Specialization of male reproductive tactics corresponds with large gonads and small brains.

Sexual selection has strong effects on gonad size, which has been proposed to shift energetic allocations, resulting in concomitant decreases in brain size. However, mixed findings leave it unclear whether negative correlations reflect direct energetic trade-offs or selection on trait combinations broadly. We tested whether male reproductive tactics impose energetic trade-offs by comparing brain and gonad sizes in Poecilia parae, a fish with discrete alternative male morphs specializing in three reproductive strategies: coercion, display, and sneaking. The obligate sneaker morph had substantially larger gonads and smaller brains than the other morphs, consistent with an energetic trade-off. However, examining individuals within morphs revealed a positive relationship, contradicting the energetic trade-off hypothesis. To resolve which morphs reflect the ancestral tissue state, we also compared gonad and brain sizes of the morphs to two closely related species whose males utilize more flexible reproductive strategies, Poecilia picta and Poecilia reticulata. Again, the P. parae obligate sneaker morph had the largest gonads and the smallest brains. Neuron-to-glia ratio (a proxy for energetic demands) showed no link to gonad size. Our results suggest that reproductive strategies shape brain evolution through correlational selection rather than direct energetic trade-offs, challenging assumptions that sexually selected traits impose constraints through direct resource allocation.