Biomedical Engineering Reference
In-Depth Information
and properties
[1, 2]
. Emergent structures are
more than the sum of the constituents in the
sense that an emergent structure has character-
istics that are not expected from looking at the
single elements brought together to make it.
Emergence is used to describe macroscopic phe-
nomena that are not seen on the microscopic
scale. An emergent structure is governed by a
combination of rules and principles at the
microscopic scale with the interaction of the
macroscopic structure and its surroundings.
Emergent properties are complex patterns or
processes derived from simple interactions
between multiple agents. There are many exam-
ples of emergent properties in biological sys-
tems, ranging from the construction of ant and
termite nests from simple individual behavioral
rules to the coordinated movements and pat-
terns of large bird flocks.
The eight phenomena described in this chap-
ter include the agile movement and navigation
in large groups like bird flocks and fish schools.
Another phenomenon is how groups of animals
coordinate the building of large constructions
such as termite mounds. In swarm intelligence,
seemingly intelligent behavior emerges from the
collective action of a large number of autono-
mous agents such as individual ants and ter-
mites. Another impressive phenomenon in large
groups is the collective decision making involved
when ants find the shortest path to food sources
and honeybees determine where to build a new
nest.
Self-sealing is the first step in the healing of
liquid transporting vessels and is seen in blood
clotting and in liana plants. Lizards and spiders
sometimes apply a drastic defense action when
they self-amputate limbs. The byssus threads
that mussels use to fasten themselves to rocks
have a remarkable self-healing capability
whereby fractures can be healed. Slime molds
are very resource efficient since they not only
build up and expand food vessels but also break
down the ones that are not needed. Finally, the
construction of a composite layered structure in
mussel nacre is examined at the end of the
chapter.
13.2 NAVIGATION IN LARGE
GROUPS
Some animals aggregate in very large groups
consisting of thousands of individuals that none-
theless act in a coordinated manner; examples
include fish schools, bird flocks, and building
and foraging tasks in social insect communities.
The sheer number of individuals and the reliance
on relatively simple behavioral rules suppress
the effects of individual variations in behav-
iors to such a degree that collective behaviors of
large groups are similar to the physical behav-
iors of inanimate objects (for example, the tide
of people exiting a busy commuter train follows
the path of least resistance and can be likened
to water flowing downhill) and can therefore be
analyzed by mathematical and statistical models
employed to describe physical phenomena
[3, 4]
.
Many birds have a tendency to fly in flocks,
and for common starlings (
Sturnus vulgaris
) the
flocks become extremely huge, as many as
100,000 birds in a single flock. Despite the fact
that the birds fly very close to each other and
at high speeds (about 70 km/h), it is rarely seen
that the birds hit each other. A spectacular phe-
nomenon called
murmuration
(aptly named
Sort sol
in Danish, meaning
black sun
) involving
such starling flocks can be witnessed on the
west coast of Denmark during the spring and
autumn migrations (
Figure 13.1
). Around sun-
set, large flocks are seen flying around looking
like quickly moving black clouds. Hunting fal-
cons attack the flock, causing rapid changes
locally in the flock so that the “cloud” changes
direction very quickly and abruptly. When the
flock is landing, a similar and apparently very
controlled behavior can be witnessed. Despite
the huge numbers of birds in the flock, all birds
land within a short distance of each other with-
out colliding.
