Higher Lifetime Fitness Research Articles

Specialized spatial cognition is associated with reduced cognitive senescence in a food-caching bird.

Senescence, the gradual reduction and loss of function as organisms age, is a widespread process that is especially pronounced in cognitive abilities. Senescence appears to have a genetic basis and can be affected by evolutionary processes. If cognitive senescence is shaped by natural selection, it may be linked with selection on cognitive abilities needed for survival and reproduction, such that species where fitness is directly related to cognitive abilities should evolve delayed cognitive senescence likely resulting in higher lifetime fitness. We used wild food-caching mountain chickadees, which rely on specialized spatial cognition to recover thousands of food caches annually, to test for cognitive senescence in spatial learning and memory and reversal spatial learning and memory abilities. We detected no signs of age-related senescence in spatial cognitive performance on either task in birds ranging from 1 to 6 years old; older birds actually performed better on spatial learning and memory tasks. Our results therefore suggest that cognitive senescence may be either delayed (potentially appearing after 6 years) or negligible in species with strong selection on cognitive abilities and that food-caching species may present a useful model to investigate mechanisms associated with cognitive senescence.

Little plant, big city: a test of adaptation to urban environments in common ragweed (Ambrosia artemisiifolia).

A full understanding of how cities shape adaptation requires characterizing genetically-based phenotypic and fitness differences between urban and rural populations under field conditions. We used a reciprocal transplant experiment with the native plant common ragweed, (Ambrosia artemisiifolia), and found that urban and rural populations have diverged in flowering time, a trait that strongly affects fitness. Although urban populations flowered earlier than rural populations, plants growing in urban field sites flowered later than plants in rural field sites. This counter-gradient variation is consistent adaptive divergence between urban and rural populations. Also consistent with local adaptation, both urban and rural genotypes experienced stronger net selection in the foreign than in the local habitat, but this pattern was not significant for male fitness. Despite the evidence for local adaptation, rural populations had higher lifetime fitness at all sites, suggesting that selection has been stronger or more uniform in rural than urban populations. We also found that inter-population differences in both flowering time and fitness tended to be greater among urban than rural populations, which is consistent with greater drift or spatial variation in selection within urban environments. In summary, our results are consistent with adaptive divergence of urban and rural populations, but also suggest there may be greater environmental heterogeneity in urban environments which also affects evolution in urban landscapes.

Promiscuous mating produces offspring with higher lifetime fitness

In many species, each female pairs with a single male for the purpose of rearing offspring, but may also engage in extra-pair copulations. Despite the prevalence of such promiscuity, whether and how multiple mating benefits females remains an open question. Multiple mating is typically thought to be favoured primarily through indirect benefits (i.e. heritable effects on the fitness of offspring). This prediction has been repeatedly tested in a variety of species, but the evidence has been equivocal, perhaps because such studies have focused on pre-reproductive survival rather than lifetime fitness of offspring. Here, we show that in a songbird, the dark-eyed junco (Junco hyemalis), both male and female offspring produced by extra-pair fertilizations have higher lifetime reproductive success than do offspring sired within the social pair. Furthermore, adult male offspring sired via extra-pair matings are more likely to sire extra-pair offspring (EPO) themselves, suggesting that fitness benefits to males accrue primarily through enhanced mating success. By contrast, female EPO benefited primarily through enhanced fecundity. Our results provide strong support for the hypothesis that the evolution of extra-pair mating by females is favoured by indirect benefits and shows that such benefits accrue much later in the offspring's life than previously documented.

Timing of Flowering: Opposed Selection on Different Fitness Components and Trait Covariation

The timing of reproduction influences how organisms interact with the environment and can have important fitness effects. In plants, the evolution of flowering phenology is often interpreted as the response to selection from mutualists, although antagonistic interactions may also be important. We examined direct and indirect phenotypic selection on the start of flowering via mutualistic and antagonistic interactions in the perennial herb Lathyrus vernus over 7 years. Flowering start influenced seed set, predispersal seed predation, and risk of grazing. These effects were in opposed directions and partly influenced different components of fitness. Combining information about effects on fitness components with information about links between fitness components and average lifetime fitness, in terms of population growth rate, showed that earlier flowering was associated with higher lifetime fitness in all years. These relationships were, however, mediated largely by variation in flower number, and direct selection on first flowering date was more variable among years. We conclude that long-term studies correcting for indirect selection and environmental covariance are needed to understand selection on reproductive phenology and that demographic approaches are necessary to assess selection mediated by several agents and influencing several components of fitness.

Song repertoire size varies with HVC volume and is indicative of male quality in song sparrows (Melospiza melodia).

Complex birdsong is a classic example of a sexually selected ornamental trait. In many species, females prefer males with large song repertoires, possibly because repertoire size is limited by the size of song control nuclei which reflect developmental success. We investigated whether song repertoire size was indicative of brain area and male quality in song sparrows (Melospiza melodia) by determining if repertoire size was related to the volume of song control nucleus HVC, as well as several morphological, immunological and genetic indices of quality. We found that males with large repertoires had larger HVCs and were in better body condition. They also had lower heterophil to lymphocyte ratios, indicating less physiological stress and a robust immune system as measured by the number of lymphocytes per red blood cell. Song repertoire size also tended to increase with neutral-locus genetic diversity, as assessed by mean d2, but was not related to internal relatedness. Our results suggest several mechanisms that might explain the finding of a recent study that song sparrows with large song repertoires have higher lifetime fitness.

Social rank, stress, fitness, and life expectancy in wild rabbits.

Wild rabbits of the two sexes have separate linear rank orders, which are established and maintained by intensive fights. The social rank of individuals strongly influence their fitness: males and females that gain a high social rank, at least at the outset of their second breeding season, have a much higher lifetime fitness than subordinate individuals. This is because of two separate factors: a much higher fecundity and annual reproductive success and a 50% longer reproductive life span. These results are in contrast to the view in evolutionary biology that current reproduction can be increased only at the expense of future survival and/or fecundity. These concepts entail higher physiological costs in high-ranking mammals, which is not supported by our data: In wild rabbits the physiological costs of social positions are caused predominantly by differential psychosocial stress responses that are much lower in high-ranking than in low-ranking individuals.

Lifetime Fitness and Evolution of Reproductive Pattern in the Herbivorous Lady Beetle

The adaptive significance of the timing of oviposition in the thistle—feeding lady beetle Epilachna niponica was investigated at two localities (site A and site F) in the northwestern part of Shiga Prefecture, central Japan. I followed cohorts produced over a season and measured four components of lifetime fitness: egg survival, larval survival, female adult survival from emergence to the reproductive season in the following year, and lifetime fecundity. These data are based on mark—recapture data for >9000 adult beetles and detailed life tables over 5 yr. Large variation in lifetime fitness was evident among cohorts within a population, but the two local populations showed a clear difference in patterns of cohort fitness. At site A, cohorts, produced early in a season had higher lifetime fitness than later cohorts, whereas at site F, later cohorts (except for the last one) tended to have higher lifetime fitness. The major causes of these between—site differences wee seasonal variation in intensity of egg and larval mortalities due to arthropod predation and host plant deterioration. Field observations revealed a significant difference in oviposition phenology between the two sites: early reproduction at site A and delayed reproduction at site F. The relatively longer reproductive life—span of females was responsible for the prolonged reproduction at site F. Results of laboratory experiment that eliminated environmental variables agreed with the field observations, and suggested a genetic basis for the oviposition schedules. Correlation of reproductive pattern at each site with higher lifetime fitness of offspring suggests selection on the timing of oviposition to improve the lifetime reproductive success of females.

QUEEN NUMBER IN COLONIES OF SOCIAL HYMENOPTERA AS A KIN-SELECTED ADAPTATION.

Using a series of kin-selection models, I examine factors that favor multiple egg-laying queens (polygyny) in eusocial Hymenoptera colonies. One result is that there is a theoretical conflict of interest between the founding queens and their daughter workers over how many and which individuals should be the extra reproductives. Both castes should prefer their full sisters. Therefore, primary polygyny (multiple related foundresses) may favor queens while secondary polygyny (related queens added to mature colonies) may favor workers. Polygyny, itself, was found to be favored by high colony survivorship and low probability of queens contributing eggs to successive broods. Polygyne colonies, however, did not need to produce more offspring per brood to be selectively favored; they could be half as productive per brood as monogyne ones and still have higher lifetime fitness under some conditions. For reproductive data from eight ant species with both monogyne and polygyne colonies, the model generates results that are consistent with a kin-selection explanation of polygyny in all of them. It is proposed that queen number is an ecologically flexible trait that is influenced by a broad set of factors but is not necessarily linked to specific habitat types. Furthermore, neither polygyny nor monogyny may be reliably considered as the primitive or ancestral Hymenopteran social system. The optimal queen number within a species may evolutionarily increase or decrease, depending on the direction of environmental change.