Substrate alters forelimb to hindlimb peak force ratios in primates

It is often claimed that the walking gaits of primates are unusual because, unlike most other mammals, primates appear to have higher vertical peak ground reaction forces on their hindlimbs than on their forelimbs. Many researchers have argued that this pattern of ground reaction force distribution is part of a general adaptation to arboreal locomotion. This argument is frequently used to support models of primate locomotor evolution. Unfortunately, little is known about the force distribution patterns of primates walking on arboreal supports, nor do we completely understand the mechanisms that regulate weight distribution in primates. We collected vertical peak force data for seven species of primates walking quadrupedally on instrumented terrestrial and arboreal supports. Our results show that, when walking on arboreal vs. terrestrial substrates, primates generally have lower vertical peak forces on both limbs but the difference is most extreme for the forelimb. We found that force reduction occurs primarily by decreasing forelimb and, to a lesser extent, hindlimb stiffness. As a result, on arboreal supports, primates experience significantly greater functional differentiation of the forelimb and hindlimb than on the ground. These data support long-standing theories that arboreal locomotion was a critical factor in the differentiation of the forelimbs and hindlimbs in primates. This change in functional role of the forelimb may have played a critical role in the origin of primates and facilitated the evolution of more specialized locomotor behaviors.     

The phylogenetic significance of strepsirhinism in Paleogene primates

Two lines of evidence reviewed here argue against the recent proposal that strepsirhinism is an apomorphous feature in primates, shared only by adapiforms and lemuriforms. These are (1) the presence of strepsirhinism in several extant taxa of nonprimate mammals, including Tupaiidae, Tenrecidae, Erinaceus,and Didelphis,and (2) the inferred presence of strepsirhinism not only in adapiforms, but also in all plesiadapiforms and omomyids for which the relevant anatomical regions are known. Therefore, strepsirhinism cannot be invoked as an adaptive innovation underlying the initial strep-sirhine/haplorhine dichotomy. Likewise, the apparent retention of strepsirhinism in omomyids suggests either that the haplorhine oronasal configuration of extant tarsiids and anthropoids was acquired independently or that tarsiids and anthropoids form a clade to the exclusion of omomyids.     

Mediolateral reaction forces and forelimb anatomy in quadrupedal primates: implications for interpreting locomotor behavior in fossil primates

The forelimb joints of terrestrial primate quadrupeds appear better able to resist mediolateral (ML) shear forces than those of arboreal quadrupedal monkeys. These differences in forelimb morphology have been used extensively to infer locomotor behavior in extinct primate quadrupeds. However, the nature of ML substrate reaction forces (SRF) during arboreal and terrestrial quadrupedalism in primates is not known. This study documents ML-SRF magnitude and orientation and forelimb joint angles in six quadrupedal anthropoid species walking across a force platform attached to terrestrial (wooden runway) and arboreal supports (raised horizontal poles). On the ground all subjects applied a lateral force in more than 50% of the steps collected. On horizontal poles, in contrast, all subjects applied a medially directed force to the substrate in more than 75% of the steps collected. In addition, all subjects on arboreal supports combined a lower magnitude peak ML-SRF with a change in the timing of the ML-SRF peak force. As a result, during quadrupedalism on the poles the overall SRF resultant was relatively lower than it was on the runway. Most subjects in this study adduct their humerus while on the poles. The kinetic and kinematic variables combine to minimize the tendency to collapse or translate forelimbs joints in an ML plane in primarily arboreal quadrupedal primates compared to primarily terrestrial quadrupedal ones. These data allow for a more complete understanding of the anatomy of the forelimb in terrestrial vs. arboreal quadrupedal primates. A better understanding of the mechanical basis of morphological differences allows greater confidence in inferences concerning the locomotion of extinct primate quadrupeds.     

The evolution and adaptive significance of heterostyly

The origin and adaptive significance of heterostylous breeding systems have fascinated evolutionary biologists since Darwin's early work on the subject. Models of the evolution of heterostyly differ in the emphasis given to different selective forces and in the sequence in which the physiological and morphological components of the polymorphism are thought to arise. Recent field studies of the population biology of heterostylous plants provide support for Darwin's hypothesis that the style-stamen polymorphism promotes disassortative pollination among the floral morphs.     

The adaptive significance of monogamy in the golden-reumped elephant-shrew

Golden-rumped elephant-shrews Rhynchocyon chrysopygus are primarily monogamous, males and females defending joint territories for long periods and probably for life. However, males may occasionally solicit extra-pair copulations from neighbouring females and annex their territories if the resident male disappears for any reason. Males provide no direct paternal care, and the adaptive significance of monogamy in this species is unclear. Although it is possible that males provide some indirect benefits to the female and her offspring, these are unlikely to be substantial. Males and females spend little time together, so males are unlikely to provide additional protection from predators, and there is no evidence that the presence of a territorial male reduces the costs of territorial defence for the female. Females were able to breed successfully when mated bigamously, demonstrating that the undivided assistance of the male is not essential for successful rearing of offspring. The fact that male elephant-shrews occasionally attempt to defend more than one female suggests that polygyny would be beneficial for males. However, defending two territories is costly, resulting in increased activity and weight loss, and higher rates of intrusion by neighbouring males. Female elephant-shrews do not occupy particularly large home ranges for their body size. The results suggest that the high costs of defending a territory large enough to encompass the ranges of more than one female, combined with the greater probability of being cuckolded, make defending more than one female a poor option for male elephant-shrews.     

The adaptive significance of amphistomatic leaves

Abstract A clear correlation between the presence of stomata on both surfaces and factors such as habitat, growth form, and physiology has yet to emerge in the literature. However, certain loose trends with these factors are evident, and these are reviewed along with evidence for hypostomaty as the primitive form. It is proposed that the effect of developing stomata on the upper surface as well as the lower is to increase maximum leaf conductance to CO₂. Plants with a high photosynthetic capacity, living in full-sun environments, and experiencing rapidly fluctuating or continuously available soil water (as opposed to seasonal or long-term soil water depletion), are identified as deriving an adaptive advantage from a high maximum leaf conductance. The correlation between groups of plants fitting the above conditions and those noted to be amphistomatic is remarkable. Plants not fitting the conditions are found to be largely hypostomatic.     

The adaptive significance of maternal effects

Recently, the adaptive significance of maternal effects has been increasingly recognized. No longer are maternal effects relegated as simple `troublesome sources of environmental resemblance' that confound our ability to estimate accurately the genetic basis of traits of interest. Rather, it has become evident that many maternal effects have been shaped by the action of natural selection to act as a mechanism for adaptive phenotypic response to environmental heterogeneity. Consequently, maternal experience is translated into variation in offspring fitness.     

Curvature of the forelimb bones of anthropoid primates: Overall allometric patterns and specializations in suspensory species

It has previously been reported that brachiating primates, particularly gibbons, are characterized by distinctively straight forelimb long bones, yet no hypotheses have been proposed to explain why straight limb bones may be adaptive to suspensory locomotion. This study explores quantitatively the curvature of the long bones in 13 species of anthropoid primates and analyzes the functional consequences of curvature in biomechanical terms. These analyses demonstrate that, whereas the humeri of gibbons and spider monkeys are functionally less curved than those of other taxa, the ulnae of brachiators are neither more nor less curved than those of other anthropoids, and the gibbon radius is far more curved than would be predicted from body size alone. The humerus is likely significantly less curved in brachiators because of its torsion-dominated loading regime and the greatly increased stress magnitude developed in torsionally loaded curved beams. The large curvature of the radius is localized in the region of attachment of the supinator muscle. Analysis presented here of muscle mass allometry in catarrhines demonstrates that gibbons are characterized by an extremely massive supinator, and the large radial curvature is therefore most likely due to forearm muscle mechanics. This study also demonstrates that the overall pattern of limb bone curvature for anthropoids is distinct from the pattern reported for mammals as a whole. This distinctive scaling relationship may be related to the increased length of the limb bones of primates in comparison to other mammals.     

The adaptive significance of inquiline parasite workers

Social parasites exploit the socially managed resources of their host's society. Inquiline social parasites are dependent on their host throughout their life cycle, and so many of the traits inherited from their free-living ancestor are removed by natural selection. One trait that is commonly lost is the worker caste, the functions of which are adequately fulfilled by host workers. The few inquiline parasites that have retained a worker caste are thought to be at a transitional stage in the evolution of social parasitism, and their worker castes are considered vestigial and non-adaptive. However, this idea has not been tested. Furthermore, whether inquiline workers have an adaptive role outside the usual worker repertoire of foraging, brood care and colony maintenance has not been examined. In this paper, we present data that suggest that workers of the inquiline ant Acromyrmex insinuator play a vital role in ensuring the parasite's fitness. We show that the presence of these parasite workers has a positive effect on the production of parasite sexuals and a negative effect on the production of host sexuals. This suggests that inquiline workers play a vital role in suppressing host queen reproduction, thus promoting the rearing of parasite sexuals. To our knowledge, these are the first experiments on inquiline workers and the first to provide evidence that inquiline workers have an adaptive role.     

The adaptive significance of autumn leaf colours

Recently W.D. Hamilton and colleagues proposed a provocative new theory to explain the adaptive significance of autumnal leaf colours. They suggested that these colours were signals produced by the trees to warn potential insect herbivores of their defensive ability and tested this theory by an analysis of data on aphid species richness on different tree species. Here we argue that the principal assumptions of their theory do not match current knowledge of plant pigment biochemistry and aphid ecology. We therefore present further adaptive explanations for autumn leaf colours and suggest alternative reasons for the reported relationship between tree leaf colour and aphid species richness.     

The adaptive significance of mandibular symphyseal fusion in mammals

The mandibular symphyseal joint is remarkably variable across major mammalian clades, ranging in adults from unfused (amphiarthrosis) to partially fused (synarthrosis) to completely ossified (synostosis). Experimental work conducted on primates suggests that greater ossification of the symphysis is a response to increased recruitment of the balancing-side (i.e. nonchewing side) jaw-adductor muscles during forceful unilateral biting and chewing, with increased fusion strengthening the symphysis against correspondingly elevated joint stresses. It is thus expected that species with diets composed primarily of foods that require high-magnitude bite forces and/or repetitive loading to process will be characterized by greater degrees of symphyseal ossification than species with relatively easy-to-process diets (i.e. food items typified by low toughness and/or low stiffness). However, comparative support for this idea is limited. We tested this hypothesis in four dietarily diverse mammalian clades characterized by variation in symphyseal fusion - the Strepsirrhini, Marsupialia, Feliformia, and Caniformia. We scored fusion in adult specimens of 292 species, assigned each to a dietary category based on literature accounts, and tested for an association between these two variables using Pagel's test for the correlated evolution of binary characters. Results indicate that greater fusion is associated with diets composed of resistant items in strepsirrhines, marsupials, and feliforms, providing some support for the hypothesis. However, no such relationship was detected in caniforms, suggesting that factors other than dietary mechanical properties influence symphyseal ossification. Future work should focus on such factors, as well as those that favour an unfused mandibular symphysis.     

Dietary change and adaptive evolution of enamelin in humans and among primates

Scans of the human genome have identified many loci as potential targets of recent selection, but exploration of these candidates is required to verify the accuracy of genomewide scans and clarify the importance of adaptive evolution in recent human history. We present analyses of one such candidate, enamelin, whose protein product operates in tooth enamel formation in 100 individuals from 10 populations. Evidence of a recent selective sweep at this locus confirms the signal of selection found by genomewide scans. Patterns of polymorphism in enamelin correspond with population-level differences in tooth enamel thickness, and selection on enamel thickness may drive adaptive enamelin evolution in human populations. We characterize a high-frequency nonsynonymous derived allele in non-African populations. The polymorphism occurs in codon 648, resulting in a nonconservative change from threonine to isoleucine, suggesting that the allele may affect enamelin function. Sequences of exons from 12 primate species show evidence of positive selection on enamelin. In primates, it has been documented that enamel thickness correlates with diet. Our work shows that bursts of adaptive enamelin evolution occur on primate lineages with inferred dietary changes. We hypothesize that among primate species the evolved differences in tooth enamel thickness are correlated with the adaptive evolution of enamelin.