The causes of the stranding of large numbers of whales and dolphins on beaches, resulting in the deaths of most if not all individuals involved, have been an unanswered question at least since the days of Greek philosopher Aristotle (who noted this curious phenomenon in his Historia Animalia in 350 BCE). Various suggestions have been made to explain this, including possible disease, parasitic infection, sympathetic mass suicide, electromagnetic disorientation, and anthropogenic activity (Cordes, 1982, Sunduram et al., 2006). It is clear that many of the animals that arrive together on a beach are not near death, as they appear to swim away competently once moved to deeper water by rescuers. Many species mass-strand, but each of these mass strandings involves only one species, and the numbers that strand together appear to reflect the size of the social groups in which these animals move around in the oceans. We have gathered evidence to evaluate what seems a likely cause of this behaviour - that they follow one or more dying or dead individuals that drift onto the shore.
These cetaceans navigate by sound, and another suggestion has been the possible disfunction of echo-location, due to the lack of sonic reflection from a gently sloping beach (Dudok van Heel, 1962, Sundaram et al., 2006), or because of the absorbtion of sound by the presence of microbubbles in shallow water (Chambers & James, 2005). These suggestions have yet to be thoroughly tested, but one might expect that animals approaching a beach with a gentle slope would have some warning because they could easily detect that the depth beneath them is small and decreasing.
Suggestions that geomagnetic topography may be having an effect can be discounted, at least in the New Zealand environment, as herd strandings have been shown to have no relationship to geomagnetic contours or magnetic minima (Brabin & Frew, 1994). Thus the most reasonable explanation appears to be that some individuals become sick and drift onto the shore and others in the group follow them, probably due to social bonds that are not yet fully understood.
Here we address this question by an examination of observations of massed stranding events in New Zealand over the past 40 years. Such events have been recorded in New Zealand dating back to the1840’s, and the record-keeping has been a legal requirement since 1978. They cover a large number of the species that have been recorded to strand in large groups. These data were provided by the New Zealand Bureau of Meteorology, via Ms. Hannah Hendricks and Project Jonah (projectjonah.org.nz). Brabin & McLean (1992), and more recently Betty et al. (2020), have provided detailed descriptions of the spatial and temporal distribution of these stranding events, identifying “hot spots” and temporal records, and focussing on the Long-finned Pilot Whale (LFPW), which is by far the most significant species for strandings in large numbers in New Zealand waters.
The New Zealand data set
The data set as used here contains some 3782 stranding events, dating from 1980 to the end of 2019. The great majority of these recorded events involve single animals, spread over the whole range of species in New Zealand waters, as described by Betty et al. (2020). We focus our analysis on events that involve 10 or more stranded animals, dated from 1980 onwards. This gives a data set of 125 massed stranding events, with numbers of animals in each event ranging from 10 to 616. For each of these massed stranding events, only one species of animal was recorded. Ninety of these strandings involved the Long-finned Pilot Whales (Globicephala melas). In addition, up to 11 may have been Short-finned Pilot Whales (Globicephala macrorhynchus) instead, or may not, due to uncertainty in identifying them at the time, but the distinction is not significant here (Short-finned Pilot Whales prefer warmer waters, and are rarely found near New Zealand, but have a similar social structure. The uncertain cases are denoted Globicephala sp in Table 1).
The main data set for analysis consists of 101 mass stranding events (10 or more animals) of Globicephala, and 24 events of other species. These other species and the number of events are: Common short-beaked dolphin (Delphinus delphis) (10), Common Bottlenose Dolphin (Terciops aduncus) (5), Sperm Whale (Physeter macrocephalus) (3), Gray’s Beaked Whale (Mesoplodon grayi) (2), Killer Whale (Orcinus orca) (1), Pygmy Killer Whale (Feresa attenuata) (1), False Killer Whale (Pseudorca crassidens) (1), and the Southern Right Whale Dolphin (Lissodelphis peronii) (1).
Common locations of the stranding events have been described by Brabyn & McLean (1992), and updated in more detail by Betty et al. (2020). For the period under consideration here (1980-2019), there were 24 events with 100 or more stranded animals, with the largest number of stranded animals in a single event being 616 in the Farewell Spit region. All of these large events involved Pilot whales.
The social cultures of cetaceans
Species of whales and dolphins vary in their social structures, many of which are not fully understood. As Pilot whales are much over-represented in these massed strandings, some details of their life-style are relevant. These animals are born in pods that may number up to several hundred animals. They take 5 years to mature to adulthood. All the animals in the pod, both male and female, are born in the pod, but none of the male members are fathers (Amos et al., 1991, 1993). Males must leave the pod to breed, apparently, with females in other pods. Females born in the pod remain there, and males may or may not return to their home pod. Members of the pod, most prominently females, generally help younger pod members, even though they may not be the parent (Amos et al.,1991; Oremus et al., 2013; Augusto et al., 2017). Thus although Oremus et al. (2013) have shown that mass strandings in New Zealand and Tasmania involve multiple matrilineal lineages, and mothers and calves are not stranded close to each other, these large groups of pilot whales might stay with a dying senior pod member due to a strong cultural relationship between young and older pod members.
Species considered here other than Pilot whales may also live for decades in pods, so that when senior members eventually die, if they have been prominent in pod social structure, younger pod members might accompany a sick or dead pod member as it drifts, due to these social bonds. In many of these other species, the groups that swim together are not closely related individuals, so that kin selection is not involved (Ball et al., 2017; Martien et al., 2014, Kobayashi et al., 2020; Patel et al., 2017; Westbury et al., 2021). But it is indisputable that competent individuals of these species do become stranded, and we are interested in determining whether this may be because they accompany one or more dying and bloated individuals. We note the increasing number of impressive studies of the genetic and the social structure of cetacean species, and this field is still developing (Moller, 2012). We expect that more evidence on this aspect will be available soon.
For all cetacean species, when the animals die it is not uncommon for their floating corpses to be deposited on nearby coasts. This is reflected by the large number of single strandings, over all relevant species in the New Zealand records. Over the 40-year period considered, there are 3063 recorded single strandings, including 180 cases of single stranded Pilot whales.
The next most common stranding number is two animals. In contrast with some other species, stranding events consisting of two Pilot Whales alone are relatively rare – over the 40-year period there are 8 recorded stranded-pairs of Pilot whales out of a total of 196. This is consistent with evidence (Amos et al., 1993) that, as a species, they tend not to form strong pair-bond relationships (as other species may do), but the males (in particular) can exist as individuals outside a pod, or have a close relationship within a large pod community.
Nor are Pilot whales over-represented in stranded groups containing 3 to 9 animals. In a total of 115 events in this range over 40 years, 16 of these events are Pilot whales, which is a number comparable with those of other species. This pattern suggests that Pilot whales strand together in the groups that live together.
The stranding process- stage 1: a floating body
We decided to examine the fit of the data, particularly the Pilot whale data, to the hypothesis that the massed strandings are due to the beaching of one or more pod members who have become ill (for whatever reason), and whose body has become buoyant (for example due to bloating of the gut), and floats on the surface. The motion of such a floating object will be subject to the effects of both the wind and waves at the surface. If a coastline is nearby, the direction of the wind and waves will largely determine whether the floating animal is driven towards the shoreline.
The stranding process- stage 2: wind and waves on a floating body
In deep water, where waves are present but not breaking, the main effect of waves on the mean motion of floating bodies is via Stokes drift. This is a mean motion of particles on the surface of the water due to the nonlinear dynamics of sufficiently large waves, which is given by (Craik, 2005)
Here us is the resultant mean speed of the floating object/body in the direction of the waves, a is the wave amplitude, λ the wavelength and T the wave period. Realistically, this gives speeds of magnitude 2 cm/s, which is approaching 2 km/day.
The above gives mean surface speeds due to non-breaking waves, but if the waves are breaking, the speed of the floating object will be much larger. This would be the case when the object reaches the wave surf zone.
The effect of the wind on floating object movement can be estimated as the drag, and this can be somewhat larger than the wave effect in the deep sea. To some extend these two factors (wind and waves) tend to be aligned, as the wind generates the waves. The wind direction can change more quickly, but there is a tendency for the two factors to be aligned, given sufficient steady wind.
Where the coastline consists of steep cliffs or rocky environments, the floating body may remain in the water, or be thrown up by heavy wave action, and the associated pod would presumably keep its distance, and there would be no subsequent record of a mass stranding event. But if it is a gently sloping beach, the body of the injured animal would be pushed close to the waterline.
The stranding process- stage 3: tides
The third part of the massed stranding process concerns the tides. If the tidal range is significant and high as a dead animal arrives, it will be driven to a level above the mean tide level on the beach. Then as the tide ebbs, it will rapidly become stranded on the beach. If the associated pod is following close behind, and spread laterally to be as close as possible, they would also be rapidly stranded by a falling tide. These animals will have no significant familiarity with tides as they usually live further offshore, and hence not be aware that the sea level may change by as much as several metres. On a gently sloping beach, a tidal range of even a metre may cause the local shoreline to retreat laterally by several metres in a matter of minutes.
In summary, our hypothesis for the massed stranding phenomenon is a three-stage process, (1) that the wind and waves at the time of the stranding would drive a floating body ashore at the stranding site; (2) that the dying or dead animal would be accompanied closely by many others from the group associated with it; and (3) that the beach at the time when the body arrives would be gently sloping, with the tide at least moderately high at the time, and the tide range sufficient so that the associated animals would be rapidly stranded.