129 cross-species questions
Explore across species
Some questions are not about an animal at all. These pages are assembled from the same evidence as the subject pages, arranged by ability, behaviour or phenomenon — so a crow, a bee and a carnivorous plant can appear side by side, each with its own scope and its own limits.
What minds do · 19
Thinking, learning and communicating
The questions that get asked of every animal and answered properly for very few. These pages are built around what has actually been demonstrated, property by property, rather than around a ranking of clever animals.
Migration
A migrating animal is one that has switched off the responses that would normally make it stop. That, and not distance, is what makes a journey a migration.
Migration is not defined by distance. It is a change of behaviour: an animal stops responding to the things that would normally stop it — food, company, a good place to be — and keeps going in one direction until a different state takes over.
Shown in 2 subjects on this site, each with its own evidenceRead on- TorporA hummingbird cannot survive a cold night at its normal metabolic rate, so it spends the night at twenty degrees cooler — deliberately, and reversibly by dawn.
- Animal behaviour“Why does it do that?” is four different questions. Most arguments about animal behaviour are two people answering different ones.
- Ambush and pursuitThe question is not fast or patient. It is how small you can make the gap before you commit — and what happens when the prey turns.
- Cooperative huntingFour wolves hunting an elk is not evidence of cooperation. Wolf hunting success stops improving at about four — because a wolf that hangs back still eats.
- Playing deadIt works because predators handle motionless prey differently — not because anybody is fooled about anything.
- Group defenceA herd need not exist for mutual protection. It can be what happens when every animal selfishly tries to put somebody else on the outside.
- Collective behaviourA starling tracks about seven neighbours — a number, not a distance. That is what holds a flock together when a predator stretches it.
- Do animals have language?Language is not one thing. Take it apart into its properties, and animals have several of them — just never all at once.
- Animal cultureA great tit that works out a better way to open the box will often abandon it, and do it the way its neighbours do.
- Social learningTeaching has a three-part definition that says nothing about intending to teach. An ant meets all three parts.
- Animal intelligenceA bee understands zero. A nutcracker remembers thousands of buried seeds. Neither could do the other’s job, and no ranking survives that.
- Seasonal survivalA hibernating insect cannot be woken by a warm November. That is the point, and increasingly the problem.
- Animal sleepA fly kept awake sleeps in longer afterwards. That, and not stillness, is what makes it sleep.
- HibernationA hibernating ground squirrel spends much of its winter fat budget rewarming — apparently in order to sleep.
- Animal memoryA jay will ignore its favourite buried food if enough time has passed for it to rot.
- Tool useNot "which animals pick things up" — which ones pay a price now for a payoff later.
- A sense of numberA bird, an insect and a plant, all responding to number — by three completely different mechanisms.
- Recognising individual humansA crow, an elephant and a bee all "recognise" people. Not one of them means the same thing by it.
What bodies do · 39
Senses, signals and abilities
Navigation without a map, hearing in the dark, growing a limb back. Each of these is a mechanism rather than an animal, so the same page can hold a bat, a bird and a turtle without any of them being the subject.
Invasive mosquitoes and range shift
Found, breeding, established, transmitting. Four different states, and most introductions stop early.
Mosquitoes carried to new places by trade and travel. The thing worth understanding is that being found, breeding, becoming established and transmitting disease are four separate states — and most introductions stop well before the last.
Read on- Smell, and how far it really worksA moth can detect a single molecule and still cannot fly straight to the source. The nose was never the limiting factor.
- Path integrationPick up an ant that has just found food, put it down a hundred metres away, and it will run the exact course home was from where you took it — into empty ground.
- How we know where animals goA ring gives you two dots on a map and nothing in between. Everything since has been an argument about how to fill in the middle.
- DiapauseAn insect entering diapause is not responding to winter. It is predicting it, from daylength, sometimes a generation before the winter arrives.
- Countercurrent exchangeRun two flows past each other in opposite directions and the gradient never collapses. That is the whole trick, and animals use it for heat, water, oxygen and salt.
- VenomThere is no such thing as “the venom of” a species. It is a mixture of many proteins, and it changes with population, age and what the animal is eating.
- Venom and poisonThe difference is not the chemical, it is the route. The same toxin can be a venom or a poison depending on how it reaches you.
- CamouflageA photograph where you cannot find the moth tells you about your eyes. The question is always: hidden from what?
- Disruptive colorationFinding an animal means finding its edge. Put a bold mark across the edge and the edge stops being obvious.
- CountershadingA rounded animal lit from above shades itself, which gives away that it is solid. Countershading paints the shadow out.
- Warning colorationA warning colour only works on an animal that has already learned what it means. Against a predator that has learned nothing, being bright is just being easy to find.
- EyespotsThe spots do work — birds really do leave the butterfly alone. They just do not seem to work by looking like an owl.
- Chemical defenceSpraying, bleeding, oozing and stinging are four different solutions. Only the ones that go through a wound are venom.
- MobbingA bird flying at an owl is doing something dangerous. The interesting question is what makes it worth the risk.
- Animal navigationA butterfly’s navigational clock is not in its brain. It is in its antennae.
- Bird migrationMove a young bird 500 km and it flies the same heading into the wrong country. Move an adult and it corrects.
- Insect migrationTrillions of insects cross overhead each year at a few hundred metres. Almost nobody has ever seen it.
- Animal sensesThe five senses are a list from ancient philosophy. Biology has found electric fields, magnetic fields, polarised light, radiant heat and the wake of a fish that swam past thirty seconds ago.
- MagnetoreceptionA bird’s magnetic compass has been demonstrated for fifty years, in laboratories all over the world. Nobody has found the receptor.
- ElectroreceptionMuscles leak electricity, and seawater conducts. A shark will attack two bare wires in the sand if the current is the size a living animal makes.
- EcholocationA bat closing on a moth fires up to two hundred calls a second. Range has stopped mattering; knowing exactly where it is has started to.
- Ultraviolet visionTwo birds identical to a human eye can be obviously different to each other. Put a filter over the window and the choosing stops.
- Polarised light visionRotate a polarising filter over an ant by forty degrees and the ant walks home forty degrees wrong. There is nothing else it could be responding to.
- Infrared sensingIt is not a second pair of eyes. It is a thermometer behind a pinhole, and it is fast enough to aim a strike in total darkness.
- The lateral lineNot hearing. A fish’s lateral line reads the water actually being pushed aside — which is why it works within a body length and nowhere further.
- Feeling the groundAn elephant call is low enough that much of its energy goes into the ground, where it travels faster and further than it does in air.
- Animal hearingSmall animals went high because high sounds can be located. Large animals went low because low sounds travel. It is the same physics read from both ends.
- BirdsongA bird raised where it can hear no other bird still sings — and sings something no wild bird would recognise. The tune has to be learned.
- How birds learn to singA young bird holding a perfect memory of its father’s song, unable to hear its own attempts, sings as badly as one that never heard anything at all.
- Vocal learningLearning what a sound means is common. Learning to make a sound you were not born able to make is rare enough to list — and humans are on a list with parrots and whales.
- Birdsong dialectsA sparrow’s song can place it within a few kilometres — and a variant that started in western Canada replaced the old one across a continent in twenty years.
- Female birdsongFemale birds sing in most songbird species. The idea that they do not was never a finding — it was a fact about where the researchers were standing.
- The dawn chorusThe puzzle is not why birds sing. It is why they all do it at the one hour of the day when they can barely see to eat.
- Birdsong in citiesCity birds really do sing higher. The shift is about a tenth of what it would need to be to escape traffic noise — they are not dodging it, they are shouting over it.
- Vocal mimicryA courting male lyrebird does not imitate a bird. It imitates an entire mobbing flock — several species and their wingbeats — at one specific moment in the display.
- How scientists listen to animalsA model can tell you who made a sound, and when, and what was happening. There is no training data anywhere for what it meant.
- RegenerationMammals do not fail to regenerate so much as succeed at something else, quickly, that makes it impossible.
- Animal communicationA vervet monkey has one call for leopards and another for eagles — and the others react correctly with nothing there.
Relationships, cycles and phenomena
Pollination, decay, mycorrhiza, seed dispersal. These are the pages that do something an encyclopaedia cannot: hold one piece of evidence that belongs to a bee, a flower and a process at the same time.
A barnacle’s upper limit is the sea. Its lower limit is another barnacle — and from a photograph the two edges look identical.
Not where a species lives, and not its job. A niche is the set of conditions and resources under which its population can keep going — and the part it actually occupies is usually smaller than the part it could, because something else is there.
- CompetitionMost competition involves no contact at all. Something was there, and then it was not.
- Competitive exclusionThe principle is not wrong. It is conditional — and a lake in summer meets almost none of its conditions.
- Niche partitioningFive warblers in one tree is the textbook case. Nobody has ever removed one to see what the others do.
- Character displacementA competitor arrived, a drought struck, and beak size shifted within one generation — watched rather than reconstructed.
- EcologyEcology’s hardest problem is not complexity. It is that the interesting systems cannot be replicated, and the replicable ones are small.
- SymbiosisSymbiosis never meant "both benefit". A tapeworm is a symbiont.
- EndosymbiosisYour mitochondria have their own genome, divide on their own schedule, and are sensitive to antibiotics. Five separate clues, one answer.
- Keystone speciesNot the biggest stone in the arch. The one whose removal brings it down.
- Trophic cascadesThe evidence for cascades is best in ponds and shores. The claims are loudest about national parks.
- Ecosystem engineersA beaver is not the dam, an earthworm is not the soil, and a termite is not the mound.
- Foundation speciesA keystone matters more than its abundance predicts. A foundation species matters because of it.
- Food websIn a chain, removing the fox has one consequence. In a web, it depends on everything else.
- Ecological successionAll three mechanisms produce a sequence. Watching the sequence tells you which one you are looking at — nothing.
- DisturbanceA forest that has not burned in eighty years is not undisturbed. It is disturbed differently.
- Resilience and regime shiftsSuppressing small disturbances can reduce the size of shock a system can survive.
- Invasive speciesMost introduced species fail. Most that survive do nothing. A minority change everything.
- FacilitationA nurse plant shelters a seedling until the seedling is big enough to kill it.
- CommensalismCommensalism is the only interaction category defined by an absence — and absence is what measurement is worst at.
- Coral and its symbiontsA reef sits in nutrient-poor water and is one of the most productive places on earth. It holds its nutrients rather than taking them.
- DetritusThe food chain everyone learns handles a minority of what a forest actually produces.
- SoilSoil is not a mixture. It is a structure, and the organisms living in it are what build it.
- Soil carbonThe leaf that rots fastest may leave the most carbon behind.
- The soil food webA springtail grazing leaf litter is mostly eating the fungus growing through it.
- The rhizosphereRoots do not so much feed the soil as leak into it. The microbes gather where the carbon is.
- Nutrient cyclingAdding dead leaves to soil can leave plants with less nitrogen, not more — for months.
- Nitrogen fixationNo plant fixes nitrogen. Legumes build an organ to house bacteria that do, and pay in sugar.
- Extreme survivalAn emperor penguin in a blizzard is about as warm as you are. Most animals in extreme places are not enduring the extreme — they are avoiding it, and that is a different story.
- AdaptationA mountaineer acclimatises in three weeks. A population adapts over generations. Neither process is the other one speeded up or slowed down.
- DormancyHibernation, torpor, aestivation, diapause and cryptobiosis were named by five different fields studying five different animals. Underneath, they are running much the same machinery.
- CryptobiosisIn the deepest dried states, no metabolism can be measured at all. Whether it has stopped or is merely below detection is still, honestly, unknown.
- Desiccation toleranceDrying out does not make a cell thirsty. It makes it collapse — because water was holding the shapes, and something has to take its place.
- Freeze toleranceA freeze-tolerant animal makes itself freeze on purpose. A slow freeze starting at −1 °C is survivable; the sudden one that follows deep supercooling is not.
- Antifreeze proteinsThe freezing point stopped matching the melting point, and that gap was the fingerprint of a protein that stops ice crystals growing rather than stopping water freezing.
- Heat toleranceThe most heat-tolerant species are often the most at risk from warming. Tolerance and the ability to raise tolerance run in opposite directions.
- Desert survivalA kangaroo rat never drinks. Its trick is not producing water — every animal does that — but losing almost none, including by condensing it back out of its own breath.
- Hypoxia toleranceA crucian carp spending winter under ice with no oxygen turns its lactic acid into alcohol and breathes it out. No other vertebrate does this.
- High-altitude adaptationThe air at altitude is still 21 per cent oxygen. It is the pressure that has changed — and Tibetans and Andeans solved that in two different ways.
- Deep-sea pressureWater barely compresses, so nothing down there is being crushed. What pressure actually does is change the shapes of proteins.
- PredationWhere hunts have actually been counted, most of them fail. The famous capabilities — the speed, the venom, the bite — are rarely what decides it.
- Predator–prey arms racesNobody is racing. Each side changes because the other changed — and where the prey can kill you, the usual asymmetry disappears.
- Suffocation was the textbook answer for a century, and had never been measured. When it was, circulation stopped in seconds — by a completely different route.
- Dynamic camouflageA cuttlefish changes pattern faster than you can blink, and by the standard test cannot see colour. Nobody has settled how the matching works.
- It is not a reflex and it is not free. What grows back is a rod of cartilage, and it cannot be shed again.
- Armour and spinesArmour does not make an animal safe. It makes attacking it a bad use of a predator’s afternoon.
- BioluminescenceBelow the depth sunlight reaches, most animals in many groups make their own light. It is the normal condition of the largest habitat on Earth.
- CounterilluminationCamouflage by being brighter. In midwater the thing that gives you away is not your colour — it is that you block the light.
- Parasitism and predationA predator eats many and kills fast. A parasite keeps its host alive. A parasitoid does one of each, which is why it needed its own word.
- Brood parasitismA host that accepts a well-matched egg is not necessarily fooled. Rejecting risks destroying its own — and where cuckoos are rare, that is the bigger mistake.
- ScavengingCarrion looks like free food. So why has no walking animal ever managed to live on it?
- Convergent evolutionVertebrate and octopus eyes are the classic case of convergence — and what makes them convincing is that they are built differently.
- MutualismA cleaner fish would rather eat its client’s mucus than its parasites. Mutualism is not friendship — it is an arrangement that needs holding in place.
- DecompositionThe ability to digest wood appears to have been invented once, by fungi, and everything a forest floor does follows from it.
- Fungal mycelium really does produce electrical spikes. The vocabulary of fifty words came from choosing where to cut the recording.
- The fungi are real, the connections are real, and carbon does move. Whether anybody is sharing is a different question, and it is still open.
- WoodA nail driven into a trunk stays at the same height for the life of the tree. Trunks do not grow upwards; they grow outwards, in a layer a few cells thick.
- RootsThe roots of the trees in a wood are not separate systems. They overlap, and they reach two or three times further than the branches above them.
- Trees and carbonAlmost every gram of a tree came out of the air. What the soil supplied is roughly one per cent of it.
- ForestsMost of the species in a forest live on the parts of it that are already dead.
- Plant communicationPlants do send chemical signals. The best evidence says they are talking to themselves, and the neighbours are listening in.
- FlowersEverything you find beautiful about a flower is an advertisement, and it was not aimed at you.
- PollinationA honey bee can sit on a tomato flower all day and pollinate nothing. It cannot buzz.
- Seed dispersalA jay is not planting an oak wood. It is burying food and forgetting some of it.
- PhotosynthesisThe oxygen you are breathing is water that light took apart. A tree is mostly air.
- GerminationA dormant seed is not asleep. It is being actively held back, waiting for evidence.
- No roots, no waterproofing. That is why a lichen can live on bare rock — and why it tells you what the air has been like.
- NectarA flower holds a sip, not a drink. Filling up would defeat the purpose.
- PollenA pollen grain is not sperm. It is a tiny organism that makes sperm and then grows a tunnel.
- Plant defenceA chewed plant smells different from a torn one. Wasps can tell, and that is not the plant asking.
- MimicryA stick insect is not mimicking a stick. Nothing is fooled into treating it as one.
- The deadliest mushroom in Europe is a plain greenish-white one that tastes fine and kills you three days later.