快猫短视频

THE brilliant red and orange hues of autumn leaves may be a warning to insect
pests to keep away, rather than just a happy consequence of ageing foliage.

Autumn leaf colours vary enormously, both within one type of tree and across
species. It was assumed that the colours were merely a by-product of temperature
changes breaking down chlorophyll in dying leaves. But the late Bill Hamilton, a
biologist from Oxford University, wondered if the flamboyant colours might be a
deliberate signal. Perhaps the most brightly coloured trees were putting on a
costly show to ward off pests.

鈥淟ike many of Hamilton鈥檚 hypotheses, one鈥檚 initial reaction is that it must
be crazy. But then you find yourself thinking . . . we don鈥檛 have any other
explanation for autumnal coloration,鈥� says Ian Owens of Imperial College in
Ascot, Berkshire.

Hamilton and his colleague Sam Brown, now at the University of Montpellier II
in France, decided to investigate. The most colourful species, they predicted,
should also be most prone to aphids鈥攑ests that usually prefer green over
yellows and reds. These trees would have most to gain by signalling loud and
clear that they would fight off their enemies.

Using field guides, they coded colours for 262 species of temperate deciduous
trees. Then they consulted an authoritative book on aphids, which are a serious
tree pest.

Brown reckons the bright colour is an 鈥渉onest signal鈥� to pests that the tree
will fight any infestation鈥攆or instance, by producing more chemical
defences or thickening its leaves. The heavier an infestation is likely to be,
the more the tree has to gain by signalling its intentions.

And that鈥檚 what they found. The brightest trees were attacked by the most
species of 鈥渟pecialist鈥� pests. Sugar maples, renowned for their luminous red
leaves, were the top target, with seven species of specialist aphids pestering
them. Brown hopes someone will now look at the effect within species. If the
hypothesis holds, the brightest individuals will fend off more pests than their
less showy neighbours.