A8 Social and Reproductive Disruptions

Topic

Social and reproductive disruptions involve the failure of the physical mechanisms of communication, coordination, and synchronization that structure the group life of mammals. Although these dysfunctions appear to belong to the realm of animal behavior, their foundations are deeply physical: vocalization is a matter of the vibratory mechanics of the vocal tract; body displays are a matter of postural biomechanics; group synchronization is a matter of mutually coupled oscillators; and unregulated physical aggression is governed by the mechanics of impact and energy transfer between bodies.

The vibratory mechanics of the vocal tract are modeled by representing the vocal folds as a vibrating taut string: the fundamental frequency at which the folds vibrate is determined by the tension to which they are subjected and by their mass per unit length, such that tighter folds vibrate at a higher frequency and more massive folds vibrate at a lower frequency. Above this fundamental frequency, the rest of the vocal tract acts as a resonant cavity that filters the sound spectrum produced by the folds, reinforcing certain frequencies over others and giving each vocalization its characteristic timbre.

Unregulated physical aggression—as well as ritualized combat between individuals of the same species—is governed by the mechanics of impact between colliding horns, antlers, or bodies: when the collision is completely inelastic—that is, when the colliding bodies or structures remain locked together after impact—the loss of kinetic energy during the collision is maximal. Conversely, when the collision is elastic and the bodies separate after impact, momentum is transferred completely from one body to the other without any loss of kinetic energy in the process. The group synchronization mentioned at the outset manifests in various ways—coordinated collective movement, choral vocalization among individuals, and seasonal synchronization of reproduction—and in every instance, it is modeled as a set of oscillators coupled by the physical signals individuals exchange: acoustic, visual, or chemical. The collective rhythm exhibited by the group emerges from the strength of the coupling between these individual oscillators: the stronger the coupling between the animals, the more synchronized their group behavior becomes.

When physical impairment in certain individuals weakens the connections—whether involving movement, vocalization, or chemical signals—that keep the group synchronized, the entire social structure may fragment; there is a minimum threshold of active social connections below which the group ceases to function as a coherent unit and splits into isolated subgroups.

When social and reproductive disruptions are severe enough to fragment the group, the consequences affect the very group life that these physical mechanisms of communication, coordination, and synchronization had sustained from the start: the group is segmented into smaller units, and each individual loses some of the protective benefit of being surrounded by others—the risk of predation, which is distributed among many individuals in a large group, becomes concentrated on each individual once the group fragments—while the group's reproductive success declines, a change directly measurable as a drop in birth rates.

ID:2625