Cannibalism in a Chrysomelid beetle, Gastrophysa viridula.
Cannibalism is a very common phenomenon throughout the animal
kingdom. Females of the green dock leaf beetle, Gastrophysa viridula, lay
clutches of eggs on the leaves of dock plants (Rumex species). Cannibalism
takes the form of larvae eating unhatched conspecific eggs. Both within-clutch
and inter-clutch cannibalism were observed. Within-clutch cannibalism was
found to be of very limited extent. However, two observations indicated that
cannibalism in G. viridula is more important than this finding might suggest.
First instar larvae proved to be much more voracious cannibals when given an
unlimited supply of newly laid eggs than when in normally hatching clutches.
Also, viable as well as non-viable eggs were eaten from within clutches.
The individual fitness benefits gained through cannibalism were studied.
Larvae fed on eggs alone were unable to complete the first instar. But larvae
given young eggs during their first instar along with dock leaves throughout all
three instars did better in terms of survival and developmental rates than larvae
fed only on dock leaves. No relationship was found between adult female size
and lifetime egg production or longevity. Females were found to lay
consecutively larger and smaller clutches throughout most of their lives.
Mathematical models were used to interpret the results of a sperm
competition experiment. The results were consistent with two mechanisms of
sperm competition: linear transfer of sperm with time from male to female and
sperm removal or repositioning with sperm mixing, or diminishing transfer of
sperm with time and sperm mixing without displacement. The individual
benefits of cannibalism in terms of survival observed in the laboratory were
sufficient to satisfy Hamilton's inclusive fitness criterion for the evolution of
selfish behaviour, even for the case of larvae eating full-siblings.
It was anticipated that mechanisms may have evolved to reduce the risks of
inter-clutch cannibalism. The distance from previously laid clutches that
females lay their eggs was found to be important in this respect. and an intrinsic
protective mechanism against cannibalism which becomes more effective with
age was found. A possible link between this mechanism and chemical defence
normally assumed to have evolved as a mechanism against inter-specific
predation is suggested. It was found that the relative concentration of total oleic
acid increased with age in G. viridula eggs, whereas the relative concentrations
of all other major fatty acids declined or remained constant. The possible
anti-cannibalistic role of oleic acid merits further investigation.
The relationships between clutch size, asynchrony of hatching and the
extent of within-clutch cannibalism were also studied. Asynchrony of hatching
was found to increase with clutch size. Although negative relationships were
found between hatching success of viable eggs and both clutch size and
asynchrony of hatching, multiple regression showed that clutch size was the
primary factor affecting the hatching success rate of viable eggs.
An ability to distinguish between related and non-related eggs as victims
was not found among first instar larvae. This finding is discussed in the light of
individual and kin selection and the probability of encountering related eggs.