Adaptation of sperm whales to open-boat whalers: Captures dropped 58pct in a few years, it appears that whales swiftly learned effective defensive behaviour

Adaptation of sperm whales to open-boat whalers: rapid social learning on a large scale? Hal Whitehead, Tim D. Smith and Luke Rendell. The Royal Society Biology Letters, Volume 17, Issue 3, Mar 17 2021. https://doi.org/10.1098/rsbl.2021.0030

Abstract: Animals can mitigate human threats, but how do they do this, and how fast can they adapt? Hunting sperm whales was a major nineteenth century industry. Analysis of data from digitized logbooks of American whalers in the North Pacific found that the rate at which whalers succeeded in harpooning (‘striking’) sighted whales fell by about 58% over the first few years of exploitation in a region. This decline cannot be explained by the earliest whalers being more competent, as their strike rates outside the North Pacific, where whaling had a longer history, were not elevated. The initial killing of particularly vulnerable individuals would not have produced the observed rapid decline in strike rate. It appears that whales swiftly learned effective defensive behaviour. Sperm whales live in kin-based social units. Our models show that social learning, in which naive social units, when confronted by whalers, learned defensive measures from grouped social units with experience, could lead to the documented rapid decline in strike rate. This rapid, large-scale adoption of new behaviour enlarges our concept of the spatio-temporal dynamics of non-human culture.

4. Discussion

While a combination of H1–H3 might produce a steep decline in strike rate, social learning of defensive measures between social units (HX) is the best-supported explanation for the rapid decline in strike rate following the first sperm whale sighting within a region. The whalers themselves wrote of defensive methods that they believed the whales were adopting, including communicating danger within the social group, fleeing—especially upwind—or attacking the whalers [17,18] (figure 1). Deep dives would also have been effective. But, perhaps the most straightforward change would be for sperm whales to cease their characteristic defensive behaviour against their most serious previous predator, the killer whale, Orcinus orca. Gathering in slow-moving groups at the surface and fighting back with jaws or flukes often works against killer whales [19,20], but will have only assisted the relatively slow-moving, surface-limited, harpoon-bearing open-boat whalers.

There are other behavioural changes that the whales may have made in response to whaling, but their impact on strike rates is less clear. There is some evidence that sperm whales formed larger groups in response to whaling [15], but this would likely have increased rather than decreased strike rates. They may have learned to avoid the whalers before the whalers detected them, but this should generally have reduced the mean detection range of the whalers and so increased the strike rate. However, if whales fleeing at long ranges made themselves more visible by blowing hard and showing their bodies forcefully, so increasing the number of sightings with groups that were not easily struck, this might have additionally decreased the strike rate.

Thus, there are learned behavioural changes that the sperm whales could have made to reduce strike rates, and some anecdotal witness that they did so. However, learning as individuals or within social units is not supported as the sole cause of the initial decline in strike rate. To achieve the observed reduction in strike rate through behavioural change, some mechanism must have allowed naive whales without the experience of whalers to receive the benefits of experience.

We suggest that naive social units learned defensive measures from grouped experienced social units and adopted them. Encounters with whalers typically lasted hours, and sperm whales through their echolocation and communication systems can probably sense and coordinate behaviour over ranges of several kilometres. Other processes could have enhanced the social learning process. If groups were particularly likely to split between or within social units after an experience with whalers, and then to join other units, this will have increased the probability that a naive animal was grouped with an experienced individual during its first encounter with whalers.

Our analysis provides substantial support for rapid (less than 20% generation time, so much too fast for genetic evolution) social learning over large spatial scales. The ability of sperm whales, or potentially other species, to rapidly change behaviour in the face of a new anthropogenic threat by making use of social learning has implications for the population significance of new threats, and their assessment. Data from the earliest exposures may not generalize to later periods, and vice versa.