Love thy mother: maternal care in cetaceans

By Lisa Hildebrand, PhD student, OSU Department of Fisheries & Wildlife, Geospatial Ecology of Marine Megafauna Lab

Last week marked the one year anniversary of the pandemic reality we have all been living. It has been an extremely challenging year, with everyone experiencing different kinds of difficulties and hurdles. One challenge that likely unites the majority of us is having to forego seeing our loved ones. For me personally, this is the longest time I have not seen my family (445 days and counting) and I know I am not alone in this situation. My homesickness started a train of thought about cetacean parental care and inspired me to write a blog about this topic. As you can see from the title, this post focuses on maternal care, rather than parental care. This bias isn’t due to my lack of research on this topic or active exclusion, but rather because there are currently no known cetacean species where paternal participation in offspring production and development extends beyond copulation (Rendell et al. 2019). Thus, this blog is all about the role of mothers in the lives of cetacean calves.

Like humans, cetacean mothers invest a lot of energy into their offspring. Most species have a gestation period of 10 or more months (Oftedal 1997). For baleen whale females in particular, pregnancy is not an easy feat given that they only feed during summer feeding seasons. They must therefore acquire all of the energy they will need for two migrations, birth, and (almost) complete lactation, before they will have access to food on feeding grounds again. For pregnant gray whales, a mere 4% loss in average energy intake on the foraging grounds will prevent her from successfully producing and/or weaning a calf (Villegas-Amtmann et al. 2015), demonstrating how crucial the foraging season is for a pregnant baleen whale. Once a calf is born, lactation ensues, ranging in length between approximately 6-8 months for most baleen whale species to upwards of one or two years in odontocetes (Oftedal 1997). The very short lactation period in baleen whales is offset by the large volume (for blue whales, up to 220 kg per day) and high fat percentage (30-50%) of milk that mothers provide for their calves (Oftedal 1997). In contrast, odontocetes (or toothed whales) have a more prolonged period of lactation with less fatty milk (10-30%). This discrepancy in lactation period lengths is in part because odontocete species do not undertake long migrations, which allows females to feed year-round and therefore allocate energy to nursing young for a longer time. 

Aside from the energetically costly task of lactation, cetacean mothers must also assist their calves as they learn to swim. Echelon swimming is a common position of mother-calf pairs whereby the calf is in very close proximity to its mother’s mid-lateral flank and provides calves with hydrodynamic benefits. Studies in bottlenose dolphins have shown that swimming in echelon results in a 24% reduction in mean maximum swim speeds and a 13% decrease in distance per stroke (Noren 2008) for mothers, while concurrently increasing average swim speeds and distance per stroke of calves by 28% and 19%, respectively (Noren et al. 2007). While these studies have only been conducted in odontocete species, echelon swimming is also observed in baleen whales (Smultea et al. 2017), indicating that baleen whale females may experience the same reductions in swimming efficiency. Furthermore, mothers will forgo sleep in the first days after birth (killer whales & bottlenose dolphins; Lyamin et al. 2005) and/or shorten their dive foraging times to accommodate calf diving ability (bottlenose dolphins [Miketa et al. 2018] & belugas [Heide-Jørgensen et al. 2001]). Females must endure these losses in foraging opportunities and decreased swimming efficiency when they are at their most nutritionally stressed to ensure the well-being and success of their offspring.

It is at the time of weaning (when a calf becomes independent), that we start to see differences in the maternal role between baleen and toothed whale mothers. Odontocetes have much stronger sociality than baleen whales causing offspring to stay with their mothers for much longer periods. Among the largest toothed whales, such as killer and sperm whales, offspring stay with their mothers in stable matrilineal units for often a lifetime. Among the smaller toothed whales, such as bottlenose dolphins, maternal kin maintain strong bonds in dynamic fission-fusion societies. In contrast, post-weaning maternal care in baleen whales is limited, with the mother-calf pair typically separating soon after the calf is weaned (Rendell et al. 2019). 

The long-term impact of social bonds in odontocetes is evident through examples of vertically transmitted behaviors (from mother to calf) in a number of species. For example, the use of three unique foraging tactics (sponge carrying, rooster-tail foraging, and mill foraging) by bottlenose dolphin calves in Shark Bay, Australia, was only significantly explained by maternal use of these tactics (Sargeant & Mann 2009). In Brazil, individuals of four bottlenose dolphin populations along the coast cooperatively forage with artisanal fishermen, which involves specialized and coordinated behaviors from both species. This cooperative foraging tactic among dolphins is primarily maintained across generations via social learning from mothers to calves (Simões-Lopeset al. 2016). The risky tactic of intentional stranding by killer whales on beaches to capture elephant seal pups requires a high degree of skill and high parental investment to reduce the associated risk of stranding (Guinet & Bouvier 1995). 

Evidence for vertical transmission of specialized foraging tactics in baleen whales currently does not exist. Bubble-net feeding is a specialized tactic employed by humpback whales in three oceanic regions where multiple individuals work together to herd and trap prey (Wiley et al. 2011). However, it remains unknown whether this behavior is vertically transmitted. Simultaneous video tags from a mother-calf humpback whale pair in the Western Antarctic Peninsula documented synchrony in dives, with the calf’s track lagging behind the mother’s by 4.5 seconds, suggesting that the calf was following its mother (Tyson et al. 2012). Synchronous diving likely allows calves to observe their mothers and practice their diving, and could offer a pathway for them to mimic foraging behaviors and tactics displayed by mothers. 

While there currently may not be evidence for vertical transmission of specialized foraging tactics among the baleen whales, there is documentation of matrilineal fidelity to both foraging (Weinrich 1998, Barendse et al. 2013, Burnham & Duffus 2020) and breeding grounds (Carroll et al. 2015). Matrilineal site fidelity to foraging grounds is not exclusive to baleen whales and has also been documented in a number of odontocete species (Palsbøll et al. 1997, Turgeon et al. 2012). 

In the GEMM Lab, we are interested in exploring the potential long-term bonds, role and impact of Pacific Coast Feeding Group (PCFG) gray whale mothers on their calves. GEMM Lab PhD student Clara Bird is digging into whether specialized foraging tactics, such as bubble blasts and headstands, are passed down from mothers to calves. I hope to assess whether using the PCFG range as a foraging ground (rather than the Arctic region) is a vertically transmitted behavior or whether environmental factors may play a larger role in the recruitment and dynamics of the PCFG. It will take us a while to get to the bottom of these questions, so in the meantime hug your loved ones if it’s safe to do so or, if you’re in my boat, continue to talk to them virtually until it is safe to be reunited.

References

Barendse, J., Best, P. B., Carvalho, I., and C. Pomilla. 2013. Mother knows best: occurrence and associations of resighted humpback whales suggest maternally derived fidelity to a southern hemisphere coastal feeding ground. PloS ONE 8:e81238.

Burnham, R. E., and D. A. Duffus. 2020. Maternal behaviors of gray whales (Eschrichtius robustus) on a summer foraging site. Marine Mammal Science 36:1212-1230.

Carroll, E. L., Baker, C. S., Watson, M., Alderman, R., Bannister, J., Gaggiotti, O. E., Gröcke, D. R., Patenaude, N., and R. Harcourt. 2015. Cultural traditions across a migratory network shape the genetic structure of southern right whales around Australia and New Zealand. Scientific Reports 5:16182.

Guinet, C., and J. Bouvier. 1995. Development of intentional stranding hunting techniques in killer whale (Orcinus orca) calves at Crozet Archipelago. Canadian Journal of Zoology 73:27-33.

Heide-Jørgensen, M. P., Hammeken, N., Dietz, R., Orr, J., and P. R. Richard. 2001. Surfacing times and dive rates for narwhals and belugas. Arctic 54:207-355.

Lyamin, O., Pryaslova, J., Lance, V., and J. Siegel. 2005. Continuous activity in cetaceans after birth. Nature 435:1177.

Miketa, M. L., Patterson, E. M., Krzyszczyk, E., Foroughirad, V., and J. Mann. 2018. Calf age and sex affect maternal diving behavior in Shark Bay bottlenose dolphins. Animal Behavior 137:107-117.

Noren, S. R. 2008. Infant carrying behavior in dolphins: costly parental care in an aquatic environment. Functional Ecology 22:284-288.

Noren, S. R., Biedenbach, F., Redfern, J. V., and E. F. Edwards. 2007. Hitching a ride: the formation locomotion strategy of dolphin calves. Functional Ecology 22:278-283.

Oftedal, O. T. Lactation in whales and dolphins: evidence of divergence between baleen- and toothed-species. Journal of Mammary Gland Biology and Neoplasia 2:205-230.

Palsbøll, P. J., Heide-Jørgensen, M. P., and R. Dietz. 1996. Population structure and seasonal movements of narwhals, Monodon monoceros, determined from mtDNA analysis. Heredity 78:284-292.

Rendell, L., Cantor, M., Gero, S., Whitehead, H., and J. Mann. 2019. Causes and consequences of female centrality in cetacean societies. Philosophical Transactions of the Royal Society B 374:20180066.

Sargeant, B. L., and J. Mann. 2009. Developmental evidence for foraging traditions in wild bottlenose dolphins. Animal Behavior 78:715-721.

Simões-Lopes, P. C., Daura-Jorge, F. G., and M. Cantor. 2016. Clues of cultural transmission in cooperative foraging between artisanal fishermen and bottlenose dolphins, Tursiops truncatus (Cetacea: Delphinidae). Zoologia (Curitiba) 33:e20160107.

Smultea, M. A., Fertl, D., Bacon, C. E., Moore, M. R., James, V. R., and B. Würsig. 2017. Cetacean mother-calf behavior observed from a small aircraft off Southern California. Animal Behavior and Cognition 4:1-23.

Turgeon, J., Duchesne, P., Colbeck, G. J., Postma, L. D., and M. O. Hammill. 2011. Spatiotemporal segregation among summer stocks of beluga (Delphinapterus leucas) despite nuclear gene flow: implication for the endangered belugas in eastern Hudson Bay (Canada). Conservation Genetics 13:419-433.

Tyson, R. B., Friedlaender, A. S., Ware, C., Stimpert, A. K., and D. P. Nowacek. 2012. Synchronous mother and calf foraging behaviour in humpback whales Megaptera novaeangliae: insights from multi-sensor suction cup tags. Marine Ecology Progress Series 457:209-220.

Villegas-Amtmann, S., Schwarz, L. K., Sumich, J. L., and D. P. Costa. 2015. A bioenergetics model to evaluate demographic consequences of disturbance in marine mammals applied to gray whales. Ecosphere 6:1-19.

Weinrich, M. 1998. Early experience in habitat choice by humpback whales (Megaptera novaeaengliae). Journal of Mammalogy 79:163-170.

Wiley, D., Ware, C., Bocconcelli, A., Cholewiak, D., Friedlaender, A., Thompson, M., and M. Weinrich. 2011. Underwater components of humpback whale bubble-net feeding behavior. Behavior 148:575-602.