Marine mammals are found in orders Cetacea, Sirenia and Carnivora, which includes the suborder Pinnipedia, families Mustelidae and Ursidae. They are found among different habitats: some are being observed in rivers, mangroves, estuarine habitats, and in coastal shallow waters, others in pelagic seas and even at the edge of the abyssal plain. Due to the colonization of various habitats, marine mammals’ diet is very heterogeneous and, therefore, they exhibit a wide range of behaviors. It is extremely important to understand the ethology of these animals, in order to develop better conservation efforts.
Wild populations that spend a part of their lives on land can be observed more easily than populations that spend their entire life cycle underwater. For instance, males of the Southern elephant seals, compete for the reproductively active females outside the water, and are more polygynous than those pinniped species that breed in the water. Another interesting example is the allo-parental care exhibited by toothed cetaceans that live in social groups. Baleen whales, such as the humpback whale, exist in cow-calf pairs, but this form of social unit along with other aspects of their mating system needs to be studied in more detail, so that conservation efforts can aim in optimal breeding conditions.
A part of marine mammal species show dramatic sexual dimorphism, with males being much larger and having a diverse feeding strategy than females. In the study of Northern elephant seals, they have found that males tend to migrate north closer to shore and forage during benthic dives, while females, which are smaller in size, move west toward the open ocean and exhibit pelagic dives interspersed with trips to the sea floor, in order to feed in waters with less predators.
In highly social species, behavior might have an important role in differentiation between units to conserve and in understanding how populations decline or persist. Whitehead and his co-workers studied the rapid decline in number of sperm whales off the Galapagos Islands due to whaling. They have discovered that between the years 1990 and 1995, the abundance of larger males had dropped and so decreased the fertility rate. Specific behaviors may also be relevant for predicting population persistence. For example, adult females of Southern sea lions off the Falkland Islands exhibit two foraging strategies: inshore in yellow and offshore in orange; it is thought that this behavior is influenced by foraging site fidelity and independent of intraspecific competition.
In terms of exploitation, whalers from the past had violated the theory of ideal free distribution, which means that the number of individuals that will aggregate in different patches of resource is proportional to the amount of the resources available in each patch.
It is difficult to determine pre-exploitation abundance. For instance, the historical population estimates for humpback whales and North Atlantic fin based on models for mDNA sequence variation are up to 20 times higher than the present day populations. An important behavioral issue of concern for exploited animals is the buffer effect, where at low densities individuals concentrate in the best habitat, but at higher densities they are more dispersed. Since some areas show high density of polar bears, this might give a false indication of abundance to hunting communities.
Adaptation can occur faster as a result of social learning. For example, humpback whale males off eastern Australia adopted the radically different western Australian song, which they heard from a few rover males. Another example is the evolution of lobtail feeding observed in whale individuals that are concentrated at the center of this network.
However, in a study on Southern right whales, more non-calf whales were observed than cow-calf pairs in historically important whaling sites, which demonstrated the calves’ reluctance to explore new areas for feeding and breeding. In addition, the abundance of killer whales that feed on Chinook salmon depends on the abundance of the salmon. As shown in the graph, the birth rate of killer whales increased with the increasing coast-wide abundance of Chinook salmon. This conformist prey specialization could limit the use of new prey-items.
One of the techniques used to reduce bycatch, for example, is operating with non-lethal deterrents which have the ability to distract, scare away or cause pain to the animal. Since these stimuli can leave severe consequences on marine mammals, these techniques could be improved.
A huge threat to the behavior of marine mammals is most certainly noise that can travel almost five times faster through water than through air, and it can interfere with communication, foraging, navigation and sociality. For instance, male blue whales (Balaenoptera musculus) have lowered the frequency of their songs due to the noise from vessels.
It is crucial to know the marine mammals’ diversity, as well as their home range, mating system and to which anthropogenic threats are they sensitive. The protected area or the critical habitat should offer enough space for growth and normal behavior, food and air, but also sites for breeding and rearing the offspring. It has been observed that killer whales are more sensitive to disturbance from vessels when they are feeding, and this may be a great indicator for developing management strategies that focus on protecting the feeding hotspots.
Another challenging quest is determining the minimum area necessary for reserves which can help mitigate different threats, like bycatch, fisheries entanglement, prey depletion, but also noise. As we can see in the graph, echolocation was displayed only when sonar frequencies were not percepted. In addition, more attention should be paid to the wider ecosystems. For example, considerations for killer whales that feed specifically on salmon should not only be inclined to their habitat, but also to the river systems where salmon populations breed.
One of the most evident environmental problems of the human era is the increase in atmospheric carbon dioxide. As a result, marine ecosystems are facing dramatic shifts, especially in relation to the rising of temperature. A consequence is the melting of sea ice coverage most notable on the north and south pole regions, where polar bears (Ursus maritimus) have become the flagship species for climate change.
Another human-caused environmental change is the use of fossil fuels, particularly the single use of plastics, which cannot be bio-degraded in the ocean. Marine mammals that inhabit coastal areas are more prone to get entangled or ingest these materials.
Aspects of behavioral ecology, such as social learning mechanisms, how is a social unit made, the role of each individual within it and their personality need to be understood in detail. Social information and fine scale social structure of marine mammals may strongly influence social dynamics and vital rates. Consistent individual differences in behavior may also influence the ability of each individual to cope with novel conditions.
While the outstanding aim for the conservation of marine mammals is maintaining genetic integrity, more focus should be put also in maintaining their sociality and behavioral diversity. In order to incorporate behavioral ecology into management models and conservation efforts for marine mammals, we have to ask ourselves: “Which are the most relevant factors?” and make our best to answer the question.