10.0 Introduction

Figure 10.1. Male seahorse and offspring, newly emerged from the male’s brood pouch.
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Pregnant males

In Figure 10.1 above, we see the miracle of birth, though it looks a bit different than we may expect. This male seahorse is shown with one of many of his offspring that he has been carrying in his brood pouch during the last couple of weeks. Unlike mammals, where the female members of the species produce placentas and give birth to the offspring, in seahorses, the males take on this intensive role of providing parental care to offspring.

Seahorses and their close relatives sea dragons and pipefish are (as far as we know) the only species in which a female deposits eggs into a male. Males therefore invest more into their offspring, while the females invest in the energetically taxing process of egg production. Through an elaborate dance, the female uses an ovipositor to deposit eggs into the male’s brood pouch. The females can then move on to attempt mating success with multiple males, while those she impregnates are left to care for the young until they emerge from the father’s brood pouch.

Many seahorse species engage in a mating system known as polyandry, where a single female mates with many males, but the males cannot mate as often due to the necessity of parental care, which only they can provide. Nature is full of species with many types of mating systems. In this chapter, we will understand the environmental pressures that can lead to these different mating systems.