Do Sarasota dolphins all have the same “lifestyle?” Do some prefer different prey, or forage in the different habitats, like grass flats as opposed to open bays?
We know a lot about the Sarasota dolphin community, but we actually know little about how individuals “make a living” in their environment. Do individuals specialize by hunting certain fish types, or by preferring certain habitats? And how does this influence their reproductive success over the course of their lives?
These are tough questions to answer, but they are important. If individual dolphin variation influences reproductive success, that might impact the population’s viability, which in turn has conservation implications.
Samuel Rossman, a PhD student in the Zoology department at Michigan State University, will is about to study this interesting problem. He is a recipient of the prestigious National Science Foundation’s Graduate Research Fellowship.
Sam’s dissertation will examine the feeding ecology of individual dolphins.
Not all members of wild animal populations use the same set of resources (habitat, prey items, etc.). In fact a growing body of evidence suggests that some individuals specialize in the use of specific resources to enhance hunting (or foraging) efficiency.
Sam will study the relationship between individual specialization and reproductive success within the Sarasota Bay bottlenose dolphin community.
He will determine which dolphins are specialists by analyzing stable carbon and nitrogen isotopes in dolphin teeth.
Hunting in different habitats (i.e. seagrass or open bay) results in different percentages of stable carbon isotopes within bottlenose dolphin tissue while stable nitrogen isotopes indicate how high in the food chain an individual is or its trophic level.
Sam’s work takes advantage of stable isotopes ratios of yearly layers in bottlenose dolphin teeth to provide repeated indicators of hunting habitat and trophic level. Because of previous work with the Sarasota dolphin community, he will be able to relate the reproductive success of individual dolphins to their hunting habitat and trophic level.
This research is critical as human populations continue to grow rapidly in coastal areas worldwide and accelerate ecosystem change. Understanding if reproductive success is dependent on specialization in foraging will open new perspectives on how the individual variation influences population viability.
The Effect of Intraspecific Foraging Specializations on Reproductive Success
(Adapted by Sam Rossman from his NSF Abstract)
Marine mammals are subject to a myriad of threats resulting from environmental perturbations due to human activity. Understanding the influence of these threats on population viability requires information on a variety of factors such as environmental change, habitat requirements, food resources, and reproductive success. Within a population, habitat use can vary such that certain members will specialize in a subset of possible habitats. How individuals using a habitat particularly impacted by human disturbance is of particular ecological concern. Yet factors that influence individual response such as survivorship are poorly known even among intensively studied populations such as bottlenose dolphins (Tursiops truncatus) resident to Sarasota Bay. Consequently, information on foraging habits (trophic level, foraging location) recorded by stable isotope values of annually deposited layers in teeth (annuli) compliments observational data by offering additional insights into individual life history strategies.
Stable isotope analysis overcomes traditional barriers to foraging studies and offers an approach to evaluate foraging habits over the lifetime of an individual. While being relatively inexpensive, stable isotope analysis provides valuable long-term information on trophic level (δ15N) and foraging habitat (δ13C). Analysis of annually deposited layers (annuli) in whale teeth provide information on foraging habits for each year of life.
I will test the relationship between individual foraging specialization and reproductive success using a community of dolphins resident to Sarasota Bay, FL. This population is the focus of the Sarasota Dolphin Research Program (SDRP), the world’s longest-running study of a dolphin population. As such, it is an excellent target population for my research. The abundance of prey fish and habitat composition have varied over the past six decades. I have access to a phenomenal data set comprising ca. 160 identifiable dolphins, including 33 females whose entire reproductive history is documented and have produced up to 9 calves each. Teeth for stable isotope analyses are available from the majority of these individuals, with some annuli dating as far back as the 1940’s (time of birth for oldest individuals). Supplementing stable isotope data are multi-decadal tagging, tracking and photo-identification efforts. This is an unprecedented opportunity. No other study of a wild marine mammal population is as data rich and the availability of teeth from a study population such as this is extremely rare.
This decade marked the first extinction of a dolphin species as a direct result of human activity. As the human population continues to grow exponentially, particularly in coastal areas, human impacts will increase environmental challenges for marine mammals. Understanding if reproductive success is dependent on specialization in foraging habits and specifically its relationship to average foraging habits across the lifetime of an individual will open new perspectives on how intra-population processes influence population viability.