A cursory glance at the two main survey areas within the 10,000 Island Dolphin Study will reveal an
obvious difference: Area One is almost entirely comprised of bays and intercoastal waterways behind barrier islands while Area Two is almost entirely coastal nearshore waters of the Gulf of Mexico.
The two areas are contiguous with each other and no obvious barriers divide them but our initial observations suggest that two distinct communities of dolphins inhabit them. The separation is significant but by no means complete; Of the 220 dolphins we have identified in these two areas, only 12 have been sighted in both; 103 have only been seen in Area One and 105 have only been observed in Area Two.
Interestingly, Anna Sellas, M.Sc. of the University of California, Santa Cruz was able to confirm a similar phenomenon in Sarasota Bay using DNA analyses. In an abstract appearing on the Sarasota Dolphin Research Program website, she writes, “Using mitochondrial DNA and nuclear microsatellite markers, significant population differentiation was detected between Sarasota Bay residents and bottlenose dolphins sampled in nearshore coastal gulf waters. This result is surprising given the short geographical distance between these areas and the lack of obvious geographical barriers to prevent gene flow.”
Bottlenose dolphins are wide ranging – found in nearly all of the tropical and temperate oceans of the world. As such, the habitats they occupy vary significantly, require different survival strategies and present different selection pressures on the populations within them. The result is that bottlenose dolphins are famously ‘plastic’ in their morphology and social attributes. In The Bottlenose Dolphin: Biology and Conservation, John E. Reynolds III, Randall S. Wells and Samantha D. Eide point out that one differentiation between bottlenose dolphin populations already recognized is that between those dolphins that ply the deep waters of the Atlantic and their inshore, coastal, shallow water brethren. (p.32)
The observable differences between these populations are dramatic: off-shore bottlenose dolphins are 30% larger and often travel in groups comprised of fifty to 100 individuals of all ages and sexes, while the near shore coastal dolphins we observe on our trips are commonly seen alone or in much smaller congregations. These differences are a response to differences in the character of the environments they occupy and the two populations “can be distinguished genetically.”(Reynolds, Wells, Eide p.33)
The authors suggest that these differences in the social structure of the respective dolphin populations are, in part, the result of differences in the distribution of both predator and prey in their habitats. Sharks, they point out, are larger and more abundant in deep water and “in shallow waters, prey are more evenly distributed than is the case in deep water, where prey tend to be found in patches.” (P.107-108).
What we are seeing in the case of the apparent differentiation between Area One and Area Two and a similar distinction between bay and coastal dolphins in Sarasota may be a more finely calibrated expression of a similar phenomenon.
It may be that it takes a slightly different set of techniques, habits and strengths to navigate and forage effectively along the coast than within the backwaters or habitats of the bays. The distribution and habits of predator and prey species may be different; the underwater geography, variations in water clarity or acoustics may require adjustments in foraging behavior. All these collectively may be different enough that the tools, know–how, etc. necessary to thrive along the coast become held by the group of dolphins that spend most of their time there, while the tool kit of behaviors and habits most effective for habitats encountered in the backwater or bay areas come to be held and transmitted to each other by the dolphins who inhabit and master those areas.
All of which is to say that the barriers, such as they are, may be cultural rather than physical or geographic. I say ‘such as they are’ because the difference may be on the order of a preference, not a rigid enough sort that would ever lead to speciation, but still significant enough to register measurable differences in the DNA of the two groups. The range of some dolphins in our catalogue, both male and female, does, after all, extend to both areas.
Anna Sellas’ analysis revealed, however, “a significant amount of male-biased disperal between the coastal gulf and Sarasota Bay,” confirming research that has shown resident males to have “wider ranging patterns and more frequent interactions with dolphins of neighboring areas than resident females.”
We have conducted too few surveys in Area 2 to say anything definitive in this regard as it relates to the population in our study area, but I was thinking about this the other day when we saw Disastre, a large dolphin from A2 in A1 following around Sparky, an A1 female who, at the time, was without a calf. He has made three brief forays into Area One, each about a year apart. Disastre may be a male that has extended his range in order to increase the number of females he contacts and hence his odds of reproductive success, but he is competing with Oscar and Sharks, a dedicated A1 male pair alliance that has spent months shadowing Sparky.
And not just Oscar and Sharks – I ran an association pattern query on Sparky and see what other males have spent time with her. Just a quick tally shows 100 interactions with Area 1 dolphins designated as male or probably male and only 1 interaction with an Area 2 dolphin. If that pattern is true of all the A1 females, it is not hard to see how noticeable genetic differences could develop over time between the two groups.
In the days and years to come we will conduct many more surveys in both Area 1 and 2. We’ll keep an eye on patterns that reveal just how distinct these two communities are and what sort of interactions occur between them.
Sources used for this article:
“Using Genetics to Examine Population Structure of bottlenose dolphins along the Central West Coast of Florida” Anna Sellas, M.SC., University of California, Santa Cruz. Website: Sarasota Dolphin Research Program / Ecology, Population Structure and Dynamics http://www.sarasotadolphin.org/Ecology/PopulationGenetics.asp
John E. Reynolds III, Randall S. Wells, Samantha D. Eide. The Bottlenose Dolphin: Biology and Conservation. University of Florida Press. 2000
Anna b. Sellas, Randall S. Wells and Patricia E. Rosel. “Mitochondrial and Nuclear DNA Analyses reveal fine scale geographic structure in bottlenose dolphins (tursiops truncates) in the Gulf of Mexico.” Conservation Genetics, 2005, volume 6, number 5, pgs. 715-728