Biology Reference
In-Depth Information
Given unparasitized hosts
Given parasitized hosts
0.6
0.5
0.4
0.3
0.2
0.1
0.0
No other
female
Female who
couldn't lay eggs
Normal
female
Other female on the patch
Fig. 10.5
The mechanism of sex ratio adjustment in the parasitoid wasp
N. vitripennis
.
Females produced a less female-biased sex ratio in response to the presence of
previously parasitized hosts and the presence of other females (photograph shows
the red eye mutant used to follow the behaviour of individuals). From Shuker and West
(2004). Photo © David Shuker and Stuart West.
had laid eggs in each nest, which they then correlated with the sex ratio of those emerging
wasps. They found a very similar pattern to Werren, with the offspring sex ratio being less
female biased in host pupae where the larger numbers of females had laid eggs.
How perfectly should we expect animals to behave? Research on LMC has proved
extremely useful for addressing this question, because theory is able to make relatively
clear predictions about how individuals should behave. One factor is the ability of
individuals to process relevant information about the environment. Within the
context of LMC, a key factor is how do individuals assess the number of females
laying eggs on a patch, and hence the extent of LMC. Shuker and West (2004)
investigated this in
N. vitripennis,
examining the relative importance of direct cues
from the presence of other females, and indirect cues from eggs laid by other females.
They were able to separate these factors by using females that could not lay eggs
because they had cut their ovipositors off, and they followed the behaviour of
individuals by using eye colour mutants. Their results showed that the shift in
offspring sex ratios with increasing number of females on a patch is primarily caused
by the presence of eggs laid by other females, and to a lesser extent by the presence of
other females (Fig. 10.5). Consequently, we should expect variation in behaviour
between females depending upon when they arrive on a patch, relative to other
females, and over time, as more eggs are laid. This shows how the answers to 'how'
(proximate) questions can help us to better understand the answers to 'why'
(ultimate) questions.
Females can
assess the number
of other females
laying eggs on a
patch by directly
observing them,
or through
indirect cues of
their presence,
such as recently
laid eggs
