For plants, sunlight is nutritious, and the soil provides vital water and minerals. This does not make life easy.
Photosynthetic plants must act to get what they need: moving, growing, and self-pruning leaves and branches that no longer meet their quota.
Being rooted to the spot with an autotrophic lifestyle is the complete opposite to the animal experience. Animals must forage or hunt for their food. Even an ambush involves putting oneself in a certain spot.
What plants and animals share in common is frequent competition to meet needs: sometimes from conspecifics, but almost always from other species.
Access to limited resources is a strong driver of evolution. As microbial biofilms illustrate, the benefits of joining forces forms the foundation upon which sociality is built.
Comprising 1/3rd of all mammal species, bats are long-lived social creatures. Colonies may number in the millions; but bats are cliquish, roosting with friends. Some species have mixed-sex social groups, while others segregate by sex.
Some bats forage in groups. Their density may become so thick that their biosonar field of view becomes clogged with conspecifics. They don’t mind. Foraging is a good time to make new friends. When one finds a nest of edibles, others come join the banquet.
A bat can detect an insect with its sonar only within 10 meters but can hear that another bat has found something to eat 100 meters away. As insects often congregate in close quarters, foraging with friends makes perfect sense.
Bats can flock at extremely close range because of their impressive reflexes and flying heuristics. They swap leader-follower roles regularly and perform coordinated flight by copying the heading of a leading neighbor 500 milliseconds earlier, which is only 4–5 wingbeats. This maneuverability is almost as fast as the blink of a human eye (300–400 ms).
Giraffes & Acacias
Among other leafy treats, giraffes on the African savanna like to forage on acacia trees. The trees don’t like it one bit. Within minutes of being treated like a snack, the acacias start pumping toxins into their leaves. The giraffes get the message and move on, passing nearby trees to move to another neighborhood of foliage.
Giraffes don’t bother munching on trees close to their assault because they know that the acacias have warned nearby trees (by wafting ethylene in the breeze). The giraffes either move upwind or father away, where the acacias’ panic scent has not reached.
Ants collectively form a highly efficient complex network. ~ German physicist and mathematician Jürgen Kurths
Ant colony foraging cascades from chaos to coherent. Initially, scout ants search until they tire, retiring to the nest for respite.
A successful scout takes a tidbit back to the colony, leaving a trail of pheromones. Others follow in the wake of the faint scent trail.
Foraging become herding, as more ants head to the food supply, strengthening the scent path to develop the shortest possible route. A self-reinforcing effect leads to optimal efficiency.
The productivity of this process is engendered by older, more experienced ants, who know the neighborhood better than fledgling foragers.
Animals forage in various ways, but all employ optimizing strategies, including search pattern improvement through learning, assessing information from others, and group foraging that results in economies. Some animals forage with other species to improve their productivity.
One of the most basic techniques is learning cues that indicate hidden food. Birds and mammals are well known for associating environmental hints with the probability of grabbing hidden grub: a mental technique called searching images.
From bees to birds to rodents, animals anticipate what has gone on in the recent past as a basis for their actions. Then, often without warning, expectations change with circumstance. With uncertainty, caution creeps in: more exploration is called for, to recalibrate expectations.
Carrion crows that live near the seashore forage for mussels at low tide: picking them up and cracking them open, often by dropping them onto a hard surface from suitable height. Mussel shells laying in the sand are normally empty, and so ignored.
In one experiment, researchers overturned that common wisdom by seeding meat in empty shells; a lesson the crows learned in a single session. The crows’ searching image was changed, at least for a time.
Another critical lesson is technique itself, which often involves a combination of physics and chemistry. Herring gulls living on Cape Cod crack shellfish shells open by selectively dropping them on rocks. Young gulls are less adept than seasoned adults, who have learned the best locations and proper height.
In human societies, cultural norms arise when behaviours are transmitted through social networks via high-fidelity social learning. ~ English zoologist Lucy Aplin et al
One of the best-known examples of birds picking up on learned tricks is the pilfering of milk from bottles left on doorsteps in Britain. Blue tits learned to open milk bottle tops to feed on the cream.
A successful technique was invented separately thrice in London in the 1920s and spread from there. The technique wasn’t acquired just by imitation. Seeing the results was enough. Chickadees, closely related to tits, learnt milk bottle top popping from just seeing an open container.
Cultural conformity is thought to be a key factor in the evolution of complex culture in humans. ~ Lucy Aplin et al
It turns out that tits are conformists: preferring the traditional technique they socially learn over any personal information they may have. If a tit relocates to a new area where a different technique is the norm, the newcomer adopts it to adhere with local practice.
Prey distribution, patch size, and the presence of conspecifics are important factors influencing a predator’s feeding tactics, including the decision to feed individually or socially. ~ Australian zoologists Maud Berlincourt & John Arnould
For most animals hunting is a solitary activity. Relatively few animals hunt in groups. Ever rarer is coordinated, cooperative hunting. Humbolt squid, little penguins, African wild dogs, cetaceans, and wolves are all exceedingly social. All hunt collectively.
Social aggregation has its benefits, but it also imposes a significant cost: concentration creates resource competition. There can be compensation for the crowding. Being in on the rumor mill yields tips on the best eating places.
Microbes move in optimized motion patterns to collect nutrients. They share intelligence with others on prime feeding locations.
Instead of scrounging for a living, many bacteria make a smart trade-off in choosing the comfort of host-based living. Life can be quite good in a microbiome: the company of fellows and regular meals.
The employment of colonial intelligence varies, depending upon the savvy and general inclination of the species. Not taking advantage of colonial network information is the norm, with notable exceptions.
Osprey form loose nesting colonies in some coastal areas. They watch their neighbors coming in with a grocery haul. A hawk seeing a colonymate fly in with schooling smelt will head out in the same direction as the other came in. Ravens that roost together take similar advantage of neighbor knowledge.
Black-headed gulls are something else in this regard. They don’t necessarily fly to forage in the same direction as successful colonymates. These noisy, gregarious birds, like many gulls, are bold, opportunistic feeders. They generally go their own way, and foraging is no exception.
When new predators – be they crabs, snakes, or modern man – emerge, their prey change. ~ American biologist Rob Dunn
As heterotrophs, all animals are predators. The smaller edible ones are also prey.
Adaptive pressures are most pronounced between predator and prey: a continuing evolutionary ballet of adaptation. Prey adjust their behaviors while predators hone their cunning.
Spiders make an essential contribution to maintaining the ecological balance of Nature. ~ Swiss zoologist Martin Nyffeler
With over 45,000 species and a population density of up to 1,000 individuals per square meter, spiders are one of the most widespread and diverse groups of animals: 7th in species diversity for all organism groups. Overall, spiders eat ~550 million tonnes of prey yearly. While 90% is insects and springtails, tropical spiders also chow down on frogs, lizards, fish, birds, and bats, and a few feed on plants.
Spider diversity provides ample opportunities for spider-on-spider predation. Jumping spiders are especially ingenious hunters of web-building spiders. A jumping spider will traipse onto the edge of a web, then vibrate the victim’s silk-spun home, making subtle adjustments to convey different signals. Once a specific signal gets a response, the hunter will repeat it, luring lunch to the edge of its web.
Seahorses are very effective predators, able to feed on one of the ocean’s greatest escape artists. ~ American marine biologist Brad Gemmell
Copepods are tiny crustaceans. Though blind, their sensitivity to water disturbance renders them an elusive prey.
Seahorses feed on copepods, capturing 90% of those stalked. A seahorse’s stealth owes to its snout, shaped to let a seahorse to slowly approach without tipping a copepod off by making a wake.
Seahorses are slow swimmers. Their prehensile tail is a great assist in hanging on to something and thereby facilitate their ambush style of predation.
Freshwater pike inhabit any water body with the promise of prey, whether a sluggish stream, weedy lake, or cold and clear rocky waters. Pike are ambush predators: stoically still for long periods, then striking with astonishing speed.
Pike are not picky. They mainly eat fish, including young pike, but are not beyond an occasional duckling or water vole.
Many fish species adjust their behavior based upon the probability of imminent danger from predation. Minnows, the tiny members of the carp family, are a potential pike snack. Minnows naturally school when feeling vulnerable. Individual minnows will temporarily swim away from a school to inspect for possible predators.
During inspections of pike, a minnow will avoid the most dangerous zone: immediately around a pike’s mouth. Stationary pike can be a bit difficult for a minnow to recognize.
Pike often stalk minnow schools by slow approach. Besides simply swimming away, minnows may react to a stalking or attacking pike via various tactics. One response is for a few school members to skitter: accelerate away from the school for short distances before returning to the school in a new position.
Sometimes the whole school will synchronously jump to a new vertical position. How such impressive coordination is achieved is not known.
Large minnow schools will open a space around an approaching pike, creating a minnow-free zone in front of the pike, but closing in around it past the pike’s path. Another maneuver is a fountain formation: a school swims ahead of the pike, then splits into 2 groups, each turning in opposite directions, but both toward the predator’s tail as it passes.
When closely approached or attacked, minnow schools sometimes spontaneously explode: rapidly swimming away from the center of the school in all directions. After splitting up, individual minnows seek cover. Pike try to flush minnows out of hiding by directing jets of water at them.
Similar scenarios of considerable variety are employed by other fish, where threat-sensitive predator avoidance escalates into multifarious escape stratagems. The parties involved are aware of each other and adjust according to local conditions and experience.
Generally, potential prey animals constantly watch for dangerous predators and keep them under close observation when they appear. A group of gazelles may even approach a predator they have spotted, to inspect and evaluate the danger.
Group behaviors evolved in many species such that individual members variously keep an intermittent lookout. This results in total group awareness while allowing individuals to get on with the business at hand, which is typically foraging for food.
When predators and prey detect each other, much time may be spent monitoring each other rather than simply attacking or fleeing. Predators watch for subtle indications of weakness or vulnerability.
Prey animals look for changes in predator behavior that indicate a greater likelihood of attack. Since it is advantageous for predators to conceal such signals, any such tell is probably inadvertent and unavoidable. Such is the makings for further adaptation.
A group of wildebeest may close in a bit on a lion that has been noticed, and line up to watch it pass. Roughly 30 meters is considered a safe distance in open country.
Usually wildebeest incessantly grunt, but when a lion appears, they go quiet. Silence is the wildebeest signal to be alert. If a nearby lion comes too close, the wildebeest back up accordingly, then turn, and return to standing watch. If the lion moves into thick vegetation, extra caution is taken.
Predators favor the very young or very old, the weak or the sick. But even when all members of a herd are in fine health, hungry predators still manage to capture some. Careful observation of prey is habitual. An easy meal beats a hard chase any day.
Top predators often prey on animals that are bigger than themselves and try to avoid being killed. It is not easy work and requires careful calculation.
Carnivores consider the reward versus the cost in pursuing prey. Puny prey are bypassed. A chase may be cut short if trying to run down a modest meal when there are other opportunities. Generally, carnivores either quickly capture or give up.
There are distinct hunting strategies. Pumas are sit-and-wait ambush hunters. This is typical of felids. Cheetahs are outliers in commonly pursuing their prey at high speeds.
Pack hunting evolved to minimize the energy each individual involved must expend to get something to eat.
The bobwhite quail is a modest-sized ground-dweller. The name bobwhite refers to its signature call.
The bobwhite is a meal ticket for many. Snakes, hawks, foxes, skunks, and raccoons consider the quail a tasty treat.
Bobwhites form coveys for protection. Groups impose costs as well as having benefits. The benefit/cost varies by group size.
The probability of a bobwhite making it through the day is highest for a covey of 11. Daily distance traveled, a cost, is lowest with a covey of 11. Most bobwhite coveys are 11. How do bobwhites know this?
In times of plenty, many animals store excess food. Hoarding takes various forms, though it most often occurs in anticipation of lean times, such as winter.
Squirrels are well known for their habit of hoarding nuts and other foodstuff in prodigious quantities, typically burying the booty. It is hard to imagine that a squirrel can remember where so many nuts in so many places are stored, but squirrels do recall the plethora of locations where food is stashed.
Squirrel nut hoard retrieval is somewhat more systematic than astounding memory alone. Environmental cues (disturbed ground) and senses (smell) play a role.
Similarly, some birds possess remarkable memory: able to recall a vast number of stored locations. Marsh tits are known to be able to remember the locations of several hundreds of stored seeds. As a risk-minimization stratagem, marsh tits take care to distribute their cache to various locations.
Clark’s nutcracker lives in the alpines of western North America. Food is quite hard to come by during the long, chilly winter. In preparation, a nutcracker gathers and stashes up to 33,000 seeds in the autumn, storing them in caches, buried or in crevices, each cache with but 2 to 5 seeds. To survive a typical snowy winter, a nutcracker must recover an estimated 1,000 caches.
Retrieval is complicated by landscape changes: fallen leaves and snow. Nutcrackers remember seed locations in reference to constant landmarks, such as sizable rocks and large trees. If necessary, a nutcracker will dig in the snow to retrieve a cache.
Pinyon jays of the southwestern United States store hundreds to thousands of pine seeds in preparation for the winter months. Those stashed pine seeds are a jay’s principal fare in the winter. Jays reclaims roughly 80% of cached seeds.
Jays and other birds are careful to stash quality seeds, as there is no reason to expend effort on seeds that won’t last. In the case of nuts, a jay may tap on a nut with its beak to assess its sonic quality, much like a person melon-thumping at the supermarket.
Through experiments of being put in separate, identifiable rooms, one where they were well fed, one where not, Western scrub jays are known to be able to relate past and future. Jays understand the concept of time continuum, as evidenced by stashing food when not hungry but anticipating that they would not be fed. While food caching has an innate knowledge component, learning from experience also plays a part.
Some predators cache prey while the hunting is good. Moles and shrews sometimes stash large numbers of prey. The short-tailed shrew will eat the first few insects caught, but then will store the rest in or near the nest.
A stashing shrew injects prey animals with a venom that paralyzes them, leaving them alive but immobile. This retards decay, leaving victims fresher.
Hoarding accelerates in winter and may bring conservation practices. Shrews sometimes shuttle stashed snails between burrow entrance and a storage chamber, keeping the snails as cool as possible to keep spoilage down.
Moles cache earthworms, often biting the anterior segment to paralyze the worms, or at least minimize movement. European moles sometimes store insect larvae and earthworms in specially excavated storage chambers near their nests.
Shrinkage is inevitable, both by spoilage and by theft. While a substantial fraction of a stash may be recovered, often weeks or months after storing, some may simply not be retrieved.
Ravens take considerable care to surreptitiously stash food, knowing that their companions are as crafty as them. Ravens manage hoarding through complex ruses, even as others are diabolical in discovering what has been concealed.