And now for something completely different: Small bees with big tongues.
Ecological fieldwork consists of fascinating questions answered with excruciating amounts of work. Take this study, by Brendan Borrell of the University of California, Berkeley. His aim: to understand nectar drinking in orchid bees ranging in size from teensy (50 mg) to just pretty small (900 mg).
Orchid bees, we learn, drink nectar by sucking it up through a straw-shaped proboscis. That makes them a breed apart from bumblebees, which lap the stuff up like dogs. The straw method works nicely over short lengths but, as anyone who has ever tried to get a jumbo Wendy’s Frostie started knows, it runs into trouble when the straw is long and the fluid is viscous (as with super-sweet varieties of nectar). The American Naturalist article puts it like this (I love it):
In suction feeding, resistance to fluid flow increases with proboscis length, but in capillary-based lapping, the amount of nectar that can be extracted per lap is proportional to the surface area of the tongue (Harder 1983a, 1986; Kingsolver and Daniel 1983, 1995). As predicted by the suction-feeding model, the relationship between energy intake rate and proboscis length was less than the direct proportionality predicted by a simple analysis of the Hagen-Poiseuille equation.
Anyway, learning the details required catching 750 bees of 32 species, getting them to stick out their tongues to be measured, weighing them before and after they fed from a “standard” nectar intake rate measurer, then dissecting them to measure individual body parts.
At the heart of the matter seems to be the abundance of small orchid bees (<100 mg) with very long tongues (up to 25 mm!). How does such a wisp of a bee work up the suction? And wouldn’t it be better off visiting shorter flowers with a shorter tongue? And, come to think of it, isn’t nectar supposed to be a reward for visiting? Shouldn’t the plant make it easier to visit, not harder?
Well, Borrell, suggests, from the bee’s point of view the answer is competition: long-tongued bees have the hassle of unspooling, slurping through and re-coiling that garden hose at each visit, but the deep flowers they visit are less likely to have been sucked dry.
The plant has its own interesting perspective. For widely dispersed plants, what good is attracting pipsqueak bees? They won’t have the stamina to make it over to the next individual. They want to attract large, athletic bees from the other end of the meadow. They do that by offering especially sweet nectar and hiding it away in deep flowers. Around those kinds of plants, if you’re a small bee with a long tongue, you’ve got it made.
But waiting at the end of all this, like a punch line, is the revelation that the flowers feeding these bees may not even be orchids. Many orchids don’t produce nectar, and orchid bees, bless their hearts, are their eternal dupes. The flowers lure the male bees (only the males) with strange scents including vanilla. As the glossy insects plunge in head-first, they get tagged with a sticky basket of pollen and sent on their way. The bees do collect some of the perfume, but no one’s completely sure what they use it for.
Read more about orchids and bees of all sorts from the master himself: Darwin wrote 300 pages on The Various Contrivances by which Orchids Are Fertilized by Insects. But it’s not for the faint of heart. From the introduction:
An examination of their [orchids'] many beautiful contrivances will exalt the whole vegetable kingdom in most persons’ estimation. I fear, however, that the necessary details are too minute and complex for any one who has not a strong taste for Natural History.
Image: N. Quiring