Morphological variability or inter-observer bias? A methodological toolkit to improve data quality of multi-researcher datasets for the analysis of morphological variation

OBJECTIVES

The investigation of morphological variation in animals is widely used in taxonomy, ecology, and evolution. Using large datasets for meta-analyses has dramatically increased, raising concerns about dataset compatibilities and biases introduced by contributions of multiple researchers.

MATERIALS AND METHODS

We compiled morphological data on 13 variables for 3073 individual mouse lemurs (Cheirogaleidae, Microcebus spp.) from 25 taxa and 153 different sampling locations, measured by 48 different researchers. We introduced and applied a filtering pipeline and quantified improvements in data quality (Shapiro-Francia statistic, skewness, and excess kurtosis). The filtered dataset was then used to test for genus-wide sexual size dimorphism and the applicability of Rensch's, Allen's, and Bergmann's rules.

RESULTS

Our pipeline reduced inter-observer bias (i.e., increased normality of data distributions). Inter-observer reliability of measurements was notably variable, highlighting the need to reduce data collection biases. Although subtle, we found a consistent pattern of sexual size dimorphism across Microcebus, with females being the larger (but not heavier) sex. Sexual size dimorphism was isometric, providing no support for Rensch's rule. Variations in tail length but not in ear size were consistent with the predictions of Allen's rule. Body mass and length followed a pattern contrary to predictions of Bergmann's rule.

DISCUSSION

We highlighted the usefulness of large multi-researcher datasets for testing ecological hypotheses after correcting for inter-observer biases. Using genus-wide tests, we outlined generalizable patterns of morphological variability across all mouse lemurs. This new methodological toolkit aims to facilitate future large-scale morphological comparisons for a wide range of taxa and applications.

Ontogeny of cranial musculoskeletal anatomy and its relationship to allometric increase in bite force in an insectivorous bat (Eptesicus fuscus)

Bite force is a performance metric commonly used to link cranial morphology with dietary ecology, as the strength of forces produced by the feeding apparatus largely constrains the foods an individual can consume. At a macroevolutionary scale, there is evidence that evolutionary changes in the anatomical elements involved in producing bite force have contributed to dietary diversification in mammals. Much less is known about how these elements change over postnatal ontogeny. Mammalian diets drastically shift over ontogeny-from drinking mother's milk to feeding on adult foods-presumably with equally drastic changes in the morphology of the feeding apparatus and bite performance. Here, we investigate ontogenetic morphological changes in the insectivorous big brown bat (Eptesicus fuscus), which has an extreme, positive allometric increase in bite force during development. Using contrast-enhanced micro-computed tomography scans of a developmental series from birth to adult morphology, we quantified skull shape and measured skeletal and muscular parameters directly related to bite force production. We found pronounced changes in the skull over ontogeny, including a large increase in the volume of the temporalis and masseter muscles, and an expansion of the skull dome and sagittal crest that would serve to increase the temporalis attachment area. These changes indicate that development of the jaw adductors play an important role in the development of biting performance of these bats. Notably, static bite force increases with positive allometry with respect to all anatomical measures examined, suggesting that modifications in biting dynamics and/or improved motor coordination also contribute to increases in biting performance.

Sex-specific aging in bite force in a wild vertebrate

The manifold differences between the sexes provide wide scope for sex differences in senescence. However, detecting physiological decline in old age and evaluating any sex difference in aging in a wild population can be challenging. This applies especially to long-lived species that require marking around birth in order to recognize elderly individuals, perhaps decades later. Here, we used bite force of known-age, long-lived Nazca boobies (Sula granti, a seabird) as a functional measure of muscle strength; surprisingly, only a single study has evaluated the possibility of senescent decline in muscle strength in a wild vertebrate. The male-biased adult sex ratio of this population constrains breeding opportunities across the lifespan for males, so we predicted that slower accumulation of reproductive costs would delay senescent decline in bite force in males compared to females, matching observed patterns in some non-muscle traits in this species. Data were collected from 349 adults using a force transducer at the start of the breeding season in November 2017 on Isla Española, Galápagos. Both sexes achieved less bite force in late life. The decline began at a later age in males, providing evidence of sex-specific schedules of decline in muscle function in a wild vertebrate.

Maternal and genetic correlations between morphology and physical performance traits in a small captive primate, Microcebus murinus

Physical performance traits are key components of fitness and direct targets of selection. Although maternal effects have important influences on integrated phenotypes, their contributions to variation in performance and to phenotypic traits associated with performance remain poorly understood. We used an animal model to quantify the contribution of maternal effects to performance trait variation, in addition to the genetic and maternal correlations between performance and the relevant underlying morphology in Microcebus murinus. We showed that bite force is heritable (h2 ≈ 0.23) and that maternal effects are an important source of variation, resulting in a medium inclusive heritability (IH2 ≈ 0.47). Bite force and head depth showed a significant genetic correlation (0.70), and other genetic correlations were generally high (0.63 for bite force and head width; 0.41 for pull strength and radius length, albeit not significant), as were the maternal correlations for bite force and head dimensions (0.44, 0.73 and 0.29). Finally, we found differences in evolvability for pull strength and bite force that were also consistent with a higher potential for evolutionary change in pull force. This demonstrates clear effects of the maternal environment on performance expression and on the relationships between morphology and performance. This illustrates the importance of accounting for maternal identity when considering the heritabilities of functional traits.

The contribution of functional traits to the understanding of palaeoenvironmental changes

Performance traits implicated in feeding interact directly with the environment and are consequently relevant ecological indicators. However, they have rarely been used to better understand palaeoenvironmental variation. Here, we evaluate the usefulness of a performance (i.e. functional) trait, estimated bite force, in reconstructing the palaeoecology of shrews. We investigate the relationships between mandible morphology, bite force estimates and the ecological context. We use geometric morphometrics to quantify mandible shape diversity in shrews of the archaeological site El Harhoura 2 (Rabat, Morocco), dated from the Late Pleistocene to the Holocene. Morphological groups were used instead of taxa as units of diversity. To explore how phenotypic traits are linked to their environment, they were compared with palaeoenvironmental inferences for the El Harhoura 2 site extracted from the literature. Morphological groups acted as phenotypic response units. Estimated bite force was related to palaeoenvironmental variation over the considered period, with a particular sensibility to arid/humid transitions. The complementarity of morphological and performance indicators allowed us to infer functional convergence and divergence among shrews. Our results suggest that functional traits may be relevant indicators of changes in palaeoenvironments. This approach opens up new possibilities to explore the impact of environmental changes on extinct organisms.

Increased performance in juvenile baboons is consistent with ontogenetic changes in morphology

OBJECTIVES

In many primates, the greater proportion of climbing and suspensory behaviors in the juvenile repertoire likely necessitates good grasping capacities. Here, we tested whether very young individuals show near-maximal levels of grasping strength, and whether such an early onset of grasping performance could be explained by ontogenetic variability in the morphology of the limbs in baboons.

MATERIAL AND METHODS

We quantified a performance trait, hand pull strength, at the juvenile and adult stages in a cross-sectional sample of 15 olive baboons (Papio anubis). We also quantified bone dimensions (i.e., lengths, widths, and heights) of the fore- (n = 25) and hind limb (n = 21) elements based on osteological collections covering the whole development of olive baboons.

RESULTS

One-year old individuals demonstrated very high pull strengths (i.e., 200% of the adult performance, relative to body mass), that are consistent with relatively wider phalanges and digit joints in juveniles. The mature proportions and shape of the forelimb elements appeared only at full adulthood (i.e., ≥4.5 years), whereas the mature hind limb proportions and shape were observed much earlier during development.

DISCUSSION

These changes in limb performance and morphology across ontogeny may be explained with regard to behavioral transitions that olive baboons experience during their development. Our findings highlight the effect of infant clinging to mother, an often-neglected feature when discussing the origins of grasping in primates. The differences in growth patterns, we found between the forelimb and the hind limb further illustrate their different functional roles, having likely evolved under different ecological pressures (manipulation and locomotion, respectively).

Movement analysis of primate molar teeth under load using synchrotron X-ray microtomography

Mammalian teeth have to sustain repetitive and high chewing loads without failure. Key to this capability is the periodontal ligament (PDL), a connective tissue containing a collagenous fibre network which connects the tooth roots to the alveolar bone socket and which allows the teeth to move when loaded. It has been suggested that rodent molars under load experience a screw-like downward motion but it remains unclear whether this movement also occurs in primates. Here we use synchroton micro-computed tomography paired with an axial loading setup to investigate the form-function relationship between tooth movement and the morphology of the PDL space in a non-human primate, the mouse lemur (Microcebus murinus). The loading behavior of both mandibular and maxillary molars showed a three-dimensional movement with translational and rotational components, which pushes the tooth into the alveolar socket. Moreover, we found a non-uniform PDL thickness distribution and a gradual increase in volumetric proportion of the periodontal vasculature from cervical to apical. Our results suggest that the PDL morphology may optimize the three-dimensional tooth movement to avoid high stresses under loading.

Proximate and ultimate drivers of variation in bite force in the insular lizards Podarcis melisellensis and Podarcis sicula

Bite force is a key performance trait in lizards because biting is involved in many ecologically relevant tasks, including foraging, fighting and mating. Several factors have been suggested to impact bite force in lizards, such as head morphology (proximate factors), or diet, intraspecific competition and habitat characteristics (ultimate factors). However, these have been generally investigated separately and mostly at the interspecific level. Here we tested which factors drive variation in bite force at the population level and to what extent. Our study includes 20 populations of two closely related lacertid species, Podarcis melisellensis and Podarcis sicula, which inhabit islands in the Adriatic. We found that lizards with more forceful bites have relatively wider and taller heads, and consume more hard prey and plant material. Island isolation correlates with bite force, probably by driving resource availability. Bite force is only poorly explained by proxies of intraspecific competition. The linear distance from a large island and the proportion of difficult-to-reduce food items consumed are the ultimate factors that explain most of the variation in bite force. Our findings suggest that the way in which morphological variation affects bite force is species-specific, probably reflecting the different selective pressures operating on the two species.

Body Size and Bite Force of Stray and Feral Cats-Are Bigger or Older Cats Taking the Largest or More Difficult-to-Handle Prey?

As carnivorans rely heavily on their head and jaws for prey capture and handling, skull morphology and bite force can therefore reflect their ability to take larger or more difficult-to-handle prey. For 568 feral and stray cats (Felis catus), we recorded their demographics (sex and age), source location (feral or stray) and morphological measures (body mass, body condition); we estimated potential bite force from skull measurements for n = 268 of these cats, and quantified diet composition from stomach contents for n = 358. We compared skull measurements to estimate their bite force and determine how it varied with sex, age, body mass, body condition. Body mass had the strongest influence of bite force. In our sample, males were 36.2% heavier and had 20.0% greater estimated bite force (206.2 ± 44.7 Newtons, n = 168) than females (171.9 ± 29.3 Newtons, n = 120). However, cat age was the strongest predictor of the size of prey that they had taken, with older cats taking larger prey. The predictive power of this relationship was poor though (r² < 0.038, p < 0.003), because even small cats ate large prey and some of the largest cats ate small prey, such as invertebrates. Cats are opportunistic, generalist carnivores taking a broad range of prey. Their ability to handle larger prey increases as the cats grow, increasing their jaw strength, and improving their hunting skills, but even the smallest cats in our sample had tackled and consumed large and potentially 'dangerous' prey that would likely have put up a defence.

Decoupled ontogeny of in vivo bite force and mandible morphology reveals effects of weaning and sexual maturation in mice

The link between performance, morphology and their sources of variation is a major target of evolutionary functional biology. In vertebrates, many studies have linked in vivo bite force to skull morphology, mostly at the interspecific level. Within species, however, the ontogeny of bite force, in relation to the development of the mandible, remains poorly known, despite its relevance for life history and for the co-evolution of form and function. Here, ontogenetic trajectories of bite force, correlated with mandible size and shape, are reported for the first time in a wild-derived colony of laboratory mice. Bite forces were measured in vivo and mandible morphology was assessed using geometric morphometrics. Most coordinated changes in morphology and in vivo bite force occur during the first stages of growth, prior to weaning. Mandible shape stabilizes after day 23. The increases in mandible size and body mass slow down around day 40, but still increase during adulthood. Despite slowing down after weaning, bite force increases through a second phase during sexual maturation (days 30–40). This may be linked to the progressive tempering of weaning stress, continued growth and synchronization of the muscular and osteological systems, together with hormonal changes, as we observed a concomitant appearance of sexual dimorphism.

Personality and performance are affected by age and early life parameters in a small primate

A whole suite of parameters is likely to influence the behavior and performance of individuals as adults, including correlations between phenotypic traits or an individual's developmental context. Here, we ask the question whether behavior and physical performance traits are correlated and how early life parameters such as birth weight, litter size, and growth can influence these traits as measured during adulthood. We studied 486 captive gray mouse lemurs (Microcebus murinus) and measured two behavioral traits and two performance traits potentially involved in two functions: exploration behavior with pull strength and agitation score with bite force. We checked for the existence of behavioral consistency in behaviors and explored correlations between behavior, performance, morphology. We analyzed the effect of birth weight, growth, and litter size, while controlling for age, sex, and body weight. Behavior and performance were not correlated with one another, but were both influenced by age. Growth rate had a positive effect on adult morphology, and birth weight significantly affected emergence latency and bite force. Grip strength was not directly affected by early life traits, but bite performance and exploration behavior were impacted by birth weight. This study shows how early life parameters impact personality and performance.

Age-Related Changes in Locomotor Performance Reveal a Similar Pattern for Caenorhabditis elegans, Mus domesticus, Canis familiaris, Equus caballus, and Homo sapiens

Locomotion is one of the major physiological functions for most animals. Previous studies have described aging mechanisms linked to locomotor performance among different species. However, the precise dynamics of these age-related changes, and their interactions with development and senescence, are largely unknown. Here, we use the same conceptual framework to describe locomotor performances in Caenorhabditis elegans, Mus domesticus, Canis familiaris, Equus caballus, and Homo sapiens. We show that locomotion is a consistent biomarker of age-related changes, with an asymmetrical pattern throughout life, regardless of the type of effort or its duration. However, there is variation (i) among species for the same mode of locomotion, (ii) within species for different modes of locomotion, and (iii) among individuals of the same species for the same mode of locomotion. Age-related patterns are modulated by genetic (such as selective breeding) as well as environmental conditions (such as temperature). However, in all cases, the intersection of the rising developmental phase and the declining senescent phase reveals neither a sharp transition nor a plateau, but a smooth transition, emphasizing a crucial moment: the age at peak performance. This transition may define a specific target for future investigations on the dynamics of such biological interactions.

Go big or go fish: morphological specializations in carnivorous bats

Specialized carnivory is relatively uncommon across mammals, and bats constitute one of the few groups in which this diet has evolved multiple times. While size and morphological adaptations for carnivory have been identified in other taxa, it is unclear what phenotypic traits characterize the relatively recent evolution of carnivory in bats. To address this gap, we apply geometric morphometric and phylogenetic comparative analyses to elucidate which characters are associated with ecological divergence of carnivorous bats from insectivorous ancestors, and if there is morphological convergence among independent origins of carnivory within bats, and with other carnivorous mammals. We find that carnivorous bats are larger and converged to occupy a subset of the insectivorous morphospace, characterized by skull shapes that enhance bite force at relatively wide gapes. Piscivorous bats are morphologically distinct, with cranial shapes that enable high bite force at narrow gapes, which is necessary for processing fish prey. All animal-eating species exhibit positive allometry in rostrum elongation with respect to skull size, which could allow larger bats to take relatively larger prey. The skull shapes of carnivorous bats share similarities with generalized carnivorans, but tend to be more suited for increased bite force production at the expense of gape, when compared with specialized carnivorans.

Proximate determinants of bite force in Anolis lizards

Performance measures associated with the vertebrate jaw system may provide important insights into vertebrate ecology and evolution because of their importance in many ecologically relevant tasks. Previous studies have shown that in many taxa, evolution toward higher bite force has gone hand in hand with the evolution of larger body size. However, independent of differences in overall body size, bite force may vary depending on head size and shape as well. Moreover, the underlying musculature may also drive variation in bite force. Here, we investigate the proximate determinants of bite force in lizards of the genus Anolis. We dissected the jaw muscles and quantified muscle mass, fibre length, and cross-sectional area. Data were analysed for both sexes independently given the sexual dimorphism detected in the dataset. Our results show that the traits that explain bite force are similar in both males and females with overall body size and muscle mass being the principal determinants. Among the different muscles examined, the adductor externus and the pseudotemporalis groups were the best determinants of bite force. However, models run for males predicted the variation in bite force better than models for females, suggesting that selection on morphology improving bite force may be stronger in males.

Reliable quantification of bite-force performance requires use of appropriate biting substrate and standardization of bite out-lever

Bite-force performance is an ecologically important measure of whole-organism performance that shapes dietary breadth and feeding strategies and, in some taxa, determines reproductive success. It also is a metric critical to testing and evaluating biomechanical models. We reviewed nearly one-hundred published studies of a range of taxa that incorporate direct in vivo measurements of bite force. Problematically, methods of data collection and processing vary considerably among studies. In particular, there is little consensus on the appropriate substrate to use on the biting surface of force transducers. In addition, the bite out-lever, defined as the distance from the fulcrum (i.e. jaw joint) to the position along the jawline at which the jaws engage the transducer, is rarely taken into account. We examined the effect of bite substrate and bite out-lever on bite-force estimates in a diverse sample of lizards. Results indicate that both variables have a significant impact on the accuracy of measurements. Maximum bite force is significantly greater using leather as the biting substrate, as compared to a metal substrate. Less forceful bites on metal are likely due to inhibitory feedback from mechanoreceptors that prevent damage to the feeding apparatus. Standardization of bite out-lever affected which trial produced maximum performance for a given individual. Indeed, maximum bite force usually is underestimated without standardization because it is expected to be greatest at the minimum out-lever (i.e. back of jaws), which in studies is rarely targeted with success. We assert that future studies should use a pliable substrate, such as leather, and employ appropriate standardization for bite out-lever.