: It turns out that humans aren't too different from blue-footed boobies, at least when it comes to age and fertility. Researchers have recently discovered that the sperm of blue-footed boobies declines with age. And unlike humans, the blue feet of the boobies also fade with age, revealing that one reason why female boobies tend to mate with brighter-footed males is to ensure the robustness of the sperm and the health of their offspring. "The study provides us with a new way of looking at what lies behind sexual signals," lead author Alberto Velando told TIME, "pointing to the importance of sexual selection in eliminating genetic mutations." How the Heck:
What's the News
The researchers traveled to a breeding blue-footed booby colony in Isla Isabel, Mexico, in 2005, and after capturing over 40 males, massaged their cloacas to collect sperm and used spectrophotometers to measure the brightness of their feet.
Incorporating their information into a database with 20 years of data on the same colony, they discovered that the sperm of the more senescent males (over 10 years old) was more degenerated than the sperm of middle-age males (3-10 years old). As boobies get older, the DNA in their germ cells undergo oxidative stress, which causes lesions that "impede further DNA synthesis."
At the same time, the color of the older males' feet was a duller blue than the younger males, suggesting that "a brighter pigment indicates a level of protection against DNA damage in the male's sperm, leading females to judge the foot colour of their mate to ensure that their progeny will have of lower risk of genetic disorders."
What's the Context:
Not So Fast: The authors concede that the boobies' feet may not change color with age: it's conceivable that colorful males---which have better sperm and are preferred by the females---just happen to die earlier than dull-footed birds, so the population of old boobies has dull feet, making it look like their feet dulled as they aged. The Future Holds: As evolutionary biologist Damian Dowling told COSMOS, "the next step is to find out how important this DNA damage is for genetic variation in nature ... We need to understand how new variations arise in nature and if new mutations are caused by senescence in the germ line." Reference: A. Velando, J. C. Noguera, H. Drummond, R. Torres. "Senescent males carry premutagenic lesions in sperm." Journal of Evolutionary Biology, 2011; 24 (3): 693 DOI: 10.1111/j.1420-9101.2010.02201.x
Known as Sula nebouxii to ornithologists, blue-footed boobies were first analyzed by Charles Darwin during his stint on the Galapagos Islands. These long-winged seabirds can also be found in Peru and on the Pacific coast of Mexico.
Blue-footed boobies are a prime animal to study links between aging and fertility because they live in remote areas, meaning that they're sheltered from many of the hazards that kill other wild animals, and that enough reach old age that researchers can actually compare significantly older boobies with younger ones.
Blue-footed boobies are perhaps best known for their courtship dance (shown in the video above), wherein males strut around to show off their dazzling blue feet to potential mates.
Before this study, scientists assumed that the germ cells of wild animals were by and large protected from DNA-damaging agents because the animals would likely succumb to diseases or predators before reaching old age. As the researchers write in their paper, "this study reports the first evidence of senescence in the germ line of a wild vertebrate."
In the animal world, there are thousands of different physical traits---from color, plumage, and others---that animals use in selecting their mates. Generally, sexually selected traits are associated with health in some way. In ducks, for example, it was recently shown that females tend to mate with males with brighter bills, which tend to have fewer STDs.
Image: flickr / HBarrison