Life histories as adaptive strategies

Optimal control theory is used to produce a general model of life history evolution in a stationary environment. Several disparate trends in current theorizing on life histories are thereby unified. An optimal life history (OLH) is defined as one which maximizes individual fitness (the Malthusian parameter in density-independent populations, the carrying capacity in density-dependent ones). Since the components of fitness depend on the phenotype, the search for an OLH is accomplished in phenotypic space. The optimization is controlled by apportioning the energy obtained at any age between conflicting processes of growth, survival and reproduction. The methods of dynamic optimization which pertain to this problem are reviewed briefly, and its results interpreted biologically. Of these, Pontryagin's method is selected and used to examine some simple models. This method leads one to define a dual variable matched to each phenotypic variable, the prospective value. This provides an indicator of the selective pressures acting at any age on a phenotypic feature to push it towards coincidence with the OLH. This also suggests that at ages in which these dual variables are low (i.e. late ages) there will be greater phenotypic variability around the OLH in any population. The problem of the optimal distribution of reproductive effort over the life history is discussed as well.     

Ageing effects in an iteroparous plant species with a variable life span

Background and Aims

Ageing effects may be due to dysfunction leading to decreasing reproduction and survival with age. In plants, however, other (physiological) causes, associated with size for example, may also play a role. Iteroparous plants with genetically variable life spans can be helpful in unravelling these two aspects of changes associated with growing older.

Methods

In a long-term experiment, Beta vulgaris ssp. maritima (sea beet) plants from the same set of populations but with different ages were compared for flowering date over several years. Flowering date, root growth and seed production were measured in a synthetic population and in progenies derived from reciprocal crosses over three consecutive years and analysed with respect to the number of years yet to live. Heritabilities of these three characters and of life span were estimated.

Key Results

Flowering occurred on average 1·3 d later each year over a plant''s whole lifetime. In the year before dying, plants flowered on average 3·3 d later and both root investment and seed production decreased significantly compared with plants that remained alive for at least 1 further year. The negative relationship (trade-off) between reproduction and root investment in early life became positive near the end of life, and the positive relationship between flowering date and root growth became negative.

Conclusions

Effects of ageing – in the sense of a decline in reproduction and root storage – combined with later flowering were particularly pronounced in the year before death. The gradual change in flowering phenology, observed over the whole lifetime, could have a physiological basis unrelated to dysfunction.Key words: Ageing, Beta vulgaris ssp. maritima (sea beet), flowering phenology, longevity, perennial, root investment, seed production, trade-offs, whole-plant senescence     

Tetanus neurotoxin: conformational plasticity as an adaptive strategy

Tetanus neurotoxin (TeNT) secreted by Clostridium tetani is the causative agent of the spastic paralysis distinctive of human tetanus. TeNT is structurally related to the family of botulinum neurotoxins (BoNTs) produced by Clostridium botulinum that cause flaccid paralysis by disabling synaptic exocytosis at peripheral cholinergic neurons. By contrast, TeNT targets the central nervous system (CNS) by hijacking receptors for neurotrophic factors to enter peripheral neurons thereby being sorted into non‐acidifying endosomes, trafficking via retrograde axonal transport organelles, and entering spinal inhibitory interneurons after transcytosis (Fig 1 A). In this issue of EMBO Reports, Masuyer et al 1 describe the structural plasticity of individual TeNT domains in the context of the holotoxin in response to environmental pH, a key factor modulating TeNT fate and action. Through the concerted use of X‐ray crystallography, single particle cryo‐EM, and small angle X‐ray scattering (SAXS), the authors provide snapshots of conformational transitions that may underlie the productive path of TeNT from its entry in the peripheral nervous system (PNS) to its ultimate site of action on central glycinergic synapses.     

Herbivore-mediated ecological costs of reproduction shape the life history of an iteroparous plant

Plant reproduction yields immediate fitness benefits but can be costly in terms of survival, growth, and future fecundity. Life-history theory posits that reproductive strategies are shaped by trade-offs between current and future fitness that result from these direct costs of reproduction. Plant reproduction may also incur indirect ecological costs if it increases susceptibility to herbivores. Yet ecological costs of reproduction have received little empirical attention and remain poorly integrated into life-history theory. Here, we provide evidence for herbivore-mediated ecological costs of reproduction, and we develop theory to examine how these costs influence plant life-history strategies. Field experiments with an iteroparous cactus (Opuntia imbricata) indicated that greater reproductive effort (proportion of meristems allocated to reproduction) led to greater attack by a cactus-feeding insect (Narnia pallidicornis) and that damage by this herbivore reduced reproductive success. A dynamic programming model predicted strongly divergent optimal reproductive strategies when ecological costs were included, compared with when these costs were ignored. Meristem allocation by cacti in the field matched the optimal strategy expected under ecological costs of reproduction. The results indicate that plant reproductive allocation can strongly influence the intensity of interactions with herbivores and that associated ecological costs can play an important selective role in the evolution of plant life histories.     

Reproductive strategy and carry-over effects for species with complex life histories

We investigated the influences of natal-pool and metamorph characteristics on juvenile survival, age-specific breeding probabilities, and dispersal of wood frogs (Lithobates sylvatica) and used this information to infer how life history strategies of short-lived species may offset risks associated with breeding in highly variable habitats. We used multistate mark-recapture data from eight wood frog populations in Maryland, USA, to investigate the influences of natal-pond and metamorph characteristics on post-metamorphic demographics. We found that post-metamorphic juvenile survival was highly variable and negatively influenced by abiotic conditions experienced during development but showed little relationship to larval density or size at metamorphosis. Estimates of recruitment and dispersal probabilities indicated that males mature earlier than females, and a small percentage of each sex disperse to non-natal pools. Survival probabilities for adults during the non-breeding season were less variable than juvenile rates, lower for females, and negatively related to mean monthly precipitation. Survival of adults during the breeding season was generally very high. We provide the first robust estimates of post-metamorphic vital rates of wood frogs that allow for variation in capture probabilities. We found little evidence for an effect of metamorph traits on juvenile survival, suggesting that wood frogs may be able to overcome initial disadvantages to have similar post-metamorphic performance. Our study suggests that variation in the age of maturity for wood frogs may mitigate risks associated with breeding in a highly variable habitat to maximize lifetime fitness without increasing lifespan, and this strategy is minimally affected by carry-over effects from the larval stage.     

Legacies in life histories

Complex life-histories are common in nature, have many importantbiological consequences, and are an important focal area forintegrative biology. For organisms with complex life-histories,a legacy is something handed down from an ancestor or previousstage, and can be genetic, nutritional/provisional, experiential,as well as the result of random chance and natural variationin the environment. As we learn more about complex life-histories,it becomes clear that legacies are inexorably linked in theshort- and long-term through ecology and evolution. Understandingthe consequences and drivers of life-history patterns can thereforeonly be understood by considering all types of legacies andintegrating legacies across the entire life cycle. Larry McEdwardwas a leader in the field of ecological physiology, and evolutionaryecology of marine invertebrate larvae with complex life-histories.Through his scientific work and publications, devotion to students,colleagues, family, and friends, Larry has left a lasting legacythat will impact the future development of the field of larvalecology and complex life-histories.     

Photoprotective pigment as an adaptive strategy in the antarctic moss Ceratodon purpureus

Summary Variation in leaf pigmentation from green to ginger is observed for Ceratodon purpureus (Hedw.) Brid. in Antarctica. Electron microscopy of ginger and green leaves reveals less thylakoid stacking, a response to greater light exposure, in the ginger leaves. In extremely exposed sites C. purpureus has low chlorophyll a/b ratios which correlate with decreased 77K chlorophyll fluorescence, indicating damage to chlorophyll a. Pigment analysis of ginger moss shows that even when the chlorophyll a/b ratio has not decreased the pigment composition differs from green moss. The increase in anthocyanin and decrease in chlorophyll concentrations largely account for the visual change from green to ginger. The ratio of total carotenoid to chlorophyll varies from 0.35 in green moss to 0.55 in the ginger moss, with violaxanthin increased preferentially. Since these changes in pigmentation are consistent with photoprotection and they are linked to light dependent variations in chloroplast structure, it appears that photoprotective pigments are a useful adaptation for the bright Antarctic environment.     

Superstition and belief as inevitable by-products of an adaptive learning strategy

The existence of superstition and religious beliefs in most, if not all, human societies is puzzling for behavioral ecology. These phenomena bring about various fitness costs ranging from burial objects to celibacy, and these costs are not outweighed by any obvious benefits. In an attempt to resolve this problem, we present a verbal model describing how humans and other organisms learn from the observation of coincidence (associative learning). As in statistical analysis, learning organisms need rules to distinguish between real patterns and randomness. These rules, which we argue are equivalent to setting the level of α for rejection of the null hypothesis in statistics, are governed by risk management as well as by comparison to previous experiences. Risk management means that the cost of a possible type I error (superstition) has to be traded off against the cost of a possible type II error (ignorance). This trade-off implies that the occurrence of superstitious beliefs is an inevitable consequence of an organism’s ability to learn from observation of coincidence. Comparison with previous experiences (as in Bayesian statistics) improves the chances of making the right decision. While this Bayesian approach is found in most learning organisms, humans have evolved a unique ability to judge from experiences whether a candidate subject has the power to mechanistically cause the observed effect. Such “strong” causal thinking evolved because it allowed humans to understand and manipulate their environment. Strong causal thinking, however, involves the generation of hypotheses about underlying mechanisms (i.e., beliefs). Assuming that natural selection has favored individuals that learn quicker and more successfully than others owing to (1) active search to detect patterns and (2) the desire to explain these patterns mechanistically, we suggest that superstition has evolved as a by-product of the first, and that belief has evolved as a by-product of the second.     

Metamorphic duration: an under-studied variable in frog life histories

Over a narrow temperature range (25.5–28 °C), metamorphic duration in 14 taxonomically diverse frog species ranged from 2.0 to 7.3 days. For 12 species, duration was significantly related to tadpole body-size parameters, especially tail length. For the remainder, duration was longer than expected. Two patterns of tail regression were seen with either rapid or slow regression at the start. Most species lost about 30% of body wet weight during metamorphosis, but two were exceptional: X. laevis lost 58% and F. fitzgeraldi 13%. The results are discussed in the context of Wassersug & Sperry's (1977 ) hypothesis that selection acts to minimize metamorphic duration because metamorphosis is a highly vulnerable stage. Several other factors are outlined as possible determinants of metamorphic duration: developmental constraints, functional trade-offs, phylogeny, predator abundance, adaptive de-coupling, body condition and metabolic and water costs. There is evidence from the data for the operation of several of these factors, especially in species utilizing specialized habitats, and a plea is made for further study of metamorphic duration as an ecologically and evolutionarily important variable in amphibian life histories. Finally, the effects of different temperatures on metamorphic duration are presented for several species, and discussed in terms of local adaptation and reaction norms.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 261–272.     

Dynamic heterogeneity in life histories

Longitudinal data on natural populations have been analysed using multistage models in which survival depends on reproductive stage, and individuals change stages according to a Markov chain. These models are special cases of stage-structured population models. We show that stage-structured models generate dynamic heterogeneity: life-history differences produced by stochastic stratum dynamics. We characterize dynamic heterogeneity in a range of species across taxa by properties of the Markov chain: the entropy, which describes the extent of heterogeneity, and the subdominant eigenvalue, which describes the persistence of reproductive success during the life of an individual. Trajectories of reproductive stage determine survivorship, and we analyse the variance in lifespan within and between trajectories of reproductive stage. We show how stage-structured models can be used to predict realized distributions of lifetime reproductive success. Dynamic heterogeneity contrasts with fixed heterogeneity: unobserved differences that generate variation between life histories. We show by an example that observed distributions of lifetime reproductive success are often consistent with the claim that little or no fixed heterogeneity influences this trait. We propose that dynamic heterogeneity provides a 'neutral' model for assessing the possible role of unobserved 'quality' differences between individuals. We discuss fitness for dynamic life histories, and the implications of dynamic heterogeneity for the evolution of life histories and senescence.     

Is priesthood an adaptive strategy?

This study examines the socioeconomic and familial background of Irish Catholic priests born between 1867 and 1911. Previous research has hypothesized that lack of marriage opportunities may influence adoption of celibacy as part of a religious institution. The present study traced data from Irish seminary registries for 46 Catholic priests born in County Limerick, Ireland, using 1901 Irish Census returns and Land Valuation records. Priests were more likely to originate from landholding backgrounds, and with landholdings greater in size and wealth than the local average. Priests were found to originate from families with more sons than the national average, but with similar numbers of daughters. These findings are discussed in relation to competition for resources and lineage survival strategies.     

Juvenile hormone as a mediator of plasticity in insect life histories

Insects display much variation in life histories mediated by juvenile hormone. We focus on the contribution of JH to variations in migratory life histories. In many migrants such as the large milkweed bug and the monarch butterfly, JH directly influences migratory flight and the relation between flight and reproduction (oogenesis-flight syndrome). In the true armyworm, JH regulates interactions among female calling, pheromone production, ovarian development, and migration with varying blends of structurally related forms of JH and JH acid. A role for JH also occurs in wing polymorphisms. Aphids regulate wing production via JH-mediated maternal effects; and in crickets, JH esterase modulates the JH influence on wing form. In addition, JH is implicated in wing muscle histolysis. The comprehensive Fairbairn model for JH regulation of wing polymorphisms in flight behavior predicts that JH action will depend on the mode of genetic control, whether single locus or polygenic. Our own studies of the soapberry bug, Jadera haematoloma, reveal a four-morph wing polymorphism in a species rapidly evolving on a new host plant. There are long- and short-winged forms, and the long-winged form displays three degrees of flight muscle histolysis. The polymorphism is subject to both genetic and environmental variations that are mediated by JH. Application of methoprene increases the frequency of the short-winged forms, but there is both within- and between-population genetic variation and genotype by environment interaction (plasticity) in the response to JH. Arch. Insect Biochem. Physiol. 35:359–373, 1997. © 1997 Wiley-Liss, Inc.     

Kinetochore life histories reveal an Aurora-B-dependent error correction mechanism in anaphase

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Environmental variability and semelparity vs. iteroparity as life histories

Research on the evolution of life histories addresses the topic of fitness trade-offs between semelparity (reproducing once in a lifetime) and iteroparity (repeated reproductive bouts per lifetime). Bulmer (1994) derived the relationship v+P(A)<1 (P(A) is the adult survival;vb(S) and b(S) are the offspring numbers for iteroparous and semelparous breeding strategies, respectively), under which a resident semelparous population cannot be invaded by an iteroparous mutant when the underlying population dynamics are stable. We took Bulmer's population dynamics, and added noise in juvenile and adult survival and in offspring numbers. Long-term coexistence of the two strategies is possible in much of the parameter region ofv +P(A)<1 when noise occurs simultaneously in all three components, or (more restricted) when it affects juvenile and adult survival or adult survival and offspring numbers. Iteroparity cannot persist when the environmental variability involves juvenile survival and offspring numbers, or when the noise acts on the three components separately.     

Sperm quality as reflected through morphology in salmon alternative life histories.

Male salmon exhibit alternative mating strategies, as both older anadromous adults and precocious juveniles (parr) participate in the spawning of a single female. This study tested the following hypotheses: 1) different intensities of sperm competition may reflect different sperm tail optima; 2) long spermatozoa are superior to short ones, with an associated cost on sperm longevity; and 3) a disfavored role in sperm competition selects for parr investing more in sperm quality. Comparisons included sperm morphological traits, whereas sperm quality was investigated by motility duration observations, measurement of the sperm adenylate system, and fertilization experiments. No evidence of different adaptive sperm dimensions between the male types was found. Positive association between spermatocrit and energy charge was, however, detected. Sperm length parameters correlated positively with ATP, energy charge, and fertilization success, whereas no evidence for an effect of sperm morphology on longevity was found. Male parr had greater spermatocrit than adults and fertilized equal proportions of eggs as adults despite a pronounced numerical subordinance in the fertilization experiments. It is concluded that a long sperm tail and midpiece may be selected to optimize energetic demands under conditions of increased sperm competition intensity.