The term animal comes from the Latin animalis, meaning “having breath.” Breath may be slight.
One nematode – Halicephalobus mephisto – thrives with scant oxygen, in deep fracture groundwater that is thousands of years old, up to a depth of 3.6 km or more. These little black roundworms, 0.5 mm long, withstand crushing pressure and searing heat to feast on the biofilm bacteria deep down. Hellish it may be, but there is a functioning biosystem there.
Other nematode species have been found 1.3 km below ground at depleted oxygen levels, warmed to 41 ºC. These nematodes may have made their way down to the depths from more surface-oriented ancestors.
Less of an exotic ode than deep-dwelling nematodes, the common breadcrumb sponge sips scant oxygen wherever it may be found in the North Atlantic Ocean or Mediterrean Sea.
Breath does not distinguish animals from plants. Plant pores (stomata) have a regulated cycle of breathing in carbon dioxide and exhaling water vapor.
Animals originated during the chilly Cryogenian period. The last common ancestor of animals arose nearly 800 MYA.
Tardigrades are tiny, nearly translucent aquatic animals, found all over the world. Tardigrades are commonly known as water bears. These little creatures – 0.2–1.2 mm long, 0.5 mm on average – may live for up to 60 years. Terrestrial species live inside the dampness of moss, lichen, leaf litter, and soil. Others are found in fresh or saltwater.
Water bears have a well-defined head and 4 pairs of legs which are fitted with claws. The legs are used for grasping and slow-motion acrobatics, not walking. Tardigrades are encased in a rugged but flexible cuticle which is periodically shed as the creature grows.
Water bears arose over 600 million years ago, after nematodes, but well before arthropods – some of which, such as insects, shed exoskeletons. Tardigrades are a distinctive branch on the tree of life.
Water bears have a physiology like larger animals, with muscles, a complete digestive system, a brain, and a nervous system.
With their mouth stilettos, some tardigrades perforate plants and absorb the sugary products of photosynthesis. Others suck the cellular contents of microorganisms or consume microbes whole.
The tardigrade body cavity is an open hemocoel that touches every cell. This affords efficient gas exchange and nutrition without the need for circulatory or respiratory systems. It is an elegantly efficient design.
Water bears generally have separate sexes and reproduce by eggs, but there are also hermaphrodites and parthenogenetic species. Fertilization is external.
Tardigrade development is direct, lacking larval stages. Water bears are born with their full complement of adult cells. Instead of cell division, water bears grow to adult size by their cells enlarging (hypertrophy). Tardigrades are eutelic: growing only by increasing the size of their cells, as the number of cells remains constant upon reaching maturity.
Through the ages, the wind carried water bears around the globe, where they adapted to niches with different environmental extremities. There are now over 1,100 species.
Tardigrades are tough. They have been subjected to temperatures as frigid as –272 Cº, or as hot as 150 Cº (well above water’s boiling point) and were able to revive after amenable conditions returned. Water bears can bear extreme pressure, suffocating gases, and seriously salty water.
Tardigrades survive extreme conditions via miraculous adaptations that allow cryptobiosis: suspended animation. Tardigrade tissue can withstand freezing crystallization, desiccation, and even the rigors of space. Water bears are inexplicably able to repair DNA damaged by radiation.
Tardigrades are as close to indestructible as it gets. ~ Brazilian physicist and astrobiologist Rafael Alves Batista
In going into suspended animation, tardigrades must dry out slowly to survive. In preparation, a water bear curls into a ball, termed a tun. (German biologist H. Baumann called the tardigrade dry husk a Tönnchenform in 1922, but it is now commonly known as a tun.)
To assist in preventing cell death, tardigrades synthesize trehalose: a sugar substitute for water, which lets cell structures and membranes remain intact when desiccated. Further, water bears have a unique genetic ability to create proteins which preserve cells during desiccation. These proteins encapsulate the molecular components of cells in glasslike matrices to preserve them, keeping them intact. Adding water melts the preservative proteins and sugars, allowing cellular revitalization.
Desiccated water bear cells are merely physical artifacts. To sustain themselves as a lifeless powder, tardigrades illustrate that living entities are principally localized energy gyres which emergently create the material mirage of physicality.
Tardigrades rehydrate and return to activity within a few minutes to a few hours. A tun may be dormant for decades and come back to life.
Those communities which included the greatest number of the most sympathetic members would flourish best and rear the greatest number of offspring. ~ English naturalist Charles Darwin
Many territorial fish, reptiles, and mammals (e.g., bears, pandas, raccoons) live largely solitary lives as adults. While mother jaguars take care of their offspring, adult jaguars don’t associate except to mate. Snakes are presumed largely solitary, though their lives are so furtive that we know little about them. Relatively few birds are inclined to solitude as a lifestyle, generally preferring the company of others.
There is no consensus on classifying animal societies. The most obvious distinction is degree of social integration. The basic bifurcation is between presocial and eusocial.
Eusocial animals are the most social. Eusocial animals have: 1) overlapping generations within a colony; 2) cooperative care of the young (alloparenting); and 3) reproductive division of labor, where only certain colony members procreate.
In providing the greatest productivity and well-being for a population, eusociality is a cogent example of evolutionary economics at its most refined. The largest long-lasting colonies are eusocial insects, which effectively function as a single superorganism. Human societies are dismally chaotic by comparison.
Presocial animals are less social than eusocial ones in 1 or more ways. Few presocial animals have reproductive division of labor or alloparenting.
There is a finer categorical slicing for presociality. Animals which practice parenting are subsocial. Animals of the same generation that live cooperatively together in a single dwelling are parasocial.
The Mirror Test
Behavior is the mirror in which everyone shows their image. ~ German writer Johann Wolfgang von Goethe
We look upon the intelligence of others with bias. This is true of other humans and even more so of non-human animals.
Religious bias long precluded serious consideration that other life could possibly be smart. Plants, microbes, and “primitive” creatures merited no investigation for intelligence. Ignorant human presumption long foreclosed intelligent inquiry – rich irony indeed.
A few birds, particularly corvids, were considered an exception. But then, the cleverness of crows has been apparent since ancient times.
As with corvids, the striking smarts of rats rendered rodents acceptable subjects of intelligence research. Their easily being caged helped.
Mammals have dominated zoological study of intelligence, especially those most like us. Darwin held a mirror up to an orangutan at a zoo and recorded the animal’s reaction. Darwin did not know what to make of the orangutan’s responses; nor did the orangutan know what to make of Darwin and his unnatural object. Hairless apes are mighty peculiar.
In the late 1960s, American psychologist Gordon Gallup, Jr., working with chimpanzees, followed in Darwin’s footsteps. Others followed.
The mirror test consists of putting a mirror in front of an animal and recording its response behavior. The mirror test often involves painting a dot on the test subject’s body, as opposed to other animals that may appear in the mirror. The surmise is that, by indicating its marked dot during a mirror test, an animal recognizes itself. Having repeatedly been exposed to mirrors, most toddlers recognize themselves in the mirror around 2 years of age; before then, they cannot.
The mirror test acts a presumed proxy for self-awareness. But mirrors don’t appear in the wild, so the supposition behind the mirror test as meaningful is silly.
Cleaner wrasse are a tiny, territorial, tropical fish with excellent eyesight. They make their living by cleaning parasites off bigger fish. Mark a spot on them and put a mirror in front of them, and a wrasse knows right away that something is amiss. How wrasse know what they naturally look like is a mystery (the inherent enigma of the mirror test).
Trained pigeons pass the mirror test but not untrained ones. So too monkeys. Wild creatures’ lack of exposure to mirrors is the difference. With familiarity comes comprehension. People score higher IQs when they are familiar with the test.
Monkeys are superior to men in this: when a monkey looks into a mirror, he sees a monkey. ~ Mauritian writer Malcolm de Chazal
Animals known to recognize themselves in the mirror include magpies, elephants, dolphins, and apes. Many different monkeys were given the mirror test over the years. Only a tiny South American monkey – the cotton-top tamarin – passed. Researchers dyed tamarins’ white topknot in brilliant psychedelic colors, a change no self-respecting tamarin could fail to notice.
Mirror test results can be ambiguous, depending upon the intelligence of the observer, not the participant. Presented with a mirror, pigs don’t care what they look like. But swine are quite willing to use the information from a mirror to find food, which is of considerable interest.
In the end, experiments that test animals’ cognition by determining when they succeed and when they fail may reveal more about human minds than other species. ~ American life-sciences writer Susan Milius