Evolution Of Life-history Strategies Research Articles

Cuckoos, cowbirds and hosts: adaptations, trade-offs and constraints

The interactions between brood parasitic birds and their host species provide one of the best model systems for coevolution. Despite being intensively studied, the parasite-host system provides ample opportunities to test new predictions from both coevolutionary theory as well as life-history theory in general. I identify four main areas that might be especially fruitful: cuckoo female gentes as alternative reproductive strategies, non-random and nonlinear risks of brood parasitism for host individuals, host parental quality and targeted brood parasitism, and differences and similarities between predation risk and parasitism risk. Rather than being a rare and intriguing system to study coevolutionary processes, I believe that avian brood parasites and their hosts are much more important as extreme cases in the evolution of life-history strategies. They provide unique examples of trade-offs and situations where constraints are either completely removed or particularly severe.

Seasonal effects on circulating leptin in the lizard Sceloporus undulatus from two populations

Characterizing leptin's structure and function in mammals has been the subject of thousands of studies since 1994. Recently, the study of leptin has expanded to include its distribution in non-mammalian taxa, and the role that leptin plays in the reproductive axis. We demonstrated in a previous study that Sceloporus undulatus, fence lizards (ectotherms), express a leptin-like protein. In the current study we quantified seasonal variation in this putative leptin among free-ranging fence lizards from two populations characterized by early and late reproductive maturation (after one or two years, respectively). Immunoblots were performed on whole blood samples to detect leptin and estimate its titer. Leptin titers were higher in the reproductive population of S. undulatus (early maturing: 2.5 ± 0.2 μg/mL; late-maturing 2.2 ± 0.3 μg/mL; mean ± 2 S.E.), but both populations showed the same seasonal pattern. Leptin titers were lowest in fall when fat stores are expected to be highest (spring: 2.6 ± 0.3 μg/mL; summer: 2.6 ± 0.3; μg/mL; fall: 1.8 ± 0.3 μg/mL), consistent with findings of seasonal variation in free-ranging mammals. Our data support previous work asserting that lizards express leptin and that it has a similar physiological function in endotherms and ectotherms. Our long-term goal is to use leptin to manipulate age at maturity and to test fundamental questions in the evolution of life-history strategies.

Growth in small dinosaurs and pterosaurs: the evolution of archosaurian growth strategies

ABSTRACT Histological evidence of the bones of pterosaurs and dinosaurs indicates that the typically large forms of these groups grew at rates more comparable to those of birds and mammals than to those of other living reptiles. However, Scutellosaurus, a small, bipedal, basal thyreophoran ornithischian dinosaur of the Early Jurassic, shows histological features in its skeletal tissues that suggest relatively lower growth rates than in those of larger dinosaurs. In these respects Scutellosaurus, like other small dinosaurs such as Orodromeus and some basal birds, is more like young, rapidly growing crocodiles than larger, more derived ornithischians (hadrosaurs) and all saurischians (sauropods and theropods). Similar patterns can be seen in small, mostly basal pterosaurs such as Eudimorphodon and Rhamphorhynchus. However, superficial similarities to crocodile bone growth belie some important differences, which are most usefully interpreted in phylogenetic and ontogenetic contexts. Large size evolved secondarily in several dinosaurian and pterosaurian lineages. We hypothesize that this larger size was made possible by rapid growth strategies that are reflected by characteristic highly vascularized fibro-lamellar bone tissues that comprise most of the cortex. Dinosaurs and pterosaurs, like other tetrapodes, generally grew more quickly in early stages and more slowly as growth neared completion. As in other vertebrate groups, taxa of small adult size may have grown at lower rates or for shorter durations than larger taxa did. Phylogenetic patterns suggest that by themselves, the low vascularity and inferred low growth rates seen in small dinosaurs and pterosaurs are not good indicators of thermometabolic regime, because they are correlated so strongly with size. They may reflect mechanical exigencies of small size rather than especially lower growth rates, tied to the process of deposition of particular kinds of bone tissues. The evolution of life history strategies in dinosaurs and pterosaurs, as they relate to rates of growth and adult body sizes, will be better understood as more complete histological studies place these data into phylogenetic and ontogenetic contexts.

Age-specific survival and reproductive probabilities: evidence for senescence in male fallow deer (Dama dama).

Survival and reproduction are key features in the evolution of life-history strategies. In this study, we use capture-mark-resighting and multi-state models to examine survival senescence and reproductive senescence in six successive cohorts of fallow bucks that were studied for 16 years. We found that the overall age-specific survival probabilities of males were highly variable and the best-fitting model revealed that fallow bucks have four life-history stages: yearling, pre-reproductive, prime-age and senescent. Pre-reproductive males (2 and 3 years old) had the highest survival. Survival declined sharply after the age of 9 years, indicating that senescence had begun. When we considered reproducing and non-reproducing males separately, there was no evidence of senescence in the former, and steadily decreasing survival after the onset of social maturity in the latter. Reproduction probability also declined in older males, and thus we provide very strong evidence of senescence. Reproducers had a greater chance of reproducing again in the following year than non-reproducers. Furthermore, there were differences in the survival probabilities, with reproducers consistently surviving better than non-reproducers. In our study population, reproducers allocate more to the effort to reproduce than non-reproducers. Therefore our results indicate the generally higher phenotypic quality of reproducing males. These results, along with earlier studies on the same population, could indicate positive relationships between fitness correlates.

FLASH FLOODS AND AQUATIC INSECT LIFE-HISTORY EVOLUTION: EVALUATION OF MULTIPLE MODELS

In disturbance ecology there is a tension between ecological and evolutionary viewpoints, because while disturbances often cause mortality in populations (an ecological effect), populations may also evolve mechanisms that ameliorate mortality risk (an evolutionary effect). Flash floods cause high mortality in the juvenile aquatic stage of desert stream insects, but these ecological effects may be mitigated by the evolution of life-history strategies that allow the terrestrial adult stage to avoid floods. Life-history theory predicts that, to balance trade-offs between juvenile growth and mortality risk from floods, (1) most individuals should emerge before the peak of the flood season, (2) optimal body size at emergence should decline as flood probability increases, and (3) a second decline in body size at emergence should occur as the reproductive season ends. These predictions were tested with data on body mass at and timing of emergence of the caddisfly Phylloicus aeneus measured in three montane Chihuahuan Desert (Arizona, USA) streams over two years. P. aeneus that had not reached the adult stage were eliminated from site-years that experienced flash floods, suggesting that timing of emergence is an important fitness component. On average 86% of emergence occurred before the long-term (∼100 yr) mean arrival date of the first seasonal flood, supporting prediction 1. The presence of two consecutive declines in body mass at emergence in most site-years was congruent with predictions 2 and 3. To test whether the two declines were associated with increasing flood probability and end of the reproductive season, respectively, maximum-likelihood methods were used to compare five body-size models: a null model that contains no parameters related to flood regime or reproductive season, a seasonal model that incorporates a reproductive time constraint, and three disturbance models that incorporate both reproductive time constraints and flood dynamics. The disturbance models outperformed the other models, suggesting that at least some of the body-mass pattern was influenced by flood dynamics. The timing of the first flood of the season was the most important determinant of observed emergence patterns. Overall, this study demonstrates that aquatic insects can compensate for flash floods by using state-dependent emergence strategies that are synchronized with long-term flood dynamics.

Flash Floods and Aquatic Insect Life-History Evolution: Evaluation of Multiple Models

In disturbance ecology there is a tension between ecological and evolutionary viewpoints, because while disturbances often cause mortality in populations (an ecological effect), populations may also evolve mechanisms that ameliorate mortality risk (an evolutionary effect). Flash floods cause high mortality in the juvenile aquatic stage of desert stream insects, but these ecological effects may be mitigated by the evolution of life-history strategies that allow the terrestrial adult stage to avoid floods. Life-history theory predicts that, to balance trade-offs between juvenile growth and mortality risk from floods, (1) most individuals should emerge before the peak of the flood season, (2) optimal body size at emergence should decline as flood probability increases, and (3) a second decline in body size at emergence should occur as the reproductive season ends. These predictions were tested with data on body mass at and timing of emergence of the caddisfly Phylloicus aeneus measured in three montane Chihuahuan Desert (Arizona, USA) streams over two years. P. aeneus that had not reached the adult stage were eliminated from site-years that experienced flash floods, suggesting that timing of emergence is an important fitness component. On average 86% of emergence occurred before the long-term (∼100 yr) mean arrival date of the first seasonal flood, supporting prediction 1. The presence of two consecutive declines in body mass at emergence in most site-years was congruent with predictions 2 and 3. To test whether the two declines were associated with increasing flood probability and end of the reproductive season, respectively, maximum-likelihood methods were used to compare five body-size models: a null model that contains no parameters related to flood regime or reproductive season, a seasonal model that incorporates a reproductive time constraint, and three disturbance models that incorporate both reproductive time constraints and flood dynamics. The disturbance models outperformed the other models, suggesting that at least some of the body-mass pattern was influenced by flood dynamics. The timing of the first flood of the season was the most important determinant of observed emergence patterns. Overall, this study demonstrates that aquatic insects can compensate for flash floods by using state-dependent emergence strategies that are synchronized with long-term flood dynamics.

EVOLUTION OF LIFE HISTORIES ALONG ELEVATIONAL GRADIENTS: TRADE-OFF BETWEEN PARENTAL CARE AND FECUNDITY

Life history responses to environmental conditions include a combination of fecundity-survival schedules and behavioral strategies that yield the highest fitness in a given environment. In this study, we examined the pattern of covariation in avian life history strategies along an elevational gradient by comparing variation in life history traits, including most components of parental care, between phylogenetically paired taxa from low- and high-elevation sites. We found that high-elevation species had significantly lower annual fecundity but provided greater parental care to their offspring. However, a strong negative relationship between offspring number and duration of parental care along the elevational gradient suggested that high-elevation species were shifting investment from offspring number toward offspring quality. Although adult survival did not differ between high- and low-elevation species, higher juvenile survival may have compensated for lower annual fecundity in high-elevation species. The elevation at which breeding occurred strong- ly influenced the partitioning of parental behavior between sexes. Male participation in nestling provisioning was significantly greater in high-elevation species. In turn, altitudinal variation in the frequency of biparental care closely covaries with the intensity of sexual selection, ultimately resulting in the strong elevational pattern of sexual dimorphism. More- over, elevational variation in costs of development and maintenance of secondary sexual traits constitutes an additional effect on fecundity-survival schedules along elevational gradients. Thus, a trade-off between fecundity and parental care, and associated interactions among morphological, life history, and behavioral traits play important roles in the evolution of life history strategies in birds.

Density‐Dependent Growth and Life History Evolution of Polycyclic Leaf Pathogens: A Continuous Time Growth Model

AbstractA density‐dependent growth model of a polycyclic leaf pathogen is analysed. Viability and fecundity of the pathogen are regulated by the current population density. Spores are produced continuously from a sporulating infection until death of the infection, and consequently, all age classes of infections are present at a certain point in time. The leaf area of the host varies with time. Evolution of life history strategies are studied by letting different pathogen genotypes compete with each other. Evolution of life history strategies and evolution of impact of disease are discussed in relation to the ecology of the host‐pathogen system. The model is exemplified by Erysiphe graminis f. sp. hordei growing on Hordeum vulgare.

Leeches (Oligochaeta?: Euhirudinea), their phylogeny and the evolution of life-history strategies

Leeches (Oligochaeta?: Euhirudinea), their phylogeny and the evolution of life-history strategies

Evolution of Life History Strategies in Ants: Variation in Queen Number and Mode of Colony Founding

Most theoretical models for the evolution of polygyny in ants assume a stable population. Here, we discuss the evolution of life history strategies with perennial life cycles, overlapping generations, and fluctuating populations. We assume two alternative strategies, i.e., monogyny = independent founding strategy (in which the survival rate of foundress queens is low but fecundity of individual queens is high), and polygyny = dependent founding strategy (in which the initial survival rate of queens is high, but queens have low fecundity and short longevity). Adopting the intrinsic rate of natural increase (r) as the fitness measurement, we compare the two strategies under various ranges of life history parameters. Our model suggests that a higher survival rate of queens may compensate for a low fecundity of queens adopting the dependent founding strategy. Since dependent foundresses can skip over the non-sexual producing founding and ergonomic stages, they can quickly initiate sexual production and gain a higher r. This implies that dependent founding, which may result in secondary polygyny, tends to be an r strategist. This early reproduction effect has not been explicitly dealt with in previous discussions of polygyny, because they have omitted population demography. We predict that dependent founding (or polygyny) would be more common in open areas where ant populations might suffer from random density independent population fluctuation than in forests where density dependent regulations could stabilize ant populations. A recent empirical data-set from Okinawa Island actually suggested that polygyny is more common in open areas than in forests. Some other published data that enabled intraspecific comparison also supported our prediction.

A critique of methods for measuring life history trade‐offs

AbstractConstraints have important effects on the evolution of life history strategies, but several difficulties have been encountered in determining constraints empirically. Here we investigate methods for measuring a specific type of constraint known as a trade‐off. A trade‐off between two traits implies a perfect negative correlation between the traits. Trade‐offs may involve more than two traits, however, and pairs of traits involved in such a higher‐dimensional trade‐off may be positively correlated. If some of the traits involved in a trade‐off are omitted from the experimental design, the trade‐off may not even be detectable. Direct measures of trade‐offs are thus complicated, and indirect means of identifying trade‐offs may often provide the only feasible measures.

Population Biology of a Desert Scorpion: Survivorship, Microhabitat, and the Evolution of Life History Strategy

The population dynamics of the vaejovid scorpion, Paruroctonus mesaensis were studied over a 4.5—yr period. A time—specific life table is presented. The shape of the survivorship curve is intermediate between Type I and Type II curves. The survivorship of both immature and mature males was significantly lower than that of females. Density—dependent regulation is evident for both immature and mature animals. Population size is stable and fluctuates around an equilibrium point. However, density—independent events can produce severe mortality. Both the net reproductive rate and the maximum rate of increase (rmax) were very low. Population and life history characteristics indicate that P. mesaenis possesses many attributes of an equilibrium or K—selected species. It is theorized that existence in a burrow predisposed the evolution of life history traits associated with equilibrium species.

Evolution of life history strategies for an asexual annual plant model

At any given moment in time a plant is partitioning total growth mass into its various component parts such as leaves, roots, reproductive material, etc. The view is taken that the plant has evolved a life history strategy to control this partitioning process. This paper illustrates the utility of optimal control theory for use in determining life history strategies which maximize fitness for a given asexual plant model. The optimal control methods are first used on a model previously analyzed by Professor Dan Cohen, who used a different method. His results of a change from 100% vegetable growth to 100% reproductive growth at a fixed switching time is again obtained. This 100% switching result is shown to be more generally applicable by using a qualitatively described model. However the switching time in general is shown to be a function of both leaf mass and time remaining to the end of the growing season. The allocation to toxin production is also considered. It is shown that under this model an inequality between system parameters must be satisfied before the plant should allocate growth to toxin production. Although the particular model explored here may rarely be realistic in nature, these same methods of optimal control theory can be applied in a similar fashion to many other proposed models of plant resource allocation.

Role of environmental variability in the evolution of life history strategies

We reexamine the role of environmental variability in the evolution of life history strategies. We show that normally distributed deviations in the quality of the environment should lead to normally distributed deviations in the logarithm of year-to-year survival probabilities, which leads to interesting consequences for the evolution of annual and perennial strategies and reproductive effort. We also examine the effects of using differing criteria to determine the outcome of selection. Some predictions of previous theory are reversed, allowing distinctions between r and K theory and a theory based on variability. However, these distinctions require information about both the environment and the selection process not required by current theory.

Evolutionarily stable strategies and the evolution of life history strategies: I. Density dependent models

The evolution of life history strategies in density dependent situations is considered in both continuous time and discrete time models. The question of equilibrium and of competitively stable strategies, the “invasion” question, are shown to be completely determined by the number of offspring left at a fixed population size. Evolutionarily stable strategies are shown to maximize population size in a certain sense.

A SEX-LINKED GENE CONTROLLING THE ONSET OF SEXUAL MATURITY IN FEMALE AND MALE PLATYFISH (XIPHOPHORUS MACULATUS), FECUNDITY IN FEMALES AND ADULT SIZE IN MALES

A sex-linked gene, P, controls the onset of sexual maturity in the platyfish, Xiphophorus maculatus. The activity of this gene is correlated with the age and size at which the gonadotropic zone of the adenohypophysis differentiates and becomes physiologically active. Immature fish of all genotypes grow at the same rate; however, as adults, males with "early" genotypes are significantly smaller than males of "late" genotypes, since growth rate declines strongly under the influence of androgenic hormone. Five alleles, P(1)... P(5), have been identified from natural populations that under controlled conditions cause gonad maturation between eight and 73 weeks. P(1)P(1) males become mature at eight weeks and 21 mm, P(2)P(2) and P(3)P(3) males between eleven and 13.5 weeks and 25 to 29 mm, and P(4)P(4) males at 25 weeks and 37 mm. Since P(5) is X-linked, no males homozygous for P(5) could be produced. The difference between P(2) and P(3) is largely based upon their interaction with P(5). P(3)P(5) males mature at 17.5 weeks and 33.5 mm and P(2)P(5) males at 28 weeks and 38 mm. The rate of transformation of the unmodified anal fin into a gonopodium, which is under androgenic control, is directly related to the age at initiation of sexual maturity, ranging from 3.2 weeks in P(1)P(1) males to seven weeks in P(2)P( 5) males. These differences may reflect different levels of circulating gonadotropic and androgenic hormones.-In two genotypes of females, initiation of vitellogenesis was closely correlated with size and this critical size was independent of age (e.g., 21 mm for P(1)P(1 )). In a third genotype (P(1)P(5)) the minimum size for vitellogenesis decreased with increasing age, so that females would mature as early as eleven weeks, provided they had attained at least 29 mm, but at 25 weeks even females as small as 23 mm possessed ripe gonads. For P(5)P(5) females, which become mature between 34 and 73 weeks of age, there is no correlation between size and initiation of vitellogenesis. In all four genotypes of females examined, egg number is strongly correlated with size, but the regression of egg number on standard length is distinct for each genotype. Late maturation of P(5)P( 5) females is not offset by an increased number of eggs; for this genotype there is a strong negative correlation between age and number of eggs. Heterozygous fish always mature later than those homozygous for the "earlier" allele. The site of action of the P locus could be the pituitary gland, the hypothalamus or higher centers of the brain where peripheral information is transduced into an appropriate signal required for the activation of the hypothalamus-pituitary-gonadal axis. The P gene could also control the peripheral information. The platyfish may be a useful model to test theories concerning the evolution of life history strategies.

Selection for Optimal Age-Dependent Schedules of Play Behavior

Positive and negative effects of play behavior are modeled in a life-historical cost-benefit framework in order to derive predictions about optimal age-dependent scheduling of play. The model predicts that not all animal species will exhibit play behavior, that play should be more frequent in K-selected species than in r-selected species, that younger animals should tend to devote more time and energy to play than older animals, that adults of some species should play, and that in the ontogeny of animals exhibiting play, behavior patterns required in adulthood should appear in play as soon as or soon after they appear in the animal's general behavioral repertory. Alternative optimal strategies, one or more including play and one or more excluding play, may frequently evolve in different subpopulations of the same species in the same environment. These theoretical results indicate that the role of random processes in the evolution of life-history strategies and life-table schedules, and in the evolution of play and perhaps of other social behaviors, may be substantially greater than supposed heretofore. Data on play behavior in additional species are of interest in the context of these predictions.

Evolution in a time-varying environment.

A simple model of competition in a time-varying environment was developed and used to discuss the evolution of life-history strategies.

Evolution of Life History Strategies: Implications of Recruitment in a Lizard Population Following Density Manipulations

Evolution of Life History Strategies: Implications of Recruitment in a Lizard Population Following Density Manipulations