Biology Reference
In-Depth Information
Single-nodule experiment
10
N
2
:O
2
Ar:O
2
8
6
4
2
0
Per nodule
mass
Per nodule
Fig. 12.10
Rhizobia which were able to fix nitrogen showed greater growth than
rhizobia who were prevented from fixing. From Kiers
et al
. (2003). Reprinted with
permission from the Nature Publishing Group. Photograph shows a split root experiment,
where one half of the root system was supplied with air and the other was supplied with
a gas mixture where nitrogen had been replaced with argon. Photo © Ford Denison.
previously produced on the territory where they help, so it was assumed that they would
often be helping raise full siblings to whom they would be relatively highly related
(
r
Helping can be
favoured due
to a lack of
opportunity
to breed
independently
0.5). A role of breeding territory availability was suggested by observations following
a conservation programme in the 1970s that led to a spectacular rise in bird numbers
on Cousin Island, from 26 to approximately 300 birds. During this time, as the
population increased, a number of individuals became helpers at the nest, rather than
attempt independent breeding. One way of thinking about this is that habitat saturation
prevents individuals from breeding, so drastically reduces the cost (
C
) of helping -
helping relatives can then be favoured to make the best of a bad situation (as with long-
tailed tits). Komdeur (1992) confirmed the role of habitat saturation experimentally by
removing 58 birds from Cousin to the previously unoccupied Aride and Cousine islands.
As predicted by the habitat saturation hypothesis, all the vacancies created on Cousin
Island were filled immediately, some within hours, by helpers from other territories, and
all the birds moved to unsaturated new islands formed independent breeding pairs, with
none acting as helpers.
However, the idea that cooperative breeding in the Sychelles warbler was driven
primarily by kin selection was overturned by a genetic analysis of who breeds and
within-group relatedness, using microsatellite markers (Richardson
et al
., 2002). These
analyses revealed three big surprises. Firstly, 40% of the offspring were sired by males
from other groups, so the chicks being raised are not usually full siblings of the helpers.
On average, the relatedness between helpers and chicks was
r
=
When territories
become available
helpers move
to fill them
and breed
independently
Almost half the
offspring are sired
by males from
other groups
013, making the indirect
benefits of helping much less than previously assumed. Secondly, female helpers often
laid eggs in the nest, producing an average of 0.46 offspring per subordinate per year.
Consequently, female subordinates gain substantial direct benefits from being helpers at
=
