American Naturalist最新文献

AbstractDehnel's phenomenon describes a seasonal and reversible winter decrease in body size, which is a trait that predicts total energy demand. However, the phenomenon remains less well studied than common energy-saving or energy-seeking strategies of mammals. Here, we explore the generality of Dehnel's phenomenon in Sorex shrews on three continents. First, we use new field sampling to document seasonal phenotypic change in masked shrews (Sorex cinereus) in North America at the lowest latitude yet investigated for this species (35.7°). This includes the first documentation of appendicular skeleton remodification in Sorex. Summer-to-winter decreases in S. cinereus body mass, braincase height, and femur length were 13%, 11.5%, and 8.7%, respectively, with subsequent increases of each in second-year individuals. Second, we compile a comprehensive dataset of studies relevant to Dehnel's phenomenon to test whether seasonal plasticity in Sorex globally is related to climate, demonstrating that body and braincase plasticity are functions of cold season temperatures. Meta-analytical models for both of these traits generalized by (a) applying at both inter- and intraspecific scales and (b) predicting the seasonal change newly observed for S. cinereus. Our results support body size plasticity as an environmentally responsive innovation in these very small homeothermic mammals.

AbstractCourtship displays of lek-mating Golden-winged Manakins (Masius chrysopterus) are context dependent. Presence or absence of a female audience and female behavior, more than male identity, determine variation in the constituent elements (repertoire) and ordering (syntax) of displays. We analyzed 422 display videos in three contexts: displays without audiences (SOLO; $n=307$), displays for female audiences that did not end in copulation (AUDI; $n=102$), and displays ending in copulation (COP; $n=13$). Using entropy and a metric we call compressibility (ratio of compressed to uncompressed display strings), we found that ordering of elements (syntax) decreased in complexity from SOLO to AUDI to COP displays. Jaro string distance, a record-linkage metric for assessing string similarity, showed that display string syntax corresponded more to audience context than to performer identity. COP displays of individual males differed more from their own AUDI or SOLO displays than from the COP displays of other males. Males responded to female behavior-her consistent positioning downslope from him on the display log-with simple COP displays. Courtship displays of Golden-winged Manakins are dynamic interactions between females and males, depending more on male response to female audience behavior than on traits intrinsic to particular males.

AbstractInbreeding depression (ID) has different components, and how these affect selection on seed size is currently unknown. Using an optimality model, I find that when pollen limitation selects for mixed mating systems, increased predispersal ID (abortion of selfed ovules) increases the optimal sizes of both selfed and outcrossed seeds, whereas increased seed size-independent postdispersal ID (reduced survival of selfed seeds) only increases the size of outcrossed seeds. The effect of decreased efficiency of maternal investment in selfed seeds depends on details of the model assumptions, but in many cases it will increase resource allocation to both types of seeds. If seed size-independent postdispersal ID is less than 0.5, predispersal ID will select for selfed seeds receiving more maternal investment than outcrossed seeds. The same is true for decreased efficiency of maternal investment. If decreased efficiency of maternal investment leads to a reduction in selfed seed size, selfed seeds may end up being smaller than outcrossed seeds. The model highlights the complex roles ID plays in determining seed size and indicates that empirical measurement of postdispersal ID may underestimate its extent. Tests of the model may help us understand whether plants can adaptively allocate resources differentially between selfed and outcrossed seeds.

AbstractSize and growth early in life are associated with physiological development, and these traits influence fitness. Life history theory predicts that the relationship between traits reflect constraints involving allocation and acquisition of resources. Using longitudinal data from 113 wild nestling barn swallows (Hirundo rustica erythrogaster), we first characterized developmental changes in glucose metabolism, a physiological trait involved in energy mobilization and response to stress. Next, we tested hypotheses from life history theory about allocation and acquisition of resources based on associations of nestling size and growth with glucose physiology and assessed whether these relationships are modified by parental care. Larger nestlings had higher baseline blood glucose and larger magnitude of change in glucose in response to a stressor than smaller nestlings. Furthermore, the relationship in which greater growth was associated with a stronger stress response, as indicated by a larger magnitude of increase in glucose levels, was most pronounced among birds in nests that received the lowest amount of parental care. These results suggest that physiological constraints may contribute to the early-life disadvantage of slow growth, especially in the context of lower parental care. While these findings are inconsistent with a trade-off involving differential allocation of resources between life history traits, they align with the differential acquisition hypothesis.

AbstractMixed-species flocks (MSFs) are an important form of animal social organization. Most studies examine MSFs at the species level, notwithstanding that social interactions occur between individuals. Empirical studies of multispecies, individual-level MSF social networks have seldom been undertaken. In this study, we use mist netting, color banding, and standardized observations to construct individual-level social networks for MSFs in the Eastern Himalaya. We describe two distinct flocktypes comprising two sets of understory species. Our social network analyses and spatial visualization suggest that the pattern of individual-level co-occurrences differs between these flocktypes and with previously described Neotropical MSFs. One flocktype has a multi-individual territorial network among individuals of its central species, while the other is led by a species with no apparent social structure. Furthermore, the addition of associating species has opposite impacts on the modularity of the two different social networks. Our study provides novel insights into MSFs at the individual level.

AbstractAlthough there are many hypothesized ecological functions of plant coloration, they have been only partly resolved by examining ecological hypotheses in isolation. Multiple ecological interactions may act in concert or in opposition to fix or maintain variation in plant coloration, that is, via ecological pleiotropy. To investigate the adaptive value of red plant pigment (anthocyanin) in a carnivorous plant, we compared insect prey capture, herbivore damage, and recruitment of specialist insect larvae in naturally occurring sympatric red and green color morphs of the pitcher plant Sarracenia purpurea. We integrated field and laboratory bioassays, visual modeling, chemical analysis of anthocyanins, and a long-term demographic study to investigate multiple ways anthocyanins mediate plant-insect interactions. In support of ecological pleiotropy, each morph performed better in one or more ecological contexts, providing evidence for ecological interactions exerting opposing selection on plant color and thus maintaining variation. The mixture of both ecological benefits and costs to anthocyanin production is further supported by stable color polymorphism and seed set data consistent with balancing selection. More broadly, this work reveals the impacts of a single anthocyanin compound on multiple key plant-insect interactions, demonstrating evidence for ecological pleiotropy maintaining intraspecific diversity in plant color.

AbstractIndividuals of some long-distance migrant shorebird species may remain on or near nonbreeding areas in place of making a breeding migration. Existing hypotheses associate oversummering with factors that impair successful migration and breeding. The hypothesis developed here takes a life history trade-off perspective. Based on the idea that survival while oversummering is higher than that of migration and breeding, it predicts that oversummering evolves when the survival advantage compensates in fitness terms for the reproduction foregone by doing so. Adults have higher reproductive success and so oversummer less readily than do yearlings. If the oversummering survival gain is similar to the threshold level of compensation required, interindividual variation in condition may place some individuals above and others below the threshold for oversummering. Partial oversummering can result. This theory accurately predicts the strong contrast in oversummering patterns observed in Peru for both adult and yearling semipalmated Calidris pusilla and western sandpipers C. mauri, otherwise very similar species. Delayed maturity (i.e., oversummering by yearlings) and intermittent breeding (partial oversummering by adults) strongly affect population productivity. These behaviors may have increased over recent decades and hence could be contributing to the steep declines being reported for some shorebird species.

AbstractSexual selection is widely hypothesized to facilitate speciation and phenotypic evolution, but evidence from comparative studies has been mixed. Many previous studies have relied on proxy variables to quantify the intensity of sexual selection, raising the possibility that inconclusive results may reflect, in part, the imperfect measurement of this evolutionary process. Here, we test the relationship between phylogenetic speciation rates and indices of the opportunity for sexual selection drawn from populations of 82 vertebrate taxa. These indices provide a much more direct assessment of sexual selection intensity than proxy traits and allow straightforward comparisons among distantly related clades. We find no correlation between the opportunity for sexual selection and speciation rate, and this result is consistent across many complementary analyses. In addition, widely used proxy variables-sexual dimorphism and dichromatism-are not correlated with the indices employed here. Moreover, we find that the opportunity for sexual selection has low phylogenetic signal and that intraspecific variability in selection indices for many species approaches the range of variation observed across all vertebrates as a whole. Our results potentially reconcile a major paradox in speciation biology at the interface between microevolution and macroevolution: sexual selection can be important for speciation, yet the evolutionary lability of the process over deeper timescales restricts its impact on broad-scale patterns of biodiversity.

AbstractThe population dynamics of autopolyploids-organisms with more than two genome copies of a single species-and their diploid progenitors have been extensively studied. The acquisition of multiple genome copies is heavily influenced by stochasticity, which strongly suggests the efficacy of a probabilistic approach to examine the long-term dynamics of a population with multiple cytotypes. Yet our current understanding of the dynamics of autopolyploid populations has not incorporated stochastic population dynamics and coexistence theory. To investigate the factors contributing to the probability and stability of coexisting cytotypes, we designed a new population dynamics model that incorporates demographic and environmental stochasticities to simulate the formation, establishment, and persistence of diploids, triploids, and autotetraploids in the face of gene flow among cytotypes. We found that increased selfing rates and pronounced reproductive isolation promote coexistence of multiple cytotypes. In stressful environments and with strong competitive effects among cytotypes, these dynamics are more complex; our stochastic modeling approach reveals the resulting intricacies that give autotetraploids competitive advantage over their diploid progenitors. Our work is foundational for a better understanding of the dynamics of coexistence of multiple cytotypes.

AbstractMany animals exhibit ontogenetic niche shifts as they grow, which strongly affects population dynamics. However, such niche shifts can be constrained by the physical environment that the population occupies. To study this, we develop a physiologically structured population model parameterized for brown trout and vary the availability of a stream used as an exclusive juvenile nursery habitat. We find fewer but large, fast-growing adults in lakes with small streams and more but smaller, slow-growing adults in lakes with large streams. We show that the mechanism behind this pattern is a reduced ability of cannibals to control juvenile survival in the lake with increasing stream availability. Juveniles emerging from the stream at larger sizes intensify competition with the lake-dwelling adults, leading to slower individual growth. These results are similar for other sources of size-dependent juvenile mortality in the lake. Field data from brown trout lakes across a stream size gradient show the same pattern: reduced trout growth and fewer large individuals in lakes with larger tributary streams. We show how ontogenetic niche shifts and stage-specific habitat availability affect population structure and dynamics through size-dependent mortality and competition. Our results provide an important foundation that may help design effective conservation and restoration strategies.