Predators and parasites
Some parasites manipulate their hosts (who they infect) in some
way in order to be eaten by their end host, who they will reproduce
in. For example, liver flukes reproduce in sheep and cattle, but
use a number of intermediate hosts, such as ants and snails, in
order to reach their final host. However, not all parasites use
this method as some only require a single host to reproduce but if
this host is killed then the parasite will die too.
This is the kind of parasite that I work on. I'm currently in
the third year of my PhD working on the nematode (parasitic worm)
Heterorhabditis bacteriophora which only requires one host
to reproduce. However, this parasite uses a range of defences to
ensure that the host it has infected is not eaten by predators. It
does this in a number of ways: it changes the colour of the host,
with most insect larvae becoming red in colour, it bioluminesces
(glows-in-the-dark), produces a foul-smelling odour and makes the
host taste bad.
What I'm interested in is how this affects different predators'
behaviour and how this is affected by different stages of
infection. In the laboratory at the main university campus I set up
infections using the parasite and soft-bodied, slow moving waxworm
larva. After two days the larvae die and start to turn the red
colour, which gradually builds up and darkens over time.
My experiments at Ness Gardens have examined the effect of
different infection stages, hence differing red colouration, on
predation by birds. Over the past few years I have been training
birds to come down to a set of trays to feed at regular times which
involved a lot of porridge! I would then place out a range of
infected prey, including uninfected prey (normally white in colour)
and observe the birds for two hours to see which they would take.
My experiments support the idea that the red colouration seems to
act as a warning signal to stop predation as birds would avoid the
infected prey, especially the latter stage, dark red infected prey.
They would tend to only eat the uninfected prey and throw the
infected prey away once they tasted it.
This year I am now trying to see how birds respond to prey which
are already quite dark in colour, such as mealworms, to see how the
prey look once infected i.e. do they still turn red, and do birds
respond to this in the same way as the waxworms. I am therefore
running the same experiment but looking at how birds respond to
infected mealworms at different infection stages, rather than
waxworms. So far the experiment has been running well and is
showing similar results to those with the waxworms but you'll have
to watch this space!
As you walk round near the pine woods, keep an eye out for my
feeding stations and feel free to come and chat to me!!!
Rebecca Jones, PhD student, Evolution, Ecology and
Behaviour, IIB, University of Liverpool