A Poke in the Eye from a Teasel

The teasels Dipsacus fullonum in my garden are beginning to shed ripe seeds, so I’m expecting the local goldfinches to show up any day now. This species’ liking for teasel seeds is legendary and I read recently that it’s only the males, which have more slender beaks than females, that can reach the seeds (see http://www.arkive.org/goldfinch/carduelis-carduelis/info.html). Can this really be true? If so, why does this sexual difference in goldfinch beak shape exist? The forest of spines on the teasel seed heads is surely a defence mechanism that has evolved to keep hungry birds at bay, by poking them in the eye, so it might be expected that natural selection, which operates to preserve features that enhance the fitness (reproductive potential) of a species would ultimately favour the evolution of teasel heads with longer spines, which would force even the slender-billed male goldfinches to keep their distance. Interestingly, plant breeders at the International Centre for Research in the Semi-Arid Tropics in India have been breeding pearl millet varieties with extra-long bristles in their seed heads for just this purpose – for keeping flocks of finches at bay by irritating their eyes and beaks when they try to eat the grain (see http://www.new-ag.info/98-1/focuson.html)

Killer Teasels?

Over a century ago there was considerable debate as to whether the teasel Dipsacus fullonum was a carnivorous plant. It’s not difficult to see why – the bases of the leaves clasp the stem to form a large cup that fills with rainwater and this soon fills with drowned insects. The criteria for a plant being truly carnivorous are that it must lure its prey, trap it, kill it, digest the victim and absorb the nutrients that it releases. Teasel fails on the fourth of these criteria and probably the fifth too – it doesn’t secrete any enzymes of its own to digest the prey and there’s no clear evidence that it absorbs any nutrients from the soup of rotting flies in its drowning pools. But it can certainly lure and trap its prey, in much the same way as a pitcher plant. This bumblebee was presumably attracted by the water in the leaf bases and as soon as it landed it slipped into the water. From that moment on it was doomed. It struggled to climb out but its wet feet slipped on the shiny, smooth surface of the stem and leaf bases. I thought about rescuing it but within less than half a minute it had fallen back in, become totally saturated and had drowned. The teasel leaf bases soon fill with dead insects and must also have a rich population of bacteria and fungi that can digest them, so maybe it’s time for another look at the eating habits of this plant, to check again at whether it derives any nutritional benefits from its decaying captures. Birds certainly do – I’ve sometimes seem blue tits and great tits raiding this gruesome larder.