The Biology of Aggression, Mating, & Arousal | Dr. David Anderson

Anderson argues that emotions are a class of internal states, alongside arousal, motivation, and sleep, that change how the brain transforms inputs into outputs. These states have properties such as persistence (outlasting the stimulus), valence (positive/negative), intensity, and generalization (spilling over into other situations). Focusing on states rather than self-reported “feelings” lets scientists study emotion mechanisms in animals where subjective report is impossible.

Electrical and optogenetic studies show that offensive, defensive, and predatory aggression use partially segregated circuits. In mice, specific neurons in the ventromedial hypothalamus (VMH) promote offensive, often rewarding aggression, while nearby but distinct neurons in the same region control fear and defensive behaviors. Predatory aggression involves other pathways. This means the label “aggression” describes behavior but not necessarily the underlying emotional state, which may be anger, fear, hunger, or others.

Within the VMH, fear-related neurons and aggression-related neurons sit millimeters apart, like different parts of a pear. Electrical stimulation in mice tended to recruit fear neurons and mask aggression, but precise optogenetic activation revealed aggression “switches” that cause rapid transitions from mating to attack or from passive exploration to glove-directed rage. Similarly, medial preoptic area (MPOA) neurons govern phases of mating, and mutual inhibition between MPOA and VMH helps the brain choose between ‘make love’ and ‘make war’ at any given moment.

In females, VMH contains distinct estrogen-receptor-positive subpopulations: one driving aggression and one driving mating. Virgin females show more activity in mating neurons; postpartum, pup-nursing females show heightened activity in aggression neurons and attack both male and female intruders. Some VMH neurons are male-specific and engaged in male aggression, while certain mating cells are female-specific. Surprisingly, estrogen and progesterone receptors in male VMH are critical for male aggression, often via testosterone’s conversion to estrogen.

Male–male mounting in mice is often dominance-related, not homosexual behavior. The same motor pattern can be driven by different internal states and circuits. Sexual mounting of females is accompanied by ultrasonic courtship vocalizations and engages MPOA mating circuits, whereas dominance mounting of males lacks these vocalizations and engages VMH aggression circuits. Weak VMH activation can induce dominance mounting; strong activation drives full attack. This shows why reading internal state from overt behavior is often misleading.