Social Interactions
Dominance hierarchy
Naked mole-rats have a linear dominance hierarchy, usually with the queen usually
as the most dominant member of the colony. The dominance hierarchy
includes both
sexes with neither sex being more dominant over the other on the whole.
The dominance hierarchy is very important in a naked mole-rat colony because
only
high-ranking males or females will become breeding individuals. Size appears
to be the most important factor in determining dominance; since naked mole
rats grow slowly age is also correlated with rank (Clarke and Faulkes,
1998). The
amount of urinary testosterone is also correlated with rank within the colony,
and with the level of aggression exhibited by naked mole-rats, it is quite
likely that the added aggression from the urinary testosterone increases
their rank
within the colony (Clarke and Faulkes, 1997).
The most common way dominance is shown, is by shoving, when two naked mole-rats
are facing one another and the more dominant individual will shove the
subordinate individual backwards in the tunnel. The queen shoves by
far the most in the
colony, and her shoves are primarily directed at other high-ranking females
in the colony
(Clarke and Faulkes, 1997). Two possible explanations for why the queen
initiates shoves: to incite “lazy” workers to work, or
to suppress the potential threats to her status as queen (Clarke and
Faulkes,
2001). The queen shoving
high-ranking females is consistent with both explanations, because these
are frequently the naked mole-rats that do the least work, and since
they are high
ranking females they would be the ones that would potentially challenge
to become the queen. While the evidence is not conclusive naked mole-rat
shoving
behavior
suggests that the queen shove to incite more work from colony members (Reeve
and Sherman, 1991).
Occasionally shoving can escalate to fighting between two high-ranking
females. In such fights the two combatants will shove for extended period
of time, frequently
locking incisors. Each female will also try to bite the other during
these fights, frequently these fights result in death for one or both
as a result
of injuries
they sustained during the fights, or infections that can follow. If the
original queen dies then the other female would likely become queen, but
their might
be
additionally
fighting
between her
and other high ranking females that also hope to become the queen (Clarke
and Faulkes, 1997).
Xenophobia
Naked mole rats are very territorial and will defend their burrow
system to the death if they find a naked mole-rat that they believe is
not a member of
their
colony. The identification of a colony member is done by smell,
which is gained through wallowing in the toilet area. Smell was shown
as the
identification mechanism in a lab when a colony member was removed
and then later reintroduced,
this colony member was attacked, even though it had been a member of
the colony a short time before and was highly related to the naked mole-rats
that were
attacking
him. In labs conspecific naked mole-rats occasionally are introduced
to
a different colony (usually accidentally) if this occurs the naked
mole-rat that first
encounters this foreigner (usually a small frequent worker)
will give an alarm call, and
back away. Quickly other larger members of the colony will go to
where the alarm call was given and defend the colony from the intruder.
The following encounters
usually
results in the foreign naked mole-rat being killed (O’Riain and
Jarvis, 1997). This could possibly be a result of trying to maintain
an inbred colony as possible, because
if a naked
mole-rat from
a different
colony infiltrated the colony and attained mating status, the offspring
would be less related to colony members.
Communication
Naked mole-rats communicate mostly via vocalizations. Colony members
make these vocalization in several different situations and serve
both in agonistic
and
cooperative functions. They have vocalizations for prior to mating,
several for defense, and a specific call in order to alert the
colony that there
is a non-colony
naked mole-rat in the colony. Breeding animals produce a specific
vocalization when they are urinating. Naked mole-rats also use
vocalizations during
reproductive conflicts, and also have general identification call.
Vocalizations serve many
important functions in the colony of naked mole rats especially
considering their limited vision (Pepper et al., 1991).
Fission/Outbreeding
New colonies appear to be formed by fission, since it seems very
unlikely that several individuals would come together to form
a new colony especially
since
they live in subterranean burrows and would have great difficulty
finding other individuals also trying to form a new colony.
One large obstacle
in starting
a new colony is finding food: since it takes a great deal of
work and energy to find food (through digging), a new colony would need
more
than just
a couple individuals to start a colony successfully. Brett
(1991a) found what appeared to be
a newly formed colony consisting of 25 individuals (including
ten
juveniles). This
colony was only 60 meters away from a molehill of another burrow,
suggesting that it split of from the larger colony in some
manner.
Brett proposed four means by which a colony might divide into two or more
separate colonies.
-Less contact with queen: as the colony expands both in number and size individuals
might have less contact with the queen and as a result not be as suppressed
reproductively, and instead of fighting the queen several individuals might
group together in a part of the burrow system, and block it off from the
rest of the burrow
system.
-High food: in the case where there was very abundant food, fewer individuals
would be needed to start a new colony and could separate themselves from the
rest of the colony more easily.
-Death of the queen: following the death of the queen, several females might
come into estrus and instead of fighting to become queen, instead block off
one another and start separate colonies.
-Chance occurrences: it is very likely that occasionally the parts of the burrow
might get blocked off from the rest of the burrow, due to their defense tactic
against snakes or as a result of external factors causing part of the ground
above to collapse. If this happened for a long enough period of time a female
in
the
blocked off area might come into estrus and begin mating (Brett, 1991a).
Another way in which the composition of a colony could be changed would
be through out breeding. It appears that some individual (usually males)
are much
more likely to leave their natal colony and try to join another colony. In
a controlled environment O’Riain et al. (1996) added an opening to
the each of colonies burrow system (they
performed this experiment with 48 colonies) and recorded which individuals
attempted to leave their colonies. Of the naked
mole rats that attempted to leave more than once, 95% of these were male.
These individuals were also one of the larger, lazier, and more fat laden
male colony members. The experimenters then exposed these “disperses” to
other naked mole-rats from other colonies, unlike the normal xenophobic
behavior naked mole-rats exhibit, the dispersers actually moved toward
individuals
of other colonies when given a choice between a foreign individual and a
fellow colony member. If this individual happened to be of the opposite sex,
the disperser
would attempt to copulate with that individual, a reaction very different
from what non-dispersing individuals would do. Additionally, these males
did not
attempt to mate with their queen, which would be expected given the
size of these males (similar sized males usually solicit the breeding female),
and
these individuals were not shoved more than other similar individuals, suggesting
that they were not driven from the colony. In addition to evidence suggesting
some individuals leave to colony willingly in attempts to breed there is
evidence showing that males will occasionally be accepted into other colonies
and breed.
Incidents of this happening seem to most often follow the death of the breeding
male in the colony the disperser was trying to enter (O’Riain et al.,
1996).
This web site was completed in partial fulfillment of the requiremtns for Biology 323, Animal Behavior at Davidson College in the Spring Semester of 2003.