Abstract

The mass stranding event is one of the most mysterious behaviors in cetaceans. Although the exact causes usually remain undetermined, one common observation noted is that most mass stranding events were reported on the species which live with tight social bonds.^2,5 The studies on the evolution forces of tight social bonds and the genetic structure within stranding groups are thus important for elucidating the causes of mass stranding events.

Pygmy killer whales (PKW) (Feresa attenuata) are often reported mass stranded.¹ However, the genetic structure within stranding events was never studied. The resident population of PKW around Hawaii was reported to live with a tight social bond.⁴ Therefore, we hypothesized that PKWs form social groups with closely related individuals, which is similar to what is known in other studied blackfish. DNA samples were collected from 8 mass stranding events and other single-stranded events (n=10) around Taiwan waters during 2010–2022. The reported mass stranded unit sizes were 3 to 27, and the individuals collected in each event ranged from 3 to 9. Mitochondria control region (1045 bp) (n=50) and nuclear DNA (n=38) obtained from high-resolution, double-digested, restriction-associated DNA (ddRADseq) were used to analyze the kinship within and between the events.

Surprisingly, some of the within-event dyads were found with relationships higher than quarter-sibling, while some within-event dyads were not. In addition, no within-event dyads with relationship higher than quarter-sibling was found in three of the eight mass-stranding events. The related dyads within stranding events were found to be male-male, female-female, and male-female. It is suggested that the genetic structure of PKW in one mass-stranding event are formed by multiple less-related clans. The driving forces of forming their tight social structure might be the differences of foraging strategies, prey preference, and habitat preference rather than simply kin-selection. Furthermore, there was no between-event dyad with relationship higher than quarter-sibling, indicating potential socially learned behavior or sampling bias from a large population. However, 90% (45/50) of the PKWs possessed the same mtDNA control region haplotype, and the PKWs in the same events possessed the same haplotype. Because this major haplotype was not reported in other regions outside Taiwan,³ it is suggested that most of the PKWs stranded in Taiwan possibly belong to a regional and comparatively newly founded population.

This is the first study of the genetic relationship within stranding events in PKWs in the West Pacific which gives insight into the social nature of this cryptic species. These stranding PKW are suggested to be the regional population, and thus, their viabilities facing stress factors and newly introduced pathogens are warranted to be studied.

Acknowledgements

We would like to thank the Marine Biology and Cetacean Research Center in National Cheng Kung University and Taiwan Cetacean Society for assistance with sample collection. All animal procedures were conducted with the approval of the Ocean Conservation Administration, Taiwan (Permit #1090002352).

*Presenting author
+Student presenter

Literature Cited

1.  Brownell, Jr. RL, Yao CJ, Lee CS, Wang MC. Worldwide Review of Pygmy Killer Whales, Feresa attenuata, Mass Strandings Reveals Taiwan Hot Spot. International Whaling Commission Scientific Committee; 2009.

2.  Hamilton LJ. Large mass strandings of selected odontocete species: statistics, locations, and relation to earth processes. J Cetacean Res Manag. 2018;19:57–78.

3.  Hancock-Hanser BL, Baird RW, Robertson KM, Rosel PE, Mignucci-Giannoni AA, Martien KK. Examining the global phylogeography of a little known species: the pygmy killer whale (Feresa attenuata). In: 21st Biennial Conference on the Biology of Marine Mammals. San Francisco, CA; 2015:14–18.

4.  McSweeney DJ, Baird RW, Mahaffy SD, Webster DL, Schorr GS. Site fidelity and association patterns of a rare species: pygmy killer whales (Feresa attenuata) in the main Hawaiian Islands. Mar Mamm Sci. 2009;25.3:557–572.

5.  Moore KM, Simeone CA, Brownell, Jr. RL. Strandings. In: Wursig B, Thewissen JGM, Kovacs K, eds. Encyclopedia of Marine Mammals. USA; 2018:945–951.