Causes and consequences of individual variation in the plasticity of incubation and embryonic heart rate in the cooperatively breeding chestnut-crowned babbler (Pomatostomus ruficeps)
Cones, Alexandra Guerin
Thesis or dissertation
University of Exeter
Plasticity is a mechanism by which organisms can alter their phenotype to match their current environment. Selection for behavioural (i.e. reversible) plasticity is expected when organisms experience costly environmental variation that can be mitigated by a phenotypic response in behavioural time. However, plasticity is costly and thus, should generally be selected for when the environment is predictably variable in time or space. It is well known that species differ in the presence and strength of plasticity in different traits, as different species (and their traits) are often under different selection pressures. However, not as much is known about how individuals within populations vary in their plasticity, which has important implications for understanding population-level evolutionary responses. Specifically, behavioural plasticity may be a key adaption for traits related to parental care and resulting offspring developmental trajectories. In this thesis, I investigate the causes and consequences of variation in plasticity in one component of parental care, incubation, and one component of offspring development, heart rate, in the chestnut-crowned babbler, a cooperatively breeding bird native to inland south-eastern Australia. In chapter 2, I analyzed the incubation behaviour of wild babbler mothers in order to understand whether they are variably plastic in their incubation schedules, and if so, what may drive that variation. I used temperature data from gauges placed in wild nests to determine incubation schedules. I found that incubation was indeed plastic in response to abiotic factors, namely temperature and wind speed, and increased with increased helper number. I found significant individual variation in the plasticity of incubation within our population and found that helper number may be a determinant of female responses to temperature variation. In chapter 3, I investigated whether individual embryos differ in the plasticity of their heart rate in response to temperature. As helper number influences developmental environment by affecting incubation, I predicted that helper number would correlate with embryonic plasticity in heart rate. I found that embryos from groups with more helpers were more plastic in their heart rates in response to temperature than those from groups with fewer helpers. This reduced plasticity in embryos from groups with fewer helpers is likely to be adaptive: embryos from groups with fewer helpers are prone to experiencing lower temperatures more frequently, which, all else being equal, will prolong the pre-hatching period. Together, my studies show that plasticity of multiple traits varies amongst individuals in our population and that such variation could potentially be adaptive. These results help to explain significant variation in both incubation schedules and offspring developmental rates, with important evolutionary and ecological implications. For example, assuming a heritable genetic component to plasticity, this population of babblers appear capable of responding in real time to environmental changes with positive repercussions for population resilience in their arid habitat.
Russell, Andrew F.
Masters by Research in Biological Sciences