The Estuarine Bumblet

Twenty-two million years have passed since the Thermocene-Pangeacene mass extinction, and ecosystems by now have largely recovered from the cataclysm. Biodiversity returns to prior levels, as new and unusual animals descended from a small but varied assortment of survivors now repopulate Serina.

One of these groups is the bumblets, the only vivas to come through the end of the Thermocene, and among the most aberrant of them as quadrupedal, live-bearing burrowing birds. With competition considerably reduced, this once restricted specialized bird lineage is able to explore a variety of new and more varied niches in the environment.

One of the new habitats the bumblets seem almost pre-adapted to make use of is bodies of water. Similar adaptations that evolved for life underground, such as paddle-shaped forelegs and a high tolerance for prolonged survival in low-oxygen conditions, are also required to dive and swim and some bumblets have now become well-adapted, almost entirely aquatic creatures in Serina’s rivers and lakes. With sprawling limbs splayed outward and considerably increased body sizes, they no longer move well upon dry land and come ashore only briefly to rest or to cross to another body of water. While most species still rear their young on land in a burrow, one species cuts ties to the land even more as it begins to raise its young right in the water, a change of upbringing made possible by a new adaptation.

The ovoviviparous bumblets were already adapted to low oxygen environments underground, the walls of their oviducts being extremely vascularized so that they were able to flood their egg pouch with oxygen to maintain a state of gas exchange within it sufficient to keep the developing young in good shape, and this also allowed to them to make short dives underwater to feed without suffocating their young. To spend greater lengths of time underwater without breathing, however, required further refinement of the system.

This bumblet has finally taken the vivas' reproductive system to its pinnacle, and become truly viviparous. The egg it now produces is much smaller than would be expected, no larger than a golf ball, and its shell is extremely thin. After a few days in the oviduct, it fuses to the wall of blood vessels and the embryo's circulatory system connects directly to that of the mother. This bumblet is the second bird lineage to develop proper pregnancy, alongside the flightless quadrupedal changelings. After hundreds of millions of years of evolution, the estuarine bumblet is now completely freed of the constraints of an egg. Its young safely developing inside the mother's body, connected to a lifeline of her air supply, she is free to dive deep in search of food, surfacing only every fifteen to twenty minutes to flood her system with fresh oxygen. While its ancestors were born very underdeveloped as a result of the loss of calcium in the eggshell so that they were softer and less fragile in the egg pouch, and therefore needed a prolonged period of care in a nest, this is not the case for the estuarine bumblet. Because the young is now attached directly to the mother's body, it has a ready supply of all nutrients and minerals it needs, including calcium, so that its skeleton can be fully developed by birth. Its single pup is thus now able to be born large, alert, and mobile from birth, able to swim alongside their mothers and learn to feed themselves within a few months.

Giving birth to just this one big baby, which is so much more precocial at birth than its ancestors, the fifty-pound estuarine bumblet is already more than 150 times as big as its Thermocene ancestor, and now totally aquatic, with no need to move onto land any longer. It is also the first bumblet to spend considerable quantities of time in saltwater, with adaptations to drink seawater and excrete the sodium via highly concentrated tears.

The estuarine bumblet lives its life in coastal waters and the mouths of rivers, where it forages for fish and crustaceans near the sediment, finding them by both touch, with sensitive whiskers, and through improved eyesight versus their ancestors. Their beak is elongated and useful to grab and keep potentially dangerous prey such as crabs and crayfish at a safe distance from its face as it thrashes them to death before swallowing. The single offspring is able to swim at birth, though it requires close protection as it is slow and rather defenseless for several weeks. These bumblets thus show increased social complexity, with females living in groups and cooperatively raising their pups. This allows each female time to forage every day while leaving her young in a creche attended by another female. Females alternate throughout the day, taking turns as babysitter and then going out to hunt, and if one does not find sufficient food for her young, another more successful female in the group may even spare a little food to an offspring other than her own. Social living is also making these bumblets smarter, as group living requires a lot of complex interactions.