Náttúrufræðingurinn - 1997, Side 25

this century is given. Then the main findings of
G. A. Parker’s great contribution lo the research
on this species is summerized. The rest of the
paper describes the results of the author who
studied size variation in the yellow dungfly,
how it arises and the effect it has on male mating
tactics.
Size variation is mainly caused by larvae
competition for food. Environmental factors
are therefore very inlluential in shaping adult
size, but the role of genetic factors cannot be
ruled out completely as Simmons and Ward
(1991) have shown. Population fluctuations are
marked with spring and autumn peaks in most
countries. In Iceland there is no summer de-
cline and density is very high. Size of males is
greatest in August and September.
Males seem to be able to assess their size
and act accordingly. They are not distributed
randomly around oviposition sites; instead, the
larger males are more likely to search and
copulate on cow pats than in the surrounding
grass, and they are more likely to guard their
females until oviposition is completed. This
distribution is not influenced by pat age, at
least up to 80 min. old droppings.
Analysis of behaviour of individual males re-
vealed that large males spend more time than
small nrales on the pat than in the grass and
increased densily makes larger males more ac-
tive while smaller males are not affected. More
large males caught females than small males in
spite of being less numerous. Density affects
males’ behaviour in many ways; they touch
each other more, make more rnoves, spend
relative more time on the pat than in the grass
and spend more time searching at each
oviposition site. Staytime (searching time) of
individual males was not dependent on size.
Their fighting tactics are shaped by their RHP
and the value of the fernale. Recent research by
Geoffrey Parker and associates has further re-
vealed that copula duration is dependenl on
phenotype and larger males are at an advan-
tage.
It is concluded that larger males are more
successful in gaining females and defending
ihem in fights. The result is that large males
are more likely than the small males to be
paired to large females. Pairing is size
assortative both in the grass and among
ovipositing pairs. The large rnales are clearly at
an advantage in competition for females while
the smaller males play the best of a bad job.
The extensive research that has been carried
out on the reproductive behaviour of this spe-
cies shows how impressive the forces of evolu-
tion can be when shaping the behaviour of dif-
ferent phenotypes.
PÓSTFANG HÖFUNDAR/AuTHORSADDRESS
Hrefna Sigurjónsdóttir
Kennaraháskóla íslands
v. Stakkahlíð,
IS-105 REYKJAVÍK
Netfang/E-mail
hrefnas@khi.is
19