Updated: Aug 17, 2021
If you’ve ever tried tying a ribbon around a gift and wished you had one more hand (at least), you’ll have an appreciation for the challenges facing a male weaver bird as he attempts the precise manipulations necessary to begin a nest! These nests, which take the form of thatched spheres, droplet-shaped pendants, or even multichambered compounds, all depend on the first secure anchor-point between nesting material and nest foundation. In the case of the Baya weaver bird, this point is a half-hitch of grass made around a tree branch, which the bird ties with its beak while holding down the springy building material with its foot. The next step for the weaver bird is to construct a strong ring of grass originating at this connection point, which will serve both as a kind of doorframe for the growing structure and as a perch for the builder as he continues work on what will eventually become a work of art, composed of over a thousand grass strips.
The weaver bird is only one of many animals around the world that fascinate us with the intricacy of their engineering and architectural feats. These species, which span the animal kingdom from insects and arachnids to birds and mammals, modify and exploit their surroundings for a variety of purposes, including energy and climate control, defense against predation, predation strategy, and courtship. These applications are wonderfully demonstrated by the amazing animals featured below.
1. Energy and Climate Control
As humans, we have the opportunity to not only delight in, but also learn from, our fellow creatures’ incredible adaptations. In fact, there is an entire field of study – biomimetics – devoted to the imitation and application of designs found in nature to human technological challenges. One group of species that scientists have looked to for inspiration are the social insects, which have been studied for the unique properties of their nest and exoskeletal materials. A particularly interesting member of this group is the Oriental hornet, or Vespa orientalis. Several fascinating adaptations allow these hornets to maintain nest temperatures within the narrow range (28 to 32°C) critical to proper pupae development, as well as moderate their own body temperature.
Oriental hornets, which are distributed throughout the Mediterranean basin and adjacent areas, rear their brood in nests built underground. The nest is made of a kind of plaster which the female of the species prepares by masticating soil particles to blend them with her rapidly-hardening saliva. Eggs are deposited in vertically oriented hexagonal tubes, sealed at the upper end by the roof of the nest and left open at the bottom end. The larva that hatches from the egg proceeds to spin a silk cocoon around itself and seals this bottom end with a silk cap, thus entering the pupal stage. The silk shields the pupa from predators and parasites, and provides it with a sterile environment in which its exoskeleton can develop without contamination by dust particles or disturbance by turbulent air currents. Perhaps the silk’s most interesting function, however, is in thermoregulation of the nest. Researchers have discovered that the silk is thermoelectric, meaning that the material is able to store excess thermal energy as electric charge, and then release that stored charge as heat when necessary. Scientists believe that silk picks up thermal energy from the sun during the hottest times of the day, causing a small electric current to flow from the silk cap to the opposite end of the cocoon. Since heat energy from air outside the cocoon is used to drive this current, the pupa inside the cocoon experiences minimal rise in temperature and is prevented from overheating. At night, the process is reversed. In a process akin to a discharging capacitor, charge begins to flow back to the opposite end of the cocoon, slowly releasing the stored energy as heat to keep the pupa warm.
Researchers have concluded that hornet silk owes its thermoelectric properties to a complex structure that makes it behave like an organic semiconductor. Semiconductors are a class of unique substances whose conductive properties lie somewhere between those of a conductor and an insulator, and can be altered in various useful ways. Importantly, the conductivity of semiconductors is highly dependent on temperature, in ways that are particular to the specific semiconductor in question. In the case of Oriental hornet silk, empirical evidence has indicated that under nest-mimicking conditions (high humidity, low light, and appropriate temperature range), temperature and conductivity are positively correlated, i.e., current flowing in the silk increases as temperature increases and decreases as temperature decreases. In other words, the hornet silk is able to be charged and discharged as needed over multiple cycles of heating and cooling.
Interestingly, Oriental hornets do not rely entirely on the intrinsic properties of their nest material to protect developing pupae. Adult hornets further enhance the thermoregulation of their nests through a number of techniques: actively fanning the cocoons with their wings, blowing hot air on them using thoracic air sacs, and even hanging water droplets on the silk cap to enable evaporative cooling of the cocoons. The exoskeletons of the adult hornets themselves have unusual electrical properties that allow them to continue caring for their nests during the high temperatures of midday. Scientists have discovered that a special pigment in the thin outermost layer of the hornets’ exoskeleton, called xanthopterin, has the ability to absorb light and convert it into electrical energy, much like a solar battery. Light rays are focused towards the xanthopterin by groove-like features on the adjacent portions of the exoskeleton. The portion of the exoskeleton that contains xanthopterin features raised bumps which increase the surface area available for light absorption, as well as antireflective properties that help maximize the amount of light reaching the pigment. Photons absorbed by the xanthopterin excite electrons and cause voltage to build up, which may be released as electric current available to power the hornets’ physical activity, as well as regulate its body temperature.