Those who travel underground have to face many hazards. One of the lesser of these is the Dungeon Tick. Almost invisible on the stone ceiling, it will wait many a year until it drops on a suitable host to feed on its blood.
Experienced adventurers who detect the tick sometimes chose to let it remain. Although the tick can sap vitality, it somehow enhance vision*, a useful tool when the light fails or utmost discression is required. Scholars have speculated that frequent exposure to dungeon ticks is what grants the underground dwellers their ability to see in the dark, and why, after a few generation of surface living, the ability fails.
Recently, scholars studying the life cycles of the dungeon ticks have made startling discoveries. Following a number of months of slow growth on its host, the tick falls off, finds a dark corner to hide and molts into a stony egg. After a month of incubation, this egg hatches.
At this point, the story becomes muddled. The wizard Lurm of Elderstone, who was trying to develop a dark-vision potion, believes that the egg hatches into a rat; while the Archbishop of Garboli claims to have proof positive that the tick becomes a pigeon, and has pressured the Pope to excommunicate Lurm over the affair. Po Befi, the Slugman Ambassador (and thus immune to threats of excommunication and far too wealthy for Lurm to bribe), was asked by the Pope to study the matter. The Slugman concluded that the egg becomes a *bat*. This, of course, did very little to settle the affair.
The three scholar agrees on what happens next however. After a period of feeding and growth, the critter molts again into an egg, which transforms into a gremlin. After some time of eating, this gremlin grows and guided by some magical instinct (or sense of smell as suggested by Po Befi), rejoins a goblin tribe (thus proving the dwarven affirmation that gremlins are immature goblins). Lurm believes that the goblin generates spores which become more ticks, while the Archbishop firmly believes that the ticks spontaneously generate. Research is ongoing, and Lurm has offered a bounty on live goblins.
Po Befi speculates that the stone underground is more nourishing, which is why surface dwelling goblins are often so wretched. Goblins do not appear to be aware of this cycle, but they do know that if they live in caves or other underground settings, the clan will get stronger over time. Too deep however and the critter - wether is be rat, pigeon or bat - fails to thrives, which explains the lack of goblins reported in the very depth of the earth.
*Adventurers wishing to let the dungeon tick feed on them to gain the 5 feet of darkvision can, but they lose 1 of their max HP, until the tick detaches, 2d4 weeks later. DM may allow PCs to develop permanent 10 feet darkvision after years of use.