Asia Pacific J Clin Nutr (1995) 4: 354-356
Asia Pacific J Clin Nutr (1995) 4: 354-356
Polynesian body size: an adaptation
to environmental temperature?
Philip Houghton
Dept of Anatomy and Structural Biology,
University of Otago Medical School, Dunedin, New Zealand
A computer simulation of exposure at sea in the
tropical Pacific supports the hypothesis that humans colonising
this region have been subject to strong directional selection for
a large muscular body. This is advanced as an explanation for the
typical Polynesian phenotype, and suggestions are made linking this
phenotype with the metabolic disorders of gout and non-insulin dependent
diabetes mellitus.
Introduction
With passage east across the Pacific, from relatively
large-islanded Melanesia to small-islanded Polynesia, there is a change
in human phenotype from rather slight to distinctively muscular. The
evidence is that the muscular phenotype was also prevalent in Micronesia
in the past, but major population change in historic time has much
altered the picture there. Within Melanesia, more muscular and robust
physiques are found on the coast and slighter physiques inland. Scrutiny
of patterns of disease (particularly malaria) and of nutrition, and
consideration of the secular trend, suggests that none of these factors
explain the phenotypic variation1.
Recently we have suggested that the basis for the
phenotypic variation has been climatic: the geographically tropical
Pacific was frequently a very cold place for Neolithic Homo Sapiens,
with a resulting strong directional selection for a large muscular
physique2,3. These studies examined survival of different
Pacific physiques under a specific set of environmental conditions
(exposure to a 16 kph wind at temperature 14.5oC) taken
to approximate common exposure to wet wind-chill conditions in the
tropical Pacific. While the advantage of a large muscular physique
for survival over several hours exposure was shown, no indication
could be given as to just how often something close to such a set
of conditions was likely to have occurred. However the factors involved
are favourable for use in a computer simulation, and the results of
such a simulation are presented here.
Materials and Methods
The variables are meteorological and biological.
Meteorological variables.
World Meteorological Organisation weather data were
obtained from 22 small-island weather stations through the South Pacific
covering a period of two years. Each weather station reports (approximately
every three to six hours) the wind speed, ambient temperature, eighths
of cloud cover, and, sometimes, rainfall.
Biological variables.
The data for body weight and surface area were the
means for males and females of a representative range of Pacific groups.
Maximum heat production by an "individual" was taken as
five times basal, augmented by up to 20% depending on th 1000 e amount
of sunshine. In the simulation, this was given by the equation:
maximum heat production (kJ/hr) = B*M* 4.6
where B = multiple of basal energy produced/kg body weight;
M = mass in kg; 4.6 is basal heat production in kJ/kg/hour.
Heat loss was based on ambient temperature and wind
speed, using the wind-chill equation:4
heat loss (kJ/hr) = 1.16 * (10 * sqrt(V) + 10.45
- V) * (33-T) * 3.6 * S
where V = wind speed in metres per second;
T = ambient temperature in degrees Celsius;
S = surface area of individual in square metres;
sqrt = square root function.
The crew became wet if wind speed reached 15 knots
(about 27 kph) or if it rained persistently. Heat loss was then taken
to increase by 60%. Body temperature change is given by the equation:
body temperature change (oC) = E/(3.5*M)
where E = energy debit or credit in kJ/hour;
M = mass of person in kg; 3.5 is the specific heat of body tissues.
A person was deemed dead when body temperature fell
below 32oC.
Several other parameters were considered in the simulation.
For example studies on several animal species show huddling to be
an effective method of reducing heat loss by up to one-third for each
animal in the huddle up to a maximum of three, compared with that
of a solitary animal5 and this doubtless was done, compatible
with the demands of keeping afloat. Heat loss from the respiratory
tract was not allowed for.
Each simulation assessed which members of the "crew"
survived the "voyage". When repeated with other sets of
weather data from the same station, the different physiques each survived
a particular proportion. If a particular physique survived in 76 of
100 simulations then the survival rate is 76/100 = 0.76. For most
weather stations at least 100 simulations were run for summer (November
to February) and winter (May to August).
Table 1. Survival
proportions for ten-day periods of exposure in summer and in winter.
The results for the extremes of Pacific physique, Karkar (small) and
Hawaiian (large), are given, and those for a group of intermediate
physique, the Lau of Malaita in the Solomon Islands.
| |
Karkar
|
Lau (Malaita)
|
Hawaii
|
| Latitude |
Female
|
Male
|
Female
|
Male
|
Female
|
Male
|
| Summer |
| 10 |
0.48
|
0.54
|
0.64
|
0.73
|
0.80
|
0.80
|
| 15 |
0.23
|
0.23
|
0.40
|
0.44
|
0.59
|
0.62
|
| 22 |
0.05
|
0.06
|
0.06
|
0.11
|
0.16
|
0.18
|
| Winter |
| 10 |
0.44
|
0.47
|
0.53
|
0.60
|
0.66
|
0.71
|
| 15 |
0.02
1000 |
0.02
|
0.04
|
0.08
|
0.11
|
0.17
|
| 22 |
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
Results and discussion
Results from simulations of ten day periods of exposure
in summer and in winter are presented (Table 1) using, for clarity,
only the results for three groups - the two extremes of physique,
being Hawaii (large) and Karkar island (off the north-eastern coast
of New Guinea, small) and an intermediate group, the Lau of Malaita
in the Solomon Islands.
For most station data at least a hundred simulations
were run. For each simulation there were only two possible consequences
for any member of the crew, survival or death. Survival was scored
as 1 and death as zero. All series of simulations were adjusted to
100 for ready comparison. Thus, for a given physique, sex and season,
a result in the table of 0.48 indicates that out of 100 simulations,
survival resulted 48 times and death resulted 52 times.
The simulations are likely to present a rather gloomier
picture of survival than the reality because human judgement is not
allowed for: craft push off and take whatever weather the computer
throws out, whereas in reality, experience and judgement on favourable
and persisting weather patterns would be significant in determining
when a voyage would start. However, being
based on human biological parameters, it seems unlikely
that these results paint a significantly distorted picture.
The results support the hypothesis that exposure would
have been a major problem for Neolithic voyagers in the Pacific, and
the likelihood of strong direction selection for a muscular phenotype.
The survival proportions for all groups were less than anticipated.
For example survival proportions in summer for males of intermediate
physique across a range of latitudes from 12oS to 25oS
are 62, 56, 19, 5 and zero. For all groups, whatever their range of
survival, there is a rapid decline in survival beyond 15o
latitude, and this association of survival with latitude was, in statistical
terms, highly significant. For any group there is an approximately
5% decline in survival for each degree movement away from ten degrees
of latitude, with an average male/female difference in survival of
5% in summer and 3% in winter, to the advantage of the males.
The inference is that the impressive Polynesian muscularity
has evolved not for locomotor purposes but as 1000 a metabolic heat
source. The particular muscle fibre type predominantly involved in
shivering (an isometric contraction) is Type IIb, or fast twitch6,7,8.
We suggest that people of Polynesian ancestry are likely to
show a genetically-determined preponderance of this muscle fibre type,
and currently we are obtaining muscle biopsies to assess this.
There are clinical studies showing a strong association
between muscle mass and abnormalities of uric acid metabolism9,10.
Other studies have shown an association between a predominance of
Type IIb fibres and obesity, and between Type IIb fibres and disturbances
of glucose metabolism and insulin response11-14. It is
hypothesised that some major metabolic health problems of modern-day
Pacific peoples thus derive from their evolutionary background. However
this background differs from that usually invoked as explanation for
high incidences of NIDDM, and we suggest that for these Pacific people
it is questionable whether the concept of a "thrifty" genotype
is relevant.
Acknowledgements.
The mathematical aspects of the computer simulation
are the work of Daniel Levy.
References
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Copyright © 1995 [Asia Pacific Journal of Clinical Nutrition].
All rights reserved.
Revised:
January 19, 1999
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