Obstract
Io study how the species organizes its time, space, and
social structure in a semi-natural situation, a group of sik
Octopus rubescens were kept in a Sm diameter tank for one month.
Observations were made of the win-loss record of each animal in
interactions with other members of the group, the time spent by
each animal in the different areas of the tank, and the various
color patterns displayed by the animals in different situations.
A pattern of dominance based on size emerged in which larger
animals won encounters with smaller animals. The establishment
and defense of a territory by at least one individual in the group
sudgested territoriality for the species. Several color patterns
were seen that did not fit previously published reports of typical
patterns in the species, and imply that the literature on body
patterning in octopuses is neither complete nor absolute.
Introduction
Octopuses are highly evolved invertebrates with a large
degree of intelligence, and have been the subject of numerous
studies of learning and nervous system function. Despite the
extensive literature that has been developed on the learning and
physiology of the genus, however, very little is known about the
behayior or ecology of octopuses. Examining the ways in which an
octopus interacts with others of its species, and how it makes use
of its time and living space in a laboratory or semi-natural
setting may provide us with clues about the ecology of the species
in the field.
There is especially little information available on the
behavior of Octopus rubescens, a small benthic cephalopod that is
found along the Facific coast of Morth America from Baja
California to British Columbia. In Monterey Bay, California,
where this study was carried out, C. rubescens is known to spend
brief period in the plankton after hatching, then settle as
juveniles in the kelp forest before migrating further oftshore to
sandy mud bottoms (Hochberg and Fields, 1980).
The only published research available on behavior in U.
rubescens was performed by Warren, Scheier, and Riley (1974), who
described the color changes observed during attacks on conditione
and unconditioned stimuli. They found many of the same patterns
described by Packard and Sanders (1971) for Octopus yulgaris, but
defined the color changes during the attack sequence as systemati
and independent of background coloration. They concluded that the
color changes that occur during feeding are so strongly associated
with the animal's motor activity as to be inevitable.
The only other study specifically involving Octopus rubescens
was detailed in an unpublished thesis by Dorsey (1976). Dorsey
studied the natural history and social behavior of the species in
Washington and concluded that the animals set up a dominance
hierarchy based on size; she found no evidence that the animals
established territorities. Because of the small size (12Ocm
SOcm icm) of her tanks, however, her results may have been
influenced by crowding. More research on the species is clearly
needed before the its social organization can be definitively
described.
Frevious research on social behavior in other Octopus specie:
has been limited, but two other studies have examined the types of
social orgainization in laboratory octopus populations. Yarnall
(1967) observed Octopus cyanea in large, semi-natural holding
ponds in Hawaii. He found that the octopuses did not appear to
defend territories, but set up a hierarchy in which
smaller animals yielded to larger ones. Mather's (1980) study of
Octopus joubini examined the use of space by the individuals in
the her tank. Occupancy of all areas were not randomly
distributed, but Mather concluded that the species was not
territorial because the animals did not exclude one another from
preferred areas of the tank. She, too, found that the octopuses
showed patterns similar to those found in dominance hierarchies.
Hanning (1972) defines a territory as 'any defended area.
In order to determine whether Octopus rubescens organizes itselt
into territories, several aspects of its behavior must be studied:
a) how much time does each individual spend in the different areas
of the tank? and b) does each animal 'defend' the area in which it
spends the most time?
Ony laboratory situation, of course, is only a poor
substitute for field research, but the use of a semi-natural
setting may provide us with a fairly accurate view of octopus
behavior. By setting up a large tank with a simulated natural
environment, obseryations can be made that may help develup an
outline of octopus social organization.
Materials and Hethods
Octopus rubescens were collected from the sandy bottom of
Honterey Bay, California by setting bottle traplines. All animale
were collected from within cne-half mile of Monterey harbor, and
from a depth of approximately 50 feet. In the laboratory, they
were weighed and tagged with colored wire. Sexes were determined
by lightly anaesthetizing the animals with a solution of 1.57
ethanol in seawater and inverting the mantle of each animal and
examining the gonads. In the case of the two largest males, this
process was not necessary because enlarged prowimal suckers and
hectocotyluses were easily visible on these animals. Four males
and three females were originally placed in the tank; one male
crauled out of the tank and died after only one day, and the study
included data from only the six animals present in the tank
throughout the study.
Animals were placed in the observation tank over a three-day
period. The tank was round, with a diameter of Sm and a depth of
SOcm, and was on a running seawater system. The bottom was
coyered with sand in an attempt to mimic the animal's natural
Habitat, and bricks, cement blocks, and jars were scattered around
the tank for homes; there were always more available homes than
there were octopuses. The octopuses were fed small crabs, a type
of prey common in their natural environment. They were fed every
day, about1.Ssmall crab per octopus per day.
Octopus rubescens is nocturnal (Hochberg and Fields, 1780).
and for ease of observation the animal's daily light cycle was
reversed, with the tank darkened during natural daylight hours
(O800 - 1800) and lit with flood lights from 1800 - O800.
The 24-hour day was braken up into 2-hour periods, and
zeveral perinds of abservations were made every day.
Opprokimately 75 hours of observations were made during the cours
of the study. Observations were made in the dark, with only a
small amount of light entering through the walls of the room --
enough light to enable the observer to identify the animals after
dark-adapting for a few minutes, but dark enough that a small
flashlight had to be used for taking notes. The observer sat on
stool beside the tank, sometimes walking quietly around the room
to clusely watch interactions. The octopuses quickly habituated
to this situation and showed mo response to the movement of the
Ebserver. A written commentary was recorded where each octopus
spent its resting time, where and when it moved, how it interactet
with ather animals, and its body patterns during activity.
Photographs were occasionally taken with a Fentax K-1000 camera
with flash.
Results
Levels of activity
For the first two weeks the octopuses were fed at the
beginning of the subjective night (0800), and a daily cycle of
activity emerged (see Figure 1). Activity levels were calculated
by measuring the percentage of each half-hour period that each
animal was moving about the tank, then averaging all animals and
all observations for that period.
When ted at 0800, octopus activity reached a sharp peak
during the half-hour of feeding, corresponding with the animals
moving about the tank hunting the crabs. Immediately following
feeding activity dropped to almost zero for about 1 1/2 hours.
presumably corresponding with the time taken to kill and eat one
or more crabs. Activity then resumed and was maintained at an
elevated level throughout the subjective night, dropped sharply
when the lights were turned on at 1800, and remained low
throughout the subjective day.
For the third and fourth weeks of the study, the animals were
fed during their subjective day at 2400 in an attempt to see to
what extent the change in time of feeding would disrupt the
activity cycle found above. On the new feeding schedule (see
Figure 2), the activity level maintained its nocturnal character.
The activity pattern associated with feeding was translocated to
the new feeding time in the subjective day, but was otherwise
unchanged. During the subjective night there was a 2-hour period
of high activity following the onset of darkness, then a drop in
activity follpwed by an increasing pulse of activity that wae
again terminated by the lights turning back on. From this data it
appears that the octopuses' daily cycle can be disrupted but not
reyersed simply by feeding during their normal resting hours.
Interactions
Ihree types of interactions were observed: a) approach -- an
octopus crauls or swims toward another, and the second octopue
crawls or zwims away before contact is made; b) touch -- an
actopus crawls or swims to another and touches the second animal
with the guter half of one or several arms, and either animal
crauls or swims away; and c) fight - an octopus crawls or swims
to another and both animals grapple arms, trying to cover each
cther with their interbrachial webs, and either animal crawis or
suims away. No discrimination was made in the analysis of the
different types of interaction; any time one animal ceded space to
another it was recorded as an encounter and went into those
animals' win-loss records.
For each animal a win-loss record was tabulated and compared
with the others (Table 1). One animal was a predominant
winner, one lost all of its encounters, and the others won every
encounter against certain animals and always lost to others. he
winner was the largest animal in the tank, and most of the other
octopuses never lost an encounter with an animal smaller than
itself. Ihis suggests a hierarchy of dominance based on sise, in
which larger animals win encounters with smaller animals (Table 2)
Only in the two smallest animals, 7 and 3, did this patterr
not hold true. This may be due to individual characteristics
(e.g. trauma) of these particular animals, to chance because of
the small number of encounters between the animals (1), or to the
fact that 47 was female and may have been evading a mating attempt
by 45, a male. In single-sex interactions, however, a linear
pattern of dominance based on size is very clear.
Territoriality
The tank was divided into 12 sectors, o around the walls of
the tank and a corresponding with homes in the center (Figure 3).
Ihe amgunt of time spent in each sector of the tank by each anima.
was measured (Table 3). No obvious pattern initially
emerged: while 1 spent all of his resting time in one sector of
the tank, the others distributed their time among several
different areas. to, however, spent most of her resting time in
sectors VIII and IX, which are adjacent to each other and could,
combined, be called a territory. Using only level of occupancy as
a criterion for territoriality, it appears that only fi and
possibly 46 spend enough time in a single area to term it a
territory.
Ongther criterion is whether or not the octopus defends the
area in which it spends the most time. To measure this, the win
loss record and the number of encounters involving the animal in
each sector were weighed against the amount of time the animal
spent in that sector (Table 4). Again, no clear pattern is
shown. It appears that the amount of time spent in an area is not
necessarily related to the number or success of encounters in that
area. However, almost half of all the encounters in the tank toom
place in sectors XI, XII, and VII -- the sector in which fi, the
dominant animal, made his home and the two sectors adjacent to it.
All of the encounters that occurred in sector XII that involved
animal 41 took place when 1 was forcing another animal from his
home, or rebutting an attempt by another animal to enter the home
while he was occupying it. In addition, although relatively tew
encounters occurred in sectors VIII and XI, where  (the second
dominant octopus) spent most of her time,  both initiated and
won every encounter that took place there. All of these facts
indicate that 1 and t6 may be defending territories. None of the
avidence shows, though, that any of the smaller animals were
actively defending territories.
Color Fatterns
Observations of color patterns were largely casual, made
during the course of observations of interactions and teeding.
Many different patterns were seen, including many of the body
patterns cataloqued by Packard and Sanders (1971) in their study
of Octopus vulgaris. Particular attention was paid to pattern
changes during interaction, but relatively few color changes were
obseryed. The octopuses were usually uniformly pale green
throughout the encounter, whether it was a simple approach, a
touch, or a fight.
Occasionally, however, the initiator and eventual winner of
an encounter assume an aggressive posture and pattern similar to
that pictured in Figure 4a. The posture is upright; the color is
uniformly green or mottled with dark bars extending from below
each eye to the upper arms. In these encounters, the submissive
animal would assume a defensive posture (Figure 4b) with arme
curled underneath it; dark rings would sometimes appear around the
eyes. These patterns were not consistent with any particular
animals and could not be predicted to occur in any particular
situation.
During feeding, the color change sequence described by Warren
et al. (1774) was often observed. In this sequence, the octopus
is crawling or sitting in the tank before detection of the prey
item, and exhibits one of many patterns. Upon detection and
during a free-swimming attack, the animal assumes a color ranging
from light orange to grey, then turns completely colorless upon
landing. While seising the prey, the octopus is spotted or
mottled, then returns to one of a variety of patterns while
eating. Sometimes, though, no color change whatsoever occurred
during the attack on a prey item, and sometimes the animal flushed
a dark red at the moment of attack rather than blanching. Again,
the occurence of the pattern sequence was not predictable.
Discussion
10
Ihe activity levels found for Octopus rubescens are those one
would expect to find in a nocturnal species, and are similar to
those found in Octopus vulgaris by Wells, O'Dor, Mangold, and
Wells (1983). The results show that the daily light cycle of the
animals can be successfully reversed. This is especially
interesting in view of the fact that nearly all of the many
studies made on learning in Octopus species have been carried out
in the daytime in normal daytime light levels, when the animal
would normally be fairly inactive. Wells et al. (1983) feel that
"behayioural experiments may be underestimating the capacities of
the animal by testing its abilities during the wrong time of day."
Heversing the daily light cycle of the octopus in the study could
dive more accurate results in behavioral or learning researcn
without reversing the diurnal cycle of the researcher.
In a semi-natural situation, D. rubescens showed interaction
patterns that could be called a dominance hierarchy and is
probably based on size. This finding is in keeping with Varnalle
(1967) obseryation of a dominance hierarchy in O. cyanea and with
Hather's (1980) similar findings in O. joubini. A hierarchy in
some form may be common to all Octopus species, as Nather
suggested, but the strength of the hierarchy may well be ditterent
in the field than it is in the semi-crowded conditions in the
laboratory.
Yarnall (1949) and Mather (1780) found that the species of
octopus they studied showed no territoriality; Dorsey concluded
11
that U. rubescens was not territorial, but her results may have
been affected by crowding. In the less crowded situation of this
study, at least one individual O. rubescens showed clear signs of
territoriality and a second showed what could be described as a
sott' territory. It's obvious that the animals didn't ewhibit a
rigid territoriality in the sense that every animal defends and it
dominant in a specific area, but the clear territoriality of one
or more animals suggests territoriality for the species as a
whole.
It should be noted that octopus 1, the dominant and
territorial animal in the tank, was also the most active and wide¬
ranging of all the octopuses. He wandered frequently all gyer the
tank, winning encounters wherever he went. It may be that his
dominance and range of activity had an inhibitory effect on the
other animals, preventing them from establishing territories.
Ihis possibility should be tested by the removal of the dominant
animal trom the tank to see how the use of space by the other
animals in the tank is affected. If the territory of animal to
becomes more rigid, or if the other animals begin to form
territories as the tank becomes less crowded, the species could be
said to be fairly strongly territorial, with the activity of the
dominant animal having a substantial influence on the
territoriality of the smaller animals.
In locking for a type of social organization in her study,
Mather (1980) assumed that dominance hierarchies and
territoriality precluded each other, that one population could not
display both types of organization: "...presumably it could be
either a territorial pattern, in which each animal defends a piece
of space, or a dominance pattern in which animals hold a ranking
relative to one another but move in common space." Manning (17/2)
states that a dominance hierarchy "relates not to a fixed area,
but to a rank order of individuals living in a common area." But
the findings in this study suggest that a dominance hierarchy and
territoriality can exist in the same population. Again, the
territoriality shown here is not rigid and is certainly a
secondary structure to the dominance hierarchy, but the two types
of organitation seem to be interacting in this situation.
It's tempting to project the results found here to the
species in its natural environment, but it's entirely possible
that the affect of crowding in the laboratory is so great that any
social structure found in the lab is a response to the artiticial
conditions and not a typical pattern at all. The population
density of O. rubescens in the wild is not known; it may be so low
that the animals never interact enough to create a need for
territories or even heirarchies. Research in the field is clearly
needed.
Ihe color patterns observed during the study were not well
quantified, but the fact that patterns occurred that did not fit
the cataloqued repertoire of patterns is significant. It appears
that some of the body patterns displayed by the species have not
yet been described, and that even those that have been described
are not as sterectypical as might be believed. The lack of the
systematic color change sequence described by Warren et al.(1774)
in at least a few individuals of Octopus rubescens casts doubt on
1
the researchers' hypothesis that the color sequence is inseparably
linked to motor activity. Further work is needed to
comprehensively document the body patterns of Octopus rubescens.
but a point to be made is that it's highly likely that none of the
studies that have so far cataloqued color patterns in octopus are
truly complete or absolute.
Literature Cited
Dorsey, E.M. 1976. Natural history and social behavior of
Octopus rubescens Berry. Master's thesis, University of
Washington: Seattle, Washington.
Hochberg, F.G. & Fields, W.G. 1980. Cephalopoda: The squids anc
octopuses. Chapter 17 in Intertidal invertebrates of
California. Morris, R.D., Abbott, D. 2 Haderlie, E., eds.
Stanford, California: Stanford University Fress.
Manning, A. 1972. An introduction to animal behavior. Reading,
Massachusetts: Oddison-Wesley Fublishing Co., pp. 97-100 and
249-250.
Mather, J. 1980. Social organization and use of space by Octopus
joubini in a semi-natural situation. Bull. Mar. Sci. 3o:
848-857.
Fackard, A. 2 Sanders, G.D. 1971. Body patterns of Octspus
vulgamis and maturation of the response to disturbance.
Anim. Behav. 19: 780-790.
Warren. L.R., Scheier, M.F. & Riley, D.A. 1974. Colour changes
of Octopus nubescens during attacks on unconditioned and
conditioned stimuli. Anim. Behav. T2: 211-219.
Wells, M.J., O'Dor, K.K., Hangold, K. 2 Wells, J. 1783. Diurnal
changes in activity and metabolic rate in Octopus vulgaris.
Mar. Hehav. Fhysiol. 7: 275-287.
Varnall, J.L.
1969. Aspects of the behavior of Octopus cyanea
Gray. Anim. Behav. 17: 747-754.
Tables
Table 1. Win-loss record of each octopus compared with each other
octopus. Reads left to right.
Iable 2. Hierarchy of dominance set up by the octopuses, with the
dominant animal at the top. Hierarchy is linearly based
on size within each se.
Table J. Fercentage of resting time spent in each sector of the
tank by each octopus.
Table 4. Fercentage of resting time (TIME), number of encounters
(E), and percentage of wins (ZW) by each octopus in the
twelve sectors of the tan.
O
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TABLE 2.
DOMINANCE
ANIMAL
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*!
*2
* 3
* 7
HIERARCHY
1209
oog
30g
909
459
709
G
O
9
6
9
TABLE 3.
III
10
VI
VII
VIII
IX
XI
XII
TIME SPENT IN SECTORS OF
ONIMAL
15
62
0
5
100
27
TANK
28
44
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Figures
Figure 1. Daily cycle of activity for the octopuses when fed at
0800. Dark line denotes subjective night.
Figure 2. Daily cycle of activity when fed at 2400. Dark line
denotes subjective night.
Figure Z. Diagram of the twelve sectors of the tank used in the
study.
(a) Oggressive posture and pattern sometimes shown by
Figure 4.
Fosture is upright:
the initiator of an encounter.
body pattern is uniform or mottled with dark bars
below the eyes.
(b) Submissive posture sometimes shown by the loser of
an entounter. Fosture is defensive, with arms
curled beneath the body; color pattern is unitorm
with dark rings occasionally appearing around the
eyes.
85
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FIGURE
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FIGURE 4.
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2
Ocknowledgments
I'd like to thank my advisor, Chuck Baxter, for his advice, for
proyiding access to his seemingly limitless knowledge of things
scientific, and for coming through with unsolicited (but much
appreciated) help at the most frustrating points in the research
process. John Kono was an invaluable ally in the construction and
reconstruction of my elaborate tanks. Carol Marzoula came through
whenever I needed help, and I appreciate her long hours on the
water pulling up octopus pots as well as her long hours in the
water looking for octopus pots. Stuart Thompson helped many
in the course by buoying spirits when stress was at its highest,
and the other professors helped make this quarter my most
satisfying at Stanford. I'm grateful, too, to the six charismatic
octopuses in my study for allowing me to put them in a tank, mees
up their cycles, and force them to make small talk.
Ond finally, I'd like to thank that "socially interactive" group,
the students of Bio 17SH 1987, for ... whatever it is you thank
yeur friends for.