Biology Reference
In-Depth Information
4
2
0
-2
0
0.5
1.0
Extent eusociality contrasts
Fig. 13.8
Fortress defence in sponge-dwelling shrimps. Plotted are the phylogentically
independent comparisons in relative abundance against the independent comparisons
in the extent of sociality, using data from 20 species. From Duffy and Macdonald (2010).
With permission of the Royal Society. Photograph shows non-breeding workers in the
shrimp
Synalpheus regalis
. Photo © Emmett Duffy.
appears to be intense, and all species are territorial and equipped with a fighting claws
that is used in communication and combat. When intruding individuals attempt to enter
an already occupied sponge, the local shrimps suddenly begin cracking their claws in
unison, producing a distinctive crackling noise that lasts for tens of seconds (Tòth & Duffy,
2004). This 'coordinated snapping' appears to be used to warn intruders away and helps
explain why these species are sometimes called 'snapping shrimp'. If fortress defence
plays an important role in favouring cooperation amongst sponge-dwelling shrimps,
then we can predict that eusociality would enhances the ability to acquire, defend and
retain limited host resources relative to less social species. Duffy and Macdonald (2010)
tested this prediction, examining how shrimp abundance correlated with sociality among
species in Belize. Consistent with a role of fortress defence, they found that eusocial
species are more abundant, occupy more sponges and have broader host ranges than
non-social sister species (Fig. 13.8). Ideally, this hypothesis would also be tested further
with experiments that manipulated host availability or shrimp density.
Experimental evidence for a role of fortress defence comes from William Foster's work
on the gall-forming aphid,
Pemphigus spyrothecae
. In the eusocial aphids, there is a soldier
caste that defends the colony against insect predators, such as ladybird and hoverfly
larvae (Fig. 13.1e; Stern & Foster, 1996). Foster (1990) manipulated the composition of
aphids in galls so that they contained either ten soldiers or ten non-soldiers, and then
introduced a single predator. The results were striking - in colonies with soldiers, the
predators were usually killed, with the loss of a few soldiers, whereas in colonies with
non-soldiers the predators survived and all the aphids were usually killed and eaten. It
has sometimes been suggested that the gall-forming aphids might have been predisposed
to eusociality because they reproduce clonally, so relatedness to siblings is given by
r
Competition for
sponges is intense
in sponge-
dwelling shrimps
…
… and eusocial
species appear to
be better at
acquiring and
keeping sponges
Soldier aphids kill
insect predators,
preventing the
entire colony from
being consumed
1.
However, the key point here is that a potential worker would be equally related to its
siblings or its own offspring, so again what is required is
B
/
C
> 1, as with monogamy.
=
