Black Pigmentation: Adaptation for
Concealment or Heat Conservation?
A recent report by Hamilton and
ward the environment. Under these
conditions,
excessive
heat loss
from
birds is prevented chiefly by the notable
Heppner (/) on some possible func-
insulative qualities of their feathers.
Interpreting the observations of Ham-
velvement
picture evoked by use of the concept
of endothermy, and in conjunction with
tions of surface coloration (albedo) in
birds once again states the possible inof physiological
processes
with varying light or energy absorption in organisms having differing coefficients of reflectivity.
Because the theoretical conclusions
that might be drawn from these data
could be interpreted as disproving or
ilton and Heppner, according to the
the insulative effectiveness of bird
plumage,I find it scarcely possible that
even the notable metabolic economy
amounting to 22.9 percent could be
attributed to the transfer into the body
of surface
heat impinging on black
face coloration, and its function under
natural selection and in evolution, I am
feathers. Such transfer is implied by
the statement, “These results indicate
that homeothermic animals can absorb
and utilize radiant solar energy and
conservation obtained (/) in insolated
process.” Is it not equally possible that
the heated surface might reverse the
at
least
as
denigrating
the
classical
views of the definitive function of sur-
constrained to argue that despite the
demonstrable 22.9 percent metabolic
blackened birds, other conclusions can
be derived from the experimental re-
sults on white andartificially blackened
Australian zebra finches (Poephila castanopsis).
Many workers have repeatedly noted
and confirmed the selective value of
concealing albedos and coloration. Of
course even extremes in albedos might
serve
one,
two,
or even
more selec-
tively effective roles in nature; nonetheless, even if one (or more) function is
served, this fact does not byitself require substitution of one for the other,
and especially not of a lesser use for
a greater one, nor especially for the
overlooked,
effects.
but
probably
definitive
In this instance difficulties chiefly
due to semantics may have contributed
to the conflicting viewpoints and interpretations of otherwise unarguable experimental observations. As long as in-
that dark pigmentation facilitates this
normal thermal gradient and thusinterpose a barrier to an otherwise extrava-
the mating activities. Blackness in conjunction with the notable penalties of
heat absorption and overheating would
processes of heat movement. In the absence of this thermal information and
with due regard to the minute amounts
of solar energy available at dawn and
dusk, when external heat is supposedly
needed and used, the feathers, regardless of color, might even occlude the
needed supplementary external heat precisely when it would be most bene-
ficial.
An additional conclusion is drawn
by the authors, “The same evidence
is applicable to the coloration of man.
Dark human skin coloration may
maximize the absorption of solar radia-
In discussions of energy conservation
used, there will probably be additional
conflicts in the interpretation of data.
and body temperature it is semantically
advantageous to substitute the dynamically expressive terms endotherm
and endothermic (2) which direct attention to the source of energy and
heat, for homoiotherm, which merely
denotes a more or less static condition resulting from endothermy. This
usage has particular value in the comparison of the internally heated mammals
and
birds
with
the
externally
heated reptiles and amphibia (the ectotherms) or with the basking heliotherms.
Under the usual conditions prevailing
in and around endotherms, and par-
ticularly in birds, the thermal gradient
usually slopes steeply from the body to1340
diurnal creatures often must generate
surface layer of feathers, it is impossible to follow and evaluate the precise
measurements of the body at the skin
surface, and outwardly to the insolated
ture, as at dawn and dusk in otherwise
hot climates.”
It seems probable that just because
confusing words
10 hours a
or more, over a period of at
several months each year. And
hot hours of intense insolation
are precisely those when many
In the absence of any temperature
gant heat loss?
tion in situations where energy must
or
burden for as many as
day,
least
these
also
internal heat while foraging, evading
enemies, defending territories, and even
in conducting respiratory cooling, or,
are
appropriate
Under the conditions in which dark
pigmentation is supposed to exert a
favorable effect it would be necessary
to have a readily changeable albedo
from light-absorbing dark hue, to a
heat-reflecting white so as to modulate
the absorption of environmental heat.
Without such a mechanism black pigment would be a distinct disadvantage,
not only in direct proportion to its
efficiency for heating during the fleeting “dawn” and “dusk” hour or so,
but also according to the number of
hot hours per day. In hot climates
where the sunlight is very intense, a
dark skin might cause a serious heat
be expended to maintain body tempera-
solar energy is so effectively absorbed
by the dark skins of most tropical
races, a low albedo (which might not
be helpful even for brief periods at
dawn and dusk) also would expose the
possessor to the handicap of an excessive external heat load for all the rest
of the daylight hours. This alternative
effect raises a serious objection to the
theoretical energy-conserving benefits of
blackness in man or finch. To meit
seems scarcely possible that maximizing the absorption of solar radiation
in animals living in hot climates in
summation could have any but a del-
eterious effect.
except for man, carrying on most of
seem to outweigh the evanescent bene-
fits by at least ten or more times.
But even the suggested benefits at dawn
and dusk of surface heating as a result
of black color seem exceedingly dubi-
ous, since the light intensities and the
total insolated energy at these critical
hours are, except for total darkness, at
their lowest levels. The facts concerning the effects of black pigmentation
on energy conservation can be ascertained only by plotting the metabolic
and external heat conditions throughout a 24-hour period under natural
radiative and behavioral circumstances.
If some other explanation for black
plumage and pelage and for the dark
skins of humanbeings in tropical countries must be sought, then that of concealment seems most plausible. Viewed
from this classical interpretation of natural selection via effective concealment, the surface coloration or albedo
which absorbs most of the incident
light and reflects the least, simultane-
ously results in providing minimum visibility in an object. Thus dark or black
bodies that reflect almost none of the
incident light rays that would be necessary for stimulation of the retinal cells
of a potential predator, will be least
visible or invisible. Furthermore, under
crepuscular conditions when predation
is usually most intense, a black object
is more apt to be overlooked than one
that reflects light. If a dark object is
SCIENCE, VOL. 158