Biology Reference
In-Depth Information
(a)
(b)
10
10
5
5
0
0
0
1
2
3
4 +
0
1
2
3
4 +6+
Post-removal group size
Pre-removal group size
Fig. 13.7
Life insurance in the tropical hover wasp,
L. flavolineata
. The relationship between the number of
small brood developing successfully to become large larvae plotted against (a) the post removal group size and
(b) the pre-removal group size. Data are shown for control (dark blue) and removal (light blue) nests. From Field
et al
. (2000). Reprinted with permission from the Nature Publishing Group. Photograph of a female marked with
dots of paint. Photo © Maurizio Casiraghi.
such potential problems, Field
et al
. (2000) experimentally removed helpers from nests
of the tropical hover wasp
Liostenogaster flavolineata
. They found that the removal of
helpers reduced the number of small larvae that were reared to a large size during the
observation period, but that groups with more helpers post-removal still reared more
larvae (Fig. 13.7). Overall their data suggest that a female could increase the number
of offspring successfully raised by 2.4-fold as a result of helping rather than breeding
independently, which is again substantially greater than 1.0.
Experimental
removal of hover
wasp helpers
shows that life
assurance
benefits could be
enough to favour
the evolution of
eusociality
The benefits of fortress defence
Eusociality could also have been favoured by the potential benefit of staying at the natal
nest and helping defend a valuable resource. This advantage of 'fortress defence' is likely
to be important in species which live in protected, expandable sites, where food is
obtained, such as the wood galleries of termites, the plant galls of social aphids and
thrips, the galleries of ambrosia beetles and the sponges of sponge-dwelling shrimps.
Remaining to help at the natal 'fortress' avoids the risk of death associated with migration
and, because food is available locally, little feeding care is required, which allows the first
worker specialists to generally be soldiers, specialized for defence (Figs. 13.1 & 13.2).
Emmett Duffy and colleagues tested the advantages of fortress defence across sponge-
dwelling shrimps in the genus
Synalpheus
. Each species within this genus appears to be
specialized to live within, and consume, one or a few species of sponge. Within the sponge,
individuals live in social groups, the nature of which ranges across species from
heterosexual pairs, to groups with multiple breeders, to eusocial colonies that contain a
single queen and more than 300 sterile workers. Because few predators can enter the
narrow canals of the sponges, the greatest competition for resources appears to come
from individuals of the same or closely related species. This competition for territories
Having a
defendable source
of food can
favour
cooperation
