Cats as Carriers of Disease

Peter P. Marra and Chris Santella provide an interesting perspective on our beloved pet cats in their book Cat Wars: The Devastating Consequences of a Cuddly Killer.  They explore the problems that arise because cats are not really domesticated animals; they still can live independently of their human keepers and continue to be effective predators for just about any animal smaller than themselves.  Cats escape from or are abandoned by their owners and become feral.  Given that they breed very rapidly and are prey for few larger animals, they multiply quickly in the wild.  While often providing a useful function in controlling the mice and rat populations near human activities, they are also proficient killers of more valued species such as birds.  The authors are driven mainly by the threat cats present to bird species that are, for a number of reasons, experiencing rapidly falling populations.  The authors conclude that house cats are not a problem if they are kept in the house, but wild cats should be completely eradicated, at least in certain locations.  A review of the authors’ argument can be found here.

As part of their argument for eradication, the authors bring up the topic of disease propagation by feral cats.  Wild cats share the same areas—and feed off of—animals that are reservoirs for serious diseases.  Consequently, they are capable of becoming infected themselves and passing on that infection to humans via bites, scratches, or even by breathing on a human. 

The plague still persists in some regions of the country and transmission to humans via cats does occur, but infrequently.  However, the illness can be fatal if not diagnosed early.

“Cases of plague transmitted from cats to humans are rare in the United States.  From 1977 to 1998 there were twenty-three cases of cat associated human plague in the country.”

A sickness more frequently transmitted is Bartonellosis, more commonly known as cat scratch disease or cat scratch fever.  Infection is likely in cats with estimates of a 40% or greater rate of occurrence.

“….cat scratch fever refers to an infection from a Bartonella bacterium that develops when an infected cat scratches or bites the human skin.  In cats themselves it is usually not a serious problem, and 40% or so of the cats that carry it are asymptomatic.  Humans, similarly, usually are not seriously harmed; a red bump forms, the lymph nodes might swell, and a mild fever may emerge.  However, more serious infections can, and have, occurred, particularly among immune-compromised individuals.”

Rabies is disease that is transmitted by several small to moderate sized mammals that are easily encountered in the wild by cats or dogs.  It is an interesting case of how a virus or bacterium can manipulate a host that has been infected in order to insure its propagation to another specimen.  When infecting an animal the virus travels along nerve fibers as it makes its way to the brain.  Once there it alters the behavior of the host, eventually making it more aggressive.  Meanwhile, the virus is also making its way to the saliva glands so that when the animal bites another animal the virus can be transferred to the next host. 

Dogs have long been the greatest threat to humans when it came to the transmission of the rabies virus.  Prior to the development of vaccines and effective post-exposure treatments, an encounter with the rabies virus was always fatal.  Fear of rabies led to enforced vaccination of pets and the near elimination of wild dogs.  Since there is less awareness of cats as a source of rabies and less popular support for the elimination of wild cats, bites and scratches from cats have become the most serious threat for rabies transmission to humans.

“Since 1988 cats have been the number one domesticated species passing rabies infections to humans.  In 2013, 53 percent of all reported rabid domesticated species were cats, followed by dogs at 19 percent.  The cause of this pattern seems clear—the presence of millions and millions of stray and unvaccinated free-ranging cats on the landscape, many of them sharing feeding stations with wildlife that are susceptible to rabies.”

The animals infected with the disease may not have observable symptoms, and a person infected by the bite or scratch of a rabid animal may go months before symptoms appear.  And once symptoms do appear it is too late and the infected person will die.  Consequently, any such interaction with an unknown or undocumented animal requires anti-rabies treatment.  Since cats can appear to be cuddly animals, especially to children, they can be particularly dangerous.

“….the vast majority of the 38,000 post-exposure rabies treatments conducted annually are the result of people interacting with a suspected rabid cat.  Each of these post-exposure prophylaxis treatments cost public health departments and U.S. taxpayers somewhere in the neighborhood of $5,000 to $8,000, amounting to at least $190 million across the United States each year.”

Cats are also the unique source of a parasite called Toxoplasma gondii.  The life cycle of this parasite requires an infected cat to spread the disease to other animals via cysts (oocysts) that are excreted into the environment where other animals can ingest them and allow the cysts to transform into tachyzoites which then multiply rapidly, and ultimately into something called bradyzoites.  These secondary hosts are then eaten by cats and the infectious cycle begins again.

Unfortunately, humans and other valued species can be infected by these cysts that infected cats distribute.

“Toxoplasmosis, the disease caused by Toxoplasma gondii, is one of the most common parasitic infections in humans.  In fact, it is estimated that approximately 30 to 50 percent of the world’s population and up to 22 percent of the U.S. population (more than 60 million Americans) are infected with Toxoplasma gondii….”

The cysts defecated by infected cats are very robust and can last for as long as a year or more in very harsh environments.  Humans can become infected by direct interaction with cat feces or by consuming foods from animals (poorly cooked meat from an infected animal for example) or by eating contaminated vegetables (cats seem to like to defecate in gardens and children’s sand boxes).  Most humans show no symptoms from an infection, but it can generate flu-like symptoms in some individuals.  The consequences of an infection can be quite severe for people with compromised immune systems.

Since so many people are infected by Toxoplasma gondii, and the vast majority of them are asymptomatic, why be concerned?  To begin with, the prime example of a compromised (or nonexistent) immune system exists in the form of a human fetus, and it has long been know that this infection can be transmitted from a pregnant woman to her fetus.

“Pregnant women and their fetuses have been known since the 1920s to be at serious risk.  If infected with toxoplasmosis in the first trimester, one in ten fetuses will be aborted or become malformed—and this is likely an underreported statistic.  Because of this problem, pregnant women have been warned for decades to avoid changing litter boxes and touching cat feces.  Despite these warnings, congenital transfer of Toxoplasma continues to happen across the world.”

Consider what is known about how the disease progresses in infected mice and rats.

“Once in the secondary host, the Toxoplasma oocysts then transform into something called a tachyzoite and multiply asexually rapidly.  Tachyzoites can be as small as one-tenth the size of red blood cells when they invade healthy cells.  There they divide quickly, causing tissue destruction and spreading of the Toxoplasma infection to the new host organism.  Eventually the infection localizes in muscle and nerve tissue—especially in parts of the brain—in the form of cysts called bradyzoites….”

The rabies virus has a strategy for modifying the host’s behavior in order to increase the probability that the host will infect another animal—Toxoplasma gondii does as well.

“Then something odd begins to happen to the newly parasitized host: its normal behavior of fear toward cats turns into attraction.  Specifically, the smell of cat urine—a smell that infected mice and rats were thought to be hardwired to fear and avoid—becomes an attractive aphrodisiac.  This is exactly how the Toxoplasma parasite wants its host to behave, because it turns infected rodents into easy prey.  Once the infected host, along with the parasites infecting its body, is eaten by a new predator, (preferentially a cat or other species of feline), the parasite can begin its sexual reproductive cycle again, infecting a new host, shedding oocysts, and expanding its reach.”

It was generally believed that these bradyzoites were a stable, latent stage that, although it remains in the infected human forever, had little if any effect.  However, if the bradyzoites are capable of altering the brain and changing behavior patterns in rats and mice, why would one assume that they were incapable of doing something similar in humans?

“Then scientists began to look a little deeper and found that the bradyzoites were actually dynamic and replicating.  In fact, one manifestation of toxoplasmosis infection is the development of ocular toxoplasmosis—basically cysts that settle in the eye.  If the cysts burst they can cause a progressive and recurring inflammation of the retina that can result in glaucoma and eventually blindness.  Regrettably, this is not the worst manifestation of toxoplasmosis infection in humans.”

Recent studies have suggested correlations between toxoplasmosis and long-term changes in physical and mental health.  Consider this summary from Wikipedia on the state of knowledge about the long-term effects of toxoplasmosis.

“Some evidence suggests latent infection may subtly influence a range of human behaviors and tendencies, and infection may alter the susceptibility to or intensity of a number of affective, psychiatric, or neurological disorders.   Research has linked toxoplasmosis with schizophrenia.”

“Latent T. gondii infection in humans has been associated with a higher incidence of automobile accidents, potentially due to impaired psychomotor performance or enhanced risk-taking personality profiles.  Moreover, correlations have been found between positive antibody titers [a measure of concentration] to T. gondii and OCD [obsessive-compulsive-disorder], Parkinson's disease, Alzheimer's disease, suicide in people with mood disorders, and bipolar disorder.   Positive antibody titers to T. gondii have been shown to be not correlative with major depression or dysthymia.  Although there is a correlation between T. gondii infection and many psychological disorders, scientists are still trying to find the cause on a cellular level.”

Some researchers are more cautious than others about drawing conclusions from these correlations; some are drawing the direst of conclusions.  The authors describe some of the work and conclusions of Jaroslov Fleger.

“Fleger believes that collectively toxoplasmosis, either through the acute stage of infection or through mental and neurotic illness manifested during the latent phase, has contributed to the deaths of hundreds of thousands of people, if not significantly more, over the last few decades.”

“Jaroslov Fleger….believes that malaria, now considered to be the most devastating protozoan killer of humans, will be ‘dethroned’ by toxoplasmosis.  As long as we continue having outdoor cats, the parasite will spread.”

More on Fleger and related research can be found in an article by Kathleen McAuliffe in The Atlantic: How Your Cat Is Making You Crazy.

For those of you who are not concerned that cats might be killing too many birds, perhaps you might wish to consider the human health consequences detailed for toxoplasmosis.  You can still have cats; you just have to keep them inside.

The interested reader might find this article informative:

Cats as Predators: The Damage They Do

  

Cats as Predators: The Damage They Do

Of all the cultural conflicts that tear our nation apart, perhaps the one that receives the least attention is that between bird lovers and cat lovers.  That is a pity because there are significant issues involved that affect all of us.  Peter P. Marra and Chris Santella inform us that there are things that require more of our attention.  They bring us up to date with their book Cat Wars: The Devastating Consequences of a Cuddly Killer.

The authors believe that the domestic cat species that have become popular as pets evolved from species of the wild cat appropriately named Wildcat. 

“Recent genetic studies corroborate the notion that today’s domestic cats evolved from several subspecies of wildcats and suggest that, of the five, the Near Eastern Wildcat is likely the domestic cat’s nearest relative.  This also confirms the hypothesis that domestication of the cat occurred somewhere in the Fertile Crescent.”

It is assumed that just as wolves and humans discovered a mutually beneficial coexistence leading to the domesticated dog, wildcats and humans did as well, resulting in the domesticated cat.  Whereas wolves and humans could share the same food, the wildcat probably learned that mice and rats and other small mammals started hanging around human food stores looking for an easy meal.  Since mice and rats are preferred meals for wildcats, it is easy to see humans concluding that letting wildcats hang around was a good idea. In this relationship, natural selection would enhance the survivability of wildcats that were less hostile to humans and would eventually give rise to versions of the domestic cat.

The attractiveness to humans of the domestic cat and its utility as a mouse and rat eliminator encouraged humans to begin carrying them wherever they travelled.  This meant that cats were introduced into environments in which they were not naturally occurring and would become an “invasive species.”  That term implies the animals would multiply beyond any natural controls and cause damage to the local ecology.  The authors begin their narrative with the experience of David Lyall who took up a position as a lighthouse keeper on Stephens Island off the coast of New Zealand and brought with him a pet cat named Tibbles who would soon deposit a litter of newborns in its new home.  Like many islands that had long been isolated, Stephens had developed unique flora and fauna—and had never experienced anything as voracious as the domestic cat.

“Cats make the perfect pet for an isolated island inhabitant, in part because they can obtain most of their own food from their surroundings.  Lizards, birds, or small mammals provide a sufficient diet.  Cats are carnivores and need to consume primarily protein and some fat to stay healthy.  They are ambush predators, sitting for long periods, motionless and quiet, waiting for the right time to pounce.  They are quick and efficient and excel at what they do—otherwise they die.  Cats have retractable, razor sharp claws that extend from their strong paws to pin down prey.  Once the prey is immobilized, cats inflict the kill bite with two sharp canines, usually to the neck, and quickly begin tearing into scales, fur, or feathers.  Cats can kill animals as large as rabbits and squirrels, but their primary prey consists of smaller rodents like mice and voles as well as birds the size of (and including) sparrows and wrens.”

“Cats do not always kill out of hunger.  They seem to be stimulated by the chase and if not hungry will still kill; cat owners who allow their cat to roam freely may have received a ‘present’ of a bird or mouse, a testament to their pet’s predatory competence.”

Cats reproduce at an astonishing rate, and in an environment where they have no significant threat from predators, their population will grow until limited by lack of food or by disease.

“Cats average three litters a year; the average number of kittens in each litter is four to six.  Kittens can come into estrus as early as four months after being born, so the numbers of cats can multiply very quickly!”

A single pregnant cat, as in the case of Stephens Island, is all that it takes.

“A female cat can produce a litter of as many as eight kittens, sometimes more, and if a male is around, she can be impregnated again within days after giving birth.  If an unrelated adult male is not around, siblings will eventually mate with one another, or offspring will mate with their mother.”

On Stephens Island, David Lyall, a naturalist at heart, had the unique experience of discovering a new species of bird on the island only to realize that within a year the proliferation of cats that he had initiated had rendered the bird extinct.  It would take 26 years of cat killing to rid the island of the predators.

Cats are an invasive species in the United States as well.  The problem with domestic cats is that most of them are feral and must live off the land by eating birds and other small mammals.

“The Loss et al. paper positioned the domestic cat as one of the single greatest human-linked and direct threats to wildlife in the United States, and emphasized that more birds and mammals die at the mouths of cats than from wind turbines, automobile strikes, pesticides and poisons, collisions with skyscrapers and windows, and other so-called direct anthropogenic causes combined.”

The numbers of predator cats and their victims are staggering.  Counting feral cats is not a simple task.

“Rough estimates do exist and include between 20 and 120 million unowned outdoor cats, with 60 to 100 million cats the most frequently cited range.”

But feral cats are not the only problem.  The estimated number of owned cats is about 84 million.  These are cats that are treated as pets and, presumably, fed and vaccinated against disease by their owner.  Unfortunately, many of these owners allow their cats to wander outside where they can hunt and contribute to the killing even though they have no need for food.

“Based on eight different studies, between 40 percent and 70 percent of owned cats were allowed outside; three additional studies suggested that between 50 percent and 80 percent of these animals actually hunted.”

The unowned, full-time hunters were the most prolific killers.

“Loss et al. estimated that each individual unowned cat annually kills 1.9 to 4.7 amphibians, 4.2 to 12.4 reptiles, 30.0 to 47.6 birds, and 177.3 to 299.5 mammals per year.”

Combining the estimates from both owned and unowned cats one arrives at the death toll.

“The final mortality numbers showed that cats killed between 1.3 and 4 billion (median 2.4 billion) birds per year, with unowned cats causing the majority of the mortality (69 percent)….The final estimates for mammal mortality were also alarming; 6.3 to 22.3 billion (median 12.3 billion) mammals were killed every year by outdoor cats.”

Since there are few organizations defending the rights of mice and rats, the focus will be on birds. 

The authors provide data indicating that numerous bird species studied have shown significant declines in number over recent decades.  Cats are not the only contributor to these declines; they compete with environmental disruption caused by the growing human population.  However, the cat problem is the one least justifiable in terms of benefits for humans—or for cats. 

The killing of billions of birds is not the only negative result.  Cats prowling around outside and interacting with animals that are susceptible to plague and rabies can acquire those maladies and infect humans.  Control of wild dogs has been effective, but uncontrolled cats are now the greatest threat for rabies transfer. 

Cats are also the unique source of a parasite called Toxoplasma gondii.  The life cycle of this parasite requires an infected cat to spread the disease to other animals via cysts (oocysts) that are excreted into the environment where other animals can ingest them and allow the cysts to transform into tachyzoites which then multiply rapidly.  These secondary hosts are then eaten by cats and the infectious cycle begins again.

Unfortunately, humans and other valued species can be infected by these cysts that cats distribute.

“Toxoplasmosis, the disease caused by Toxoplasma gondii, is one of the most common parasitic infections in humans.  In fact, it is estimated that approximately 30 to 50 percent of the world’s population and up to 22 percent of the U.S. population (more than 60 million Americans) are infected with Toxoplasma gondii….”

The infection will sometimes produce only mild flu-like symptoms in healthy individuals, but the invasive parasite does not go away; it lodges itself—hopefully in a dormant state— in tissue and will remain there as long as the individual is alive.  This latent stage was long thought to be benign, but recent studies have suggested correlations between infection and long-term changes in physical and mental health.

Infection in a person with a compromised immune system can be dangerous and even fatal.  The unborn fetus has no active immune system making infection via the mother a serious concern.

“Pregnant women and their fetuses have been known since the 1920s to be at serious risk.  If infected with toxoplasmosis in the first trimester, one in ten fetuses will be aborted or become malformed—and this is likely an underreported statistic.  Because of this problem, pregnant women have been warned for decades to avoid changing litter boxes and touching cat feces.  Despite these warnings, congenital transfer of Toxoplasma continues to happen across the world.”

Ardent bird lovers and cat lovers can both agree that there are too many unsupervised cats in the environment, feral cats spread disease, and that life for a cat in the wild is short and brutish.  Cat owners often assume that cats are quite capable of living on their own outdoors, making it easy to abandon them when it becomes convenient.  They may live outdoors, but few of them will thrive, and none will die of old age.

“Unowned cats without veterinary care are prone to disease (including feline leukemia, renal failure, feline panleukopenia, plague, rabies, and toxoplasmosis….).  They are vulnerable to predation by other animals, especially Coyotes and, to a lesser extent, eagles, owls, foxes, and Raccoons.  And they are frequently hit by cars—the most common cause of demise in outside cats.  Such are the hazards if they survive to adulthood, but estimates suggest that 50 to 75 percent of kittens born outdoors do not, dying from exposure, parasites and disease.  If they do reach adulthood, the life expectancy of an outdoor cat without caregivers providing regular feeding, water, and sometimes makeshift shelter is two years.  Outside cats that receive such care have a much longer life span, averaging 10 years.  The average life span of an inside cat is thirteen to seventeen years, depending on the breed.”

It would seem that a cat owner has a moral responsibility to keep her beloved pet inside and safe from harm.

So if everyone can agree that there are too many cats running around outside, what does one do about it?  The place to start is to keep from adding more cats to the environment.  Cat abandonment should be a crime.  There are leash laws for dogs, why not leash laws for cats.  Allowing an owned cat to run free could become a crime at some level.  But what to do with cats already running wild?

The main response from those concerned with protecting cats from harm is to foster trap-neuter-return (TNR) programs.  The assumption was that if a wild cat was trapped, taken to a facility and neutered, and then allowed to return into the environment, this would lead to a decrease in the population of feral cats.  Unfortunately, the data does not support this hypothesis.  Studies of wild cat colonies suggest that one would need to neuter 70-90 percent of the population before the numbers would begin to decline.  This level of TNR efficiency would be extremely difficult to reach.

The authors conclude that there is no alternative to the total elimination of the wild cats.  It can be done—at least locally.  As an example, they describe a program put in place on Ascension Island, a British territory.  Combined poison baiting and trapping (plus euthanasia) efforts began in 2002 as a means of ridding the island of cats.  By 2006 all the cars were gone.  The total cost of the effort was $1.3 million.

“Whether you consider $1.3 million an outrageous sum to pay to save a few birds or a wise investment in biodiversity will depend on your philosophical stance.  But from a purely financial perspective, there is little question that eradication—at least on a local level—will trump endangered species remediation every time.  A breakdown of per species dollars invested in conservation efforts for endangered species from 2004 to 2007 shows that $60.5 million was spent to resuscitate populations of the Southwestern Willow Flycatcher, $67.4 million to protect Red-cockaded Woodpeckers, and nearly $83 million to protect Bald Eagles.”

Brains, Energy, and Humanity’s Most Important Achievement: Learning to Cook

Suzana Herculano-Houzel created a revolution in our knowledge of the mammalian brain and its evolution when she discovered a simple technique that would, for the first time, allow measurement of the numbers of various cells that make up brain material.  She presented results from studies using this technique in her book The Human Advantage: A New Understanding of How Our Brain Became Remarkable.  A brief summary of her findings can be found here.

Not surprisingly, the most interesting results from her studies centered on what could be determined about the human brain.  Humans are primates and are considered to be one of what are often referred to as the great apes.  When Herculano-Houzel evaluated her data on the neuron cell count in the brains of various classes of mammals, she discovered what she referred to as “the primate advantage.”  Taking the number of neurons in a brain as a proxy for the cognitive capacity of the brain, the primate advantage consisted in the fact that primates had evolved the means to control neuron cell size so that for a given brain size, a primate brain would contain more neurons than the brains of other classes of mammals, whose neurons gained size as brain mass increased.  Primates then had greater cognitive capacity per unit of brain mass, but not necessarily greater cognitive capability.  Capacity becomes capability when the neurons are used to learn things that provide an advantage to the possessor of the brain. 

An example was provided to demonstrate the size of this “primate advantage.”

“Once we knew the neuronal scaling rules for rodent brains, in 2006, we could already do some rough calculations.  With the equations relating the number of neurons in a rodent brain to the mass of the brain and of the body, we could estimate that a rodent brain that had anything in the order of 100 billion neurons, as the human brain was supposed to have, would weigh more than thirty kilograms [66 pounds] and belong in a body that weighed more than 80 tons.  In other words: if we were rodents, we would look like a blue whale, have to live in water, and carry an impossibly large brain, one that would likely collapse under its own weight.”

Herculano-Houzel eventually determined that the human brain contained about 86 billion neurons.

She was always puzzled by the assumption that humans were somehow exceptional to an extent that seemed inconsistent with the notion that humans evolved in the same manner as other animals.  The fact that humans were primates closely related to gorillas and chimpanzees (the great apes) that were comparable in size but only had about a third the brain mass of a human supported this notion of human exceptionality.  Once she had data on primate species similar to that for rodents she could apply scaling rules to the number of neurons versus brain size, and brain size versus body size.

“According to the neuronal scaling rules that apply to primates, we would expect a generic primate brain with a total of 86 billion neurons to weigh about 1,240 grams (2.75 pounds) in a body weighing about 66 kilograms (145 pounds).  These numbers are just about right for us humans, with our, on average, 1,500 gram (3.3 pound) brains and 70 kilogram (155 pound) bodies.  The conclusion should come as no surprise to a biologist: we are that generic primate with 86 billion neurons in its brain.  Our brain is made in the image of other primate brains.”

If that explanation is correct, then the other great apes must be the exceptions because they have much smaller brains than the scaling rules suggest.  Therein resides an interesting tale of how humans, in fact, became exceptional.  It was not the brain we were born with that makes us exceptional, it is what we did and what we continue to do with that brain that made us unique.

It is necessary then to explain why the other great apes ended up with smaller brains in order to understand why we ended up different.  Consider that the amount of energy required to keep a neuron functioning is believed to be about constant and independent of neuron cell size, although it is a function of the brain region.

“What our findings indicate, however, is that within each of these neuronal types, cerebellar or cortical, variations in cell size across species are accompanied by neither an increased nor a decreased energy cost: neurons of the same type still cost the same across species, regardless of their size, with a fixed average energy budget per neuron across species.”

Nonprimate species have brains that increase with body size, but their neurons also become larger as brain size increases. Consequently they end up with many fewer neurons for a given brain size than primates and avoid creating an excessive energy demand to support their brains.

On the other hand, primate brains, with their large number of neurons in a relatively small size, consume a large amount of energy.  A human requiring a 2000 calorie diet to maintain its body weight would be devoting 500 of those calories to keeping its brain functioning.  A large brain with a lot of neutrons produces a large energy requirement that must be satisfied.  Herculano-Houzel provides an argument to support the hypothesis that given the environment in which the great apes evolved, their available food sources, and their physical capabilities, it is not possible for gorillas and other great apes to greatly exceed their current caloric intake and thus could not support a much larger brain.

Field studies of apes in the wild tally the amount of time they spend foraging for food.  Since apes tend to be on the lean side and generally do not gain excess weight, one can assume that they forage until they attain their needed nutrition.  What was discovered was that the bigger the ape, the more food that could be gathered and consumed in a given time period.  However, as body weight increased, the energy demand of the body increased faster with weight than the ability to consume more calories.  That meant that there was, in principle, a maximum mass that an ape could attain.  It also means that there was no extra energy available to support larger brains.  These apes could have evolved to a smaller size and, perhaps, could have acquired a larger brain as compensation, but natural selection seems to have decided against that strategy for those apes in their particular environments. 

“….the lineages that remained on all fours (and gave rise to modern apes) seemed to have invested any additional kilocalories [what we refer to as calories are technically kilocalories] amassed per day from longer times foraging and feeding into growing larger bodies.  For knuckle-walking species that are, for anatomical reasons, not very mobile, becoming as large as they could afford must have been advantageous, earning larger animals higher social status and thus greater access to food, among other privileges.”

Opting for a larger brain can be a risky evolutionary decision.  The brain always gets the energy it needs—and it always needs about the same amount no matter what the animal is doing—or the animal dies.

“Because an individual brain always uses the same amount of energy, no matter whether the rest of the body is starving, having too many neurons is clearly a liability when a species lives close to the limit of its caloric intake possibilities.”

Humans then had to have done something to escape from this brain size dilemma.  The starting point of a new direction was probably becoming bipedal with increased mobility and access to different and more extensive feeding zones, and new classes of survivability issues.

“….for our newly bipedal and suddenly highly mobile australopithecine ancestor, who some 4 million years ago diverged away from the lineage that would give rise to the modern chimpanzee and bonobo, investing the additional kilocalories it amassed per day in a greater number of brain neurons housed in a leaner, lighter body must have proved a much better investment strategy.”

Knowledge of how brain size must have increased over time from archeological evidence suggests that pre-humans adapted physically to the new environment and its challenges by growing slightly larger and increasing brain size as well.  However, the increase in brain size was small until about 1.5 million years ago, when it began to rapidly increase.  The hypothesis is that even though pre-humans learned not only to gather food but also to hunt it, it still faced an energy constraint due to the time required to obtain and consume food.

If one needed a dramatic increase in nourishment one could either figure out a way to acquire much more food, or one could figure out a way to obtain more nourishment from the food supply on hand.  The archeological evidence suggests that around 1.5 million years ago, when brain growth really began to rise rapidly, humans also began to cook their food.  Richard Wrangham’s book Catching Fire: How Cooking Made Us Human took note of this occurrence and formulated a “cooking hypothesis.”

“In a nutshell, the cooking hypothesis proposes that it was the invention of cooking by our direct ancestors and the resulting availability of cooked food that offered the larger caloric intake that allowed the brain of Homo to increase in size so rapidly in evolution.  The circumstantial evidence of the drastic reduction in tooth and cranial bone mass, expected for a species that no longer had to use much effort to chew, was all there, along with the fossil record that put the use of fire with transforming of foods between 1.0 and 1.5 million years ago.  What Wrangham did not have then was an indication that cooking, or some other way to increase caloric input from food, was not simply a bonus, but rather an essential requirement for their brains to become any larger.”

Why was a diet of raw foods so limiting for primates?

“….cooking presupposes the use of heat to denature proteins, break carbohydrate chains, and otherwise modify the macromolecules of food, turning foodstuffs into smaller, softer, more easily chewable and enzymatically digestible versions of their former selves.  Cooking with heat breaks down the collagen fibers that make meat tough and softens the hard walls of plant cells, exposing their stores of starch and fat.  Cooked foods yield 100 percent of their caloric content to the digestive system because they are turned into mush inside the mouth, then digested completely by enzymes in the stomach and small intestine, where, once converted into amino acids, simple sugars, fatty acids, and glycerol, they are quickly absorbed into the blood stream.  In contrast, the same foods may yield as little as 33 percent of the energy in their chemical bonds when eaten raw because these harder foods are swallowed while still in pieces, and thus are broken down and digested only partially.  Only the surface of the raw food crumbs is exposed to digestive enzymes in the stomach and small intestine; most of the unbroken starch finally gets digested in the large intestine by bacteria that keep the energy for themselves.”

Cooking available foods increased the obtained caloric content of those foods by a factor of up to three.  That is clearly a revolutionary occurrence, but the additional energy did not immediately lead to a bigger brain.  First, our ancestors would have to encounter or create situations in which the increased cognitive capacity was needed in order to obtain a survival advantage.  Natural selection would take over at that point.

The human advantage was not so much its bigger brain, but the ways in which our ancestors put that brain to use.  Our brains at birth are rather empty, useless things, but with a lot of potential.  It is the fulfilling of that potential that makes humans unique.

Now that we have learned how critical cooking food has been to human development, it should come as no surprise to learn that returning to eating only raw food has become a bit of a movement in some areas.  The terms crudivore and crudivorisme (for those comfortable with the French language) have arisen.  The English version of wikipedia files it under “raw foodism.”

Herculano-Houzel provides this comment on the topic.

“….despite the amenities of the modern, technological world, obtaining enough kilocalories from raw foods  remains so difficult that the crudivore diet is the ‘tried-and-true” way to lose weight—although not without its drawbacks: the drastic weight loss that ensues, with a constant feeling of hunger, is often accompanied by malnourishment to the point that women on the diet stop menstruating.”

Interestingly, other animals can still be smarter than humans on occasion.  Most animals that have been fed cooked food will gladly trade their raw food for the new stuff.

The interested reader might find these articles informative:

Suzana Herculano-Houzel Explains How Our Brain Became Remarkable

Change in Human Brain Size, Natural Selection, and Evolution

Suzana Herculano-Houzel Explains How Our Brain Became Remarkable

A revolution has occurred in our understanding of how our brain and those of other animals have evolved.  This revolution has produced a profound change in how we should consider ourselves in relation to other species, and, in particular, to how we should compare ourselves to the other great apes.  While areas in science where great amounts of money are involved, in terms of products to be produced or in the quest for research dollars, tend to be argued in the mainstream media, others that aren’t associated with a large revenue stream can rattle around in obscure (to most of us) journals for many years before they are revealed to the general public.  That has been the case with the research started by Suzana Herculano-Houzel and summarized in her fascinating new book The Human Advantage: A New Understanding of How Our Brain Became Remarkable.  Her work began in 2003, was first published in 2005, and after 10 years of additional research is now being presented to the rest of us in her book.

The author, a native of Brazil, had an exemplary academic career with a stint at Case Western Reserve University in Cleveland before obtaining a Ph.D. in neurophysiology at the Max Planck Institute for Brain Research in Frankfurt.  For personal reasons, she returned to Brazil and worked at the Museum of Life in Rio for three years.

“For the first three years back in Rio I designed hands-on activities for the children who visited the museum, created a website, and wrote my first book on Neuroscience for the general public, which landed me a job at her alma mater.”

The year before she initiated her research she was named an associate professor at the Federal University of Rio de Janeiro whose specialty was assumed to be science communication.  However, she was told she could perform research as well if she wished. 

Herculano-Houzel had always been troubled by the quandary that arose when true-believers in evolution reasoned that humans had to be produced by evolution, yet they continually evaluated humans in a manner that placed them beyond the bounds of what could be explained by evolution.  If we are the end point of evolution of the great apes, how could we have evolved to have a brain that was about three times larger in mass?  Why are we so unique?  Her great contribution was to demonstrate that, in terms of evolution, we are not unique.

It was believed that cognitive abilities in a species would be related to the number of neurons contained within the brain.  If that was the case, then how can one discuss the relative brainpower of a given species if this number was not available?  This is the problem Herculano-Houzel wanted to solve.  Prior attempts at measuring neuron numbers utilized a technique known as stereology in which thin slices of a brain would be sampled in depth at a few locations and the results applied to the entire section.  She needed a method both simpler and more powerful for what she intended.

“The highly heterogeneous distribution of neurons across different structures of the brain, however, makes stereology impractical for determining numbers of cells in whole brains.  Neuronal densities vary by factors of up to 1,000 across brainstem structures and the cerebellum.  Even within a single structure, like the cerebellum, different layers have neurons packed in widely varying densities….[Stereology] would be prohibitively expensive even for a well equipped lab.  And it was even more so for myself, with no lab and no funding.”

Her solution to the problem required some trial and error, but no exotic new equipment or materials.  She merely produced what she gleefully refers to as brain soup.  Sections of brain material to be analyzed where sloshed around in a detergent solution that dissolved the cell boundaries but left the cell nuclei intact.  This soup had to be homogenized so that samples of the fluid could be used to visually count the number of nuclei.  This approach was aided by applying chemicals that would attach to the nuclei and produce a blue color.  This provided a total cell count.  To get a separate count of the number of neurons there was a known molecule that would attach itself to neuron nuclei and produce a red color. 

“Counting 500 nuclei (which took around 15 minutes at the microscope) was enough to determine the percentage of neurons with a certainty of 0.2 percent.  Applying the percentage of neurons to the total number of cells in the structure of origin yielded an estimate of the total number of neurons in it.  By subtraction, I had the total number of other cells, presumably mostly glial cells, in the tissue.  Summing the results for the various brain structures—and I started with entire brains subdivided simply into cerebral cortex, cerebellum, and rest of the brain—yielded for the first time, direct estimates of the total number of neurons and other cells in the whole rat brain.  And the entire process took less than a day.”

“Luckily for us, however, there were a few stereological estimates available in the literature for the rat cereberal cortex and cerebellum—and our estimates matched them.”

This picture will help with the nomenclature.

The rather small cerebellum is clearly indicated, as is the brainstem.  The rest of the visible parts make up the cerebral cortex, which is often divided into sections that have more to do with physical structure than function.

 Herculano-Houzel was now off and running.  She had a credible technique.  Now she needed specimen brains—lots of them.  Her studies were restricted to mammalian brains which all have similar structures.  One can postulate that mammals all derive from a single ancestor and the various families of mammals (clades) broke off from that original line and evolved according to its newly determined evolutionary rules.  Determining the manner in which species in a clade added neurons to their brains as the species increased in adult size would be of great interest.

After 10 years of research, Herculano-Houzel and her collaborators had accumulated and published data for six different mammalian families.  What they discovered was that for most species the number of neurons did not scale proportionately with brain size, whether in the cerebral cortex or the cerebellum.  As the family evolved to larger animals with larger brains, the size of the neuron cells increased and thus the number of neurons increased but did not keep up with the gain in brain size.  The primate family was the great exception to that rule.  The small primates studied had somehow developed a mechanism that kept the size of neurons nearly constant as larger species with bigger brains evolved.  Thus a primate brain would pack more neurons per volume than a brain of a different family but of the same size.  This is referred to as “the primate advantage.”

Humans are primates.  The greatest enlightenment provided by these studies was placing the human brain in context.  Previously, it was generally concluded that the human brain was exceptional in size because it was so much larger than those of the other great apes.  However, the scaling rules determined for primates indicated that the human brain is exactly what it should be for a primate of that size.  The other great apes were the outliers.  They possessed brains much smaller than they should have had.  What happened?

Herculano-Houzel attributes the hypothesis that can explain this to Richard Wrangham who presented his thoughts in his book Catching Fire: How Cooking Made Us Human.  Her results corroborate the hypothesis.  It must be recognized that a brain requires an exceptional amount of energy to keep it running.  There appears to be a constant energy requirement per neuron; a bigger brain, or just a rain with more neurons, requires more energy.  For humans, the brain utilizes about 25% of the calories in a normal diet.  For an ape to have developed three times the brain size (as scaling suggested) that ape would have had to have a plentiful supply of calories to support it.  Observational studies of the other apes in the wild suggest that food is not plentiful for them; they work hard foraging for enough food to maintain their weight.  Given such a situation, it seems that natural selection for these apes determined that a larger body and a smaller brain was the more advantageous situation.

How did humans avoid this evolutionary trap?  First of all, they came down from the trees and began to walk on their hind legs.  This made them more mobile and put them in new environments that might change the tradeoff between body size and brain size.  But the issue of energy had to be solved.  Foraging for food might or might not have been more efficient on flat land.  What the historical record suggests is that the brain grew slowly in pre-humans after they became bipeds until about 1.5 million years ago when brain size began to grow dramatically.  This is also about the time that the historical record suggests the pre-humans began to cook their food.

Why is cooking food so important?  The process of cooking makes food much more easily digestible and allows the body to absorb about 100% of the caloric content.  Raw food, on the other hand, provides only about a third of the potential caloric content.  Cooking also saved the time and caloric output required to chew raw food sufficiently that it could be swallowed.

Cooking then could have relieved pre-humans of the energy constraint and allowed them to develop larger brains, assuming that conditions favored a larger brain.

“And once the energy afforded by cooking turns a larger number of neurons from being a liability into being an asset, it becomes easy to envision a rapidly ascending spiral where larger numbers of neurons are selected for, given the cognitive advantage conferred to those individuals who have them and who now also have the time available to use them to hunt in groups, navigate the environment, look for better homing, hunting, and gathering grounds, and care for the wellbeing of their group, protecting it and passing on knowledge about where to find food and shelter.”

The human brain itself is not exceptional in an evolutionary sense—it is basically just another primate brain, albeit a large one.

“Those pieces of science writing which hail the human brain as a wonder often forget to mention that it never, ever starts life as a wonder, but rather as a 300-gram mass that doesn’t do much yet—although it certainly holds great promise.”

The human advantage then is not so much our physical brain as the knowledge that we have accumulated and have learned to impart to our brains.

Many thanks go to Suzana Herculano-Houzel for providing us such interesting insight on a fascinating topic.

The interested reader might find it curious to learn that the human brain has actually begun to decrease in size over recent millennia: Change in Human Brain Size, NaturalSelection, and Evolution