Module 10: Sexual Dimorphism

Pipefish Courtship

Sexual dimorphism evolves for a few reasons but the main cause of these differences in males and females is due to the fact that each sex has a different roll when it comes to how they experience selection. Males can develop characteristics like horns, larger size, ornamental feathers...etc. due to competition with other males and the need to impress females and pass on their genes. Some of these characteristics/ competitions allow the animal to compete with others and in some cases, the ones with the best fitness will pass on their genes.

Pregnancy is very costly for females of any species. Extra sustenance and is required due to the energetic costs of it. However, pipefish roles are reversed and the males gestate the offspring until birth. In pipefish, seahorses, and sea dragons, the females pass the eggs on to the males in a pouch or on their tails. Then the males fertilize the eggs in the designated pouch or tail, where they are kept until they hatch. Carrying offspring is a very costly and takes a lot of extra resources and stress on the body but the females of these fish only have to expend the energy it takes to make the eggs, which would usually be the males role. Instead the males take on the physiologically taxing role of carrying the offspring. Sexually dimorphic characteristics like the pipefish sail, antlers, and intricate peacock feathers are costly too so the roles in pipefish are generally reversed; the females court and the males carry young. Female courtship probably evolved in pipefish due to this reason. The males are able to choose a mate who they think will produce good offspring that are worth the cost to carry.

Similar to pipefish, male seahorses and sea dragons also carry offspring; but unlike pipefish, they have generally common sex roles in mating and courting. The males generally compete for female choice. Mating rituals involve mirror dancing and tail grabbing in seahorses. They are sexually dimorphic in some ways, like body and tail length, but they don't have flashy costly displays like the female pipefish. Maybe pipefish sail is a good indicator of health and that is how the female specific characteristic evolved. It also probably helps male pipefish make what they see as the best choice, to help choose the fittest female and have the fittest offspring.

Sources

BBC - Earth News - Ghostly 'dance of a sea dragon' - this short article explains the dancing courtship ritual of sea dragons and seahorses