How Male Seahorses Evolved to Give Birth

Male pregnant seahorses give birth to around a thousand babies. Seahorses come from the family Syngnathidae, which also includes seadragons and pipefish. They're the only animals in which the males get pregnant. After females transfer eggs, the males then fertilise them. But why do male seahorses give birth? And how did this way of reproduction evolve?

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References:
Whittington CM & Friesen CR. The evolution and physiology of male pregnancy in syngnathid fishes. Biol Rev Camb Philos Soc 2020;95:1252–1272.
Roth O, et al. Evolution of male pregnancy associated with remodeling of canonical vertebrate immunity in seahorses and pipefishes. Proc Natl Acad Sci U S A 2020;117:9431–9439.
Wilson AB, et al. Male pregnancy in seahorses and pipefishes (family Syngnathidae): rapid diversification of paternal brood pouch morphology inferred from a molecular phylogeny. J Hered 2001;92:159–166.

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