Murdoch McAllister, Bernard Roitberg and Laurence Weldon of Simon Fraser
University in Canada have shown that pea aphids, Acyrothosiphon pisum, infected
with the eggs or larvae of a parasitic wasp, Aphidius ervi, opt for a deliberately
risky escape strategy if they are confronted by a predatory ladybird (Animal
Behaviour, vol 40, p 167). Aphids carrying the parasite are likely to drop
off the plant on to the ground, where the chances of death because of heat
stress and desiccation are higher than the chances of being eaten on the
plant. The aphid tends to choose a response that is potentially suicidal.
And if it dies, the parasite dies too.

If an infected aphid can still reproduce, however, it seems to be less
likely to choose the risky option. An aphid that picks up the parasite early
in life is more likely to opt for the suicidal escape than one infected
later. The researchers suggest that this is because aphids that gain parasites
early on are unlikely to produce any young before the parasite kills them.
Those that acquire eggs or larvae later in their life cycles can still produce
young before they die. These aphids risk losing potential offspring if they
choose the likely suicide.

The researchers have also shown that infected aphids living in cooler
habitats do not show an increased tendency to drop off the plant when faced
with a hungry ladybird. In these conditions, dropping off the plant is no
more risky than staying near the predator.

Stella Brewer and Bill McGrew of the University of Stirling have studied
wild chimpanzees (Pan troglodytes) in the Gambia. Their observations are
the first of a chimpanzee actually using a ‘tool set’ (Folia Primatologica,
vol 54, p 100).

Brewer and McGrew observed a chimpanzee named Katie who was trying to
gain access to a source of honey. Katie used four different tools in all.
The first was a large branch with a sharp end. She used this as a ‘chisel’
to jab at and crumble the strong waxy covering of the bees’ nest. Katie
then discarded this tool in favour of a shorter, thinner stick, which was
more sharply pointed. This she used to chisel deeper and more accurately
into the hole she had made.

Next, Katie took up a third tool – a green branch, which was about a
centimetre in diameter and which she had trimmed to a length of about 30
centimetres. She inserted this into the hole and then pushed it with considerable
force to puncture the nest’s seal. Finally, she dipped a thin green vine
through the hole and used this to extract quantities of dripping honey from
the nest.

Zoologists have managed to induce such a ‘sequential’ use of tools –
in which the use of each tool depends on the action of its predecessor –
in chimpanzees in captivity. But Brewer and McGrew are the first to report
such a ‘tool set’ being used by a wild chimpanzee.