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Stress and weavers in the Kalahari night
By Dominic Cram - University of Exeter
Balancing precariously atop a ladder, I reach into the darkness, slowly lining up my trusty capture-bag with the grass roosting chamber. I pause to contemplate this strange moment, balanced between the endless stars above, and the red Kalahari sands below, and then – STRIKE! The flapping in my capture-bag tells me I've been successful, and I sprint back to the car. From there, my camera’s long exposure tells the story.
The wobbly light-trail of my head-torch enters the frame, revealing that my ‘sprint’ is now a feeble jog. At the back of our 4×4, I inspect the indignant bird. “Who have we here? Oh – blue/lilac/metal/black.” An infamously ferocious male. My head-torch paints a frenetic scribble as the bird mauls my knuckles.
Our 4×4 is, in truth, more of a mobile laboratory in the desert. At the back, I measure the bird, the tools of the trade laid out before me. Next, I take a small blood sample. Within moments, the centrifuge in the foot-well whirrs into action, and the blood samples are ready for state-of-the-art analysis back at the University of Exeter.
My NERC-funded PhD took me to the Kalahari Desert in South Africa, to study the rather dull-looking white-browed sparrow weaver. However, what they lack in flamboyance, they more than make up for in fascinating behaviour: these birds breed cooperatively, with a number of ‘helpers’ caring for the dominant pair’s nestlings. Who benefits from this peculiar strategy? Do dominants with helpers enjoy a life of leisure, allowing them to remain healthy and breed more?
My PhD examines these questions by investigating the impact of cooperative breeding on a particular physiological component in the weavers - oxidative stress. Oxygen free radicals are produced as a by-product of respiration, and under normal conditions their harmful effects are minimized by antioxidant defences. However, increased activity – such as frenzied feeding of ever-hungry nestlings – risks over-powering antioxidant protection, allowing free radicals to cause cellular damage, reduced reproductive success and shortened lifespan. So, does reproduction promote oxidative stress in our weavers? Do helpers reduce dominants’ oxidative burden? My research aims to find out.
After a final check-up, I return the bird to his Acacia tree. In the darkness, I replace him in his roosting chamber, to peacefully pass the rest of the night, unaware of his contribution to our understanding of oxidative stress physiology and the evolution of cooperation.
Our 4×4 mobile laboratory in the South African Kalahari desert. At the rear of the vehicle, I measure and blood-sample a ferocious weaver, next to my trusty capture-bag (a pillow-case on a stick). At the front, a car-battery powered centrifuge whirrs, and my assistant busily processes blood samples.