How “Bachelor Groups” of Sperm Whales die together

Sperm whales (Physeter macrocephalus) are amongst the most specialized and highly adapted marine mammals for deep diving foraging and hunting. Sperm whales are critically endangered across all global extant populations. The species endangerment risk has increased during historically low global species abundance. Within the past decade sightings have documented an increased frequency of juvenile male Sperm Whales aggregated together in ‘bachelor groups’ beached together. To date, there is no conclusive evidence that has determined the physiological conditions that lead to Physeter m. recurring beachings, nor any conclusive evidence to support why groups of juvenile males have repeatedly been found beached together. High rates of beaching across all age groups have contributed to the species' slow recovery after the ban on whaling. Incorporating sociality data available for documented cases of juvenile male pods is necessary for future conservation, specifically how behavioral decisions have influenced diving responses to anthropogenic disturbances. 

An emerging field of behavioural science has begun to investigate what differences may exist between subpopulations of social species (Cantor 2015). This research is significant for evaluating existing fitness benefits that influence directional selection of sociality, foraging, and preferential food differences. The differences between populations may contribute to subpopulation crashes if socially bonded groups are cohesive in decisions that physiologically harm the individuals and lead to the death of all members. There is a significant gap within existing literature that requires the integration of behavioural science of Sperm Whale social age and sex-specific temporal aggregation, diving physiology, and considerations to conservation efforts as an endangered species. Several studies have documented Sperm Whale clans (amongst countless other marine species) beaching with the primary suspect of cause of death determined to be slow starvation from accumulated ingestion of plastic debris (Jacobsen 2010, Jaquet 2000). Existing literature supports the hypothesis that sociality influences food preferences in Male clans of P. catodon (Christal 1998). The cause of mass strandings could be a result of clan learning detrimental food preferences, including polluted prey and garbage mistaken for prey. Sperm Whales, similar to Orcas, differ in hunting behavioural decisions depending on their pod, influenced by geographic region, prey abundance, and abiotic environmental factors (Cantor 2015, Christal 1997). Similarly, rapid behavioral decisions made in response to sonar exposure and boating noise pollution have been documented to cause osteonecrosis and decompression sickness as a result of rapid divergence from typical diving behaviour (Moore 2004). Bachelor groups of over ten individuals have repeatedly been found beached together, but no conclusive evidence has been published within the field explaining this recurring phenomenon (Jaquet 2000, Levettevall 2002). The repeated loss of juvenile males before reaching reproductive age is a significant concern for maintaining the species' historically low global abundance and has even worse prospects for conservation efforts to remove the species from high-risk endangered status (Taylor 2008). This paper aims to disentangle the existing theories attributing causes of Sperm Whale beaching, the physiological effects leading to the cause of death documented, and social aggregation factors that may be contributing to the species extinction threat. 

Sperm whales are the deepest divers amongst the odontocetes, documented to dive as deep as 3000 feet, in an evolutionary arms race of diving capacity with their largest prey species Giant Squid (Architeuthis) into the deepest depths of the sea (Pabst 2016). Watwood et al (2006) found P. macrocephalus spent the majority of their dive time foraging in deep prey patches and suggested that across regions with tagged whales (Gulf of Mexico, Ligurian Sea, Atlantic Ocean) this expanded foraging time was the result of the species highly evolved long-range echolocation and strong aerobic diving capacity compared to other marine mammals. Neurophysiology analysis has revealed the highly developed cortex of Physeter m. in which the evolution of sociality and complex communication can be attributed to the development of, supported by the spermaceti organ which facilitates the long-range echolocation necessary for deep dives. Sperm Whales' geographic range is one of the broadest across species, spanning globally and has been documented in every ocean and sea (Pirotta 2020, Whitehead 2009). Individual preferences and social pods have been suggested to influence habitat use in Physeter m (Gero 2015). Increased anthropogenic disturbances bring a significant rise in stress-related response, interrupting patterns of migration and mating life history cycles (Whitehead 2016). 

Most mammalian sociality has evolved under directional selection pressures towards intraspecific competition, often the small gamete sex (in mammals, mostly males) is more likely to select for behavioural traits that enhance an individual's reproductive success winning over other male individuals within a polygynous population. However, some papers suggest Physeteridae have evolved higher cognition and cooperative behaviour amongst non-related kin. High sociality is a characteristic life history trait amongst clans of P. macrocephalus, which consists of females (mothers and offspring) and typically one mature (age 30+) male (Richard 1996). Male offspring upon reaching the age of 5 leave their natal clan to form ‘bachelor groups’ that isolate from clans and spend up to ten years together before returning to breeding territory (Richard 1996). These bachelor groups have been observed diving, foraging, migrating, and sleeping together forming long-term associations (Kobayashi 2020). Temporal aggregates have been suggested to be less socially bonded than female clan bonds, likely due to females shared offspring rearing (Christial & Whitehead 2001). Male-only group clan interactions are seen in other highly social species, such as African elephants (Loxodonta africana) that may be a result of beneficial interaction preparing males of similar age groups for intraspecific competition past juvenile years (Chiyo et al 2011). Other marine mammals such as Bottlenose Dolphins (Tursiops) have also evolved socially bonded male clans, illustrating that although rarely present in mammals, highly intelligent and social species have evolved male bonds alongside male-male competition (Connor et al 1992). Recent studies have argued against previous conclusions that P. catodon males do not form social bonds, publishing findings that supported strong social bonds between individuals in bachelor groups that could benefit cooperative predation avoidance and foraging strategies (Kobayashi et al. 2020).

A recent study found incidental exposure to sonar noise to Physeter m. significantly disturbed the pods’ diving response, changing from U shape to V shape dive pattern as the sonar frequency and duration increased marking a shift to non-foraging deep dives (Isojunno 2016) This same pattern shift is observed in Harbor Seals, however, due to seal’s position in the trophic pyramid as prey to many larger predators in their environment do not display this beaching phenomenon (Kooyman 1981). The high mortality of a top predator has many potential consequences for trophic structure, leaving many questions still unanswered concerning the impact of their loss across habitats globally. Despite many countries banning whaling, other significant anthropogenic pressures on whales have yet to be regulated (Richter 2006, Taylor 2009). Without regulation, Physeter m. are currently forced to make unfavourable tradeoffs between physiological safety (noise pollution) and ecological needs (prey abundance, diving strategies) that may force more pods into vulnerable positions (Joyce 2017). Juvenile males are crucial to mating structure and the continued loss of these pods endangered populations towards extinction vortices. Incorporating sociality data available for documented cases of juvenile male pods will be essential for marine conservation efforts, for Sperm Whales, their prey species, and the habitats they have historically occupied.

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