The human foot and heel-sole-toe walking strategy: a mechanism enabling an inverted pendular gait with low isometric muscle force?

Mechanically, the most economical gait for slow bipedal locomotion requires walking as an 'inverted pendulum', with: I, an impulsive, energy-dissipating leg compression at the beginning of stance; II, a stiff-limbed vault; and III, an impulsive, powering push-off at the end of stance. The characteristic 'M'-shaped vertical ground reaction forces of walking in humans reflect this impulse-vault-impulse strategy. Humans achieve this gait by dissipating energy during the heel-to-sole transition in early stance, approximately stiff-limbed, flat-footed vaulting over midstance and ankle plantarflexion (powering the toes down) in late stance. Here, we show that the 'M'-shaped walking ground reaction force profile does not require the plantigrade human foot or heel-sole-toe stance; it is maintained in tip-toe and high-heel walking as well as in ostriches. However, the unusual, stiff, human foot structure--with ground-contacting heel behind ankle and toes in front--enables both mechanically economical inverted pendular walking and physiologically economical muscle loading, by producing extreme changes in mechanical advantage between muscles and ground reaction forces. With a human foot, and heel-sole-toe strategy during stance, the shin muscles that dissipate energy, or calf muscles that power the push-off, need not be loaded at all--largely avoiding the 'cost of muscle force'--during the passive vaulting phase.

Postural strategies employed by orangutans (Pongo abelii) during feeding in the terminal branch niche.

Obtaining food in an arboreal habitat is complex due to the irregular and flexible nature of the supports available. As the largest predominantly arboreal primate, orangutans are expected to have developed particular postural strategies to enable them to feed successfully. In particular, they need to be able to cope within the terminal branch niche (TBN) as this is where the smallest, most compliant supports are, and also where the majority of the fruit and leaves are situated. We recorded feeding posture, along with a number of ecological and behavioral variables from different age-sex classes to enable analysis of the interactions between these and the compliance of the supports (as estimated from stiffness score). Suspensory postures with a pronograde orientation were used on the most compliant supports for all age-sex classes and appeared to play a particular role in facilitating safe use of the TBN by distributing body weight and using limbs for balance across multiple supports. This contradicts the idea that orthograde suspension evolved in response to the demands of feeding in the TBN. Adult males appear to use the same postures and feeding zones as the other age-sex classes, but appear to use stiffer supports where possible due to their larger body mass. Feeding method differed between the age-sex classes in relation to support stiffness, with larger adult males taking fewer risks due to their larger size, compared to infants and juveniles.

A new method for recording complex positional behaviours and habitat interactions in primates.

In an arboreal habitat, primates have to cope with a complex meshwork of flexible supports in order to obtain food, find mates and avoid predators. To understand how animals interact with such complex environments we can study their positional behaviour. However, due to the intricate variation in locomotion and posture it can be difficult to capture details such as limb use (i.e. weight and balance), limb flexion and substrate use. This paper presents a suitable method replicable for any primate species, based on the movement notation technique, Sutton Movement Writing (SMW), aiming to record the spatial arrangement of limbs during positional behaviours on multiple, compliant supports. This method was piloted during a year-long field study of wild orangutans (Pongo abelii) and validated and tested for inter- and intraobserver reliability using videos from the field. Overall, SMW shows considerable promise for increasing the resolution with which positional behaviours can be recorded under field conditions and provides a way to extract numerical data for use in statistical analyses. This will facilitate our understanding of how behaviours vary in response to the environment, and the capabilities of primates to perform key tasks in their distinct niches.

Selection for cuticular melanism reveals immune function and life-history trade-offs in Spodoptera littoralis.

Several insect species show an increase in cuticular melanism in response to high densities. In some species, there is evidence that this melanism is correlated with an up-regulation of certain immune system components, particularly phenoloxidase (PO) activity, and with the down-regulation of lysozyme activity, suggesting a trade-off between the two traits. As melanism has a genetic component, we selected both melanic and nonmelanic lines of the phase-polyphenic lepidopteran, Spodoptera littoralis, in order to test for a causative genetic link between melanism, PO activity and lysozyme activity, and to establish if there are any life-history costs associated with the melanic response. We found that, in fact, melanic lines had lower PO activity and higher lysozyme activity than nonmelanic lines, confirming a genetic trade-off between the two immune responses, but also indicating a genetic trade-off between melanism and PO activity. In addition, we found that lines with high PO activity had slower development rates suggesting that investment in PO, rather than in melanism, is costly.