The Bumblets: Quadrupedal Burrowers

Bumblets are the smallest of the vivas (ovovivavian aardgeese), most species weighing only a few ounces, and since their appearance over one hundred million years ago, they have become some of the most specialized of all birds. They are now highly adapted to burrowing underground. Though their common ancestor was only a part-time burrower and fed predominately above ground, the single surviving family of the late Thermocene is completely subterranean. Unlike the beak-driven molebirds, they dig their tunnels with paddle-like wings, the underlying skeletal structure of which are almost totally vestigial except for the very large wrists, which attach almost directly to the shoulder girdle; the upper and lower arm bones are extremely reduced in size and almost immobile. The wrist, however, has a wide range of circular movement and functions as a shovel; a large claw on the thumb loosens soil while the rest of the hand pushes it backwards and to the sides. The short tail of the bumblet's ancestors has been totally lost again and to make up for the space needed in its abdomen to carry its eggs, the body of the bumblet has elongated. The feet are very small and are now positioned at the extreme end of the body, able to help push it along in its burrows, but not to walk or really to stand at all. The larger wings now do most of the work in locomotion and are used by the bumblet to crawl through its burrows. Projected outwards to its sides, the bumblet is capable of walking quadrupedally by putting weight on the interior side of its wrists, which then move alternately forward and back, with the approximate gait, speed, and grace (or lack thereof) of a baby sea turtle as it dashes out to sea.

Unlike molebirds, bumblets are carnivorous, and despite their cute and pudgy appearance are fierce predators for their size. Though a bumblet's beak is small and usually thin, it is armed with tiny keratin teeth on both jaws which help to secure a grip on prey, usually insects and earthworms, but occasionally other vertebrates and even other birds. Once food has been captured, it is cut apart by slicing motions of the the tongue which is also lined with teeth as in other vivas. The food is swallowed in large chunks and then further broken down in the stomach. Bumblet metabolisms are fast and they must spend most of the day feeding, even in winter, for they cannot hibernate.

Bumblets have independently evolved true viviparity from the serilopes, through different means. Whereas the highly muscular oviduct of serilopes toughened so that the hard eggshell fragments from hatched eggs could simply be voided without causing injury after the chick was born, like afterbirth, the bumblets are the first birds to evolve away from hard-shelled eggs entirely. The calcium content in their eggshells are low, an adaptation that leaves their eggs soft and pliable; this allows these smallest of vivas to carry multiple eggs at a time without the risk of them bumping together and breaking in the mother's body. This increases the female's potential fertility, allowing her to raise broods of as many as five chicks at a time compared to the single offspring of almost every other viva, which can be very beneficial, for compared to larger animals her average lifespan as a small prey animal is often much shorter. It comes with considerable costs, however; to still fit in her body cavity - made even smaller by her lack of a tail - her eggs are proportionally much smaller than those of other vivas relative to her body weight, and her chicks hatch out in a much more dependent and vulnerable state. Indeed, they are not only blind, featherless, and completely dependent on her for food and warmth for the first few weeks of their lives, but they hatch out even with under-developed skeletons composed almost totally of cartilage; without calcium in the eggshell, bone development is delayed until after hatching, when the mother begins to feed her chicks a rich secretion of crop milk high in the minerals necessary for their growth. Shortly before giving birth, a mother bumblet must thus create a suitably safe and secure nest underground where her young will spend the first few weeks of their lives.

Another issue with life underground is a lack of oxygen. The adult bumblet itself manages to get by with proportionally large lungs and a highly efficient system of air sacs. The developing eggs in its oviduct, however, are at a high risk of suffocation in this stagnant environment. Most vivas living above ground rely on rhythmic pumping of their abdominal walls in order to produce negative pressure, sucking air into the cloaca and oxygenating the oviduct similarly to lung, but oxygen concentrations in the burrows inhabited by the bumblet are often extremely low. To keep its eggs oxygenated, bumblets have had to adopt a new strategy which would not work for larger animals; their oviducts are completely closed off to the cloaca by a valve and instead the tissue of the oviduct itself is extremely oxygenated. Gases absorbed through the mother's respiratory system are diffused directly from her blood into the chamber where her eggs are incubated, and waste gases - mainly carbon dioxide - are expelled occasionally as a puff of flatulence from her backside.

For most of the year bumblets don't normally come to the surface unless their tunnels are flooded, in which case they may be seen scurrying for higher ground in large numbers. They must do so in order to find a mate, however, and these few nights a year are likely the only reason that the bumblet retains an acceptable sense of vision, for it certainly doesn't have much function underground. Females sit at the openings to their tunnels while males leave theirs to find them. Both genders emit ultrasonic chirps, forming a duet which becomes faster as the partners come closer until the calls join into a single hum. There is no courtship and it seems females are not very particular, usually accepting any male that comes to their doorstep, allowing the gauntlet of predators that capitalize on the emergence of the males from their tunnels to do all the thinning necessary of the weakest genes in the pool. After mating, the female returns underground and the male continues on his trek to find another partner. In most bumblets, he'll try his luck until sunup and then return to the burrow to feed and restore his energy reserves. For the male of the sacrificial bumblet, however, this is not enough. Even if he mates several females, he is instinctively driven to continue, putting himself at risk more and more the longer he stays above ground. The harsh reality is, however, that no matter what he does once he leaves his burrow, his fate is sealed. Even the fastest and sliest males, which manage to avoid predators, will not return home. Spending as long as three days on a sexual binge and foregoing all feeding, even the most fortunate will eventually die of exhaustion.

Only the female sacrificial bumblet ever makes it into her second year of life; males almost invariably die after mating, either from predators or starvation. By providing the ultimate sacrifice - which provides their common name - by removing themselves from their burrows and territory, their mates and developing offspring will have more resources for themselves.

above: the sacrificial bumblet. This species is typical in size and form of most of its family; it grows to about seven inches in length and weighs about four ounces. Females are marginally larger than males.