Greggans possess three sexes, but socially class themselves into five, at least three of which are necessary for viable offspring.

A-type carry sperm but not ovum.

B-type carry ovum and sperm, but cannot carry the eggs to term (though they can carry them in a fertilized state for some time).  Self-fertilization can happen but results in all D-types.

C-type can produce ovum but in limited numbers; they usually have fertilized ovum implanted into them by B-types.

D-type are a sterile neuter sex; these are considered failed A-types and typically grow to the largest sizes.  This type occurs due to an over-abundance of a testosterone-like chemical during development that destroys their sperm-producing organs even as it develops their musculature and skeletons to larger sizes.

E-type are another sterile neuter sex; they may be similar to any other type, but are generally considered to be more diminutive than the others in Greggan society.  This particular mutation occurs due to a breaking of their complex reproductive system, rendering them sterile.  In reality they may be any size that is not larger than the average A-type (if they do grow as large or larger, they are classified as D-type, not E, regardless of the cause of their sterility).

The complexity of Greggan reproduction has led to very complex social systems, which have changed greatly over time.  In the common current of most modern Greggans, however, a strict caste hierarchy is observed.

Greggan social relationships are complex; while theoretically an A and C or a B and C can form a two-partner relationship, usually an A takes several B-types as partners, and each B takes one C-type to carry their ovum.

A-type are generally large and powerfully-built.  They are socialized to be aggressive and dominant.

B-types are not as large as A-types and are trained to be loyal subordinates to A-types, though still with a higher status over all other types.

C-types have historically been viewed as chattel and are raised to be submissive and obedient.

D-types are viewed as excellent menial laborers or soldiers.  They are generally not educated, giving them a reputation for being stupid, though they are not inherently less intelligent.

E-types frequently suffer poorer health due to the mutations that relegate them to this type.  However, they are also frequently given less (in historical times) or lower-quality food, which figures heavily into this.  They may be artisans or perform similar delicate, if menial, jobs.

Greggan society has been historically dominated by civilizations led by A-types.  While they economically and socially evolved, they never fully left behind their caste system.  Improvements in food production and science allowed them to more precisely control the balance of their society, but a period of wars brought on by intense competition led to a near-collapse of their civilization.  After this, a period of relative enlightenment occurred, where substantial social gains were made for all castes.

As their populations can grow very quickly in times of peace and plenty, they pressed into space at a lower technological level than might be expected, utilizing brute force and numbers to create their first extra-planetary colonies.  These colonies were often built by D and E-types, who, already being sterile, were viewed as hardier against long-term radiation.  Their populations being unable to replenish themselves, however, left them beholden to the home world for replenishing their numbers.

Utilizing brute-force tech they colonized a dozen adjacent star-systems at sublight over a matter of five centuries, each one consolidating power around a primary planet that housed most reproductively-capable populations.  Great tensions and internal strife ensued as the non-reproductive castes tried to leverage their control of resources against the ability to replace population losses that the primary planets controlled.

How this might have turned out is unknown; during this period they were encountered by the Aeena, who already possessed zerospace travel.  Though the Greggans were cautiously open to friendly relations at first, the xenophobic nature of the Aeena quickly led to a breakdown of relations that led to war.

The Aeena quickly realized the vulnerability of the Greggans having all their reproductive potential on only a handful of worlds, and focused on bombarding the planets to annihilate this potential.  In some cases they were aided by short-sighted D and E-type rebels who thought this would lead to some sort of liberation (in some cases being promised superior cloning technology by the Aeena, which would solve their own problems; these promises were lies, however).

After destroying the spawnworlds, the Greggan resistance quickly collapsed.  Numbers of them fled, though still lacking FTL.  Only scattered groups, and very few reproductive members of their species survived, leading to their population rapidly declining even after finding safe harbors in the territories of other species.

The damage to their society and the loss of their homeworlds, have led the Greggans to widely turn nihilistic; many of those alive are themselves of a sterile type and see little reason to think of or plan for the long-term.  As a result, many engage in illegal activities such as piracy or mercenary work, and seek to simply enjoy what is left of their lives.