When ecologists study populations of animals, they commonly round off the individuality of individuals, treating animals of the same species, sex, and age like identical units. This has practical utility for studies focused on how populations change in size and composition and how they respond to their environment.
Rémi Fay, a student at Université de La Rochelle, in Villiers-en-Bois, France, is interested in the peculiarities that make some animals more successful than others. Unrecognized differences in performance between individuals can sometimes have demographic effects that skew the interpretation of data at the scale of whole populations, if the differences are not due to chance but to an underlying variability in “individual quality.” If, for example, low-quality individuals die young, the population as a whole would appear to gain in performance with age.
In a study published in the Ecological Society of America’s journal Ecological Monographs, Fay and his colleagues pursued the elusive individual quality in a population of wandering albatross, a singular bird possessed of several helpful characteristics for distinguishing the influence of individual quality: they live a very long time, grow to adulthood slowly, and breed infrequently, investing all of their parental energy into a single egg every few years.
Fay and colleagues observed that some birds consistently performed better throughout their lives on distinct measures of survival and procreative success. Birds that began reproducing early also had more chicks, and were more likely to successfully rear their chicks to fledge and take flight. These high-performing birds were more likely to live a long time. Failed breeders were more likely to fail again.
The concordance in success in several life history traits, the authors say, is an indication that these individuals have an intrinsic quality that makes them successful, rather than occasional good luck. Quality varied strongly between birds born in different years. This observation suggests that the early-life environment has a powerful, lifelong influence, Fay says. Birds born in warm years, when there is less food, were smaller. Birds born in years of high population density also performed more poorly throughout their lives.
Although fundamental to natural selection, individual quality is notoriously difficult to pin down. Teasing out performance differences due to inherent individual quality from the luck of the environmental…