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Social Organisation of the Short-finned Pilot Whale, Globicephala macrorhynchus, with Special Reference to the Comparative Social Ecology of Delphinids
Abstract
As a contribution to the understanding of comparative social trends within the cetacean family Delphinidae, a 22-month study was conducted on the shortfinned pilot whale, Globicephala macrorhynchus, which has been suggested to have a unique social system in which males and females in the same group are related and mating occurs outside of the group. The individual identification of 495 pilot whales, analysed in daily group association patterns, allowed identification of 46 pods. They were classified as productive or non-productive based on the presence or absence of immature animals. Productive pods were a significantly larger, although 12% of them lacked adult males. Two classes of whales (residents and visitors) were defined by patterns of occurrence,suggesting differential patterns of habitat use. Resident pods occasionally travelled together (41% of all groups) and associations between age and sex classes showed that in mixed-pod groups, the highest ranked associations of the reproductive females were with males from other pods, while within pods, adult males and females associated less. During summer, the proposed peak conception period, pilot whale groups were significantly larger and contained individuals from a significantly greater number of pods. These findings support the hypothesis that males and females mate when associating with individuals from other pods. A comparative analysis of sexual dimorphism, brain size, and testes size, habitat, prey and group size within the 17 delphinid genera identified a correlation between sexual dimorphism and body size, but relative measures of brain size and testes size did not correlate with broad ecological or social classifications. However, a comparison of three delphinid societies identified two distinct male mating systems: males of the small, mono-morphic Tursiops truncatus live in age/sex segregated groups and mate with a number of discrete female communities. Males in the large sexually dimorphic Glob icephala spp. and Orcinus orca mate with associated female pods and yet remain with their female kin. This corresponds to the avunculate social system described in some human societies. It could evolve from a promiscuous mating system where there is little guarantee of paternity and where males that live with their kin increase their inclusive fitness.
... As observed in other deep-diving species of cetacean, such as the sperm whale (Physeter macrocephalus), differential deposition of FA in blubber according to sex is common and primarily depends on the reproductive stage of females (Jackson et al., 2022). Several pilot whales' social units in the Webbnesia ecoregion are known to be stable and composed of animals of both sexes and age classes (Heimlich- Boran, 1993;Alves et al., 2013a;Servidio et al., 2019;Esteban et al., 2022;Betty et al., 2023), so the sex bias in the number of biopsy samples (with males outnumbering females) in our study could be due to males' defending the group during the boat approach and/or to the fact that we avoided sampling females in the vicinities of calves. ...
... Some species, like the short-finned pilot whale (Globicephala macrorhynchus, hereinafter referred to as pilot whale), have island-associated animals that are known to move between archipelagos (Alves et al., 2019). Some pilot whales in the Webbnesia, a subregion inside Macaronesia formed by Madeira and the Canary Islands (Freitas et al., 2019), are known to be highly associated with the Southeast of Madeira Island and the Southwest of Tenerife Island (Canaries), yet likely with different levels of site fidelity (Heimlich-Boran, 1993;Alves et al., 2013a;Servidio et al., 2019;Fernandez et al., 2021;Esteban et al., 2022). These are among the few known island-associated populations worldwide and strongly support local whale-watching activities (Sambolino et al., 2022;Sousa et al., 2023). ...
... Cañadas and Sagarminaga 2000;de Stephanis et al. 2008b;Hartny-Mills 2015). Possible drivers of these patterns include prey availability (Shane 1995;de Stephanis et al. 2008c), as well as breeding (Heimlich-Boran 1993;Cañadas and Sagarminaga 2000;Alves et al. 2013) and calving (Hartny-Mills 2015) behaviour. ...
... Tenerife island (Spain) holds one of the few resident populations of pilot whales worldwide, jointly with other oceanic islands such as Madeira and Hawaii (Alves et al., 2013;Heimlich-Boran, 1993;Servidio et al., 2019;Shane & McSweeney, 1990). Previous works on adult pilot whales from Tenerife using motion and digital acoustic recording tags (DTAGs) (Johnson & Tyack, 2003), have shown that these animals feed at average depths between 810 ± 92 m during the day, and 141 ± 81 m at night (Aguilar de Soto et al., 2008). ...
This study reveals early prey eating by a short-finned pilot whale (Globicephala mac-rorhynchus Gray, 1846, Cetacea: Delphinidae) in the Canary Islands. Stomach contents , trophic markers, skin isotopic ratios of nitrogen (δ 15 N: 15 N/ 14 N) and carbon (δ 13 C: 13 C/ 12 C), and fatty acid profiles of the blubber of a short-finned pilot whale of 213 cm size euthanized in free-ranging conditions were analyzed. A total of 15 species of oegopsid squids, mostly diel vertical mesopelagic migrant species of the families Enoploteuthidae, Ommastrephidae, and Histioteuthidae, as well as mother's milk, were identified in the stomach contents. Asperoteuthis acanthoderma (Lu, 1977, Cephalopoda: Chiroteuthidae) was found as first time in this area, suggesting the possibility of its presence on both sides of the subtropical Atlantic, extending its current known distribution. The δ 15 N value (11.55‰) was higher than expected based on the size range of squid ingested, but lower than that of adult pilot whales, suggesting that mother's milk intake has a significant effect on these values in calves. Similarly, the δ 13 C values (−17.99‰) were shifted to those of adult pilot whales rather than the ingested squids, also due to the ingestion of high-fat breast milk. The fatty acid (FA) composition of blubber showed a clear stratification. Long-chain polyunsaturated fatty acids (LC-PUFA) were mainly present in the inner layer, while most relevant ≤C20 monounsaturated fatty acids (MUFA) were more abundant in the outer layer.
In the Northeast Atlantic, it is unclear whether short-finned pilot whales, Globicephala macrorhynchus (SFPWs), are transient or resident above 40ºN. We used Anisakis spp. to identify the latitudes recently visited by a SFPW pod stranded in NW Spain (43ºN) in 2020. Analysis of cox2 gene in 30 nematodes from 6 SFPWs revealed the presence of A. simplex sensu stricto (s.s.) (93.3%) and A. pegreffii (6.7%). Morphological analysis of 972 nematodes corroborated species molecular identification and relative proportions; in males (n = 66), the estimated proportions of A. simplex s.s. and A. pegreffii were 86.4% and 13.6%, respectively. These percentages resembled those reported in fish/cetaceans above 40ºN, and L4 larvae or adults of A. typica (a tropical-temperate species found in SFPWs up to 38ºN-39ºN) were not detected. Population structure of A. simplex s.s. + A. pegreffii in SFPWs suggested a continuous recruitment of nematodes starting at least 3 weeks before stranding. We interpret that either the SFPW pod was within the range of the species (perhaps as a recent northern shift) or represented a vagrant group that visited waters off Northwest Spain for a protracted period. Future analysis on nematode assemblages could shed light on movements and climate-driven shifts in cetacean distribution.
In the Northeast Atlantic, it is unclear whether short‐finned pilot whales, Globicephala macrorhynchus (SFPWs), are transient or resident above 40°N. We used Anisakis spp. to identify the latitudes recently visited by a SFPW pod stranded in NW Spain (43°N) in 2020. Analysis of cox2 gene in 30 nematodes from 6 SFPWs revealed the presence of A. simplex sensu stricto (s.s.) (93.3%) and A. pegreffii (6.7%). Morphological analysis of 972 nematodes corroborated species molecular identification and relative proportions; in males ( n = 66), the estimated proportions of A. simplex s.s. and A. pegreffii were 86.4% and 13.6%, respectively. These percentages resembled those reported in fish/cetaceans above 40°N, and L4 larvae or adults of A. typica (a tropical‐temperate species found in SFPWs up to 38°N–39°N) were not detected. Population structure of A. simplex s.s. + A. pegreffii in SFPWs suggested a continuous recruitment of nematodes starting at least 3 weeks before stranding. We interpret that either the SFPW pod was within the range of the species (perhaps as a recent northern shift) or represented a vagrant group that visited waters off Northwest Spain for a protracted period. Future analysis on nematode assemblages could shed light on movements and climate‐driven shifts in cetacean distribution.
Aerobic dive limits (ADLs) are a useful paradigm for assessing marine mammal diving ability. Given the allometry of total body oxygen stores and metabolic rate, larger animals should have increased diving capacities and thus elevated ADLs. The short-finned pilot whale Globicephala macrorhynchus is a deep-diving species with pronounced sexual size dimorphism, and individuals are regularly found in size-mixed groups. Therefore, we asked how body size constrains dive durations in this species and whether behavioral ADL (bADL), estimated as the 95th percentile of dive duration, is a useful measure of physiological ADL. We analyzed 30169 dives from 45 animals tagged with satellite-linked recorders off Cape Hatteras, North Carolina, and Jacksonville, Florida, USA, and determined a species-level bADL of 18.8 min and individual bADLs ranging from 13.9 to 22.1 min. To assess the influence of size on bADL, we estimated the body lengths of 19 whales from dorsal fin measurements. Body length did not fully explain intraspecific bADL variation, but similar dive distributions and lower bADL variance between animals tagged together indicated a potential effect of group membership. Moreover, individuals in Cape Hatteras had a significantly lower median bADL than those in Jacksonville, suggesting location may influence dive durations. These results indicate the potential impact of social and location-specific factors on bADL estimates in a deep-diving, sexually dimorphic species.
Context
Strandings are an important source of information for estimating marine mammal biodiversity, particularly in data-sparse ocean basins such as Oceania.
Aims
Here, we report on knowledge acquired from 218 stranding events recorded in the waters of New Caledonia (1877–2022).
Methods
We investigated spatio-temporal distribution, stable isotope signatures, trace element concentrations, biometry measurements, genetic diversity, and diet, for the four most commonly stranded taxa (dugongs, 35% of events; sperm whales, 19%; Delphinidae, 18%; pygmy and dwarf sperm whales, 14%).
Key results
Beginning in 1991, reports of stranding events increased (183 events, 322 individuals, 20 species from seven families: Dugongidae, Physeteridae, Delphinidae, Kogiidae, Ziphiidae, Balaenopteridae, Otariidae), with hotspots identified on the west coast (Bourail, Ouano, Nouméa) and in Prony Bay. Causes of death were not determined in 84% of stranding events, but were identified in the majority of expert-led necropsies (24 of 29 individuals from 10 species). Yet, valuable information regarding the impact of anthropogenic activities was gathered for some species of concern, such as the endangered dugong (28% human-caused). Since 2016, training and outreach have been provided to rangers, veterinarians, and various public safety officers to support their engagement in the scientific monitoring of marine mammal strandings. A website (www.rescue.ird.nc) was developed to facilitate standardised data collection and storage, and to provide public access to stranding records.
Conclusion
Although the number of individuals reported here remains modest, this study provides new information on poorly documented species in New Caledonia.
Implications
Long-term monitoring of strandings can help design effective conservation measures.
Two species of pilot whales are globally distributed, the long-finned ( Globicephala melas ) in cold-temperate waters and the short-finned ( G. macrorhynchus ) in tropical and warm-temperate latitudes. Two subspecies of the long-finned pilot whale are recognized, G. m. melas in the North Atlantic and G. m. edwardii in the Southern Hemisphere. In addition, three types have been proposed in short-finned pilot whales. In general, it is assumed that pilot whales live in matrilineal societies composed of stable units/pods displaying bisexual natal philopatry, but inter- and intraspecific variabilities in the sociality of these units have been described worldwide. Moreover, there is inter- and intraspecific heterogeneity in life history and reproductive parameters, which supports geographic variation. To investigate life history parameters, sociobiology, and reproductive strategies within different populations of pilot whales, we reviewed the current literature and compiled novel data. We cover populations from both hemispheres and combine life history characteristics from strandings with field-/behavioral-based information such as long-term photographic-identification, social analysis with molecular sexing, and drone technology. This chapter contributes to improving our knowledge of the life history parameters between sexes and populations, interactions between animals of different sexes within units, social structures, and reproductive strategies in pilot whales. We explore pilot whales’ sexual group dynamics and social system and discuss whether they are strictly matrilineal in comparison with other “matrilineal” species.
