hummingbirds & the high cost of s*x

One of the nice things about reading books by great science writers is that I just know I’m going to learn lots. I’ve just got back into Nick Lane’s latest book Life Ascending (it’s been my lunchtime reading at work & recently other things have intruded…). Lane has a lovely lyrical way of writing that I really enjoy, & I thought I’d share a couple of paragraphs with you. They’re from the chapter titled Sex: the greatest lottery on Earth. in which he’s discussing how & why sexual reproduction evolved. At the heart of this excerpt is the idea that the biological costs of sex are quite hard to measure, but can be very high. The example here is hummingbirds – the go-betweens for sex in many species of tropical plant.

Rooted to the spot, plants are the most implausible of sexual organisms, yet the overwhelming majority of them are exactly that; only dandelions, along with a handful of other species, cock a snook at sex. The rest find a way, the most spectacular being the exquisite beauty of flowering plants, which swept through the world some 80 million years ago, turning the dull green forests into the magical painted glades we know today. Although they first evolved in the late Jurassic, perhaps 160 million years ago, their global takeover was long delayed, and ultimately tied to the rise of insect pollinators like bees. Flowers are pure cost to a plant. They must attract pollinators with their flamboyant colours and shapes, produce sweet nectar to make such visits worthwhile (nectar is a quarter sugar by weight), and distribute themselves with finesse – not too close (or inbreeding makes sex pointless). Having settled on a pollinator of choice, the flower and pollinator evolve in tandem, each imposing costs and benefits on the other. And no cost is more extreme than that paid by a tiny hummingbird for the static sex life of plants.

The hummingbird must be tiny, for no larger bird could hover motionless over the deep throat of a flower, its wings beating at 50 beats a second. The combination of tiny size and colossal metabolic rate needed to hover at all means that hummingbirds must refuel almost incessantly. They extract more than half their own weight in nectar every day, visiting hundreds of flowers. If forced to stop feeding for long (more than a couple of hours), they fall unconscious into a coma-like torpor: their heart rate and breathing plunge to a fraction of that in normal sleep, while their core temperature goes into free fall. They have been seduced by the enchanted potions of plants into a life of bondage, moving relentlessly from flower to flower, distributing pollen, or collapsing into a coma and quite possibly dying.

With costs like that, the benefits of sex have got to be significant. That, and the evolution of this complex practice, occupy the rest of the chapter. I am so enjoying this book 🙂

N.Lane (2009) Life Ascending: the ten great inventions of evolution. Norton.

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