雄性怀孕仅限于海马、海龙和它们的近亲——海龙科动物（在这个科的动物中，幼仔是在雄性的育仔囊中抚育的）。
正如Kimberly Paczolt 和Adam Jones在一项新的研究中所发现的那样，这些抚育幼仔的雄性所做的事情并不只是充当一个被动的孵化者。人们已经知道，育仔囊是一个抚育发育中幼仔的复杂结构。但在抚育的同时，它还会除杂。
在“海湾海龙”（Syngnathus scovelli）中，雄性能够选择性地使来自雌性的、被认为缺乏吸引力的胚胎夭折，以便将资源留给以后会碰到的更有希望的胚胎。这是在一个性逆转物种中有关交配后性冲突的惟一已知例子。(生物谷Bioon.com)
生物谷推荐原文出处：
Natue doi:10.1038/nature08861
Post-copulatory sexual selection and sexual conflict in the evolution of male pregnancy
Kimberly A. Paczolt1 & Adam G. Jones1
Department of Biology, 3258 TAMU, Texas A&M University, College Station, Texas 77845, USA
Male pregnancy in seahorses, pipefishes and sea dragons (family Syngnathidae) represents a striking reproductive adaptation that has shaped the evolution of behaviour and morphology in this group of fishes1, 2, 3, 4. In many syngnathid species, males brood their offspring in a specialized pouch, which presumably evolved to facilitate male parental care5, 6. However, an unexplored possibility is that brood pouch evolution was partly shaped by parent–offspring or sexual conflict, processes that would result in trade-offs between current and future pregnancies. Here we report a controlled breeding experiment using the sexually dimorphic Gulf pipefish, Syngnathus scovelli, to test for post-copulatory sexual selection within broods and for trade-offs between successive male pregnancies as functions of female attractiveness. Offspring survivorship within a pregnancy was affected by the size of a male’s mate, the number of eggs transferred and the male’s sexual responsiveness. Significantly, we also found that embryo survivorship in a current pregnancy was negatively related to survivorship in the prior pregnancy, clearly demonstrating fitness trade-offs between broods. Overall, our data indicate that post-copulatory sexual selection and sexual conflict occur in Gulf pipefishes. The conflict seems to be mediated by a strategy of cryptic choice in which males increase rates of offspring abortion in pregnancies from unattractive mothers to retain resources for future reproductive opportunities. Hence, the male brood pouch of syngnathid fishes, which nurtures offspring7, 8, 9, also seems to have an important role as an arbiter of conflict between the sexes
