If you’d like to live a long life, it might be time to practice using your non-preferred hand or foot, at least if you identify with game birds. Some pheasants always use the same leg when undertaking certain tasks, while others are more even-footed, and which category a bird falls into plays a big part in its life expectancy in the wild. The applicability to humans is questionable, but it may explain why more animals don’t show a preference for one side of their body.
Humans are not the only animals to use limbs on one side more than the other for certain tasks, a tendency known as lateralization. "Lateralisation is common in nature, with animals ranging from parrots and chimpanzees to bees and nematode worms showing evidence of favouring one side for specific tasks," said Dr Mark Whiteside, of the University of Exeter, in a statement.
Whiteside led a study, published in Scientific Reports, which looked at 103 captive-bred pheasants released into the wilds of Devon in England at 10 weeks old. Being a young pheasant is a dangerous business, and barely half the birds were still alive 223 days later. Survival rates were greatly shaped by lateralization. Among birds that strongly favored one foot when stepping over obstacles at 16 days old, less than 45 percent were still alive at the end of the study. The figure was 60 percent among those that used both feet equally.
The finding contrasts with other animal studies, which have found benefits to lateralization. “Chimpanzees that consistently favour a particular hand, regardless [of] whether it is the right or the left, are better at fishing for termites,” Whiteside said. “Likewise, strongly lateralised chickens are better able to detect food and model predators compared to their weakly lateralised companions.”
Lateralized male sage-grouse even manage to put on better mating displays. However, previous studies have been limited to the effects of lateralization on specific tasks, not survival.
Whiteside admitted the reasons for the pheasants' increased death rates are unknown. The paper raises several speculative explanations, including that lateralization slows down certain cognitive processes, making the birds more vulnerable in a crisis, or that there is a “U-shaped” effect, where a small amount of lateralization increases survival, but too much is harmful.
The work does, however, hint at why lateralization is not stronger and more widespread. While many animals are lateralized, few show preferences as strong as most humans. If there is a price to pay for lateralization in some circumstances, species will be in a constant evolutionary tug-of-war between the costs and benefits.
In this particular study, most pheasants used each foot almost exactly equally, but 39 showed strong lateralization, with slightly more left-footers than right among this group.