M9 Social and Reproductive Modes

Topic

Ritualized combat between males—such as wrestling and shoving—allows mammals to establish social hierarchies without the fights resulting in lethal injuries for either contender.

The mechanics of these ritualized bouts are modeled as a collision between two masses connected elastically, each representing one of the contenders; in combat involving horns, the contact force at the moment of impact can exceed the animal's body weight several times over. However, the elastic nature of the collision—where horns and antlers lock, yield, and separate rather than piercing—is precisely what prevents the impact energy from causing a lethal injury.

Alongside direct combat, courtship displays offer another way for an individual to demonstrate its quality without the need to fight. These serve as honest signals precisely because their energy cost is so high that only high-quality individuals can sustain them; consequently, a low-quality individual cannot fake such a display without becoming exhausted before completing it.

Beyond courtship between mates, mammals also synchronize physiological processes at the group level—such as reproductive cycles among females in the same group or heart rates between partners. This phenomenon is modeled as a system of mutually coupled oscillators: a parameter ranging from zero to one measures the group's level of synchronization at any given moment, with a value of one indicating perfect synchronization and a value of zero indicating a complete lack of collective synchronization. This synchronization does not emerge gradually; instead, it arises abruptly once the coupling force between individuals exceeds a critical threshold, which depends on the degree of dispersion among the individuals' natural, intrinsic rhythms. Once the reproduction facilitated by this physiological synchronization is complete, physical parental care—such as carrying the offspring or keeping them warm by huddling—imposes quantifiable energetic costs on the parents.

The energetic cost of carrying an offspring increases with the ratio of the offspring's mass to that of the parent carrying it: the heavier the offspring relative to the carrier, the greater the energetic cost of transport.

In many rodent species, keeping offspring warm is achieved by forming a huddle of tightly packed bodies; by pressing against one another, each individual in the huddle reduces the body surface area directly exposed to the environment, thereby minimizing heat loss to the surroundings.

The ultimate investment each parent makes in this form of physical care is not a fixed amount but rather the result of an optimal parental investment model: the level of care provided by each parent is predicted by the difference in their relative investments, stabilizing at an equilibrium point where neither parent gains any advantage by unilaterally altering their investment level while the other maintains theirs.

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