When Dolphins Emigrate

While most mammals tend to have a "home range" in which they spend most of their adult lives, there are many reasons why they might wish to move elsewhere. Perhaps most obviously, there is the movement of juvenile individuals leaving home for the first time, and so travelling out of their parent(s) home range to establish their own territory. There's also the issue, mostly with males, of travelling about in the hope of encountering fertile members of the opposite sex. On a larger scale, there can be regular migration events, including short-range movements such as goats moving down mountain slopes in winter to avoid the worst of the weather.

But large-scale movements on a one-off basis are relatively rare. In social animals, individuals may move from one herd (or other group) to another for all sorts of reasons, although, in many species, even that can carry risks. Incidents in which a large portion of a herd ups sticks and moves to a place already occupied by another herd, before trying to integrate with the locals, are relatively rare. Which also makes them difficult to study, since it largely relies on the luck of happening to already be looking at a given group when it happens to occur. It's perhaps particularly hard to do so for cetaceans, which live underwater, and can be difficult to track.

Nonetheless, one long-term study in the Caribbean has been looking at some of the local dolphin communities over a 30 year period, and has been able to gather some detailed information on (among many other things) where and when they travel. The Caribbean is home to a number of dolphin species, but this particular project focuses on two of them: the common bottlenose dolphin and the Atlantic spotted dolphin (Stenella frontalis).

This a fairly typically sized dolphin, closely related to a number of other species in the area, and lives in social groups spread across most of the warmer waters of the Atlantic, from Massachusetts  and Morocco to Uruguay and Angola. It has the advantage that individuals are relatively easy to age, because of the way that the spots develop as they age. For the first few years of life, the dolphins are fairly bland in colour, but at around the age of four they begin to develop black spots, which become clearer, and larger, as the animal becomes progressively older. By the time they are fully adult, at about sixteen years, the dark spots have merged to the point that the animal is now more accurately described as black with white spots, rather than the other way around. Females, and possibly also males, are able to breed for a number of years prior to this happening, but the development is regular enough that sexual maturity can be gauged with some accuracy merely by looking at the dolphin's pattern.

While such dolphins are found throughout the Caribbean, the study mainly deals with one particular pod that lives primarily on the Little Bahama Banks. These stretch to the northwest of the island of Grand Bahama, about 40 miles (65 km) off the Miami coastline. They consist of submerged sand banks, typically 6 to 16 metres (20 to 50 feet) below the waves. The edges of the bank are steep, with the seafloor suddenly plunging hundreds of metres into the depths, making for a well-defined area suitable for shallow-water dolphins to live within.

Throughout the 1990s, the population of Atlantic spotted dolphins in this area consistently numbered around a hundred individuals, with new births keeping pace with overall mortality rates. They were divided into three sub-groups that nonetheless regularly interacted with one another, and had overlapping territories along the banks. Then, in September 2004, Hurricane Frances hit the area. By the time it reached the Bahamas, it had already weakened to a 'mere' category 2, but it still destroyed thousands of homes and took out 75% of the electricity grid.

Which was obviously bad enough for the locals... except that three weeks later, it was followed up by the category 3 Hurricane Jeanne. Taken in combination, this wasn't just unfortunate for the human population, but for the dolphins, too. Over a third of their population was wiped out, leaving the survivors reeling. The basic social structure, however, seems to have survived, with the same three sub-groups as before, although there was apparently a stronger tendency for individual dolphins to stick with their close friends and avoid relative strangers in the hurricanes' aftermath.

The community slowly recovered over the next eight years, and by 2012 was about half-way back to its former level. This was mostly due to new births, with, as before, relatively few newcomers arriving from elsewhere to take up the slack. In September of that year, the researchers studying the group packed up and went home, returning for another field season in May of 2013. Only to discover that half of the dolphins had vanished.

It turned out, however, that this wasn't due to some horrible catastrophe. The missing dolphins were safe and well... on an entirely different sand bank off the island of Bimini about 75 miles (120km) to the south. The largest of the three sub-groups had moved in its entirety, taking a few members of the other two groups with it. The new banks were larger than the old ones, but otherwise much the same oceanographically, and, perhaps unsurprisingly, they were already inhabited by other dolphins of the same species.

The social consequences, both for those dolphins who had chosen to stay, and those who had moved to a new home, were more significant than anything following the hurricanes years previously. The two sub-groups that remained at the Little Bahama Banks both continued to be distinct, suggesting that they are quite a fundamental part of how those dolphins lived their lives. But, within the groups, individuals were much more open, showing no preference for close friends, and interacting more or less equally with every other dolphin they knew. Similar changes have been seen in chimpanzees in response to sudden population crashes, as the whole community gathers together in the face of an increased threat to their survival.

Those that took part in the exodus completely failed to integrate with the existing population, interacting with them as little as possible. Among themselves, they too, banded together, helping everyone, rather than just close friends. This, it's interesting to note, must include the small number of dolphins from the other two sub-groups who abandoned their original friends (and, in the case of some females, even their sisters, which is unusual for dolphins) to head south with the others. So, clearly, they can integrate with relative strangers if they really want to, and are treated as equals once they do.

But why bother to move in the first place? What do they gain out of all of this. One benefit to the dolphins that moved is apparent. While there were enough of them already to provide a viable population going forward, they now have the opportunity to mate with members of a large resident population in their new home, increasing genetic diversity. While it's true that they don't seem to be forming social bonds with the locals, that sort of thing doesn't normally discourage male dolphins, and we'd reasonably expect some degree of mating between the two groups - which, may of course, have knock-on effects in future years.

Those that stayed behind face the opposite situation, being only able to breed among themselves, and with a population that's lower both in genetic diversity and raw numbers than that which survived the hurricane. There were, after all, few immigrants from outside before the sudden exodus, and that's assuming that whatever caused the others to leave doesn't also discourage newcomers from arriving.

Indeed, when similar opportunities opened up after the hurricane, it was new bottlenose dolphins that moved in, not the spotted sort. If that happens again, it has further implications for genetic diversity, since the two species have been observed to mate with one another. Although some suspiciously spotted bottlenose dolphins have been seen around the Bahamas, we don't have definitive proof that this can result in healthy offspring, let alone fertile ones, but it's not impossible, since bottlenose dolphins are known to be able to breed successfully with some other species. Given that male hybrids are more likely to be sterile in mammals in general, this could lead to a sex imbalance and longer term breeding problems.

But, since the population was stale and viable to start with, it's unlikely that the emigrants moved to obtain new mating opportunities; they should have had enough where they were. Something else must have spurred them to move, and, here, it may be relevant that the sub-group of dolphins that moved were much more likely to feed at the edges of the sand bank, on slightly deeper water squid and fish. Over the last twenty years, even before the hurricanes, the amount of algae in the water over the sand banks decreased, and this may well have had a knock-on effect on the fish population.

The effect was presumably gradual, with food resources becoming more difficult for the dolphins to obtain, and the slightly different feeding habits of one group of dolphins perhaps put them more at risk than their kin. Perhaps, at some point in the winter of 2012, things reached a crisis point, tipping over some invisible line that made that group of dolphins suddenly give up, and cross the deep sea to the other bank - where the algal levels have not been observed to change over the same time period.

This is speculation, of course; we don't know for sure why so many dolphins suddenly left their home for new pastures. We do know that, as of the end of 2015, they were still there, and have shown no inclination to head back. But something made them move, and, while it was probably the right decision for them, it may have created problems for those that didn't. For now, though, you may be much less likely to see spotted dolphins off the shores of Grand Bahama.

[Photo by Wayne Hoggard, on behalf of NOAA. In the public domain.]