- Charles
Darwin
:
- Many of actions as
hiccough & yawn are probably merely
coorganic as connexion of mammae & womb. We
need not feel so much surprise at male animals
smelling vaginae of females. when it is
recollected that smell of ones own pud, not
disagree. Ourang outang at Zoolog Gardens
touched pud. of young male & smelt its
fingers. Seeing
a dog & horse & man yawn, makes me feel
how much all animals are built on one
structure.-
-
- the passage
comes from Darwin's NOTEBOOK M
846-89
- It is
transcribed in: Barrett et al, Charles Darwin's
Notebooks, 1836-1844:
- Geology,Transmutation
of Species, Metaphysical Enquiries. p.
540.
- Dr
John van Wyhe
Director The
complete work of Charles
Darwin
University of Cambridge
-
- Evolutionary analyses of human behavior have
illuminated a number of fundamental questions
(i.e., foraging, mating systems, communication,
etc.), and a wide array of behaviors have been
targeted for investigation (infanticide,
polygyny, child abuse, homicide, etc.). In
general, investigation has centered on complex
behaviors organized into broader functional
units based on their context, or in some cases,
their outcome. Little attention has been given
to more basic behavioral units (Scott, 1950)
(e.g., ingestive, eliminative) that serve as the
foundation for all more complicated patterns.
The ubiquity of these basic patterns across
animal species may have contributed to their
perceived lack of importance in understanding
higher order and more complex behavior patterns.
On the other hand, the systematic analysis of
basic motor/behavior patterns [e.g., fixed
action pattern (Lorenz, 1954; Lorenz &
Tinbergen, 1938) or modal action pattern
(Barlow, 1968)], characterized by: (1) being
species typical (within classes of animals the
behavior is typically found in all individuals);
(2) consisting of movements that occur
simultaneously or sequentially with a high
degree of predictability; and (3) being
repeatedly recognizable (Slater, 1978, p. 14)
may be quite productive.
-
- Evolutionary theory has rendered some
aspects of human behavior, that were previously
thought to be solely influenced and directed by
cultural specific patterns of behavior, to be
influenced by a long evolutionary history. It is
true, however, that there are still a number of
human behaviors that remain puzzling in spite of
the analytical power of Darwinian evolutionary
theory. Relatively little attention has been
paid to the study of fixed or modal action
patterns in humans, with one notable exception.
One of the frequently overlooked behaviors in
the study of human behavior, yawning is a
virtually ubiquitous behavior among all
vertebrate species (Heusner, 1946). Alcock
(1993, p. 26) noted that a human yawn is one of
the best examples of a fixed or modal action
pattern in our species. The study of yawning,
particularly in humans, is important because ( 1
) it is a behavior pattern that we share with
all vertebrates, (2) it occurs in several
different contexts in essentially the same form,
and (3) it is contagious (Moore, 1942; Provine,
1996), unlike sneezing, coughing or crying.
-
- Previous studies of yawning in humans have
centered on yawning and activity levels of
individuals (Baenninger, Binkley &
Baenninger, 1996; Provine, Hamernik &
Curchack, 1987). In general, little attention
has been paid to the evolution of yawning and in
particular the identification of its ultimate
function, although quite a bit of work has
focused on its proximate or more immediate
causation (Alcock & Sherman, 1994).
Moreover, the ontogeny of yawning has been
little studied, even though it has been reported
to regularly occur in utero by the 15th week of
pregnancy in humans (de Vries, Visser, &
Prechtl, 1982; Egerman & Emerson, 1996;
Sepulveda & Mangiamarchi, 1995; Sherer,
Smith & Abrainowicz, 1991). Taken together
these observations suggest that the evolution of
yawning is a potentially important and largely
overlooked behavior.
-
- Description and Classification
- A yawn is a very deep inspiration, taken
with jaws wide open which ventilates all alveoli
(not the case with normal quiet breathing)
(Marieb, 1995). Yawning is characterized by a
long inspiration followed by shorter expiration
of air. Yawns are a strong prolonged reflex (311
seconds) which invoives a strong and coordinated
contraction of a complex array of pharyngeal and
associated muscles (Barbizet, 1958; Provine,
Hamernik & Curchack, 1987). Yawning is
closely related to the gape, but differs in a
number of crucial components. The principle
difference is the combination of two components:
a respiratory and a mouth-gape component. Gaping
simply involves a large opening of the mouth and
stretching of the muscles of the mandible and
maxilla, and thus is differentiated from yawning
behavior seen in rodents, carnivores, primates
and possibly birds and herbivores (Heusner,
1946). Gaping has been reported in a wide
variety of vertebrate species (fish)
(Baenninger, 1987; Baerends & Baerends-van
Roon, 1950; Morris, 1954; Peiper, 1932; Rasa,
1971), reptiles and amphibians (Cramer, 1924;
McCutcheon, 1970)] but may only be analogous
to yawning and not a true homologue. According
to Heusner (1946), it is unclear whether birds
(Delius, 1967; Sauer & Sauer, 1967) as well
as herbivores (Barbizet, 1958; Cramer, 1924)
actually yawn. Nevertheless, yawning has been
described in carnivores (Bekoff, 1974;
Leyhausen, 1979) and in a variety of primate
species (Anderson & Wunderlich, 1988;
Deputte, 1994; Hadidian, 1980; Hinde &
Rowell, 1962; Scucchi, Maestripieri &
Schino, 1991; van Lawick-Goodall, 1968; Wolfheim
& Rowell, 1972).
- Investigators have attempted to identify and
classify yawns, but have been largely
unsuccessful in categorizing yawning behavior
into clearly defined types based on fundamental
motoric differences in yawning patterns.
Attempts at classification have largely relied
on functional differentiation of context to
define the different types of yawns (Hadidian,
1980). Deputte (1994) recognized two contexts
for yawns, the 'rest yawn' observed in
transitions from rest to waking states and is
synonymous with 'true yawns' (Altmann, 1967;
Angst, 1975), and the 'emotion yawn' or the
'tension yawn' (Bertrand, 1969; Hinde &
Rowell, 1962; Redican, 1975). Deputte (1994)
notes that the 'emotion yawn' could also be
called the social yawn since it is elicited by a
number of social signals. Charles Darwin
recognized that yawning occurred in several
different contexts. He noted that, "... baboons
often show their passion and threaten their
enemies in a very odd manner, namely, by opening
their mouths widely as in the act of yawning ...
Some species of Macacus and Cercopithecus behave
in the same manner" (Darwin, 1872, p.
136-7).
-
- A cross-specific analysis of yawning
suggests two broad functional explanations. One
centers on the immediate physiological
circumstances that are implicated in yawning;
and the other, a possibly secondarily derived
social communicatory function. Investigators do
not agree on the functions of yawning, nor on
its importance in the behavioral repertoire of
any particular species. van Hooff (1967), in an
important early review of facial displays in Old
World monkeys and apes, does not include yawning
as a social communicatory behavior. While
Redican (1975), in a exhaustive review of facial
expressions in nonhuman primates, devotes an
entire section (pps.147-53) to the discussion of
yawning,
- Based solely on observational data, these
two types of yawns (rest yawn and true yawn) are
motorically identical. In macaques, yawns are
described as having three basic components:
oral, postural and respiratory. These components
are distinguished on behavioral bases since
there is no temporal disruption in a yawn. In
general, prior to the yawn the head is lifted
backward and rotated sideways, the mouth is
slightly open, but the teeth are not exposed.
The second phase of the yawn is characterized by
continued upward motion of the head, the mouth
is fully open, often exposing the teeth and
gums. A deep inhalation is also part of this
phase. In the final phase, the head is lowered,
the mouth closed rapidly and the lips covered
with the teeth (Deputte, 1994). In a discussion
of baboon behavior, Altmann (1967) suggests that
one can distinguish true yawns from those with
socially implied functions, but only by context.
true yawns are presumably stimulated by a
lowered oxygen tension in the blood, while
social yawns express conflict, anxiety or
threat. The external manifestations of these two
types of yawns are quite similar. The entire set
of teeth including the canines may bc exposed,
and as the yawn reaches a climax the head may bc
thrown back (Redican, 1975).
-
- While yawning, the mouth opens widely and
roundly, usually fairly slowly, typically
closing more swiftly. A swelling of the throat
is usually visible, accompanied by a deep breath
and closing of the eyes and lowering of the
brows (Brannigan & Humphries, 1972, p. 58).
Yawns have an average duration of 6 seconds, are
difficult to stop midperformance and are
infectious, stimulating yawning in other humans
that observe or even hear the yawner. One of the
most interesting characteristics of human
yawning behavior is its high degree of
contagion. After observing, hearing, reading, or
thinking about yawning evokes a yawn (Provine,
1986,1989a,b). Oddly, this contagious feature of
yawning has not been observed in nonhuman
primates (Deputte, 1978). In addition to its
contagious nature, yawning has been shown to
have a true circadian cycle in both humans and
laboratory animals (Anfas, et ai., 1984; Greco,
et al., 1993; Provine, et al. 1987).
-
- Suggested Functions
- Although overlooked in the human
evolutionary literature, yawning has been used
as a behavioral indicator of various types of
experimental neurochemical interventions in
laboratory animals for quite a while. Since it
is clear that yawning occurs in at least two
vastly different contexts (true vs. emotional
yawns), any discussion of the functional or
evolutionary responses must keep these
behavioral variants clearly separated. Table 1
lists the suggested functions of yawning
behavior and distinguishes between true and
emotional yawns. One of the interesting aspects
of Table 1 is the conspicuous absence of
empirical verification of any of the
propositions about yawning [except for the
work of Provine and his colleagues (1986,
1987)]. In spite of the lack of research
into the evolutionary basis of yawning, many of
the proximate mechanisms implicated in the
behavior have been identified. A variety of
studies have shown that cholinergic,
dopaminergic as well as serotonergic systems are
implicated in the induction of yawning in
laboratory animals (Brown, et al., 1990, 1991;
Mogilnicka & Klimek, 1977; Mogilnicka,
Boissard & Delini-Stule, 1984;
Urbà-Holmerren, Holmgren & Anias,
1982; Urbà-Holmgren, et al., 1979; Yamada
& Furukawa, 1980; Zarrindast &
Poursoltan, 1989; Zarrindast, Fatehi &
Mohagheghi-Badi, 1995; Zarrindast, Toloui &
Hashemi, 1995; Zarrindast, et al., 1995). While
these studies have enhanced our knowledge of the
brain mechanisms and neurochemical pathways
implicated in yawning behavior, none have
attempted to link these neural mechanisms to the
behavior of free-ranging animals. We are
therefore still left with no satisfactory
enipirical answer to our earlier question of the
evolution of yawning.
-
- Throughout this discussion we have assumed
an evolutionary basis for such a widespread
behavior like yawning. It is possible that
yawning bas no immediate adaptive value, and is
merely a byproduct of selection for other types
of behavior. This possibility seems unlikely,
however, given its highly contagious nature. It
is reasonable to expect that a behavior that
elicits a similar behavior in conspecifics so
frequently and with no intention by the yawner
has been the object of strong selective
pressure. One of the reasons why yawning may
have been overlooked and investigated only on a
limited basis is the perception that the costs
and benefits of yawning are relatively trivial.
This may not be the case, at least in some
situations. Tesfaye and Lal (1990), Tesfaye,
Skorzewska & Lal (1990), Warner & Warner
(1990), and Lurie (1990) all note that the costs
of yawning can be quite high and can result in
subluxation of the lower jaw. Subluxation of the
temporomandibular joint results in the jaw being
locked open, deviated either left or right,
often leaving the affected individual unable to
speak. Certainly, in a minority of cases,
yawning may be very costly.
-
- The interesting question still remains, i.e.
the ultimate causation of yawning. As previously
noted, in nonhuman species, yawning seems to
serve two distinctly different ends. While there
are marked similarities between the yawns of
humans and nonhuman primates, the functional
differentiation of human yawns is not so clear.
Did yawning originate as a direct response to
some physiological process and then subsequently
co-opted as signal of underlying emotional
state? Did yawning arise as a communicatory act
and subsequently associate with an entirely
different underlying physiological process? It
is unlikely that we will ever know the answer to
these questions with any degree of certainty for
either humans or nonhumans. What we are left
with are a set of best guesses based on an
examination of the behavior as it manifests
itself in modern humans, coupled with
comparative observations of other vertebrate
species.
- The most compelling characteristic of human
yawning that calls for an evolutionary
explanation is its contagious nature coupled
with the absence of this contagion in other
yawning species. These observations suggest that
yawning in humans has evolved as a fitness
enhancing behavior pattern. Identification of a
precise function for yawning is difficult, but
perhaps the coordination of activity, whether it
is preparing for sleep, increasing alertness
(Askenasy, 1989), warding off respiratory
infection (McKenzie, 1994) or preventing alveoli
collapse (Forrester, 1988) are among its most
important and enduring features. Clearly, our
understanding of yawning as a coordinated human
behavior would benefit from additional research
and this research would likely yield a clearer
picture of its adaptive nature.
-
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