Elevational trends in life histories: revising the pace-of-life framework.

Life-history traits in birds, such as lifespan, age at maturity, and rate of reproduction, vary across environments and in combinations imposed by trade-offs and limitations of physiological mechanisms. A plethora of studies have described the diversity of traits and hypothesized selection pressures shaping components of the survival-reproduction trade-off. Life-history variation appears to fall along a slow-fast continuum, with slow pace characterized by higher investment in survival over reproduction and fast pace characterized by higher investment in reproduction over survival. The Pace-of-Life Syndrome (POLS) is a framework to describe the slow-fast axis of variation in life-history traits and physiological traits. The POLS corresponds to latitudinal gradients, with tropical birds exhibiting a slow pace of life. We examined four possible ways that the traits of high-elevation birds might correspond to the POLS continuum: (i) rapid pace, (ii) tropical slow pace, (iii) novel elevational pace, or (iv) constrained pace. Recent studies reveal that birds breeding at high elevations in temperate zones exhibit a combination of traits creating a unique elevational pace of life with a central trade-off similar to a slow pace but physiological trade-offs more similar to a fast pace. A paucity of studies prevents consideration of the possibility of a constrained pace of life. We propose extending the POLS framework to include trait variation of elevational clines to help to investigate complexity in global geographic patterns.