Does stress mess with rodents' heads? Influence of habitat amount and genetic factors in mandible fluctuating asymmetry in South American water rats (Nectomys squamipes, Sigmodontinae) from Brazilian Atlantic rainforest remnants

Does mercury affect morphology, developmental stability and canalization of the skull in the Common wall lizard (Podarcis muralis)?

Environmental pollution by metals and metalloids can have a detrimental effect on the fitness and development of organisms. Studies on the influence of metals and metalloids as environmental stressors on developmentally and functionally complex morphological structures of reptiles are important, as this group of vertebrates is highly threatened and is an important component of food webs. To assess the effects of chronic mercury exposure on cranium morphology and post-natal development in a model species of lizards, we analysed the concentration of this metal in liver tissues in the population of the Common wall lizard (Podarcis muralis) from the mercury mine tailings and the control population, and possible differences in skull size, shape, developmental stability and canalization between the two groups. Patterns of variation and asymmetry of the cranium shape were analysed using geometric morphometrics. The mercury concentration was significantly higher in the population from the polluted locality, but had very little to no effect on the cranial morphology. Juveniles and females from both sites had the same size and shape of the dorsal and ventral cranium, while males showed small differences in ventral cranium shape, reflected in slightly longer maxillae and wider crania at the polluted site. The pattern of sexual dimorphism remained constant at both localities. Both static and ontogenetic allometry were significant in both groups, and allometric trajectories did not differ between the two sites. The differences in fluctuating asymmetry (FA) between localities were not statistically significant for the dorsal and ventral cranium. The results do not support the idea that FA can be used as an early indicator mercury exposure at the population level.

Habitat fragmentation increases the risk of local extinction of small reptiles: A case study on Phrynocephalus przewalskii

Habitat fragmentation represents a multifaceted global conservation threat, exerting both direct and indirect effects on individual animals and communities. Reptiles, particularly smaller species with limited migratory abilities, are especially vulnerable to these changes. This study examines how small reptiles adapt their life history strategies in fragmented habitats and determines whether their responses are primarily due to phenotypic plasticity or genetic adaptation. We focused on Phrynocephalus przewalskii, analyzing body size, reproductive strategies, genetic structure, and diversity across three habitats with varying degrees of fragmentation. This study investigated the response of P. przewalskii to varying degrees of habitat fragmentation in Wuhai City, Inner Mongolia. The study compared morphological characteristics, reproductive investment, genetic diversity, and fluctuating asymmetry of P. przewalskii individuals from the sampling areas. Results showed that individuals in highly fragmented areas exhibited smaller body size, higher reproductive investment, and lower genetic diversity. Genomic analyses revealed significantly fewer SNPs, INDELs, and SVs in the highly fragmented area compared to others, with the slowest linkage disequilibrium decay rate, indicating strong environmental pressure. Furthermore, individuals in the highly fragmented area had significantly higher fluctuating asymmetry, serve as an important ecological and biological indicator reflecting habitat fragmentation. These findings demonstrated that habitat fragmentation negatively impacts the survival and reproduction of P. przewalskii, reducing its genetic diversity and adaptive potential, and increasing the risk of local extinction. This study provides crucial data for understanding the mechanisms of animal adaptation to fragmented habitats and aids in assessing species extinction risks and developing conservation strategies.

Exploring fluctuating asymmetry in two recovering populations of the Eurasian otter

The Eurasian otter is a wide-ranging semi-aquatic mammal that underwent a significant population decline in the last century, leading to local extinctions, reduction and fragmentation of populations. The individuals of populations exposed to both external and internal stress may present the inability to produce a specific developmental outcome, generating developmental 'noise' (developmental instability (DI)). Factors contributing to DI include inbreeding depression, population bottlenecks, habitat loss and exposure to pollution. We analysed fluctuating asymmetry (FA) as a proxy of DI in two European otter populations that experienced a major decline in the 1990s. Using three-dimensional geometric morphometrics methods on skull samples of otters from the UK and Sweden, we compared the degree of FA both between populations before, during and after the decline. We found a higher FA in the UK populations compared with Sweden. The level of asymmetry differed significantly over time only in the UK population, where it was higher during the decline phase. FA in the UK populations can be attributed to the specific impact of polychlorinated biphenyls pollution that caused a bottleneck. More generally, our study suggests that habitat loss, pollution and limited gene flow may contribute to DI in declining otter populations, highlighting the need for continued investigation to identify and quantify the specific stressors behind this trend in local populations.

Does the primate face cue personality?

When looking at others, primates primarily focus on the face - detecting the face first and looking at it longer than other parts of the body. This is because primate faces, even without expression, convey trait information crucial for navigating social relationships. Recent studies on primates, including humans, have linked facial features, specifically facial width-to-height ratio (fWHR), to rank and Dominance-related personality traits, suggesting these links' potential role in social decisions. However, studies on the association between dominance and fWHR report contradictory results in humans and variable patterns in nonhuman primates. It is also not clear whether and how nonhuman primates perceive different facial cues to personality traits and whether these may have evolved as social signals. This review summarises the variable facial-personality links, their underlying proximate and evolutionary mechanisms and their perception across primates. We emphasise the importance of employing comparative research, including various primate species and human populations, to disentangle phylogeny from socio-ecological drivers and to understand the selection pressures driving the facial-personality links in humans. Finally, we encourage researchers to move away from single facial measures and towards holistic measures and to complement perception studies using neuroscientific methods.

Skeletal age during hurricane impacts fluctuating asymmetry in Cayo Santiago rhesus macaques

As natural disasters become more frequent due to climate change, understanding the biological impact of these ecological catastrophes on wild populations becomes increasingly pertinent. Fluctuating asymmetry (FA), or random deviations from bilateral symmetry, is reflective of developmental instability and has long been positively associated with increases in environmental stress. This study investigates craniofacial FA in a population of free-ranging rhesus macaques (Macaca mulatta) that has experienced multiple Category 3 hurricanes since the colony's inception on Cayo Santiago, including 275 individuals from ages 9 months to 31 years (F = 154; M = 121). Using geometric morphometrics to quantify FA and a linear mixed-effect model for analysis, we found that sex, age, and decade of birth did not influence the amount of FA in the individuals included in the study, but the developmental stage at which individuals experienced these catastrophic events greatly impacted the amount of FA exhibited (p = .001). Individuals that experienced these hurricanes during fetal life exhibited greater FA than any other post-natal developmental period. These results indicate that natural disasters can be associated with developmental disruption that results in long-term effects if occurring during the prenatal period, possibly due to increases in maternal stress-related hormones.

The skull variation of the olive field mouse Abrothrix olivacea (Cricetidae: Abrotrichini) is localized and correlated to the ecogeographic features of its geographic distribution

The relationship between phenotypic variation and landscape heterogeneity has been extensively studied to understand how the environment influences patterns of morphological variation and differentiation of populations. Several studies had partially addressed intraspecific variation in the sigmodontine rodent Abrothrix olivacea, focusing on the characterization of physiological aspects and cranial variation. However, these had been conducted based on geographically restricted populational samples, and in most cases, the aspects characterized were not explicitly contextualized with the environmental configurations in which the populations occurred. Here, the cranial variation of A. olivacea was characterized by recording twenty cranial measurements in 235 individuals from 64 localities in Argentina and Chile, which widely cover the geographic and environmental distribution of this species. The morphological variation was analyzed and ecogeographically contextualized using multivariate statistical analyses, which also included climatic and ecological variation at the localities where the individuals were sampled. Results indicate that the cranial variation of this species is mostly clustered in localized patterns associated to the types of environments, and that the levels of cranial differentiation are higher among the populations from arid and treeless zones. Additionally, the ecogeographical association of cranial size variation indicate that this species does not follow Bergmann's rule and that island populations exhibit larger cranial sizes compared to their continental counterparts distributed at the same latitudes. These results suggest that cranial differentiation among the populations of this species is not homogeneous throughout its geographic distribution, and that the patterns of morphological differentiation are also not completely consistent with the patterns of genetic structuring that have been described recently. Finally, the analyses performed to ponder morphological differentiation among populations suggest that the contribution of genetic drift in the formation of these patterns can be ruled out among Patagonian populations, and that the selective effect imposed by the environment could better explain them.

Differential Impact of Forest Fragmentation on Fluctuating Asymmetry in South Amazonian Small Mammals

(1) Background: Southern Amazonia encompasses some of the most human-impacted and deforested regions of South America, resulting in a hyper-fragmented landscape. In this context, by using a geometric morphometrics approach, we aimed to examine the effect of forest fragmentation on developmental instability (DI) of the mandible, assessed by variation of fluctuating asymmetry (FA), in four neotropical small mammal species inhabiting the municipality of Alta Floresta (Brazil). (2) Methods: The impact of fragment area, fragment shape, isolation, and edge length on DI were assessed by measuring variation in mandibular FA in the long-tailed spiny rat (Proechimys longicaudatus), the hairy-tailed bolo mouse (Necromys lasiurus), the woolly mouse opossum (Marmosa demerarae), and the Amazonian red-sided opossum (Monodelphis glirina). Mandibles from a total of 304 specimens originating from different-sized fragments (ranging from 5 to 900 ha) were used. Twelve homologous landmarks were digitized in photographs of the mesial view of each hemi-mandible. (3) Results: The two largest species, P. longicaudatus and M. demerarae, exhibited significantly higher levels of FA in mandibular shape in small fragments (5–26 ha) in comparison to large ones (189–900 ha). Edge length negatively impacted M. demerarae, the only arboreal species, reinforcing its strongest dependence on core forest habitats. (4) Conclusions: For small mammal communities, we propose that fragments >~200 ha should be the focus of conservation efforts, as both resilient and more sensitive species would benefit from their more preserved biotic and abiotic conditions. Conversely, fragments <~25 ha seem to lead to a significant increase in stress during developmental stages.