Biology Reference
In-Depth Information
Banded mongooses
Banded mongooses,
Mungos mungo
, live in mixed-sex groups of 8-40 adults. Each
group contains one to five older, dominant females and varying numbers of young,
subordinate females (Fig. 6.20b). In a study in Uganda, Michael Cant and colleagues
(2010) found that a dominant female had greatest individual reproductive success in a
group of four to five breeding females. Beyond this, pup survival declined, most likely
because of competition for food. There was no evidence for reproductive restraint by
subordinates; all females in the group entered oestrus around the same time and mated.
However, dominant females were increasingly likely to evict pregnant subordinates
when there were more than five breeding females in the group. Some of these evicted
subordinates left for good, while others aborted their litters and were then re-accepted
in the group. This option of abortion and re-acceptance may explain why subordinates
showed no initial pre-emptive restraint.
The main message from the fish hierarchies and mongooses is that we need to assess
individual pay-offs from group living and to consider how grouping can be stable despite
a skew in benefits within the group. In some cases, dominant individuals can control
subordinates by threat (of attack or eviction) or by action (egg eating, infanticide,
interference with mating, aggression). In other cases, the conflict is not resolved and
subordinates fight back, so groups may not be stable. For example, in banded mongooses
there are sometimes mass evictions or departures of subordinates and, hence, large
fluctuations in group size (Cant
et al
., 2010).
Dominants may
not be able to
control
subordinate
reproduction
Group decision making
Local rules and self-organized groups
We now turn from the costs and benefits of grouping to a discussion of the mechanisms
involved in group movements. The complex, coordinated movements of bird flocks
and fish shoals have been a source of wonder to human observers ever since ancient
times. Early philosophers imagined some unseen leader must be controlling the
group, or perhaps coordination was achieved by 'thought transference' or other
mystical powers. Recent work has shown how complex group movements and
leadership can emerge from individuals adopting simple local movement rules
(Couzin & Krause, 2003).
Traffic lanes in humans and ants
Consider a crowd of humans moving in opposite directions along a street or corridor.
When no-one else is nearby, an individual will accelerate to a desired speed and orientate
towards its destination. If it meets another individual head on, it slows down and moves
aside to avoid collision. At first, this occurs frequently but then individuals who begin to
follow others find that they are less likely to have to perform time-wasting manoeuvres
and they, in turn, protect others who follow them. These local rules produce a crowd
which soon self-organizes into lanes; when new individuals arrive, they tend to fall
behind others who move in 'their' direction.
Local rules may
lead to complex
group movements
