Human Physiology and Detective Stories

Detective stories combine accidents of evolution (blood can be ID'd, someone who's been running breathes faster, etc) with universal themes of conflict and desire.

If our evolution was different, the physics of murder would be different, and stories would change, too.

Identifying the Killer


We look different, and are very attuned to even tiny differences in appearance. (1 inch of height difference is less than 2% of a height of 6 feet, but is really noticeable. The difference between extremely short (5') and tall (6'2") is less than 15%. Tiny differences in facial structure are noticable, and change in characteristic ways over time. We have somewhat recognizable style/body shape (although strength/fatness can change a lot). There are alleged plietropic relationships between hormone levels and facial structure, hair growth, voice deepness etc. It's not accepted widely, but nearly every culture traditionally asserts that there is a link between facial appearance and someone's character and emotions.


To us, everyone mostly smells the same - even if they are distinctive, it can't really be used for identification. But to dogs, we are really distinct.


We have recognizable gaits, recognizable faces (except for dopplegangers). We can disguise our face somewhat, but intimates would not be fooled. Our gait is a giveaway to animals - dogs can recognize owners by gait much farther than they can recognize by face - but humans don't usually pay much attention to it.

Gait is probably consistent over a lifetime. Gait reveals historical wounds. Gait reveals muscular development - not usually used, but allegedly a subtle mating symbol - Leg strength a good proxy for overall muscular development.


We have unique voices, and can distinguish between any of maybe 10,000 voices. Other forms of communication also give off information - the particular language we understand, our accent/locality where we learned it. Then there's word choice, handwriting, vocabulary etc.


After shooting up to full height, we stay almost the same height, decreasing maybe 0.1% per year for the rest of our lives. This is unlike some animals which continuously grow their entire lives - for them, size & age are interchangeable. Plants mostly just keep growing.


Our sex is set at birth and can't really be changed. The sex of an individual is pretty easy to identify - although again in old age, both sexes lose their distinctive features and become more and more similar.

This is unlike some animals, where different genders are difficult to tell apart visually, or where animals spend most of their lives neuter - or even ones in which most of the population is neuter and works to increase the reproduction of blood relatives.

There are also animals where the different sexes have grossly different body size.


We might lose a limb here and there but it's pretty unusual, and usually lethal in primitive environments (compared to the octopus, which can lose / regrow tentacles pretty easily). Swarms of bees can grow/shrink intentionally.


We are pretty constant in our identity - we don't merge, or split at all unlike some animals. If you are a detective trying to catch a protozoan murderer, and arrive only as he's in the process of splitting in two, who do you arrest?


The one complication in human murders is when the criminal has a twin. Whether it's a secret one, or an evil one, it really mixes things up. Imagine an intelligent species which reproduced by cloning - very tough for a detective to distinguish between a thousand cloned lizards.

Life Strategy

There are not any clearly defined life strategies for humans, unlike in some animals which have specific types - i.e. the lizard species where males are either tough and try to win conflict to mate, thieves and try to secretly mate, or masqueraders who pretend to be female to get close to other lizards and mate. In such a society, what would be the social status of these life strategies (which do have biological components and can't easily be changed).

Body Parts

In modern times, the tiniest part of the body can be traced to the original through DNA. But for most of human history, blood was just blood. Skin and hair somewhat serve as individual identifiers.


There are physical problems that affect just one half of the body - and there are ones which affect all of the body below a certain height (head may be ok, or head+shoulders, or head+above waist, etc.) But not the reverse.

Physiological Changes

Many emotional states have fairly visible physical symptoms. But it's not too exact, and fades pretty quickly. Fertility has no visible signs.

Capture & Escape


Humans travel fastest while alone. (compared to birds / fish / bicyclists, which travel faster in packs) Imagine if two or three guys traveling together would have a big advantage following one guy - they could draft for eachother and save energy while the target got worn out.

Tying People up

It's possible to tie people up fairly well. But it takes time, and it's hard to do in a primitive environment. Vines don't work that well, and won't last long. Ropes work well, and in modern times, people can definitely be disabled.

It's almost impossible to tie up an octopus.

Whales can't be tied up at all - but what would it even mean, since they don't actually do anything. Being tied up is uncomfortable, but doesn't kill us as it would a whale or a hummingbird.

In Hummingbird legal systems, ropes (and wing clippers) would be murder weapons.

Blocking the Senses

We can voluntarily turn off sight, but no other sense. Some drugs can turn off the sense of touch, and some kinds of alcohol can cause blindness.


Certain sitting positions can make people temporarily numb, limp, or unable to walk.

The Murder


We can't even fall our own height safely. Cats can fall 10x their height pretty easily - and ants / insects can fall as far as they want. Some species might not fall at all - fish, jellyfish

Eating / Drinking

We can go days without eating, but just a day or two without drinking.


We need it, and fast


There are tons of types of poison, some detectable, some not.

Getting Away with It

(Apparent) Sanctity of Memory

There are things which change what we think (alcohol, drugs, false memories, amnesia) - but no agent can make us think of particular people, or implant specific memories. Our factual memory is pretty solid (it can fade, but it's hard to change specific things about the past). There is a particular condition which knocks out all personal specific memories, while leaving skills & personality somewhat intact (amnesia).

Fragility of Emotion

But our emotional state is much more fragile - there are drugs, music, people who can make us feel all sorts of emotions. We don't have very good conscious control.

Forced Forgetfulness

There's no way to make someone forget something! But we can only remember short snippets of certain types of information - yet we can remember faces, places, smells forever. A lot of the information we can recall well is perfectly reproducible using paper or words - but the things we can remember well, can't be very exactly communicated this way.

Forced Sleep

Some animals automatically fall asleep when some stimuli happen - I think chickens just pass out when you put their head under their wing.

Communicating Sense Experiences

Using words, we can communicate sound very well, history very well, smell barely at all, touch, heat/cold only roughly. Faces can barely be described at all with words -but can be fairly accurately described with paper (by a skilled artist) Smells can't be described at all, either way. Places can be described fairly well with words, and drawn well.

Mental Illusions

We have a huge subconscious that frequently makes us do insane things. Even when we are trying to be rational, every decision runs through the black box of the subconscious, which can change our view without us even knowing. We also have a big list of problems even within the rational mind.

Sense Illusions

We have some optical illusions related to the way our eyes work - pattern-finding neurons in the back of our eyes that don't work right for certain types of input. There are other weird physical illusions or crosslinked expressions such as sneezes, hiccoughs, funny bones, crying with happiness.


All senses have possible negative inputs - including rare ones like heat/cold, vertigo, emotions. But most are never used. People don't often try to torture someone by making them taste bad-tasting food, or smell horrible smells, or watch depressing movies.

Just getting by


We have physiological signs of lack of sleep, and need to sleep about 1/3 of every day.


We desperately need to breathe, all the time. Exercise causes us to make noise - loud breathing.


Humans have no hibernation, and our minds will drive us to insanity if we don't receive physical & social stimulation.


1 week without food, 3 days without water

~6 weeks without vitamin C


Life Cycle

Most of life is a process of decay, and there is a huge span of years at the end. Lots of animals aren't like this - some insects bigger & better throughout their lives, until the moment of reproduction - which also causes death.

Other animals are fully regenerative, and don't really age.