For social animals, status is extremely important. The highest-ranking individuals in a social group will enjoy a myriad of benefits, such as better mating success, more food resources, and better access to shelters. However, if group members are constantly bickering over social status, it wastes time and energy for everyone in the social group and risks unnecessary injury. In established groups where individuals interact with one another frequently, it is more efficient for animals to communicate their social status through a signal, rather than constantly fighting to establish who is the strongest.
Animals often communicate their rank through physical traits called badges of status. These status symbols can be brightly coloured feathers on a bird, or big and powerful claws on a crab. Badges of status are often costly to produce and maintain and so these signals must be honest. For example, only the strongest deer can grow large and elaborate antlers, and so antlers are an honest signal of quality. However, other badges of status are not necessarily linked to some costly physiological process. In these cases, what keeps animals honest? How do animals know what status signal to produce?
Cody Dey and his colleagues from McMaster University, Canada, and Massey University, New Zealand, decided to investigate social status signaling in the pukeko (Porphyrio porphyrio melanotus), a bird with a complex social system. Pukeko are cooperative breeders, and multiple males and females live together as a family unit. These birds have strict social hierarchies. While all of the birds in the group defend the territory and take care of the offspring, only the top-ranking birds reproduce.
These blue and black birds also have distinctive red bills and red facial shields. The researchers hypothesized that this bright red facial shield might be a badge of status. First, the scientists measured shield size in a large number of wild pukeko, and watched the birds to determine their position within their social hierarchies. They found that shield size was highly correlated with rank, even when controlling for body size and sex.
The scientists then took the study a step further. If the red facial shields are badges of status, how is honesty maintained? How do pukeko know what size their red shield ought to be? To answer these more complex questions, the researchers cosmetically altered the size of the red frontal shields in some of the pukeko. They watched the birds within their social groups, and after a week, they recaptured the same birds and re-measured their true shields.
The scientists found that birds with cosmetically reduced shields were treated as more subordinate by other group members, and were aggressively challenged more often. And after a week of such treatment by their social group, the birds with cosmetically reduced shields had actually reduced the size of their true shields.
This study lends insight into the dynamic nature of status signaling in animal societies. Goethe may have been more right than he thought when he said, ‘If you treat an individual as he is, he will stay as he is; but if you treat him as if he were what he ought to be, he will become what he ought to be.’ This study suggests that a status signal not only reflects an animal's intrinsic strength or health but is also influenced by how the animal is treated by other members of the social group.