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.
So what can scientists learn from the energy and climate control adaptations of these amazing animals? In an age of increasing awareness of natural resource conservation, more attention is being given to developing thermoelectric and photovoltaic means of converting heat and solar radiation into useful electricity, as well as energy-efficient climate control technology to reduce energy consumption. In terms of fuel consumption, one of the costliest endeavors of human society is thermoregulation; in fact, carbon dioxide emissions attributable to maintaining thermal comfort in buildings has been reported to be almost 40% of the world-wide total! It is hoped that understanding hornet exoskeleton and silk structure will help engineers design building materials, solar batteries and thermoelectric generators that are renewable, easily recyclable, and responsive to their environment. Hornet silk is particularly applicable to building design, which may integrate fibers and composite materials with thermoelectric properties into building elements to increase their functionality. An example? Multilayered walls mimicking silk structure that act as capacitors or electrical generators. The house of the future may indeed be more than just four walls and a roof!
2. Predation Strategy
While engineers inspired by the Oriental hornet may be pondering how to implement multi-functional elements into the buildings of the future, an unusual insect known to science as Acanthaspis petax continues to reap the benefits of a multi-tasking strategy its species has been using all along. A. petax is a species of “assassin bug” – a large family of insects primarily composed of ambush predators – and inhabits the jungles and savannahs of East Africa and Southeast Asia, preying on ants, beetles and flies. Like other members of its family, A. petax consumes its prey by first piercing it with its sharp proboscis, then injecting it with a tissue-dissolving enzyme, and finally sucking out the prey’s innards. As subtly terrifying as its method of predation may seem, the bug in question measures less than half an inch, making it difficult to distinguish from the curious load it is often seen carrying on its back – the carcasses of ants it has preyed upon! A. petax can carry as many as twenty ant exoskeletons at a time, often combining them with an underlying coat of soil and dust particles. (This feat is especially impressive considering that only the nymphs (immature insects) of the species display “backpacking” behavior.) As a finishing touch, the particles of the assassin bug’s coat are all bound together by fine elastic threads secreted by specialized glands on the bug’s back, creating a disguise that often exceeds the size of its own body.
Scientists have actively debated the exact purpose of this insect’s strange portable construction; research indicates that the benefits are actually two-fold. Firstly, it appears to provide A. petax with defense against its primary predator, spiders of the Salticidae family. These “jumping spiders” are known for their acute vision (some of the best among the arthropods) and great agility, both of which they employ in hunting. In experiments carried out by a team of New Zealand scientists in 2007, jumping spiders were placed in the presence of both “masked” and “naked” assassin bugs, i.e., bugs carrying or devoid of an ant carcass disguise. The spiders were found to attack “naked” bugs about ten times more often than “masked” ones, suggesting that the spiders much less readily identify masked bugs as prey items. To control for the effects of movement and behavior present in live bugs, researchers repeated the experiments with dead, preserved bugs.
The results were the same, in that spiders preferentially predated on unmasked bugs, strengthening the idea that the assassin bug’s disguise fundamentally alters their predator’s response. Scientists suggest that the mound of ant carcasses alters the bug’s shape beyond recognition by jumping spiders. These observations in turn suggest that predation by visually acute animals may be triggered by movement, but is dependent on an initial recognition of form. The mechanism of this recognition, as well as the exact level of deviation from a recognized morphology sufficient to render the prey unrecognizable, are fascinating areas for future study.
Another interesting aspect of the assassin bug’s behavior is its nearly exclusive use of ants in disguise construction, despite the fact that it preys upon several kinds of insects. It may be that ants not only allow the assassin bug to change shape, but provide the added benefit of being an inherent deterrent to predatory spiders. Because of their tendency to swarm and their ability to secrete chemical defenses, ants are not a typical prey item for these spiders, which tend to avoid them. Given their formidable nature, it may be surprising that ants comprise the main food source for A. petax. Here enters the second benefit of the assassin bug’s multitasking “backpack” of soil and ant exoskeletons: the ability to take on ants. The dead ants piled up on its back allow A. petax to act like the proverbial wolf in sheep’s clothing – except that in this case, the “sheepfold” our predator is trying to penetrate is an ant nest swarming with organisms that are themselves highly perceptive and potentially lethal. Since ants rely on chemical receptors in their antennae to identify potential threats, masking its “smell” allows the assassin bug to enter unchallenged into the nest.
Studies performed by German researchers in 2002 verified that the assassin bug’s mask significantly compromised their chemical and tactile recognition by worker ants, which responded aggressively towards naked bugs more often than towards those wearing masks. Interestingly, the research distinguished between the effects of individual elements of the assassin bug’s covering; bugs wearing the soil and dust component alone were afforded as much protection against recognition by ants as bugs wearing a dust coat and ant exoskeletons.
This observation would seem to weaken support for one hypothesized mechanism of disguise, namely, that the assassin bug is using dead ants to fool living ants into mistaking it for one of their own. Unfortunately, the researchers apparently did not perform tests to isolate the ant component of the bug’s construction, which would seem necessary in order to conclusively rule out the hypothesis above. Even without this additional evidence, however, it is not unreasonable to believe that organisms as perceptive as ants would be able to distinguish between dead and living members of their species, in which case carrying around dead ants only would not be sufficient camouflage. Additionally, the exoskeletons that make up the bug’s backpack are usually very dry and many are old, given the insect’s habit of reassuming its old pack after successive molts. This might mean that most of the dead ants have lost the volatile components of any odor which live members of their species could recognize. All in all, therefore, it seems that the different components of A. petax’s fantastic construction work together to provide its builder with the dual benefits of visual protection from predators, and chemical/tactile protection from a challenging prey item. Now that’s killing two ants – or a lot more – with one load!