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Brain-behavior relationship
Midshapman fish have two forms of males: the nest-building Type I and "sneaker male" or "satellite male" Type II. Type I males attract females to their nests with their humming, coax them to lay eggs, and guard them. In contrast, Type II males do not build nests or attract females on their own. Instead, they sneak up to the Type I's nests and deposit their eggs. These behavioral differences can be seen in the differences in the structure and function of the nervous system.

Neurons and Muscles
Type I males and Type II males and females grow along different growth paths when it comes to neurons and muscles that determine morph-specific vocal behavior. For Type I males, for example, sexual maturation is preceded by growth of the mate-calling circuit and sonic muscle. Specifically, before the transformation from juvenile to type I male, the size of the motoneurons and volume of the sonic motor nucleus increases twofold and the number of sonic muscle fibers increases fourfold. At the start of sexual maturation, the motoneurons increase in size again, although not as much as before. The pacemaker neurons also increase in size at this time but not as much as the motoneurons in general. The sonic muscle also increases fivefold in the size of muscle fibers. In contrast, there is little dramatic change seen before the transformation from juvenile to type II male or adult female. In fact, the vocal neurons and muscles change little or not at all.

Hormones
Differences between the reproductive strategies of Type I and Type II are also reflected in hormonal differences during sexual maturation. The three different morphs – Type I, Type II, and females – also produce different levels of various hormones. Type II males produce the highest levels of testosterone, followed by females and then Type I males. Females only have estrogen in the form of 17β-estradiol but at much lower levels than testosterone.

Type I males also have five times more 11-ketotestoterone, which is a form of testosterone common to teleosts, than type II males and females. 11-ketotestosterone is likely to be more potent than testosterone in supporting courtship bevahiors such as humming.