Body Temperature and the AAT/H: Does the human condition indicate an aquatic past?
AAT/H proponents often make
several claims about mammalian body temperatures:
What we find when we look at the
facts (which are readily available) is that the body temperature of normal, healthy humans certainly does fluctuate.
In fact, it does so on a regular basis even without exercise.
With exercise it fluctuates even more.
It is very similar to our primate relatives, even those -- like baboons -- which live in open savannahs.
I've got some facts and figures below about body temperatures of several types of mammals.
The body temperature of normal,
healthy humans doesn't fluctuate, while that of terrestrial mammals does.
Normal human body temperature
is like that of whales, rather than our primate relatives or other terrestrial
This leads to a fascinating
question: why are AAT/H proponents so often surprised that we are like our primate relatives?
For instance, some terrestrial mammals, such as antelope,
can allow their body temperature to rise while the temperature of their head (and brains) stays lower.
They can manage this because they have a network of blood vessels which flow around the nose so the blood to the brain is cooled by the mouth and nostrils.
Humans and other primates, even those which
live on open savannahs, cannot do this to any great degree.
Also, some terrestrial mammals -- such as horses, cows, goats, and rabbits -- generally
have higher body temperatures than humans.
AAT/H proponents consider these facts to be evidence that we did not evolve on land; they typically state that we would have a thermoregulatory system just like that of distantly related animals like antelope rather than the type of system seen in other primates.
Yet our human system is shared with our primate relatives -- even those which live on open
savannahs -- so it is exactly what we should expect if we evolved as terrestrial mammals.
Lastly, our body temperature
is not like that of whales, although all mammals' body temperatures are
relatively close ("normals" being between about 34-40 degrees C).
So this is the AAT/H question regarding our thermoregulation (this question was actually asked of me online):
If we were on the savanna for the millions of years as you say, and did not have an intervening period of "aquatic-ness", instead evolving with the other savanna creatures, why then did we not end up evolving the same way?"
First, note that I'm not talking about our earliest hominid features necessarily arising on savannahs.
Second, why do AAT/H proponents generally insist that all savannah animals deal with heat in the same way, when in
fact they utilize a variety of approaches to this problem?
Baboons and patas monkeys, for instance, use a far different system than antelope just like humans do.
Third, as a prelude to answering this, let me present another question along the same lines.
Sea gulls and many other sea birds excrete salt through nasal glands; crocodiles do so through salt
glands on their tongues (although it should be noted that some
scientists believe that these glands were developed to counteract the
effects of dehydration, a problem which crocodile, paradoxically, are susceptible to).
Why don't they both use the same method?
Why don't whales do it the same way?
(Instead they use the usual mammalian solution of the kidneys plus hormonal regulation of urine formation and concentration.)
They're all in essentially the same environment (marine) and are dealing with the same problem (too much salt).
Why do they use very different methods to do the same thing?
The answer is the essence of evolutionary theory: relatedness.
Evolution is not like catalog shopping.
You can't just "pick and choose" from the Big Book O' Adaptations: I'll have
one of those, and, oh, that looks good! just doesn't work in the real physical world.
You have to start with what you have, and consequently we shouldn't be surprised to find that we resemble primates in our body
temperature and thermoregulatory needs.
We've been separated from other animals for tens of millions of years, and just because they can let
their temperatures rise by 10 degrees on a hot day doesn't mean we can
forsake our primate heritage just because it seems like a handy thing to be able to do.
Just as we see with the problem of dealing with too much salt, we see that not all savannah animals deal with heat in the same way.
So why should we be expected to be different from other primates, even savannah-dwelling primates, and instead be like distantly related animals like antelope and warthogs, when in fact in evolution distantly related animals generally utilize a variety of approaches to any given problem?
Back to our questioner:
We ended up taking a different evolutionary path from the majority of animals there, now the question is why?
There aren't primates, for one, and more specifically, they aren't apes.
We've been separated from even baboons for probably about 20-25 million years.
We are far more closely related to apes, and especially to African apes, than to anything else on the planet.
That we resemble them and not antelope, cattle, aardvarks, and warthogs is not something
to be wondered at; it is to be expected.
I'm not saying that we first evolved our hominid characteristics on savannahs,
although it is obvious that we we're able to live there, as other primates, chimpanzees for example, also can.
I am pointing out that it is silly to
think that we would evolve the same thermoregulatory system as antelope
instead of using similar systems to what we see in other primates, including
And that in regard to our body temperature
we resemble other primates, including savannah-dwelling primates, unlike the
oft-repeated claims of AAT/H proponents.
Why do people harbor the incorrect idea that
human body temperature doesn't fluctuate?
Anthony Smith, in his 1985 book,
The Body, provides an answer:
"The word normal, as applied
to human body temperature, is a splendidly misleading description.
The little arrow on the clinical thermometer, doggedly directed at 98.4 degrees
F (36.9 degrees C), gives extra support to it and the exactness of that
point four of a decimal emphasizes it even further". (Smith 1985: 365-366)
[My note: Great Britain uses
"98.4 degrees F" as "normal", while the French and North Americans use "98.6 F (37 C)".]
"Normally the majority of us do not have a 'normal' temperature.
Infants have a higher temperature; older people have a lower one.
Ordinary adults wake up with their temperature
below normal and go to sleep with it above normal." (Smith 1985: 366)
He then elaborates:
"To confound the picture of
'normality' still further there is the daily cycle.
Babies acquire it quickly,
and then follow the traditional pattern of a peak in the evening and a
trough at dawn.
An average between maximum and minimum is 2 degrees F (1
degree C), but individuals can go up and down by 3 degrees F (1.5 degrees C) or more.
The menstruation of women is a further complexity; during it the temperature is low.
This increases slightly with a small but pronounced
rise (perhaps as much as a degree) during ovulation when an ovary releases an egg.
A hot bath, reasonably enough, send the bather's temperature up
to 100 degrees F (38 degrees C) or so.
Exercise does even better.
After three miles a runner can read 105 degrees F (40.5 degrees C) rectally,
but less than normal on his skin due to all the sweating there". (Smith
So the idea a "normal" human body temperature is an exact number like "98.6" is oversimplified quite a bit; in fact, Black's Medical Encyclopedia even lists humans' "normal" body temperature as being between 36.7 and 37.2 degrees C (98-99 degrees F).
Human body temperature compared to primates
So we can see that the body
temperatures of normal, healthy humans do vary, contrary to AAT/H claims.
We can also see that, contrary to AAT/H claims, our temperatures are like
those of our relatives, including open country baboons (Papio, listed below):
Macaca mulatta (Rhesus
macaque) 36-40 degrees C
(Crab-eating macaque) 37-40 degrees C
Papio hamadryas (Hamadryas
baboon) 36-39 degrees C
(From 1987 The Care and
Management of Laboratory Animals Trevor Poole, ed. Longman Scientific
and Technical: Harlow, Essex)
Also note that these temperature
ranges are likely slightly higher than those of wild, resting individuals,
since in order to take their temperature, these primates must be forcibly
"In most cases, they are likely
to represent normal ranges, but normals are difficult to establish for
animals which readily become excited when restrained". (Poole 1987: 602)
Human body temperature compared to whales
As for the supposed similarity
in body temperatures between humans and whales, let's just look at what
an expert on cetaceans has to say about it:
From 1979 Whales (first
pub 1958; revised 1962), by Dr. Everhard J. Slijper (Professor, Zoological
Laboratory, University of Amsterdam). Hutchinson of London: London.
pg. 301 (after giving the body
temperatures from many studies of whales): "For the time being, at least,
we may take it that the average body temperature of Cetaceans in general
is about 95.9 degrees F. -- a very low figure indeed for a mammal".
"This figure is 2.5 degrees F. below that of man, whose
temperature is low in turn when compared with
that of horse (100.4 degrees F.), of cows and guinea-pigs (101.3 degrees
F.), of rabbits, sheep and cats (102.2 degrees F.), and of goats (103.1 degrees F.).
Only hedgehogs are known to have an average summer temperature
equal to that of cetaceans, while sloths, opossums, and duck-bills (89.6
degrees F.-93.2 degrees F.) are even more cold-blooded.
But then the last-named species occupy such a special position among mammals in so many respects,
that we may say that compared with terrestrial mammals, whales have a very low temperature.
Seals and related species certainly have higher temperatures,
for Clarke has measured 98 degrees F. in an elephant seal.
The hippopotamus, on the other hand, has a temperature similar to cetaceans (96 degrees F.),
and sea-cows probably have a lower temperature still".
A few other mammalian body temperatures
From 1991 Environmental and Metabolic Animal Physiology 4th edition.
C. Ladd Prosser, ed. Wiley-Liss: New York.
pg. 111 (from Table 1):
Man 37 degrees C
Baboon 38.1 degrees C
Mountain sheep 37.9 degrees C night
Mountain sheep 39.8 degrees C
Goat 37-40 degrees C
Fur seal 38 degrees C
Humpback whale 36 degrees C
Bat (Dobsonia) 37 degrees C
Did you know our body temperature is similar to that of at least some bats?
Does this mean we used to fly?