Protein pheromone expression levels predict and respond to the formation of social dominance networks.

Communication signals are key regulators of social networks and are thought to be under selective pressure to honestly reflect social status, including dominance status. The odours of dominants and nondominants differentially influence behaviour, and identification of the specific pheromones associated with, and predictive of, dominance status is essential for understanding the mechanisms of network formation and maintenance. In mice, major urinary proteins (MUPs) are excreted in extraordinary large quantities and expression level has been hypothesized to provide an honest signal of dominance status. Here, we evaluate whether MUPs are associated with dominance in wild-derived mice by analysing expression levels before, during and after competition for reproductive resources over 3 days. During competition, dominant males have 24% greater urinary MUP expression than nondominants. The MUP darcin, a pheromone that stimulates female attraction, is predictive of dominance status: dominant males have higher darcin expression before competition. Dominants also have a higher ratio of darcin to other MUPs before and during competition. These differences appear transient, because there are no differences in MUPs or darcin after competition. We also find MUP expression is affected by sire dominance status: socially naive sons of dominant males have lower MUP expression, but this apparent repression is released during competition. A requisite condition for the evolution of communication signals is honesty, and we provide novel insight into pheromones and social networks by showing that MUP and darcin expression is a reliable signal of dominance status, a primary determinant of male fitness in many species.

Octopaminergic gene expression and flexible social behaviour in the subsocial burying beetle Nicrophorus vespilloides.

Flexible behaviour allows organisms to respond appropriately to changing environmental and social conditions. In the subsocial beetle Nicrophorus vespilloides, females tolerate conspecifics when mating, become aggressive when defending resources, and return to social tolerance when transitioning to parenting. Given the association between octopamine and aggression in insects, we hypothesized that genes in the octopaminergic system would be differentially expressed across different social and reproductive contexts. To test this in N. vespilloides, we first obtained the sequences of orthologues of the synthetic enzymes and receptors of the octopaminergic system. We next compared relative gene expression from virgin females, mated females, mated females alone on a resource required for reproduction and mated females on a resource with a male. Expression varied for five receptor genes. The expression of octopamine ß receptor 1 and octopamine ß receptor 2 was relatively higher in mated females than in other social conditions. Octopamine ß receptor 3 was influenced by the presence or absence of a resource and less by social environment. Octopamine α receptor and octopamine/tyramine receptor 1 gene expression was relatively lower in the mated females with a resource and a male. We suggest that in N. vespilloides the octopaminergic system is associated with the expression of resource defence, alternative mating tactics, social tolerance and indirect parental care.

The influence of foot posture on the cost of transport in humans.

Although humans appear to be specialized for endurance running, the plantigrade posture of our feet, in which the heel contacts the substrate at the beginning of a step, seems incompatible with economical running. In this study, we tested the hypothesis that plantigrade foot posture reduces the energetic cost of transport (COT) during walking in humans. When human subjects walked with their heels slightly elevated in a 'low-digitigrade' posture, COT increased by 53% above that of normal plantigrade walking. By contrast, there was no difference in COT when subjects ran with digitigrade versus plantigrade foot posture. Stride frequency increased and stride length decreased when subjects switched to digitigrade walking; however, this change did not influence the COT. Additionally, we found that possible reductions in postural stability appear not to have caused the elevated cost of digitigrade walking. Digitigrade walking, however, did (1) increase the external mechanical work performed by the limbs; (2) reduce the pendular exchange of kinetic and potential energy of the center of mass; (3) increase the average ground reaction force moment at the ankle joint; and (4) increase the recruitment of major extensor muscles of the ankle, knee, hip and back. These observations suggest that plantigrade foot posture improves the economy of walking. Relative to other mammals, humans are economical walkers, but not economical runners. Given the great distances hunter-gatherers travel, it is not surprising that humans retained a foot posture, inherited from our more arboreal great ape ancestors, that facilitates economical walking.