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What causes interindividual variation in fitness? Evidence of heritability of latent individual fitness traits has resparked a debate about the causes of variation in life histories in populations: neutralism versus empirical adaptationism. This debate about the processes underlying observed variation pits neutral stochastic demographic processes against evolutionarily relevant differences among individual fitness traits. Advancing this debate requires careful consideration of differences among inference approaches used by proponents of each hypothesis. Here we draw parallels between several disciplines focusing on processes generating variation in individuals' life-course, and we contrast methodologies to disentangle these processes. We draw on other disciplines to clarify terminology, risks of flawed inference, and expand the panel of hypotheses and formalizations of processes generating variation in life histories. Trends Evidence of heritability of individual fitness traits in wild populations has reopened a debate about the relative contribution of neutral, stochastic demographic processes to observed variations in life histories. There are conceptual differences among published studies documenting heterogeneity in life histories; differences so fundamental that they led to misunderstandings between schools of thought. The question of the processes generating heterogeneity in longitudinal trajectories has stimulated a large body of work in econometrics, political, social and biomedical sciences, which have highlighted risks of flawed inference; these risks have been overlooked in biology. Other disciplines offer useful frameworks for future work on life histories in three areas: terminology, the characterization of the diversity of processes underlying variation in life histories, and the methods of statistical inference to disentangle these processes.

Aspidoscelis neotesselatus (Colorado Checkered Whiptail) is a hybrid-derived triploid parthenogenetic lizard with a natural range overlapping with six counties in southeastern Colorado, USA. It has also become established by anthropogenic causation in Grant County, Washington State, approximately 1,600 km northwest of its range in Colorado. Large parts of its natural range are within military reservations. Reduced genetic variation in all-female species makes them especially susceptible to environmental disturbances, such as military activities. At Fort Carson (FC), we estimated an abundance index via a catch-per-unit estimator, weekly survival using Cormack-Jolly-Seber models, and body condition and clutch size as indicators of population health across three low-impact training areas (TA; 45, 48, and 55). Abundance estimates varied across TAs from a low of 0.99 to a high of 6.12 females per hectare. Body condition only marginally varied by age class and TAs. Apparent monthly survival was relatively low in all areas and even lower at TA 55 than at TA 48 (0.638 versus 0.771); however, the uncertainty around those estimates was large. Results suggest that TA 48 supported a large fraction of reproductive females that were successful in producing eggs, providing further insight into where monitoring and conservation efforts should be concentrated within FC.

Harvest indices are used by state wildlife management agencies to monitor population trends and set harvest quotas for furbearer species. Although harvest indices may be readily collected from hunters, the reliability of harvest indices for monitoring demography and abundance of the harvested species is rarely examined, particularly amongst large carnivores. The overall objective of this study was to assess whether cougar (Puma concolor) harvest statistics collected by wildlife managers were correlated with changes in cougar demography, mainly survival rates and abundance. We estimated key demographic parameters for 2 cougar populations in Utah over 17 years during which we monitored 235 radio-collared cougars. We then compared these demographic parameters to harvest statistics provided by the Utah Division of Wildlife Resources over the same time period for the Oquirrh-Stansbury (lightly harvested population) and Monroe (heavily harvested population) harvest management units. In the Oquirrh-Stansbury unit, the percent of harvested cougars >6 years old was positively correlated with annual survival, indicative of a population experiencing several years of high survival resulting in an older age structure. Percent of permits filled and cougar abundance were also significantly correlated, suggesting higher hunting success with increased density. In the Monroe management unit, the annual percent of permits filled was correlated with changes in overall annual survival and male and female annual survival. Of utmost importance, pursuit success (cougars treed/day) increased with the number of cougars on the unit suggesting that pursuit indices may be an informative metric for wildlife managers to determine cougar population trends. Because both management units were subjected to contrasting mortality regimes, results provided by this assessment could potentially be applied to additional management areas sharing similar ecological characteristics and harvest metrics. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.