American Naturalist最新文献

AbstractMilk is not cheap. The energetic cost for mammalian mothers to provide and sustain milk production makes it a finite resource. Offspring are therefore expected to wean before sexual maturity and reproductive activity, whether on their own or through termination by their mothers. Weaning delays should result in a reproductive trade-off for the mother: the possibility of begetting a fitter pup at the cost of a longer interbirth interval. Using 20 years of data, we show the occurrence of repeated suckling events between female Galápagos sea lions (GSLs; Zalophus wollebaeki) and their adult (≥5 years) biological offspring well beyond the average age of independence and when the offspring are themselves already reproductively active. This behavior, "supersuckling," suggests that GSL mother-offspring relationships are more complex and longer lasting than previously thought. To our knowledge, this is the first long-term documentation of known mother-offspring pairs repeatedly performing this behavior in any marine mammal species.

AbstractNavigation to and from a familiar site is a common animal behavior called homing. Many species use olfactory environmental landmarks or scents deposited in the environment by themselves or their conspecifics for homing. Birds regularly commute to and from their nest, and olfaction, an underappreciated sense in birds, may facilitate this behavior. Burrow-nesting seabirds, for example, rely on olfaction to locate their breeding colony and to identify their burrow, but the specific chemical information they use is unclear. We examined the chemical profiles of the colony landscape and its avian occupants at a breeding island of Leach's storm petrels (Hydrobates leucorhous) to determine whether place-specific chemicals, bird-produced chemicals, or both enable homeward navigation in this burrow-nesting species. We found that the colony contains spatial gradients of chemicals that may facilitate multiple stages of homing. We also show that burrows possess unique odors owing to chemicals deposited by their occupants. Moreover, the burrow shapes the odor of the birds such that individuals carry the scent of their nest and mated pairs possess similar chemical profiles. The bidirectional transfer of compounds between burrows and birds may enable burrow recognition in this species and potentially functions as a means of communication between conspecifics.

AbstractAccording to optimal foraging theory, mesopredators should forage in areas where their prey is abundant while avoiding high predation risk. Here, we investigate how environmental factors influence mesopredators' abilities to minimize spatiotemporal overlap with predators while increasing spatiotemporal overlap with prey. We paired 30 western diamond-backed rattlesnake (Crotalus atrox) 3D printed replicas with game cameras in West Texas for 2 years to quantify several spatiotemporal factors affecting prey availability and predation risk. Concurrently, 25 C. atrox were radiotracked at the same site to gather activity and microhabitat selection data regarding free-ranging individuals. Random forest algorithms were trained using data obtained from the game camera and applied to predict the probability of predation and the probability of prey encounter for each radiotracking event. Time of day, month, vegetation structure, and concealment percentage all had a significant association with the probability of predation and the probability of prey encounter. Our results suggest that rattlesnakes choose to be active when and where the probability of prey encounter was significantly higher than the probability of predator encounter, thus following optimal foraging theory. Our results demonstrate that mesopredators increase chances of prey capture while reducing predator detection in natural settings.

AbstractUnderstanding how temperature affects adaptation of cell size is challenging because cell size mediates numerous physiological and ecological trade-offs. While physiological mechanisms can lead to decreases in cell size with warming (the temperature-size rule [TSR]), it is unclear how ecological processes (competition, predation) combine to modify the TSR. Here, we evaluate how ecological interactions affect thermal adaptation of phytoplankton cell size. We perform an eco-evolutionary analysis of a nutrient-phytoplankton-zooplankton model. The model assumes that phytoplankton experience size-dependent constraints on resource allocation that cause small cells to sacrifice investment in growth machinery, thereby reducing maximum growth rate but increasing competitive ability. We find that trophic interactions strongly impact the evolutionarily stable cell size across temperatures. Without zooplankton, cell size declines monotonically with temperature, consistent with the TSR. With zooplankton, cell size varies unimodally with temperature, due to temperature-dependent shifts in the grazer's capacity to ease nutrient competition by controlling phytoplankton biomass. Size-selective grazing does not qualitatively alter this result but can facilitate coexistence of phytoplankton via a competition-predation resistance trade-off. Trophic interactions therefore can produce temperature-size responses in phytoplankton that differ qualitatively from the canonical TSR, and an understanding of how temperature affects cell size is incomplete without this ecological component.

AbstractEvaluating the evolutionary impacts of anthropogenic activity on populations is key to understanding species resiliency and to designing effective conservation strategies. Sequencing DNA from historical specimens provides the opportunity to establish a historical baseline and empirically assess changes in genetic diversity, changes in effective population size, and selection over time. Here, we sequenced historical and contemporary samples of the cardinalfish Taeniamia zosterophora collected in 1908 and in 2021-2022 across two sites with differing human impact in the Philippines. At both sites, genetic diversity increased over time, with contemporary samples having significantly higher Watterson's θ than historical samples. This diversity increase was primarily attributable to positive selection on low-frequency alleles such that they increased toward intermediate frequencies through time. For the putatively neutral fraction of the genome, in contrast, there was a slight but significant decline in Watterson's θ at both low and high human impact sites, suggesting that drift strengthened and effective population sizes declined through time. There was more evidence for selection and greater loss of neutral diversity at the site with higher human impact. Our results provide empirical evidence for the surprising preservation of genetic diversity through the action of natural selection in the face of anthropogenic impacts.

AbstractRensch's rule posits that sexual size dimorphism increases with overall body size among animal species with larger males and that it decreases with body size among species with larger females. The rule, originally based on patterns observed in a limited dataset rather than theory, has attracted much attention in the sexual selection literature. However, evidence for the rule has been equivocal. We test Rensch's rule with a recently published dataset on sexual size dimorphism in mammals using linear regressions with phylogenetic controls. We find that neither male-biased nor female-biased dimorphic species conform to Rensch's rule across mammals. When the analysis is restricted to within-family comparisons, as Rensch originally intended, the rule applies only to three of the 21 mammalian groups tested. We find very limited support for the "rule" in mammals and suggest that it is unlikely to be the general phenomenon that Rensch proposed.

AbstractVariation in colonization ability (comprising the dispersal and successful establishment of lineages in new regions) and its connection to species diversification may be one of the major reasons why clades vary widely in standing diversity. Diversity variation driven by colonization ability can be generated under two scenarios. Under a neutral model, high colonization ability enables some clades to colonize unoccupied areas and over time diversify into more species. Under a nonneutral model, some competitively superior clades are able to rapidly diversify into already occupied niches, both on continents they already occupy and on continents they are invading. Because entire lineages occasionally become extinct, including those that have colonized other landmasses, it can be difficult to distinguish between these models based on extant species. Here, we test these two alternatives using a species-level phylogeny of all extant and extinct species of the mammalian order Carnivora and related extinct groups. We find that species that colonize new continents leave more descendant species than noncolonizers and that colonizing species belong to clades that were diversifying faster than noncolonizers at the time of colonization. Our results suggest that variation in diversification may be partly driven by nonneutral processes with variable competitive ability between lineages. Our study highlights the importance of including extinct species in phylogenies when trying to understand evolutionary and biogeographic patterns.

AbstractMutualistic interactions between plants and pollinators play an important role in supporting biodiversity and ecosystem stability. However, these interactions are increasingly threatened by climate change, which can alter the phenology of species and cause temporal mismatches between interacting partners. Leveraging historical and contemporary datasets collected more than a century apart, we investigated phenological shifts in plants and pollinators and the impact of changes in temporal overlap of the interaction partners on the persistence of their interactions. We found that the onset of flowering and insect activity generally started earlier and has lasted longer in the present. We also found that greater temporal overlap of plant and pollinator species predicted a higher probability of persistence of their interaction between time periods. Our results document phenological shifts over a century and emphasize the importance of maintaining phenological matching for the persistence of plant-pollinator interactions. This illustrates the value of historical datasets for understanding long-term ecological dynamics in the face of accelerating environmental change.

AbstractFunctional diversity is expected to decrease following land conversion. Empirically, however, the consequences of such changes are highly variable. One possible explanation is that the magnitude and direction of functional diversity change depend on how agricultural land conversion interacts with the original determinants of community assembly (e.g., temperature and elevation gradients). We compared the functional structure of 50 Anolis lizard communities on the island of Hispaniola in both forested and deforested habitats along an elevation gradient, as elevation often determines community composition. We used morphological measurements of body size, limb and tail length, and toepad width to capture ecomorphological aspects of functional diversity. These traits are strongly linked to habitat use, which has been shown to be the primary axis of niche partitioning in anoles. We found that deforestation had little effect on functional (morphological) richness at low elevations but increased functional richness and evenness at high elevations, where natural communities are depauperate owing to thermal constraints. Simultaneously, deforestation reduced spatial turnover and eliminated morphologically peripheral species. These results suggest that how land conversion affects communities depends on whether it relaxes or reinforces a community's dominant environmental filters: at high elevations, as deforestation increases daytime temperatures, the filters that typically shape these communities are relaxed, allowing them to functionally resemble low-elevation communities. While this enriches high-elevation communities, it also removes morphologically unique species and homogenizes diversity across elevations. Our results highlight that how land conversion reorganizes the functional structure of a community depends on environmental context.

AbstractSexual size dimorphism (varying body sizes between males and females) and the operational sex ratio (ratio of sexually active males to receptive females) are key demographic traits influenced by complex selective pressures. Two hypotheses explain their relationship: the mating competition hypothesis posits that male-biased sexual size dimorphism intensifies with increasingly male-skewed adult sex ratios, while the mating opportunity hypothesis proposes that female-biased sexual size dimorphism escalates with greater male-biased adult sex ratios. We tested these hypotheses across 101 Chinese anuran species. Our results support the mating opportunity hypothesis, with enhanced female-biased sexual size dimorphism at more male-skewed operational sex ratios, particularly in monogamous species. We further explored the role of ecological factors and life history traits in shaping sexual size dimorphism and operational sex ratio. We found predation pressure to covary negatively with the male bias in operational sex ratios, while temperature variation, likely reflecting seasonal differences, negatively influenced both sexual size dimorphism and operational sex ratio. Our findings highlight the interplay between sexual selection, ecology, and life history in driving the evolution of sexual size dimorphism and operational sex ratio in anurans. Understanding these mechanisms is crucial for predicting how species may respond to future environmental changes.