Currently, our efforts focus on sound-producing/vocalizing species because they afford us the opportunity to directly relate divergent brain phenotypes to both male-female and male morph differences in behavior. In particular, we concentrate on one species, the plainfin midshipman fish (Porichthys notatus), for which we discovered two male morphs with divergent spawning and vocal phenotypes. Type I, "singing" males build and guard nests and acoustically court females. Type II, "sneaking" males neither build nests nor sing to females, but instead sneak spawn from positions inside or just outside a type I male's nest. Neuroanatomical studies have now delineated a "song" control pathway that extends from forebrain to hindbrain levels. This vocal network also has extensive linkages to central auditory pathways. A hindbrain vocal motor network includes pacemaker neurons with rhythmic, oscillatory-like activity that establish the characteristic traits of species and sex-typical vocalizations (e.g. fundamental frequency and duration). These studies utilized intracellular recording and staining methods to also demonstrate sexual dimorphisms in single, physiologically-identified neurons of the vocal circuit between the two male morphs as well as between males and females. (Females and type II males are convergent, but divergent from type I males, in their vocal motor and vocal behavior traits.) This is the only example where intracellular studies have shown that individual hindbrain neurons directly establish the physical attributes of a vocalization. The results of these studies were also the first explicit demonstration for any one species that: (1) alternative mating tactics can be paralleled by alternative phenotypes for the neurons that determine tactic-specific types of behavior, and (2) reproductive maturation is not obligatorily linked to the expression of "typical" male neuronal secondary sex characteristics.
Other studies of the vocal motor system have established that: (1) type I and type II male brain phenotypes arise from distinct developmental trajectories established early in development, (2) testosterone, but not estrogen, can induce type I male-like vocal traits (both anatomical and physiological) in juveniles, (3) brain levels of aromatase, the enzyme responsible for the conversion of testosterone to estradiol, are several fold lower in the vocal hindbrain region of type I males compared to type II males (and females), and (4) the neurophysiological properties of the hindbrain pattern generator is directly influenced by the action of the neuropeptides arginine vasotocin, AVT and isotocin, IT (the fish equivalents of arginine vasopressin and oxytocin in mammals) in the preoptic area-anterior hypothalamus, a brain region implicated in the control of social/vocal behaviors across all vertebrates. Most importantly, the pattern of AVT and IT modulation is dependent on social/vocal behavior, not gonadal sex.
Together, these findings provide a unique opportunity to investigate the separate and/or interactive effects of peptides and steroid hormones on brain circuitry that determines vocalization behaviors.
See also this report in ABC Science News, which includes audio samples of the fish songs.
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