Predictions Of Life-history Theory Research Articles

Trait-dependent associations between early- and late-life reproduction in a wild mammal.

Early- versus late-life trade-offs are a central prediction of life-history theory that are expected to shape the evolution of ageing. While ageing is widely observed in wild vertebrates, evidence that early-late trade-offs influence ageing rates remains limited. Vertebrate reproduction is a complex, multi-stage process, yet few studies have examined how different aspects of early-life reproductive allocation shape late-life performance and ageing. Here, we use longitudinal data from a 36-year study of wild Soay sheep to show that early-life reproduction predicts late-life reproductive performance in a trait-dependent manner. Females that started breeding earlier showed more rapid declines in annual breeding probability with age, consistent with a trade-off. However, age-related declines in offspring first-year survival and birth weight were not associated with early-life reproduction. Selective disappearance was evident in all three late-life reproductive measures, with longer-lived females having higher average performance. Our results provide mixed support for early-late reproductive trade-offs and show that the way early-life reproduction shapes late-life performance and ageing can differ among reproductive traits.

Convergence of life history and physiology during range expansion toward the phenotype of the native sister species

In our globally changing planet many species show range expansions whereby they encounter new thermal regimes that deviate from those of their source region. Pressing questions are to what extent and through which mechanisms, plasticity and/or evolution, species respond to the new thermal regimes and whether these trait changes are adaptive. Using a common-garden experiment, we tested for plastic and evolutionary trait changes in life history and a set of understudied biochemical/physiological traits during the range expansion of the damselfly Ischnura elegans from France into a warmer region in Spain. To assess the adaptiveness of the trait changes we used the phenotype of its native sister species in Spain, I. graellsii, as proxy for the locally adapted phenotype. While our design cannot fully exclude maternal effects, our results suggest that edge populations adapted to the local conditions in the newly invaded region through the evolution of a faster pace-of-life (faster development and growth rates), a smaller body size, a higher energy budget and increased expression levels of the heat shock gene DnaJ. Notably, based on convergence toward the phenotype of the native sister species and its thermal responses, and the fit with predictions of life history theory these potential evolutionary changes were likely adaptive. Nevertheless, the convergence toward the native sister species is incomplete for thermal plasticity in traits associated with anaerobic metabolism and melanization. Our results highlight that evolution might at least partly contribute in an adaptive way to the persistence of populations during range expansion into new thermal environments and should be incorporated when predicting and understanding species' range expansions.

Changes in the body size of black-veined white, Aporia crataegi (Lepidoptera: Pieridae), recorded in a natural population in response to different spring weather conditions and at different phases of an outbreak

Changes in body size in response to environmental factors (especially temperature) is one of the crucial traits studied in connection with insect adaptation to climate change. However, current data on the strength and direction of temperature-size responses in Lepidoptera are inconsistent and the reasons for this are unclear. This study investigates the relationship between the adult size of Aporia crataegi L. (Lepidoptera: Pieridae) and spring weather conditions (temperature and rainfall) at different phases in its outbreak cycle (low population density or high abundance). The forewing area of A. crataegi, a univoltine and irruptive Lepidopteran species, was used as a proxy for overall body size. It was found that temperature in the last month before imago emergence (May) and temperature in the larval growth period following overwintering (April) had differing effects on imago size. The fact that the wing size of both male and female A. crataegi increased following higher temperatures in May corresponds with the converse temperature-size rule and is consistent with the predictions of life history theory for univoltine species. Conversely, while imago size decreased following higher temperatures in April, increased rainfall had a slightly positive effect on imago size. The wings of A. crataegi at the peak of abundance were larger than when sampled during periods of low population density, contradicting available data on changes in the body weight of A. crataegi recorded during outbreaks.

Positive early-late life-history trait correlations in elephant seals.

Correlations between early- and late-life performance are a major prediction of life-history theory. Negative early-late correlations can emerge because biological processes are optimized for early but not late life (e.g., rapid development may accelerate the onset of senescence; "developmental theory of aging") or because allocation to early-life performance comes at a cost in terms of late-life performance (as in the disposable soma theory). But variation in genetic and environmental challenges that each individual has to cope with during early life may also lead to positive early-late life-history trait correlations (the "fixed heterogeneity" or "individual quality" hypothesis). We analyzed individual life-history trajectories of 7,420 known-age female southern elephant seals (Mirounga leonina) monitored over 36yr to determine how actuarial senescence (a proxy for late-life performance) correlate with age at first reproduction (a proxy for early-life performance). As some breeding events may not be detected in this field study, we used a custom "multievent" hierarchical model to estimate the age at first reproduction and correlate it to other life-history traits. The probability of first reproduction was 0.34 at age 3, with most females breeding for the first time at age 4, and comparatively few at older ages. Females with an early age of first reproduction outperformed delayed breeders in all aspects we considered (survival, rate of senescence, net reproductive output) but one: early breeders appeared to have an onset of actuarial senescence 1 yr earlier compared to late breeders. Genetics and environmental conditions during early life likely explain the positive correlation between early- and late-life performance. Our results provide the first evidence of actuarial senescence in female southern elephant seals.

Optimal control of root nodulation – Prediction of life history theory of a mutualistic system

Legumes produce root nodules containing symbiotic rhizobial bacteria that convert atmospheric molecular nitrogen into ammonia or related nitrogenous compounds. The host plant supplies photosynthetic products to root nodules forming a mutualistic system. Legumes have physiological mechanisms for regulating nodule production with chemical signals produced in leaves, called the autoregulation of nodulation. In this paper, we discuss the optimal number of root nodules that maximizes the performance of the host plant. Here, we study two models. In the stationary plant model, the acquired photosynthetic products minus cost and loss are used for reproduction. In the growing plant model, the excess material is invested to produce leaves, roots, and root nodules, resulting in the exponential growth of the whole plant. The analysis shows that having root nodules is beneficial to the plant for a high leaf nitrogen content, faster plant growth rate, a short leaf longevity, a low root/shoot ratio, and low soil nutrient concentration. We discuss the long-distance control of nodulation-autoregulation and dependence on the environmental conditions of terrestrial plants considering these results.

Age-dependent plasticity in reproductive investment, regeneration capacity and survival in a partially clonal animal (Hydra oligactis).

Asexual reproduction diversifies life-history priorities and is associated with unusual reproduction and somatic maintenance patterns, such as constant fertility with age, extensive regeneration ability and negligible senescence. While age-dependent plasticity in relative allocation to sexual versus asexual reproductive modes is relatively well studied, the modulation of somatic maintenance traits in parallel with age-dependent reproduction is much less well understood in clonal or partially clonal animals. Here, we asked how age-dependent investment into sexual and asexual reproduction co-varies with somatic maintenance such as regeneration in a partially clonal freshwater cnidarian Hydra oligactis, a species with remarkable regeneration abilities and experimentally inducible sex. We induced gametogenesis by lowering temperature at two ages, 1 or 4weeks after detachment from an asexual parent, in animals of a male and a female clone. Then we measured phenotypically asexual and sexual reproductive traits (budding rate, start day and number of sexual organs) together with head regeneration rate, survival and the cellular background of these traits (number of reproductive and interstitial stem cells) for 2 or 5months. Younger animals had higher asexual reproduction while individuals in the older group had more intensive gametogenesis and reproductive cell production. In parallel with these age-dependent reproductive differences, somatic maintenance of older individuals was also impacted: head regeneration, survival and interstitial stem cell numbers were reduced compared to younger polyps. Some of the traits investigated showed an ontogenetic effect, suggesting that age-dependent plasticity and a fixed ontogenetic response might both contribute to differences between age groups. We show that in H. oligactis asexual reproduction coupled with higher somatic maintenance is prioritized earlier in life, while sexual reproduction with higher maintenance costs occurs later if sex is induced. These findings confirm general life-history theory predictions on resource allocation between somatic maintenance and sexual reproduction applying in a partially clonal species. At the same time, our study also highlights the age-dependent integration of these resource allocation decisions with sexual/asexual strategies. Accounting for age-related differences might enhance repeatability of research done with clonal individuals derived from mass cultures.

Carry-over effects of the larval environment on the post-metamorphic performance of Bombina variegata (Amphibia, Anura)

Metamorphs of the yellow-bellied toad, Bombina variegata, vary widely in size at metamorphosis in the field. We performed a replicated outdoor mesocosm study to simulate the environmental factor combinations in permanent and ephemeral breeding sites and to quantify their effects on tadpole development (duration of the larval period, metamorph size and body condition). Looking for potential carry-over effects of the larval environment, we quantified locomotor performance of all metamorph phenotypes originating from the mesocosms immediately after metamorphosis under controlled conditions. In contrast to the prediction of life-history theory, tadpoles were unable to adjust developmental rate to water availability, but metamorphs originating from the ephemeral pond treatment were smaller and had a lower body condition than those from the permanent pond treatment. Size-dependent carry-over effects included the length of the first jump following tactile stimulation, burst performance (total length of spontaneous jumps) and endurance (total distance covered in 10 forced jumps). A size-independent effect of larval environment was the prolonged locomotor effort to escape (5.7 consecutive jumps following initial stimulus) of metamorphs from the ephemeral pond treatment compared to same-sized ones (3.7 jumps) from the permanent pond treatment. Thus, we demonstrate that carry-over effects of larval environment on metamorph phenotype and behaviour cause a considerable variation in fitness in the early terrestrial stage of B. variegata. Informed conservation management of endangered populations in the northern range should therefore include the provision of small permanent breeding ponds promoting larger and fitter metamorphs.

The effects of age at menarche and first sexual intercourse on reproductive and behavioural outcomes: A Mendelian randomization study.

There is substantial variation in the timing of significant reproductive life events such as menarche and first sexual intercourse. Life history theory explains this variation as an adaptive response to an individual’s environment and it is important to examine how traits within life history strategies affect each other. Here we applied Mendelian randomization (MR) methods to investigate whether there is a causal effect of variation in age at menarche and age at first sexual intercourse (markers or results of exposure to early life adversity) on outcomes related to reproduction, education and risky behaviour in UK Biobank (N = 114 883–181 255). Our results suggest that earlier age at menarche affects some traits that characterize life history strategies including earlier age at first and last birth, decreased educational attainment, and decreased age at leaving education (for example, we found evidence for a 0.26 year decrease in age at first birth per year decrease in age at menarche, 95% confidence interval: -0.34 to -0.17; p < 0.001). We find no clear evidence of effects of age at menarche on other outcomes, such as risk taking behaviour. Age at first sexual intercourse was also related to many life history outcomes, although there was evidence of horizontal pleiotropy which violates an assumption of MR and we therefore cannot infer causality from this analysis. Taken together, these results highlight how MR can be applied to test predictions of life history theory and to better understand determinants of health and social behaviour.

Illness, Social Disadvantage, and Sexual Risk Behavior in Adolescence and the Transition to Adulthood.

This study investigated the influence of illness on sexual risk behavior in adolescence and the transition to adulthood, both directly and through moderation of the impact of social disadvantage. We hypothesized positive effects for social disadvantages and illness on sexual risk behavior, consistent with the development of faster life history strategies among young people facing greater life adversity. Using the first two waves of the National Longitudinal Study of Adolescent to Adult Health, we developed a mixed-effects multinomial logistic regression model predicting sexual risk behavior in three comparisons: risky nonmonogamous sex versus safer nonmonogamous sex, versus monogamous sex, and versus being sexually inactive, by social characteristics, illness, interactions thereof, and control covariates. Multiple imputation was used to address a modest amount of missing data. Subjects reporting higher levels of illness had lower odds of having safer nonmonogamous sex (OR = 0.84, p < .001), monogamous sex (OR = 0.82, p < .001), and being sexually inactive (OR = 0.74, p < .001) versus risky nonmonogamous sex, relative to subjects in better health. Illness significantly moderated the sex (OR = 0.88, p < .01), race/ethnicity (e.g., OR = 1.21, p < .001), and childhood SES (OR = 0.94; p < .01) effects for the sexually inactive versus risky nonmonogamous sex comparison. Substantive findings were generally robust across waves and in sensitivity analyses. These findings offer general support for the predictions of life history theory. Illness and various social disadvantages are associated with increased sexual risk behavior in adolescence and the transition to adulthood. Further, analyses indicate that the buffering effects of several protective social statuses against sexual risk-taking are substantially eroded by illness.

Increased allocation to reproduction reduces future competitive ability in a burying beetle.

The existence of a trade-off between current and future reproduction is a fundamental prediction of life history theory. Support for this prediction comes from brood size manipulations, showing that caring for enlarged broods often reduces the parent's future survival or fecundity. However, in many species, individuals must invest in competing for the resources required for future reproduction. Thus, a neglected aspect of this trade-off is that increased allocation to current reproduction may reduce an individual's future competitive ability. We tested this prediction in the burying beetle, Nicrophorus vespilloides, a species where parents care for their offspring and where there is fierce competition for resources used for breeding. We manipulated reproductive effort by providing females with either a small brood of 10 larvae or a large brood of 40 larvae and compared the ability of these females, and virgin females that had no prior access to a carcass, to compete for a second carcass against a virgin competitor. We found that increased allocation to current reproduction reduced future competitive ability, as females that had cared for a small brood were more successful when competing for a second carcass against a virgin competitor than females that had cared for a large brood. In addition, the costs of reproduction were offset by the benefits of feeding from the carcass during an initial breeding attempt, as females that had cared for a small brood were better competitors than virgin females that had no prior access to a carcass, whilst females that had cared for a large brood were similar in competitive ability to virgin females. Our results add to our understanding of the trade-off between current and future reproduction by showing that this trade-off can manifest through differences in future competitive ability and that direct benefits of reproduction can offset some of these costs.

Parasite burden in a short-lived chameleon, Furcifer labordi

Life history theory predicts that species with shorter lifespan should show higher investments into growth and reproduction at the expense of immune defenses. Labord's chameleon (Furcifer labordi) is the tetrapod with the shortest known life span. To investigate to which extent immunosenescence influences the die-off of these chameleons when they are only about 6 months old, we examined the gastrointestinal-, blood- and ectoparasite burden in F. labordi in Kirindy Forest (western Madagascar) and compared them with sympatric and longer living F. cf. nicosiai. Moreover, we included data from wild F. labordi that were singly housed under ambient conditions with daily food and water supply. Gastrointestinal parasite prevalence of wild F. labordi increased dramatically during the last 3 months of their lives, which include the reproductive period. Furcifer cf. nicosiai was found to have a belated increase in gastrointestinal parasites compared to F. labordi. In F. cf. nicosiai higher prevalence of blood parasites were found, which probably result from the longer exposure to the arthropod intermediate host. Both species showed infestations with ectoparasites, which peaked in the rainy season but disappeared towards the dry season. Male F. labordi showed a significantly higher prevalence of gastrointestinal - and ectoparasites and higher intensities of coccidians and ectoparasites than females. Males of F. cf. nicosiai exhibited higher prevalence of blood- and ectoparasites, as well as higher intensities in ectoparasites. Caged individuals of both sexes showed delayed senescence, reduced parasite burden and lived longer than their wild conspecifics. Overall, the increase in the prevalence in gastrointestinal - and blood parasites towards the disappearance of the wild population of F. labordi indicates that this species invests comparatively less energy in efficient immune system function, supporting the prediction of life history theory.

The life history of European Middle Pleistocene equids: first insights from bone histology

ABSTRACT Evolutionary trends in body size are a central issue of study in Palaeontology. However, and despite body size being one of the most important life history traits of an animal, iconic size-decrease trends such as the one experienced by Equus during the European Pleistocene have never been analysed under a life history framework. Here, we studied the metapodial bone histology of two large Middle Pleistocene species (Equus mosbachensis and Equus steinheimensis) to reconstruct key life history traits that correlate with body size (e.g. size at birth, growth rate), and compare them with that of smaller extant Equus (Equus grevyi, Equus quagga, Equus zebra and Equus hemionus). Our results show that neonatal size of these Middle Pleistocene equids fits predictions from body mass scaling. We estimate a similar age of epiphyseal fusion for the metapodia of E. mosbachensis and E. steinheimensis and that of extant equids. Our findings also reveal that extinct equids grew at higher rates than extant Equus. This result conforms to the predictions of life history theory on environments with different levels of resource availability and provides a new framework of study for body size shifts on European Pleistocene equids.

Enlarged Egg Size Increases Offspring Fitness of a Frog Species on the Zhoushan Archipelago of China

Egg size represents maternal investment and is an important life-history trait. It also is assumed to have an effect on offspring fitness. Life-history theory predicts that oviparous animals on islands will produce enlarged eggs because of increased maternal investment to improve offspring performance to facilitate intra-specific competition. The life-history theory, developed during the 1950s, provides a possible explanation for the “island rule”, but this rule has seldom been tested. Although several studies have detected a positive relationship between egg size and offspring fitness, it is difficult to exclude the covarying effects on offspring performance, such as genetic variation and developmental plasticity; predictions made using the life-history theory on the islands have not been tested. In this study, we have evaluated the relationship between egg size and offspring fitness on 20 islands in the Zhoushan Archipelago and two nearby mainland sites. To exclude covarying effects, we compared larval performance among different egg sizes in three levels: among siblings within clutches, among clutches within populations, and among different islands. The results showed that frogs on most of the islands did produce enlarged eggs and that their larvae had improved larval fitness. Additionally, at all three levels, the offspring that evolved from enlarged eggs had increased offspring fitness. The results of this study indicate that, for the first time, the life-history theory predictions concerning egg size and offspring fitness are supported.

Behaviourally specialized foragers are less efficient and live shorter lives than generalists in wasp colonies

A widely held assumption in ecology is that specialists are more efficient than generalists. However, empirical evidence for this fundamental assumption is surprisingly scarce and often contradictory. Theoretically, the evolution of alternative life history strategies is underpinned by a trade-off between activity levels and survival. We investigated the consequences of specialization in a foraging context, by comparing the performance and longevity of closely related individuals in a social insect, the common wasp (Vespula vulgaris). Using radio-frequency identification technology, we monitored the lifetime foraging activity of individual wasps from three colonies kept under natural foraging conditions. Returning foragers were video-recorded as they passed the nest entrance so that their foraging load could be assessed. There were substantial differences in foraging activity and survival within and between colonies. At the colony level, foraging specialization was weak. Yet, workers within each nest demonstrated a remarkable range of foraging specialization levels (defined as the degree of overlap between individual and colony-level task allocation) and efficiencies (defined by the number of successful trips and trip duration). We found that specialist foragers were less efficient than generalist siblings within the same colony. Behavioural specialists accomplished fewer successful trips per foraging day, and their trips were typically relatively longer. Specialized foragers also showed reduced life expectancy. The mortality risk was higher for individuals spending relatively more time in the field, yet we found no link between the level of specialization and relative field exposure. Our extensive dataset of unprecedented detail provides strong empirical evidence that behavioural specialization is not associated with a better lifetime performance, on the contrary, the opposite appears true for the common wasp. We also show that the survival of genetically similar individuals can be linked to life-long differences in behaviour according to classical life-history theory predictions.

Timing is everything: Evaluating behavioural causal theories of Britain's industrialisation.

Baumard's thesis that the English Industrial Revolution can be explained by Life History Theory's predictions for psychological development is a progression of much literature in economic and social history. However, the theory suffers from its reliance on increasingly fragile data indicators for "wealth" and its focus on "innovation" as new research begins to explore sectoral dynamics in long run growth.

Effects of aging on timing of hibernation and reproduction

Small hibernators are long-lived for their size because seasonal dormancy greatly reduces predation risk. Thus, within a year, hibernators switch between states of contrasting mortality risk (active season versus hibernation), making them interesting species for testing the predictions of life-history theory. Accordingly, we hypothesized that, with advancing age and hence diminishing reproductive potential, hibernators should increasingly accept the higher predation risk associated with activity to increase the likelihood of current reproductive success. For edible dormice (Glis glis) we show that age strongly affects hibernation/activity patterns, and that this occurs via two pathways: (i) with increasing age, dormice are more likely to reproduce, which delays the onset of hibernation, and (ii) age directly advances emergence from hibernation in spring. We conclude that hibernation has to be viewed not merely as an energy saving strategy under harsh climatic conditions, but as an age-affected life-history trait that is flexibly used to maximize fitness.

Brood rearing has an immediate survival cost for female Chinese Grouse Tetrastes sewerzowi

Reproductive activities can incur various costs to breeding individuals in birds. One cost is that reproduction decreases survival probabilities of attendant individuals, which may have a major effect on population demography. During brood rearing, adults of precocial species usually make extensive movements to lead their young to sites with adequate food resources and dense cover. However, few studies have evaluated the effects of brood movement on attendant precocial adults. In this study, we tracked female Chinese Grouse during brood-rearing periods using radiotelemetry at Lianhuashan Nature Reserve, Gansu, China, during 2010–2012, to evaluate the effects of brood rearing and movement distances on females’ survival probabilities using known fate models in program MARK. All 41 females attempted to breed, and 30 females successfully hatched at least one chick; 11 failed during the incubation period. Although females with broods moved more extensively than females without broods, movement distances did not influence survival probabilities of attendant females. Seven females with broods were killed by predators, resulting in a lower survival probability (0.958 ± 0.016 weekly survival and 0.679 ± 0.099 through the 9-weeks brood-rearing periods) than for females without broods, which all survived the brood-rearing periods. Our results agree with the prediction of life history theory, which assumes that cost must correlate with reproduction. The extensive movement patterns found in this study might reflect a lack of suitable brood-rearing habitat near nest sites, which might be detrimental to chick survival and influence the persistence of this population.

Fitness implications of sex-specific catch-up growth in Nephila senegalensis, a spider with extreme reversed SSD.

BackgroundAnimal growth is often constrained by unfavourable conditions and divergences from optimal body size can be detrimental to an individual’s fitness, particularly in species with determinate growth and a narrow time-frame for life-time reproduction. Growth restriction in early juvenile stages can later be compensated by means of plastic developmental responses, such as adaptive catch-up growth (the compensation of growth deficits through delayed development). Although sex differences regarding the mode and degree of growth compensation have been coherently predicted from sex-specific fitness payoffs, inconsistent results imply a need for further research. We used the African Nephila senegalensis, representing an extreme case of female-biased sexual size dimorphism (SSD), to study fitness implications of sex-specific growth compensation. We predicted effective catch-up growth in early food-restricted females to result in full compensation of growth deficits and a life-time fecundity (LTF) equivalent to unrestricted females. Based on a stronger trade-off between size-related benefits and costs of a delayed maturation, we expected less effective catch-up growth in males.MethodsWe tracked the development of over one thousand spiders in different feeding treatments, e.g., comprising a fixed period of early low feeding conditions followed by unrestricted feeding conditions, permanent unrestricted feeding conditions, or permanent low feeding conditions as a control. In a second experimental section, we assessed female fitness by measuring LTF in a subset of females. In addition, we tested whether compensatory development affected the reproductive lifespan in both sexes and analysed genotype-by-treatment interactions as a potential cause of variation in life-history traits.ResultsBoth sexes delayed maturation to counteract early growth restriction, but only females achieved full compensation of adult body size. Female catch-up growth resulted in equivalent LTF compared to unrestricted females. We found significant interactions between experimental treatments and sex as well as between treatments and family lineage, suggesting that family-specific responses contribute to the unusually large variation of life-history traits in Nephila spiders. Our feeding treatments had no effect on the reproductive lifespan in either sex.DiscussionOur findings are in line with predictions of life-history theory and corroborate strong fecundity selection to result in full female growth compensation. Males showed incomplete growth compensation despite a delayed development, indicating relaxed selection on large size and a stronger trade-off between late maturation and size-related benefits. We suggest that moderate catch-up growth in males is still adaptive as a ‘bet-hedging’ strategy to disperse unavoidable costs between life-history traits affected by early growth restriction (the duration of development and adult size).

Environmental cues influence parental brood structure decisions in the burying beetle Nicrophorus marginatus

Parents evaluate multiple extrinsic and intrinsic cues when making decisions associated with reproduction. These decisions often reflect classic trade-offs between the cost of a strategy and its perceived fitness payoff. Life history theory predicts that when parents experience austere conditions, reproductive success is increased by producing fewer but larger offspring with a competitive advantage in this environment. Conversely, parents experiencing favorable conditions are expected to increase current reproductive success by favoring quantity over quality of offspring. We tested the predictions of life history theory using Nicrophorus marginatus (Coleoptera: Silphidae), a burying beetle species that exhibits infanticide during biparental care and hypervariable adult size across populations, by employing a factorial design that manipulated density and nutritional quality of food. We measured (1) the average number of offspring produced, (2) the average individual size of offspring, and (3) the sex ratio of the offspring. We found no effect of density or food quality on offspring sex ratio, but mean offspring size and number differed between low and high-density treatments. Nutritional environment interacted with density effects such that parents with access to high quality diets were able to modulate offspring size and number to match the perceived competitive environment, whereas those in poor nutritional condition appeared to exhibit physiological constraints to producing optimal brood structures.

The Female Perspective of Personality in a Wild Songbird: Repeatable Aggressiveness Relates to Exploration Behaviour

Males often express traits that improve competitive ability, such as aggressiveness. Females also express such traits but our understanding about why is limited. Intraspecific aggression between females might be used to gain access to reproductive resources but simultaneously incurs costs in terms of energy and time available for reproductive activities, resulting in a trade-off. Although consistent individual differences in female behaviour (i.e. personality) like aggressiveness are likely to influence these reproductive trade-offs, little is known about the consistency of aggressiveness in females. To quantify aggression we presented a female decoy to free-living female great tits (Parus major) during the egg-laying period, and assessed whether they were consistent in their response towards this decoy. Moreover, we assessed whether female aggression related to consistent individual differences in exploration behaviour in a novel environment. We found that females consistently differed in aggressiveness, although first-year females were on average more aggressive than older females. Moreover, conform life history theory predictions, ‘fast’ exploring females were more aggressive towards the decoy than ‘slow’ exploring females. Given that personality traits are often heritable, and correlations between behaviours can constrain short term adaptive evolution, our findings highlight the importance of studying female aggression within a multivariate behavioural framework.