperatively breedingrelatives. I compared the duration of parental care betweencooperatively and noncooperatively breeding species using phylogeneticallyindependent contrasts and matched pairs. The incubation andnestling periods did not differ between the two parental caresystems, but the duration of postfledging offspring care wassignificantly longer in species that regularly breed cooperatively.This relationship remained when other factors that are thoughtto affect the duration of fledgling care (breeding habitat,body size, latitude of breeding, diet) were controlled statistically.Cooperative breeders appear to provide more prolonged postfledgingcare because additional care providers reduce the costs of parenting, offspring have less incentive to become independent,and a division of labor can develop during reproduction—helperscontinue to feed fledglings while breeders initiate the nextnesting attempt.     

Tolerance of a perennial herb, <Emphasis Type="Italic">Pimpinella saxifraga</Emphasis>, to simulated flower herbivory and grazing: immediate repair of injury or postponed reproduction?

Perennial, polycarpic herbs can respond to herbivory either by (1) regrowth in the same season in order to compensate for lost reproductive structures or by (2) postponing reproduction until the following growing season. We tested these response patterns with the perennial umbellifer Pimpinella saxifraga by simulating flower herbivory and shoot grazing both in the field and in a common garden experiment. In the field, both simulated flower herbivory and grazing effectively suppressed current reproduction, whereas no statistically significant effects of previous-year treatments on growth or reproduction were found in the following year. In the common garden, in the first year the species fully compensated for simulated flower herbivory in vegetative parameters but seed set was reduced by 26%. After 2 years of flower removal, the plants overcompensated in shoot and root biomass by 47 and 46%, respectively, and compensated fully in reproductive performance. Simulated grazing resulted in 21% lower shoot biomass in the first season, but the root biomass was not affected. In the second season the root biomass increased by 43% as compared to the control plants. However, regrowth following simulated grazing resulted in a significant delay in flowering with the consequence that the seed yield of fertile plants was reduced by 55% as compared to the control plants. These results suggest that in resource-rich garden conditions P. saxifraga may immediately repair injuries caused by flower herbivory, but repairs more extensive shoot injury less successfully. Delayed phenology decreases the benefits of immediate repair. In resource-poor conditions, the benefits of regrowth can be negligible. Accordingly, in our field population, the plants postponed their reproduction until the following year in response to simulated grazing and frequently in response to flower removal. When the plants gain very little from regrowth, the costs of reproduction would select for postponed reproduction in response to injury.     

Short- and long-term effects of litter size manipulation in a small wild-derived rodent

Iteroparous organisms maximize their overall fitness by optimizing their reproductive effort over multiple reproductive events. Hence, changes in reproductive effort are expected to have both short- and long-term consequences on parents and their offspring. In laboratory rodents, manipulation of reproductive efforts during lactation has however revealed few short-term reproductive adjustments, suggesting that female laboratory rodents express maximal rather than optimal levels of reproductive investment as observed in semelparous organisms. Using a litter size manipulation (LSM) experiment in a small wild-derived rodent (the common vole; Microtus arvalis), we show that females altered their reproductive efforts in response to LSM, with females having higher metabolic rates and showing alternative body mass dynamics when rearing an enlarged rather than reduced litter. Those differences in female reproductive effort were nonetheless insufficient to fully match their pups’ energy demand, pups being lighter at weaning in enlarged litters. Interestingly, female reproductive effort changes had long-term consequences, with females that had previously reared an enlarged litter being lighter at the birth of their subsequent litter and producing lower quality pups. We discuss the significance of using wild-derived animals in studies of reproductive effort optimization.     

Corticosterone,testosterone and life-history strategies of birds

Steroid hormones have similar functions across vertebrates, but circulating concentrations can vary dramatically among species. We examined the hypothesis that variation in titres of corticosterone (Cort) and testosterone (T) is related to life-history traits of avian species. We predicted that Cort would reach higher levels under stress in species with higher annual adult survival rates since Cort is thought to promote physiological and behavioural responses that reduce risk to the individual. Conversely, we predicted that peak T during the breeding season would be higher in short-lived species with high mating effort as this hormone is known to promote male fecundity traits. We quantified circulating hormone concentrations and key life-history traits (annual adult survival rate, breeding season length, body mass) in males of free-living bird species during the breeding season at a temperate site (northern USA) and a tropical site (central Panama). We analysed our original data by themselves, and also combined with published data on passerine birds to enhance sample size. In both approaches, variation in baseline Cort (Cort0) among species was inversely related to breeding season length and body mass. Stress-induced corticosterone (MaxCort) also varied inversely with body mass and, as predicted, also varied positively with annual adult survival rates. Furthermore, species from drier and colder environments exhibited lower MaxCort than mesic and tropical species; T was lowest in species from tropical environments. These findings suggest that Cort0, MaxCort and T modulate key vertebrate life-history responses to the environment, with Cort0 supporting energetically demanding processes, MaxCort promoting survival and T being related to mating success.     

Differential reproductive responses to stress reveal the role of life-history strategies within a species

Life-history strategies describe that ‘slow’- in contrast to ‘fast’-living species allocate resources cautiously towards reproduction to enhance survival. Recent evidence suggests that variation in strategies exists not only among species but also among populations of the same species. Here, we examined the effect of experimentally induced stress on resource allocation of breeding seabirds in two populations with contrasting life-history strategies: slow-living Pacific and fast-living Atlantic black-legged kittiwakes. We tested the hypothesis that reproductive responses in kittiwakes under stress reflect their life-history strategies. We predicted that in response to stress, Pacific kittiwakes reduce investment in reproduction compared with Atlantic kittiwakes. We exposed chick-rearing kittiwakes to a short-term (3-day) period of increased exogenous corticosterone (CORT), a hormone that is released during food shortages. We examined changes in baseline CORT levels, parental care and effects on offspring. We found that kittiwakes from the two populations invested differently in offspring when facing stress. In response to elevated CORT, Pacific kittiwakes reduced nest attendance and deserted offspring more readily than Atlantic kittiwakes. We observed lower chick growth, a higher stress response in offspring and lower reproductive success in response to CORT implantation in Pacific kittiwakes, whereas the opposite occurred in the Atlantic. Our findings support the hypothesis that life-history strategies predict short-term responses of individuals to stress within a species. We conclude that behaviour and physiology under stress are consistent with trade-off priorities as predicted by life-history theory. We encourage future studies to consider the pivotal role of life-history strategies when interpreting inter-population differences of animal responses to stressful environmental events.     

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

1. 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.