Tails and toxins: Exploring life history traits and predator-induced defenses in Eastern Red-spotted Newts (Notophthalmus viridescens viridescens)

Open Access
Author:
Brossman, Kelly Helene
Graduate Program:
Ecology
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 26, 2013
Committee Members:
  • Tracy Lee Langkilde, Thesis Advisor
  • Victoria Anne Braithwaite Read, Thesis Advisor
  • Robert Brooks, Thesis Advisor
  • James Harold Marden, Thesis Advisor
Keywords:
  • Eastern Red-spotted Newt
  • life history trait
  • locomotion
  • predator-induced defenses
  • stress
  • tetrodotoxin
Abstract:
Environments can be dynamic due to fluctuations in localized biotic and abiotic factors. Some environmental changes occur rapidly, which result in organisms having traits that are mismatched with current conditions, incurring costs. To cope with changing conditions, many organisms exhibit phenotypic flexibility, or the ability to alter phenotypes given the same genotype. In doing so, organisms benefit by altering their phenotype to keep up with their environment. Such flexibility of traits helps to ameliorate costs associated with maintaining or producing traits that are not optimal to current conditions. Environmental variation can be predictable, such as changes that occur seasonally. Animals that utilize different habitats during the breeding and non-breeding season, for example, need to adjust to predictable changes in their environment. Trait flexibility can help these organisms improve their performance in these different environments. If traits carry-over between environments, however, traits providing benefits in one setting might impose costs in the other setting. Other environmental factors can vary unpredictably over short time scales, such as predation threat; an important selective pressure that can shape individuals, populations, and communities. Inducing predator defenses (e.g. chemical or morphological defenses) only when a predator is present can minimize costs of production and maintenance for these traits. Behavioral defenses, such as refuge seeking and alarm signaling, are often driven by an organism’s physiological response to stress. The role of “stress” (including the production of glucocorticoid stress hormones) in driving morphological and chemical responses to predators is poorly understood, and will provide important insight into predator-prey interactions. Here, I explored the flexibility of traits in Eastern Red-spotted Newts (Notophthalmus viridescens viridescens). In Chapter 1, I examine whether tail size in adult newts carries-over from the aquatic (breeding) to the terrestrial (non-breeding) phase, and whether this imposes costs to locomotor performance across environments. I found that tail size did carry-over – newts with larger tails in the aquatic phase also had larger tails in the terrestrial phase. Larger tails were beneficial in the aquatic phase as they increased swim speed, but did not impede locomotion in the terrestrial phase. In Chapter 2 and 3, I investigate predator-induced morphological (tail size) and chemical defenses (tetrodotoxin, TTX) in larvae and adult Eastern Red-spotted Newts. Larval newts increased tail size in the presence of a predator cue, but did not alter chemical defenses. By contrast, adults showed evidence of chemical but not morphological defense flexibility during the course of the trials. To evaluate the relationships between the stress hormone, corticosterone (CORT), and the production of defensive traits, I validated a water-borne hormone collection technique for adults. A positive relationship between CORT levels and TTX was found, but no relationship was identified between CORT and tail size. Overall, the results of this research reveal the flexibility of traits in shifting environments. My work has revealed that flexible traits may carry-over between habitats, and further research examining potential costs of this would be informative for understanding the evolution and maintenance of these traits. I found that an organism’s morphological and chemical defenses can be modified, and the degree of flexibility can vary seasonally and across developmental stage. Flexibility of traits can be triggered by predator presence (morphological traits) and may be linked to stress hormone concentrations (chemical defenses). Further exploration into the factors regulating flexible traits will shed light on the evolution of complex life histories and anti-predator defenses.