Year of Award


Document Type

Dissertation - Campus Access Only

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Fish and Wildlife Biology

Department or School/College

College of Forestry and Conservation

Committee Chair

Thomas E. Martin

Commitee Members

Michael S. Mitchell, H. Arthur Woods, Bret W. Tobalske, Erick Greene


University of Montana


Age-specific mortality is a dominant driver in the evolution of life history strategies and reproductive effort. Species with low adult mortality probability, such as those commonly found in the tropics, are expected to prioritize themselves over their offspring which is manifested in their relatively low parental effort. Indeed, if we take tropical birds as an example, we can observe that parents do not devote effort to their young to the best of their ability. I found that one species decreased effort in food provisioning when the brood decreased in size, and I showed experimentally that provisioning was not limited by adult quality or food availability. Instead, this species with low mortality, may be reducing effort to enhance chances of future reproduction. Thus, mortality selection during the adult stage can influence parental effort. But selection on adult mortality can also interact with offspring mortality risk to affect reproductive success.

When offspring predation risk increases, parents may reduce parental effort in order to decrease the probability of actual predation. Yet, these reduction in parental effort can have negative consequences for reproductive success. Theory suggests that the degree of these reductions and their associated demographic costs will be greater for species with low adult mortality, such as those in the tropics. I performed an experiment increasing perceived offspring predation risk and found evidence for an alternative. When perceived offspring predation risk increased, some tropical species maintained parental effort and incurred less of a demographic cost than temperate species. Parental effort of tropical species may already be at a minimum and further reductions may not be favored by selection.

When changes in predation risk are directed towards adults instead of offspring, species with low adult mortality (i.e. long lives) are expected favor their own survival over that of their offspring. I tested this idea experimentally across diverse species and latitudes using direct measures of adult mortality probability. I found that species with lower adult mortality were more risk averse, favoring self-preservation more than temperate species. The influence of adult mortality was observed both within and across latitudes. This experimental result confirms and observational pattern, where adult mortality is positively correlated with parental effort across latitudes.

Lastly, mortality selection can also influence the evolution of offspring strategies. Young of many species produce loud calls to solicit food from their parents, but these calls can be costly as they can inadvertently attract predators. I found that young from nests of species under high nest predation produced calls that have characteristics that makes them harder to locate by predators. Together, the work presented here furthers our understanding of the influence of mortality during different life stages on parental care behaviors and offspring strategies, at both proximate and ultimate levels.

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