Are you thinking of bed bugs? They don’t dump their eggs, but have lower lifetime egg output because they are mitigating the attack. Seed beetles? Female gets injured, but the male’s spines are linked to other traits that make her offspring do better. The net effect on her reproduction is actually positive despite the harm.

With ducks, the coercer quality hypothesis is an important hypothesis. But the evidence has caused the hypothesis terrain to shift to the indirect genetic benefit or reinforcement of existing preference hypothesis. Here’s some situations that complicate the coercer quality hypothesis.

1. Coercers aren’t reliably the “leftover” or inferior males as expect in the coercer quality hypothesis. It’s mostly already-paired males doing the forcing, or, put another way, already screened and selected males which is arguably an indicator for genetic fitness.

2. If the tract exists to filter for good genes via female choice, you’d expect some positive relationship between choice-related investment and tract complexity. Instead, egg size (a proxy for female investment) is negatively correlated with both penis length and vaginal spiral count according to a 2020 study. This is the reverse of the prediction. Some in the research community argue female reproductive investment is doing more of the explanatory work. And not mate quality screening.

So the current leading hypothesis isn’t “the tract detects and blocks bad genes,” it’s closer to “the tract protects a choice she already made through courtship, before force ever entered the picture.” Coercion gets filtered because he isn’t the one she picked. But this isn’t settled yet and there are ongoing debate and more research to be done.