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Under the Gwynne et al. model, smaller females produce smaller eggs than larger females and these smaller eggs are less likely to result in viable progeny2. However, Gwynne et al. go on to explain that this model alone is inadequate, because it does not predict a trade-off between offspring survivorship in subsequent broods, one of our key findings. Thus, they add a second layer to their model, which posits that the “total costs of being pregnant with a larger brood” are higher than those for a smaller brood2, without specifying how these costs may arise. In fact, if the total cost of being pregnant reduces future reproductive opportunities, then this total cost is clearly a form of parental investment3. Under the Gwynne et al. model, then, males experience an increase in parental investment for larger broods originating from larger females. As we discuss in the paper, this increase in parental investment must either be a consequence of males using a strategy in which they increase investment in broods from larger females, or a female (or offspring) strategy in which the broods of larger females somehow take resources from the male1. As we indicate in the paper, only the former strategy is consistent with all of our observations1.

Further support for the idea that the male is playing an important role in post-copulatory sexual selection in Gulf pipefish can be seen in our path diagram (figure 3 in ref. 1), which indicates what appears to be a counterintuitive negative relationship between male size and brood size1. This observation, which cannot be explained easily by a female-mediated strategy, is easily explained by a male-mediated strategy of cryptic choice. We paired males with females at random with respect to male body size. Consequently, a larger male was more likely than a smaller male to be paired with a female smaller than himself. If males assess female attractiveness relative to their own body size, as our data indicate (figure 2 in ref. 1), then the larger males should have been the least attracted to their assigned mates, which would explain a reluctance to accept eggs. Female control of egg transfer would predict either a positive relationship between male length and brood size or no relationship, but not a negative relationship. Hence, males appear to control brood size, which is the key variable in the Gwynne et al. model2 and must be controlled by females to successfully dismiss a role for males in post-copulatory sexual selection.

In summary, the Gwynne et al. model still requires a mechanism for trade-offs between broods, and our data indicate that the most likely explanation in Gulf pipefish is that males adjust parental investment1. Observations by other scientists documenting nutrient transfer from pregnant male to brood4,5,6 and from brood to pregnant male7 lend additional credence to this interpretation. However, we agree with Gwynne et al. that further research is warranted. Moreover, we would be surprised if intrinsic egg quality and female-mediated effects play no role in determining egg or offspring survivorship within male pregnancies. Nevertheless, our results strongly support the conclusion that male-mediated processes play a significant role in post-copulatory sexual selection in pipefish.