20 animal behaviors scientists still can't fully explain

For some migratory birds, the first major journey begins without the benefit of having traveled the route before. Innate directional programs and cues from the sun, stars, landmarks, odors, and Earth's magnetic field can all contribute.

The picture differs considerably among species, however: some young birds travel independently, while others learn routes by following experienced individuals. A study of reintroduced whooping cranes found that groups flying with older, more experienced birds stayed closer to the most direct migration route.

How these inherited and learned components interact remains an active area of research.

Purring is closely associated with a relaxed cat, but the same behavior also occurs during fear, injury, illness, and other stressful situations.

Communication is among the better-supported functions. A study led by Karen McComb at the University of Sussex found that cats seeking food can embed a cry-like sound within a "solicitation purr" that people perceive as more urgent.

Self-soothing has also been proposed, while hypotheses involving physiological effects from low-frequency vibrations remain far less established. No single explanation yet accounts for why cats purr across such different circumstances.

Elephants have been observed lingering near carcasses, investigating bones and tusks, and returning to remains after death. In field experiments in Kenya's Amboseli National Park, African elephants showed far more interest in elephant skulls and ivory than in the skulls of other large mammals or in natural objects.

Those encounters are well documented; what they mean to the animals is much harder to establish. Their behavior may resemble human mourning from the outside, but researchers cannot simply equate it with grief without knowing more about the elephants' internal experience.

Experiments with mirrors have produced evidence consistent with self-recognition in dolphins, placing them among a small group of animals that show such responses. In a landmark 2001 study by Diana Reiss and Lori Marino, bottlenose dolphins marked on their bodies spent time at a mirror examining the marked areas.

The larger question concerns what that accomplishment actually measures. Passing a mirror test may reveal something important about how an animal distinguishes itself from others, but whether it demonstrates human-like self-awareness remains debated.

Thousands of ants can divide labor, locate food, defend a nest, and respond collectively to changing conditions without an individual directing the operation. Chemical communication, local interactions, and relatively simple behavioral rules explain much of this self-organization.

Stanford biologist Deborah Gordon, for example, has spent decades studying how harvester ant colonies regulate foraging through the rate at which individual ants encounter one another.

Researchers are now working to understand how those mechanisms scale across entire colonies and why collective responses differ between species and environments.

A dog's nose can distinguish extraordinarily subtle chemical differences, and controlled studies have shown that trained dogs can detect odors associated with certain diseases and physiological changes. Volatile organic compounds and other odor signatures are leading candidates for what they perceive.

As a review in BMC Infectious Diseases explores, performance varies with the condition, individual dog, training, and testing environment, leaving researchers to determine exactly which cues produce reliable detection.

Male humpback whales produce complex songs that change over time, and new versions can spread through populations through cultural transmission. Research tracking South Pacific humpbacks over more than a decade found new song types repeatedly moving eastward from Australia across the region.

Their connection to the breeding season strongly suggests a reproductive role, but that doesn't reveal exactly what the songs accomplish. Whether they primarily attract females, mediate competition among males, serve several purposes at once, or do something else remains unsettled.

Puzzle solving, object manipulation, escape behavior, and apparent forms of play have all made octopuses fascinating subjects for cognition research.

Their intelligence also followed an evolutionary path radically different from that of vertebrates, with roughly two-thirds of their neurons located in their arms. How this distributed nervous system integrates information into coordinated decisions is still yielding surprises.

Female sea turtles can spend years traveling across vast stretches of ocean before returning to reproduce near the area where they began life.

One well-supported hypothesis, developed by Kenneth Lohmann's lab at the University of North Carolina, holds that young turtles imprint on distinctive features of Earth's magnetic field and later use that information for natal homing.

Magnetic cues are clearly important, but exactly how they are encoded, remembered, and combined with other navigational information is not yet a complete story.

A swarm of synchronous fireflies can appear to blink as one, but that coordination emerges from interactions among individual insects rather than a central signal.

A study of Photinus carolinus fireflies found that individuals flash irregularly when isolated but fall into rhythm in groups. A firefly that sees another flash tends to start flashing too, so the fastest responders effectively set the beat.

Researchers are still working out how these visual interactions play out across the hundreds to thousands of fireflies in a natural swarm.

The monarch's annual migration spans generations, so individuals can travel toward destinations they have never personally visited. A time-compensated sun compass and other sensory cues help them orient.

A recent review by Eric Warrant and Ryszard Maleszka proposes that monarchs reach their destination through successive flight segments guided by compass cues, favorable winds, and innate responses to local cues.

How this migratory program is passed across generations remains unresolved.

Peacock trains are one of biology's textbook examples of sexual selection, so the existence of female mate choice isn't particularly mysterious. Pinning down which features influence that choice has proved trickier.

Studies have produced mixed results on the relative importance of ornamentation, movement, visual presentation, and other signals. For example, one study of free-ranging peafowl found no evidence that females preferentially mated with males with more elaborate trains, adding to conflicting findings about which characteristics matter most in mate choice.

Ballooning allows small spiders to travel through the air on strands of silk, sometimes over considerable distances. Wind has an obvious influence, but experiments by researchers Erica Morley and Daniel Robert also found that atmospheric electric fields can trigger ballooning behavior.

The remaining challenge is understanding how spiders integrate electrical, aerodynamic, and other environmental information when deciding whether conditions in the wild are right for takeoff.

Faced with unfavorable conditions, some tardigrades can enter cryptobiosis, reducing detectable metabolic activity to extraordinarily low levels.

Protective proteins and other cellular adaptations help explain how their tissues withstand severe dehydration and other stresses. In 2017, for example, researchers led by Thomas Boothby identified tardigrade-specific disordered proteins that help protect cells as they dry out.

Scientists have uncovered several pieces of that survival machinery, though the molecular sequence that preserves cells through extreme dormancy and successful revival is still being pieced together.

Beginning in 2020, Iberian orcas drew international attention for repeated interactions with vessels, particularly sailboats, sometimes damaging their rudders.

Play, social learning, and other motivations have all been proposed, but none has emerged as a definitive explanation. The spread and persistence of the behavior offer researchers an unusual chance to examine how a novel activity develops and moves through a wild population.

A dead crow can draw other members of the species to the scene, where they may gather, vocalize, and inspect their surroundings.

Research on wild American crows has shown that these encounters can provide useful information about threats, allowing the birds to learn about dangerous people and places associated with dead crows.

That gives the behavior a substantial learning component, although whether something resembling an emotional response to death accompanies that practical function is much harder to demonstrate.

With numerous photoreceptor types and sensitivity to forms of polarized light invisible to humans, mantis shrimp possess one of the animal kingdom's most unusual visual systems.

Surprisingly, research led by Hanne Thoen at the University of Queensland found that more receptor types don't give them finer color discrimination than humans.

Their strategy appears to process visual information differently, and scientists are still exploring how that specialized system translates sensory input into rapid behavioral decisions.

Lose a limb, and an axolotl can build another one, complete with bone, muscle, nerves, and other tissues arranged in the right places. Axolotls can also regenerate portions of structures such as the spinal cord to a degree that mammals cannot.

Many cellular and molecular players are now known. Research by Otsuki and colleagues identified a Hand2–Shh feedback loop that helps preserve positional memory in regenerating axolotl limbs.

How these signals work together to reconstruct complex anatomy remains an active question.

Electric eels don't simply shock prey with maximum force. During hunting, they can produce different patterns of electrical discharge.

Experiments by Vanderbilt biologist Kenneth Catania showed that their high-voltage pulses can activate prey's motor neurons, causing involuntary muscle contractions that reveal hidden prey or freeze it in place.

The basic physiology is well established, shifting the more interesting questions toward how sensory feedback and neural circuits allow the eel to adjust the timing and pattern of its discharges during rapidly changing encounters.

Among bonobos, sexual behavior appears in far more situations than reproduction alone. Researchers have documented it around food, during periods of tension or excitement, in reconciliation, and across a range of social relationships.

A study of bonobos at the Lola ya Bonobo sanctuary in the Democratic Republic of Congo found that reconciliation after conflicts was almost entirely sexual.

Rather than pointing to one evolutionary explanation, the evidence has generated competing hypotheses about how sociosexual behavior developed and what roles it plays in bonding, conflict management, cooperation, and group dynamics.