The research program reviewed here focuses, in the broadest sense, on the co-evolution of neural and neuroendocrine mechanisms of social behavior among vertebrates, in this case for species exhibiting alternative reproductive tactics (ARTs). I undertook such a program not so much to study sex differences in brain and behavior, but rather to better understand the basis of intraspecific variation in “processes that determine when behavior will occur and what form it will take” (p. vii, Marler and Hamilton, 1966). Alternative male phenotypes present such an opportunity. Like “hormonally regulated sets of male and female traits, functionally and morphologically differentiated larval and adult characters, and even the contrasting tissue and organ systems of a multicellular individual … different alternative phenotypes of the same species may show dramatic differences in morphology, behavior and ecological niche” (West-Eberhard, 1986). As the assessment of male and female phenotypes, hormonal manipulations within the brain or at systemic levels have been useful tools for identifying characters and mechanisms underlying intrasexual variation in behavioral phenotypes.

I first launched an ART research program together with a postdoctoral associate, Matthew Grober, to investigate neuro-hormonal plasticity among sex reversing fishes, namely in the number and size of GnRH-synthesizing neurons in the preoptic area (POA) of the two male morphs (initial and terminal phase males) of the blue-headed wrasse, Thalassoma bifasciatum (Grober and Bass, 1991; Grober et al., 1991). Before long, however, efforts concentrated on a readily accessible, highly sonic species of toadfish (Batrachoididae), the plainfin midshipman, Porichthys notatus Girard, aka the “California singing fish” (Greene, 1924; simply referred to below as midshipman). This strategic switch was largely driven by two sets of findings. First, we discovered that midshipman exhibit two adult male morphs that follow alternative reproductive tactics (ARTs, Fig. 1) (Bass and Marchaterre, 1989a, Bass and Marchaterre, 1989b; Brantley and Bass, 1994). Second, the relative simplicity of the midshipman's vocal system presented a distinct opportunity to directly relate the properties of single neurons to the acoustic features of natural vocalizations (Fig. 2, Bass and Baker, 1990; see Bass et al., 1994 for application of the term vocalization to this group of sonic fish). These findings gave rise to two overarching hypotheses that we continue to test: (1) ARTs are paralleled by suites of vocal, auditory and neuroendocrine characters and mechanisms (Bass, 1992, Bass, 1996; Feng and Bass, 2017), and (2) highly vocal teleost fish, in this case midshipman, have a vocal control network comparable to that of birds and mammals (Bass, 2014; Schuppe et al., 2024; see Bass et al., 2015 for other sonic fish species). Fortunately, a research program led by Sigal Balshine of McMaster University complements ours by identifying ecological characters and mechanisms influencing the reproductive fitness and survival of midshipman fish (e.g., Bose et al., 2018; Brown et al., 2021; Pepler et al., 2021).

Parenthetically, I note that species within the genus Porichthys (Walker and Rosenblatt, 1988) are commonly referred to as midshipman because they have rows of pearl-like, bioluminescent photophores along the head and trunk, typically parallel to mechanosensory lateral line receptors (Greene, 1899; Tsuji et al., 1972). These photophores were reminiscent of the buttons on a naval academy midshipman's uniform, hence, the common name midshipman (Jordan and Bollman, 1890). A brief note suggests that photophores flash on and off during spawning (Crane, 1965), but this has yet to be rigorously tested.