Determinants of pull strength in captive grey mouse lemurs

Effect of the habitat and tusks on trunk grasping techniques in African savannah elephants

Among tetrapods, grasping is an essential function involved in many vital behaviours. The selective pressures that led to this function were widely investigated in species with prehensile hands and feet. Previous studies namely highlighted a strong effect of item properties but also of the species habitat on manual grasping behaviour. African savannah elephants (Loxodonta africana) are known to display various prehensile abilities and use their trunk in a large diversity of habitats. Composed of muscles and without a rigid structure, the trunk is a muscular hydrostat with great freedom of movement. This multitasking organ is particularly recruited for grasping food items while foraging. Yet, the diet of African savannah elephants varies widely between groups living in different habitats. Moreover, they have tusks alongside the trunk which can assist in grasping behaviours, and their tusk morphologies are known to vary considerably between groups. Therefore, in this study, we investigate the food grasping techniques used by the trunk of two elephant groups that live in different habitats: an arid study site in Etosha National Park in Namibia, and an area with consistent water presence in Kruger National Park in South Africa. We characterised the tusks profiles and compared the grasping techniques and their frequencies of use for different foods. Our results show differences in food-grasping techniques between the two groups. These differences are related to the food item property and tusk profile discrepancies highlighted between the two groups. We suggest that habitat heterogeneity, particularly aridity gaps, may induce these differences. This may reveal an optimisation of grasping types depending on habitat, food size and accessibility, as well as tusk profiles.

Quantitative assessment of grasping strength in platyrrhine monkeys

OBJECTIVES

Despite the longstanding importance of grasping adaptations in theories of primate evolution, quantitative data on primate grasping strength remain rare. We present the results of two studies testing the prediction that callitrichines-given their comparative retreat from a small-branch environment and specialization for movement and foraging on tree trunks and large boughs-should be characterized by weaker grasping forces and underdeveloped digital flexor muscles relative to other platyrrhines.

METHODS

First, we directly measured manual grasping strength in marmosets (Callithrix jacchus) and squirrel monkeys (Saimiri boliviensis), using a custom-constructed force transducer. Second, we reanalyzed existing datasets on the fiber architecture of forearm and leg muscles in 12 platyrrhine species, quantifying digital flexor muscle physiological cross-sectional area (i.e., PCSA, a morphometric proxy of muscle strength) relative to the summed PCSA across all forearm or leg muscles.

RESULTS

Callithrix was characterized by lower mean and maximum grasping forces than Saimiri, and callitrichines as a clade were found to have relatively underdeveloped manual digital flexor muscle PCSA. However, relative pedal digital flexor PCSA did not significantly differ between callitrichines and other platyrrhines.

CONCLUSIONS

We found partial support for the hypothesis that variation in predominant substrate usage explains variation in empirical measurements of and morphological correlates of grasping strength in platyrrhines. Future research should extend the work presented here by (1) collecting morphological and empirical metrics of grasping strength in additional primate taxa and (2) extending performance testing to include empirical measures of primate pedal grasping forces as well.

Jumping performance in tree squirrels: Insights into primate evolution

Morphological traits suggesting powerful jumping abilities are characteristic of early crown primate fossils. Because tree squirrels lack certain 'primatelike' grasping features but frequently travel on the narrow terminal branches of trees, they make a viable extant model for an early stage of primate evolution. Here, we explore biomechanical determinants of jumping performance in the arboreal Eastern gray squirrel (Sciurus carolinensis, n = 3) as a greater understanding of the biomechanical strategies that squirrels use to modulate jumping performance could inform theories of selection for increased jumping ability during early primate evolution. We assessed vertical jumping performance by using instrumented force platforms upon which were mounted launching supports of various sizes, allowing us to test the influence of substrate diameter on jumping kinetics and performance. We used standard ergometric methods to quantify jumping parameters (e.g., takeoff velocity, total displacement, peak mechanical power) from force platform data during push-off. We found that tree squirrels display divergent mechanical strategies according to the type of substrate, prioritizing force production on flat ground versus center of mass displacement on narrower poles. As jumping represents a significant part of the locomotor behavior of most primates, we suggest that jumping from small arboreal substrates may have acted as a potential driver of the selection for elongated hindlimb segments in primates, allowing the center of mass to be accelerated over a longer distance-and thereby reducing the need for high substrate reaction forces.

Three toes and three modes: Dynamics of terrestrial, suspensory, and vertical locomotion in brown-throated three-toed sloths (Bradypodidae, Xenarthra)

Living sloths exhibit numerous anatomical specializations towards inverted quadrupedalism, however, previous studies have noted a more varied locomotor repertoire than previously anticipated. In this study, we present spatiotemporal gait characteristics and triaxial kinetic data from the brown-throated three-toed sloth (Bradypus variegatus) across three locomotor modes: terrestrial quadrupedal "crawling", suspensory walking, and vertical climbing. Compared to quadrupedal crawling and suspensory walking, B. variegatus adopted longer contact times and stride durations, larger duty factors, and greater speed during vertical climbing. Net fore-aft impulses were significantly greater during vertical climbing in both limb pairs than in quadrupedal crawling and suspensory walking. Functionally, during quadrupedal crawling and vertical climbing, both limb pairs served propulsive roles, while differentiation between a propulsive forelimb and braking hindlimb was observed during suspension. Net tangential forces differentiated vertical climbing kinetics from the other modes of locomotion, with the introduction of bidirectional pulling and pushing forces in the forelimb and hindlimb, respectively. The net mediolateral impulses were similar in vertical climbing and quadrupedal crawling as both limb pairs directed forces in one direction, whereas during suspensory walking, the laterally dominant forelimb was opposed by the medially dominant hindlimb. In total, this study provides novel data on the diverse locomotor dynamics in a slow-moving arboreal tetrapod and posits new testable hypotheses about the neuroplasticity and ease of transitioning between locomotor behaviors. The strikingly similar kinetic profiles of quadrupedal crawling and suspensory walking compared to vertical climbing suggest shared neuromuscular and mechanical demands between these mirrored locomotor modes.

The influence of substrate size upon pulling and gripping forces in parrots (Psittaciformes: Agapornis roseicollis)

The ability to securely grasp substrates of variable diameter is critical to arboreal animals. Arboreal specialists have emerged across several vertebrate lineages - including mammals, lizards and amphibians - and several attempts have been made to quantify their grasping performance, by measuring either gripping (i.e. forces generated about an object or substrate enclosed within the digits) or pulling (i.e. the ability to resist being removed from a substrate) forces. In this study, we present data on both pulling and gripping performance across a range of substrate diameters (0.5-17.5 mm) within a model parrot species (Agapornis roseicollis). Parrots represent an ancient arboreal lineage, allowing us to compare their abilities with those of arboreal specialists within other tetrapod groups. Data were collected using 3D-printed perches of variable diameter, and forces were registered using either an AMTI low-load force plate (grip force) or a Harvard Apparatus portable strength tester (pull force). Gripping forces peaked at a 5 mm diameter perch, while pulling forces were greatest at a 2.5 mm diameter. All forces strongly diminished above 10 mm size, suggesting grip force is optimized when utilizing small perches, a finding which corresponds to observational studies of preferential perching habits among free-ranging parrots. Relative grasping performance (adjusted for body size) in parrots is roughly equivalent to that of other arboreal specialists from other tetrapod lineages, but low when compared with that of raptorial birds that utilize their feet during aerial prey capture. Further taxonomic sampling is encouraged to contextualize how grasping performance varies in an adaptive evolutionary context.

Overview of age-related changes in psychomotor and cognitive functions in a prosimian primate, the gray mouse lemur (Microcebus murinus): Recent advances in risk factors and antiaging interventions

Aging is not homogeneous in humans and the determinants leading to differences between subjects are not fully understood. Impaired glucose homeostasis is a major risk factor for cognitive decline in middle-aged humans, pointing at the existence of early markers of unhealthy aging. The gray mouse lemur (Microcebus murinus), a small lemuriform Malagasy primate, shows relatively slow aging with decreased psychomotor capacities at middle-age (around 5-year old). In some cases (∼10%), it spontaneously leads to pathological aging. In this case, some age-related deficits, such as severe cognitive decline, brain atrophy, amyloidosis, and glucoregulatory imbalance are congruent with what is observed in humans. In the present review, we inventory the changes occurring in psychomotor and cognitive functions during healthy and pathological aging in mouse lemur. It includes a summary of the cerebral, metabolic, and cellular alterations that occur during aging and their relation to cognitive decline. As nutrition is one of the major nonpharmacological antiaging strategies with major potential effects on cognitive performances, we also discuss its role in brain functions and cognitive decline in this species. We show that the overall approach of aging studies in the gray mouse lemur offers promising ways of investigation for understanding, prevention, and treatments of pathological aging in humans.

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.

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).

Ontogeny of locomotion in mouse lemurs: Implications for primate evolution

The environment of juvenile primates is very challenging. They have to forage and move on the same substrates as adults do and escape the same predators, despite their immature state. In this study, we explore the developmental strategies that may provide effective locomotor abilities early in life. This could provide new insights into the selective pressures acting on juvenile primates and into evolution of primate locomotion. We conducted an ontogenetic study of 36 arboreal gray mouse lemurs from birth to adulthood (6 months of age). The investigated parameters were, for both limbs, (1) grasping behavior during locomotion (i.e., grip postures), (2) grasping performance (i.e., pull strength), and (3) motor coordination (i.e., rotarod test). Our results show that 8-day-old babies are able to climb substrates of various slopes and diameters outside of their nest. Although juveniles cannot successfully complete a motor coordination test before 30 days of age, young individuals display relative pull strengths that are very high or even on par with adults, guaranteeing stability on narrow substrates. These powerful grasps highlight the importance of the grasping function for these juveniles that are not carried and move independently on arboreal substrates shortly after their first week of life. Moreover, the pedal grasping provides a secure grasp on all substrates across ontogeny; however, manual secure grasps decrease during development, being highly used only shortly after birth on vertical and narrow substrates. These results first suggest different functional roles of the hands and feet, with the hind limbs ensuring body balance on the substrates, freeing the upper limbs for manipulation. They further show vertical and narrow branches to be especially challenging, requiring strong grasps, which suggests that they may drive the evolution of strong grasping abilities in primates.

Heritability and genetic correlations of personality, life history and morphology in the grey mouse lemur (Microcebus murinus)

The recent interest in animal personality has sparked a number of studies on the heritability of personality traits. Yet, how the sources variance these traits can be decomposed remains unclear. Moreover, whether genetic correlations with life-history traits, personality traits and other phenotypic traits exist as predicted by the pace-of-life syndrome hypothesis remains poorly understood. Our aim was to compare the heritability of personality, life-history and morphological traits and their potential genetic correlations in a small primate (Microcebus murinus). We performed an animal model analysis on six traits measured in a large sample of captive mouse lemurs (N = 486). We chose two personality traits, two life-history traits and two morphological traits to (i) estimate the genetic and/or environmental contribution to their variance, and (ii) test for genetic correlations between these traits. We found modest narrow-sense heritability for personality traits, morphological traits and life-history traits. Other factors including maternal effects also influence the sources of variation in life-history and morphological traits. We found genetic correlations between emergence latency on the one hand and radius length and growth rate on the other hand. Emergence latency was also genetically correlated with birth weight and was influenced by maternal identity. These results provide insights into the influence of genes and maternal effects on the partitioning of sources of variation in personality, life-history and morphological traits in a captive primate model and suggest that the pace-of-life syndrome may be partly explained by genetic trait covariances.

An integrative modeling approach to the age-performance relationship in mammals at the cellular scale

Physical and cognitive performances change across lifespan. Studying cohorts of individuals in specific age ranges and athletic abilities remains essential in assessing the underlying physiological mechanisms that result in such a drop in performance. This decline is now viewed as a unique phenotypic biomarker and a hallmark of the aging process. The rates of decline are well documented for sets of traits such as running or swimming but only a limited number of studies have examined the developmental and senescent phases together. Moreover, the few attempts to do so are merely descriptive and do not include any meaningful biological features. Here we propose an averaged and deterministic model, based on cell population dynamics, replicative senescence and functionality loss. It describes the age-related change of performance in 17 time-series phenotypic traits, including human physical and cognitive skills, mouse lemur strength, greyhound and thoroughbred speed, and mouse activity. We demonstrate that the estimated age of peak performance occurs in the early part of life (20.5% ± 6.6% of the estimated lifespan) thus emphasizing the asymmetrical nature of the relationship. This model is an initial attempt to relate performance dynamics to cellular dynamics and will lead to more sophisticated models in the future.

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.