Sexuality as Conflict in Fiction

The pteranthropans on my world reproduce by what amount to threesomes, plus a symbiotic relationship with another species that originated as brood parasitism.

The pteranthropans are amphibious creatures. They lay eggs in water, and the larvae develop in a purely aquatic, very-low-intelligence, form. Brain development is delayed until the larvae develop the necessary internal support for large brains.

There are two "castes", or morphs, of the species, which begin diverging in the juvenile phase (I haven't figured out what determines the caste yet), the aquatics, who never leave water (but do become air-breathing), and the terrestrials, who do leave the water and develop flight. They are both intelligent, the terrestrials more than the aquatics.

Reproduction involves two terrestrials - a male and a female, and one aquatic. The male terrestrial courts the female for much of the year leading up to breeding season. As breeding season approaches, the pair finds a good nesting spot, and the male helps the female gather food to produce very large eggs (rich in yolk) As they establish their nest, the pair enters courtship with an aquatic, and the three form a short-term bond lasting a few weeks. When it comes time to reproduce, the female terrestrial lays several large eggs, each consisting of a single ovum and several large support cells, yolk-filled haploid cells. The male terrestrial deposits sperm fertilizing the eggs (external fertilization). If the aquatic is male, he also releases sperm, so that some of the eggs are fertilized by the terrestrial (the majority of the eggs), and some are fertilized by the aquatic. If the aquatic is female, she produces several small ova without the support cells, which share support cells with the female terrestrial's ova. Male terrestrials also produce some support cells, but not as many as the female, their main contribution being in procuring food and defending the nest.

After the eggs have been laid and fertilized, the terrestrials leave the nest. The aquatic remains to guard the eggs until they hatch. After the eggs hatch, they find "nurses" for the larvae. Nurses are an unrelated species that raises the larvae as if they were their own young. Originally it was brood parasitism, not unlike cuckoos on Earth. It has since evolved into a form of symbiosis, with the aquatics defending nurses from large predators, and also keep competitors away from their food sources. The nurses, in turn, protect the larvae from small predators and feed them. Pteranthropan larvae retain a morphological resemblance to nurse larvae, a vestige of their earlier brood parasitism, but have evolved distinctive color patterns, to which the nurses respond by preferential treatment.

The aquatics tend to preferentially assist - and to preferentially choose as hosts - those nurses that had raised them, or the daughters of their host-mothers, thus creating selective pressure for more effective nurses. In fact, the transition from brood parasitism to symbiosis split the proto-nurses into two species, the nurses and a related one that ignores pteranthropan larvae and are ignored by the same.

Once the pteranthropan larvae reach a certain size, they begin to transition to the juvenile phase, acquiring a resemblance to their eventual adult forms, their development at this point splitting into those who become terrestrials and those who become aquatics, and their brains begin developing. They start to seek out their own kind, and leave their host-mothers, eventually either joining a loose band of aquatic juveniles (adults are typically solitary, or, at most, form small bands of 3 or 4 adults), or head to the land, where they emerge from the sea and seek out bands of terrestrials (terrestrials form long-term bands and societies, and are of roughly human-level intelligence)