Biology Reference
In-Depth Information
110
D
100
C
90
R
80
B
70
50
Hours to death from starvation
40
30
20
10
0
Fig. 12.7
In vampire bats, weight loss after feeding follows a negative exponential
decline, with death from starvation occurring at 75 per cent of pre-fed weight at dusk.
Therefore a donation of 5 per cent of pre-fed weight when at weight D should cause a
donor to lose C hours but will provide B hours to a recipient at weight R. From Wilkinson
(1984). Reprinted with permission from the Nature Publishing Group. Photo © Deitmer
Nill/naturepl.com
death. However, this same amount can bring an enormous benefit to a starving
individual, moving its position considerably to the left along the time axis. The blood
meal, therefore, has little cost to the donor and great benefit to the recipient; indeed,
it may save the recipient's life and enable it to survive until it has the chance to
forage again for itself the next night.
(3)
Individuals adjust their help to others dependent on the extent to which they have helped
them previously - that is they must be able to recognize cooperators and cheats, feeding
those that had fed them and refusing to feed previous recipients who fail to reciprocate
.
Wilkinson carried out some clever experiments in the laboratory where he formed
a group of bats, some individuals (all unrelated) coming from one roost and
others (also unrelated) from another roost. In a series of trials, one bat, chosen at
random, was removed and kept hungry while all the others had access to blood.
The hungry bat was then reintroduced. It was found that 12 of the 13
regurgitations occurred between individuals from the same roost in the field, in
other words individuals which were familiar with each other. Furthermore, the
starved bats which received blood later reciprocated the donation significantly
more often than expected had the exchanges occurred randomly. However, these
experiments just show that individuals are more likely to feed bats that they
usually associate with, which can have alternative explanations (as explained in
the next section). Most importantly, it was not shown that individuals
preferentially fed those who had previously fed them, or refused to feed those who
had not fed them.
(4)
Cooperation cannot be explained more simply via some mechanism that does not rely
on reciprocation
. While Wilkinson's results are consistent with reciprocity, it is
also possible to come up with a number of simpler explanations. One possibility
… and the
benefits far
outweigh the cost
…
… but there is no
evidence that
cooperation is
preferentially
directed at more
helpful
individuals …
