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.     

Evolution of unstable and stable biparental care

Evolutionarily stable strategy models suggest that biparentalcare will be stable when parents partially compensate for changesin care by the other parent. Previous work has emphasized therelationship between parental expenditure and the current componentof fitness (e. g., offspring survival and fecundity) in causingpartial compensation. This study shows that partial compensationdepends critically on the effect of current parental expenditureon a parent's future fitness (e. g., survival to and fecundityin subsequent breeding seasons). Partial compensation is favoredand biparental care is stable when future fitness is a concave-downfunction of expenditure (i. e., each increment of expenditureis more costly than the previous). However, when future fitnessis a convex-down function of expenditure (i. e., each incrementof expenditure is less costly) biparental care is unstable.(BehavEcol 7: 490–493(1996)]     

Sex,long life and the evolutionary transition to cooperative breeding in birds

Long life is a typical feature of individuals living in cooperative societies. One explanation is that group living lowers mortality, which selects for longer life. Alternatively, long life may make the evolution of cooperation more likely by ensuring a long breeding tenure, making helping behaviour and queuing for breeding positions worthwhile. The benefit of queuing will, however, depend on whether individuals gain indirect fitness benefits while helping, which is determined by female promiscuity. Where promiscuity is high and therefore the indirect fitness benefits of helping are low, cooperation can still be favoured by an even longer life span. We present the results of comparative analyses designed to test the likelihood of a causal relationship between longevity and cooperative breeding by reconstructing ancestral states of cooperative breeding across birds, and by examining the effect of female promiscuity on the relationship between these two traits. We found that long life makes the evolution of cooperation more likely and that promiscuous cooperative species are exceptionally long lived. These results make sense of promiscuity in cooperative breeders and clarify the importance of life-history traits in the evolution of cooperative breeding, illustrating that cooperation can evolve via the combination of indirect and direct fitness benefits.     

Parental conflict in birds: comparative analyses of offspring development, ecology and mating opportunities

Parents often conflict over how much care to provide to their offspring. This conflict is expected to produce a negative relationship between male and female parental care, the strength of which may be mediated by both ecological and life-history variables. Previous studies have observed such trade-offs, but it is not known how generally they occur. Traditional views of sexual conflict place great importance on ecological factors in determining levels of parental care, whereas alternative views propose that the key determinant is mating opportunity. We carried out a broad-scale comparative study of parental conflict using 193 species from 41 families of birds. Using phylogenetic comparative analysis, we establish the generality of intersexual parental care conflict. We also show that parental conflict, as indicated by the disparity in care between the male and the female, depends on offspring development and mating opportunities, since in precocial species both males and females responded to increased mating opportunities. Altricial birds, however, failed to show these relationships. We also found little influence of breeding climate on parental conflict. Taken together, our results suggest that sexual conflict is a key element in the evolution of parental care systems. They also support the view that the major correlates of the intersexual conflict are mating opportunities for both sexes, rather than the breeding environment.     

Evolution of life histories and yields in experimental populations of Daphnia magna

An experiment on life-history evolution is described in which replicated populations of the Cladoceran Daphnia magna , made up of a standard mixture of clones, were subjected to two contrasting culling regimes, involving removal of small or large individuals. After approximately 150 days of culling, analysis of the life histories showed that genetic differences had emerged between culling regimes. Clones selected by culling small sized individuals grew rapidly through small size classes, whereas those selected by culling large sizes grew slowly through small size classes, with the result that the age at which they became vulnerable to harvesting was delayed. In addition, there was some redistribution of reproduction towards size classes that were not culled. This evolution is consistent with a major decline observed in the yield from populations in which large individuals were culled. We argue that changes of this general kind are likely to take place in exploited populations and that serious consideration should be given to evolutionary aspects of the management of such populations.     

Life history evolution in cichlids 1: revisiting the evolution of life histories in relation to parental care

Empirical links between egg size and duration of parental care in fishes have generated a considerable amount of theory concerning life history evolution. However, to date, this link has not been investigated in relation to other important life-history traits such as clutch size and body size, or while controlling for shared ancestry between species. We provide the first phylogenetically based tests using a database with information on egg size, clutch size, body size and care duration in cichlid fishes (Cichlidae). Multiple regression analyses, based on independent contrasts on both the species and the genus level, showed that clutch size is the variable most closely related to duration of care. This pattern appeared to be driven by post-hatch care relationships. Our results show that, contrary to expectation, there is no positive link between egg size and care duration in Cichlidae. Instead, greater reproductive output through increased clutch size investment appears to have coevolved with greater care of offspring. We suggest that re-evaluation of the generality of current models of the evolution of egg size under parental care in fishes is needed.     

Ecological immunology: life history trade-offs and immune defense in birds

There has been considerable recent interest in the effects oflife-history decisions on immunocompetence in birds. If immunocompetenceis limited by available resources, then trade-offs between investmentin life-history components and investment in immunocompetencecould be important in determining optimal life-history traits.For this to be true: (1) immunocompetence must be limited byresources, (2) investment in life-history components must benegatively correlated with immunocompetence, and (3) immunocompetencemust be positively correlated with fitness. To gather such empiricaldata, ecologists need to be able to measure immunocompetence.We review techniques used to measure immunocompetence and howthey are applied by ecologists. We also consider the componentsof the immune system that constitute immunocompetence and evaluatethe possible consequences of measuring immunocompetence in differentways. We then review the empirical evidence for life-historytrade-offs involving immune defense. We conclude that thereis some evidence suggesting that immunocompetence is limitedby resources and that investment in certain life-history componentsreduces immunocompetence. However, the evidence that immunocompetenceis related to fitness is circumstantial at present, althoughconsistent with the hypothesis that immunocompetence and fitnessare positively correlated. We argue that future work needs toexamine the fitness effects of variation in immunocompetenceand suggest that artificial selection experiments offer a potentiallyimportant tool for addressing this issue.     

The evolution of parental care in shorebirds: life histories, ecology, and sexual selection

Parental care is expected to evolve according to a trade-offbetween the benefits of increased survival of offspring andcosts of reduced survival and future reproduction of adults.Here we investigate the components of this life-history trade-offin shorebirds (Charadriides, excluding Laroidea), an avian infraorderdisplaying an unusual diversity in extent of care by each sex.We show that evolutionary increases in the duration of carein one sex are associated with decreased care by the other.We found no evidence that various hypothesised benefits of careprovide a general explanation for the duration of care by eitheror both sexes, although parental feeding of the young was tooconservative for comparisons. Sexual dimorphism in body sizehad a similar relationship to parental care in both sexes: reductionsin duration of care by either sex were matched by increasesin the size of that sex relative to the other. Whereas thispattern could be explained by sexual selection in males, itwas retained within socially monogamous females. Reduced carein males (but not in females) appears to have facilitated theevolution of greater migration distances. These results suggestthat parental care has had different causes and consequencesin each sex. Benefits of desertion due to sexual selection aremore clearly demonstrable for males, whereas correlates of careare less clear for females     

Evolutionary perturbations of optimal life histories

Summary An optimal age-structured life history is perturbed by increasing the mortality factors specific to an agek. These can be density dependent (DD) or independent (DI), avoidable or unavoidable. The last two refer to whether their effect on any individual depends or not on how much energy it devotes to defence. Agespecific trade-offs between the allocation of energy to defence and fecundity exist: survival probabilities through each agex, P_x, are concave decreasing functions of the fecundity per unit size at that age,b_x. These are constraints for the optimal life history. The changes induced by perturbation are evaluated by equations that predict whether some extra energy is diverted towards survivorship at the expense of fecundity or vice versa. The model predicts that for DI environments the degree of avoidability of the mortality source perturbed, is a decisive factor for the strategy selected at agek, but not for any other age class. DD environments are more complex since all ages are simultaneously embedded in density effects. The perturbations not only act directly — as in the DI situation — but also indirectly through their effect on equilibrium density,N^*. When any kind of mortality source becomes more intense at agek, N^* always decreases and all ages react in consequence according to the effect of density on each age-specific trade-off. Either coincidental or opposing reactions can be expected from direct and indirect effects. The resultant strategy for any age would be a matter of magnitude comparisons. Some possible general patterns are discussed.     

Sex-biased survival predicts adult sex ratio variation in wild birds

Adult sex ratio (ASR) is a central concept in population demography and breeding system evolution, and has implications for population viability and biodiversity conservation. ASR exhibits immense interspecific variation in wild populations, although the causes of this variation have remained elusive. Using phylogenetic analyses of 187 avian species from 59 families, we show that neither hatching sex ratios nor fledging sex ratios correlate with ASR. However, sex-biased adult mortality is a significant predictor of ASR, and this relationship is robust to 100 alternative phylogenetic hypotheses, and potential ecological and life-history confounds. A significant component of adult mortality bias is sexual selection acting on males, whereas increased reproductive output predicts higher mortality in females. These results provide the most comprehensive insights into ASR variation to date, and suggest that ASR is an outcome of selective processes operating differentially on adult males and females. Therefore, revealing the causes of ASR variation in wild populations is essential for understanding breeding systems and population dynamics.     

Evolution of displays within the pair bond

Although sexual selection is an important cause of display evolution, in socially monogamous species (e.g. many birds), displays continue after formation of the pair bond. Here, we consider that these displays evolve because they stimulate the partner to increase investment in offspring. Our study is motivated by elaborate mutual displays in species that are largely monomorphic and have long-term pair bonds (e.g. the great crested grebe, Podiceps cristatus) and by many empirical results evidencing that display manipulation affects parental investment. Using population genetic models, we show that a necessary condition for the permanent establishment of mutual displays in the pair bond is that the benefit of investment by the pair is more than twice that resulting from investment by a single individual. Pre-existing biases to respond to displays by increased investment are a necessary component of display evolution. We also consider examples where one sex (e.g. males) stimulates increased investment in offspring by the other sex. Here, display and additional investment cannot evolve permanently, but can increase and linger at high frequency for a long time before loss. We discuss how such transient effects may lead to the evolution of permanent displays as a result of evolution at additional loci.     

Parent age differentially influences offspring size over the course of development in Laysan albatross

Offspring growth and survival are predicted to be higher for older parents, due to a variety of mechanisms, such as increased breeding experience or greater investment favored by low residual reproductive value. Yet the extent to which parent age affects offspring viability is likely to vary between different aspects of growth and survival, perhaps being most pronounced at the most stressful stages of reproduction. We studied the link between parent age and nestling growth and survival in the Laysan albatross, a long-lived seabird with a mean first breeding age of 8 years. Offspring of older parents were more likely to survive to fledging. Among those that did fledge, nestlings of older parents grew more rapidly. However, parent age did not influence the eventual asymptotic size that nestlings reached before fledging: fast-growing nestlings of older parents reached 90% of asymptotic size roughly 1 week sooner, but slow-growing nestlings of younger parents eventually caught up in size before fledging. Older parents bred c . 2 days earlier than younger parents, but hatch date did not explain observed variation in offspring success. The extent to which parent age accounted for variation in size of individual nestlings was not constant but peaked near the midpoint of development. This could reflect a time period when demands on parents reveal age-based differences in parental quality. Overall, growth and survival of offspring increased with parent age in this species, even though the late age of first breeding potentially provides a 7-year period for birds to hone their foraging skills or for selection to eliminate low-quality individuals.     

A meta-analysis of parasite virulence in nestling birds

Parasitism is a common cause of host mortality, but little is known about the ecological factors affecting parasite virulence (the rate of mortality among infected hosts). We reviewed 117 field estimates of parasite-induced nestling mortality in birds, showing that there was significant consistency in mortality among host and parasite taxa. Virulence increased towards the tropics in analyses of both species-specific data and phylogenetic analyses. We found evidence of greater parasite prevalence being associated with reduced virulence. Furthermore, bird species breeding in open nest sites suffered from greater parasite-induced mortality than hole-nesting species. By contrast, parasite specialization and generation time of parasites relative to that of hosts explained little variation in virulence. Likewise, there were little or no significant effects of host genetic variability, host sociality, host migration, host insular distribution or host survival on parasite virulence. These findings suggest that parasite-induced nestling mortality in birds is mainly determined by geographical location and to a smaller extent nest site and prevalence.     

Parasite prevalence and the worldwide distribution of cognitive ability

In this study, we hypothesize that the worldwide distribution of cognitive ability is determined in part by variation in the intensity of infectious diseases. From an energetics standpoint, a developing human will have difficulty building a brain and fighting off infectious diseases at the same time, as both are very metabolically costly tasks. Using three measures of average national intelligence quotient (IQ), we found that the zero-order correlation between average IQ and parasite stress ranges from r = −0.76 to r = −0.82 (p < 0.0001). These correlations are robust worldwide, as well as within five of six world regions. Infectious disease remains the most powerful predictor of average national IQ when temperature, distance from Africa, gross domestic product per capita and several measures of education are controlled for. These findings suggest that the Flynn effect may be caused in part by the decrease in the intensity of infectious diseases as nations develop.     

Annual plant life histories and the paradigm of resource allocation

Summary Much of life history theory follows from the idea that natural selection acts on the allocation of resources to competing and independent demographic functions. This paradigm has stimulated much research on the life histories of annual plants. Models of whole-plant resource budgets that use optimal control theory predict periods of 100% vegetative and 100% reproductive growth, sometimes with periods of mixed growth. I show here that this prediction follows from the assumption of independence of the competing vegetative and reproductive compartments. The prediction is qualitatively unchanged even after relaxing important simplifying assumptions used in most models. Although it follows naturally from the assumptions of the models, this kind of allocation pattern is unlikely to occur in many plants, because it requires that (1) leaf and flower buds can never simultaneously be carbon sinks; and (2) organs that accompany flowers, such as internodes and bracts, can never be net sources of photosynthate. Thus while resources are doubtless important for annual plants, an exclusively resource-based perspective may be inadequate to understand the evolution of their life histories. Progress in research may require models that incorporate, or are at least phenomenologically consistent with, the basic developmental reles of angiosperms.     

Evolution and spectral tuning of visual pigments in birds and mammals

Variation in the types and spectral characteristics of visual pigments is a common mechanism for the adaptation of the vertebrate visual system to prevailing light conditions. The extent of this diversity in mammals and birds is discussed in detail in this review, alongside an in-depth consideration of the molecular changes involved. In mammals, a nocturnal stage in early evolution is thought to underlie the reduction in the number of classes of cone visual pigment genes from four to only two, with the secondary loss of one of these genes in many monochromatic nocturnal and marine species. The trichromacy seen in many primates arises from either a polymorphism or duplication of one of these genes. In contrast, birds have retained the four ancestral cone visual pigment genes, with a generally conserved expression in either single or double cone classes. The loss of sensitivity to ultraviolet (UV) irradiation is a feature of both mammalian and avian visual evolution, with UV sensitivity retained among mammals by only a subset of rodents and marsupials. Where it is found in birds, it is not ancestral but newly acquired.     

Evolution of Strategies to Stay in the Game

Life-history evolution is a complexprocess. Life-history theory covers the fundamentallevel of the process, the evolution of life-historytraits. Life-history traits interact; thosecoevolving as a response to the same selectionpressure form life-history tactics. Top level of thehierarchy, life-history strategy, is formed bygenetically interconnected tactics. Our aim is toexpand the traditional view to life-history evolutionby considering what boundary conditions a successfullife-history strategy has to fulfil. We claim thatthe most fundamental condition successful strategieshave to meet is to minimize the risk of evolutionaryfailure. Here the risk of failure refers to failurein transferring practitioners of the strategy to thenext time point, either through survival, or byreproduction. We make an attempt to classify types ofrisks as they lead to evolutionary failure, anddiscuss how risk minimization ideas may be approachedempirically. We conclude that understanding howtraits evolve may not cover all aspects of howstrategies evolve. We emphasize that bookkeeping ofthe actual causes of failure might help in developinglife-history theory that uses causes of selection topredict responses to selection.     

The reproductive tactics of dace in central Siberia: evidence for temperature regulation of the spatio-temporal variability of its life history

Major life history characteristics of dace Leuciscus leuciscus from the R. Yenisei at Mirnoye (66° N, Central Siberia, Russia) were determined in June 1993 and were compared with those available throughout the distribution range of the species. Differences between populations located 25° further south and 8000 km away are smaller than those described for other freshwater fishes, whose distributions fall within that of dace. For all the populations, the growth rates ( K ) are inversely correlated with latitude but these rates for the actual growing season are faster in northern dace. The latitudinal (spatial) variations in the growth rates resemble the temporal variation described for the R. Frome (U.K.). Also, the variations in fecundity between populations are comparable to the temporal variations reported for this British river. Fecundity of the Yenisei dace was correlated with female length [log F=– 3–6284+4–0424 x log L ] but these females spawned lower numbers of eggs than other populations; and the size of their eggs, like those of the Siberian Ust'Ilim dace, did not vary with female length. We hypothesize that a similar spatio-temporal response to low water temperature, coupled with limitations of energy for reproduction, may result in a constant egg size in Siberian dace. The effects of other selective forces cannot, however, be excluded.     

Unrestricted migration favours virulent pathogens in experimental metapopulations: evolutionary genetics of a rapacious life history

Understanding pathogen infectivity and virulence requires combining insights from epidemiology, ecology, evolution and genetics. Although theoretical work in these fields has identified population structure as important for pathogen life-history evolution, experimental tests are scarce. Here, we explore the impact of population structure on life-history evolution in phage T4, a viral pathogen of Escherichia coli. The host–pathogen system is propagated as a metapopulation in which migration between subpopulations is either spatially restricted or unrestricted. Restricted migration favours pathogens with low infectivity and low virulence. Unrestricted migration favours pathogens that enter and exit their hosts quickly, although they are less productive owing to rapid extirpation of the host population. The rise of such ‘rapacious’ phage produces a ‘tragedy of the commons’, in which better competitors lower productivity. We have now identified a genetic basis for a rapacious life history. Mutations at a single locus (rI) cause increased virulence and are sufficient to account for a negative relationship between phage competitive ability and productivity. A higher frequency of rI mutants under unrestricted migration signifies the evolution of rapaciousness in this treatment. Conversely, spatially restricted migration favours a more ‘prudent’ pathogen strategy, in which the tragedy of the commons is averted. As our results illustrate, profound epidemiological and ecological consequences of life-history evolution in a pathogen can have a simple genetic cause.