Closely related species are expected to diverge in foraging strategy, reflecting the evolutionary drive to optimize foraging performance. The most speciose cetacean genus, Mesoplodon, comprises ...beaked whales with little diversity in external morphology or diet, and overlapping distributions. Moreover, the few studied species of beaked whales (Ziphiidae) show very similar foraging styles with slow, energy-conserving movement during long, deep foraging dives. This raises the question of what factors drive their speciation. Using data from animal-attached tags and aerial imagery, we tested the hypothesis that two similar-sized mesoplodonts, Sowerby's (Mesoplodon bidens) and Blainville's (Mesoplodon densirostris) beaked whales, exploit a similar low-energy niche. We show that, compared with the low-energy strategist Blainville's beaked whale, Sowerby's beaked whale lives in the fast lane. While targeting a similar mesopelagic/bathypelagic foraging zone, they consistently swim and hunt faster, perform shorter deep dives, and echolocate at a faster rate with higher frequency clicks. Further, extensive near-surface travel between deep dives challenges the interpretation of beaked whale shallow inter-foraging dives as a management strategy for decompression sickness. The distinctively higher frequency echolocation clicks do not hold apparent foraging benefits. Instead, we argue that a high-speed foraging style influences dive duration and echolocation behaviour, enabling access to a distinct prey population. Our results demonstrate that beaked whales exploit a broader diversity of deep-sea foraging and energetic niches than hitherto suspected. The marked deviation of Sowerby's beaked whales from the typical ziphiid foraging strategy has potential implications for their response to anthropogenic sounds, which appears to be strongly behaviourally driven in other ziphiids.
Bottlenose dolphin stock structure in the northeast Atlantic remains poorly understood. However, fine scale photo-id data have shown that populations can comprise multiple overlapping social ...communities. These social communities form structural elements of bottlenose dolphin (Tursiops truncatus) corrected populations, reflecting specific ecological and behavioural adaptations to local habitats. We investigated the social structure of bottlenose dolphins in the waters of northwest Ireland and present evidence for distinct inshore and offshore social communities. Individuals of the inshore community had a coastal distribution restricted to waters within 3 km from shore. These animals exhibited a cohesive, fission-fusion social organisation, with repeated resightings within the research area, within a larger coastal home range. The offshore community comprised one or more distinct groups, found significantly further offshore (>4 km) than the inshore animals. In addition, dorsal fin scarring patterns differed significantly between inshore and offshore communities with individuals of the offshore community having more distinctly marked dorsal fins. Specifically, almost half of the individuals in the offshore community (48%) had characteristic stereotyped damage to the tip of the dorsal fin, rarely recorded in the inshore community (7%). We propose that this characteristic is likely due to interactions with pelagic fisheries. Social segregation and scarring differences found here indicate that the distinct communities are likely to be spatially and behaviourally segregated. Together with recent genetic evidence of distinct offshore and coastal population structures, this provides evidence for bottlenose dolphin inshore/offshore community differentiation in the northeast Atlantic. We recommend that social communities should be considered as fundamental units for the management and conservation of bottlenose dolphins and their habitat specialisations.
Animals are distributed relative to the resources they rely upon, often scaling in abundance relative to available resources. Yet, in heterogeneously distributed environments, describing resource ...availability at relevant spatial scales remains a challenge in ecology, inhibiting understanding of predator distribution and foraging decisions.
We investigated the foraging behaviour of two species of rorqual whales within spatially limited and numerically extraordinary super‐aggregations in two oceans. We additionally described the lognormal distribution of prey data at species‐specific spatial scales that matched the predator's unique lunge‐feeding strategy.
Here we show that both humpback whales off South Africa's west coast and blue whales off the US west coast perform more lunges per unit time within these aggregations than when foraging individually, and that the biomass within gulp‐sized parcels was on average higher and more tightly distributed within super‐group‐associated prey patches, facilitating greater energy intake per feeding event as well as increased feeding rates.
Prey analysis at predator‐specific spatial scales revealed a stronger association of super‐groups with patches containing relatively high geometric mean biomass and low geometric standard deviations than with arithmetic mean biomass, suggesting that the foraging decisions of rorqual whales may be more influenced by the distribution of high‐biomass portions of a patch than total biomass. The hierarchical distribution of prey in spatially restricted, temporally transient, super‐group‐associated patches demonstrated high biomass and less variable distributions that facilitated what are likely near‐minimum intervals between feeding events.
Combining increased biomass with increased foraging rates implied that overall intake rates of whales foraging within super‐groups were approximately double those of whales foraging in other environments. Locating large, high‐quality prey patches via the detection of aggregation hotspots may be an important aspect of rorqual whale foraging, one that may have been suppressed when population sizes were anthropogenically reduced in the 20th century to critical lows.
A free Plain Language Summary can be found within the Supporting Information of this article.
A free Plain Language Summary can be found within the Supporting Information of this article.
To understand the effects of anthropogenic disturbance on the nutritional health of animals, it is important to measure and understand the morphometrics, allometrics, and body condition of the ...species. We examined the body shape, allometric relationships, and body condition of short-finned pilot whales (Globicephala macrorhynchus) in three locations across the North Atlantic. Using unmanned aerial vehicles, the body length (BL) and width (along the body axis) were measured from photographs of the dorsal side, while body height (dorso-ventral distance) was measured on the lateral side. Seventy-seven pilot whales were measured (mean ± SD), including 9 calves (BL 2.37 m ± 0.118), 31 juveniles (2.90 m ± 0.183), and 37 adults (3.72 m ± 0.440). The body shape was similar among reproductive classes, with the widest point being anterior of the dorsal fin (at 30–35% BL from the rostrum). The cross-sectional body shape of the whales was flattened in the lateral plane, which increased towards the peduncle and fluke. The rostrum-blowhole distance and fluke width increased linearly with BL. The estimated volumes of pilot whales ranged between 0.15 and 0.32 m3 for calves, 0.25 and 0.64 m3 for juveniles, and 0.46 and 1.13 m3 for adults. The body condition (residual of log-volume vs. log-length) ranged from −34.8 to +52.4%. There was no difference in body condition among reproductive classes or locations.
Social deep-diving odontocetes face the challenge of balancing near-surface proximity to oxygen and group members with foraging in the deep sea. Individuals rely on conspecifics for critical life ...functions, such as predator defence, but disperse during foraging to feed individually. To understand the role of social acoustic mediation during foraging in deep-diving toothed whales, we investigated the context of social burst-pulse call production in Risso’s dolphin ( Grampus griseus ) using biologgers. Dolphins produced context-specific burst pulses predominantly during daytime foraging, preceding or following foraging dives and in the early descent of daytime deep dives. Individuals applied differential short and long burst-pulse calls intended for either near-surface receivers (horizontal transmission) or deep-foraging receivers (vertical transmission). Our results show that deep-diving toothed whales are reliant on acoustic communication during certain foraging contexts, to relay information including foraging conditions or an individual’s location. Moreover, they accentuate the importance of maintaining acoustic contact with conspecifics, specifically when dispersed during deeper foraging. It also signifies that our oceanic top predators may be specifically vulnerable to the current strong increase in anthropogenic noise. Potential masking of the signals from group members communicating at a distance could undermine their social cohesion, and hence their capacity to maintain vital life functions.
Monitoring the abundance of mobile and wide‐ranging cetacean populations for conservation management is challenging, especially when the management is focused on static protected areas. Where ...abundance estimates are derived from mark–recapture data, such as photo‐identification of naturally marked individuals, unpredictable movements of animals in and out of the survey area can reduce ‘capture’ probabilities and affect the precision and accuracy of resulting estimates.
A Bayesian hierarchical log–linear model was applied to photo‐identification data collected in summer 2014 to derive a multi‐site abundance estimate for a population of bottlenose dolphins, Tursiops truncatus, ranging widely throughout the coastal waters of western Ireland. In addition, the effects of varying levels of sampling effort on the minimum detectable decrease in population size were examined.
The abundance (median) of dolphins was estimated as 189 (coefficient of variation (standard deviation/mean), 0.11; 95% highest‐posterior density interval, 162–232). Over 50% of the well‐marked dolphins encountered throughout the study were sighted in more than one distinct coastal area, thus displaying high mobility. In addition, it was found that it would require biennial surveys to detect a 25% decline in abundance within the six‐year reporting period of the EU’s Habitats Directive.
Given that the Special Area of Conservation designated for these dolphins consists of two separate areas covering a substantial portion of the west coast of Ireland, the multisite approach is appropriate for monitoring this population. It produces a more precise estimate and is well suited for sparse recapture data collected opportunistically at multiple sites, when the lack of resources prevents large‐scale surveys or when concentrating surveys on smaller localized areas fails to capture the broad range and unpredictable occurrence of the animals. The Bayesian multi‐site approach could be applied to the management of other wide‐ranging marine or terrestrial taxa.
Long-finned pilot whales (Globicephala melas) are highly social cetaceans that live in matrilineal groups and acquire their prey during deep foraging dives. We tagged individual pilot whales to ...record their diving behaviour. To describe the social context of this individual behaviour, the tag data were matched with surface observations at the group level using a novel protocol. The protocol comprised two key components: a dynamic definition of the group centred around the tagged individual, and a set of behavioural parameters quantifying visually observable characteristics of the group. Our results revealed that the diving behaviour of tagged individuals was associated with distinct group-level behaviour at the water's surface. During foraging, groups broke up into smaller and more widely spaced units with a higher degree of milling behaviour. These data formed the basis for a classification model, using random forest decision trees, which accurately distinguished between bouts of shallow diving and bouts of deep foraging dives based on group behaviour observed at the surface. The results also indicated that members of a group to a large degree synchronised the timing of their foraging periods. This was confirmed by pairs of tagged individuals that nearly always synchronized their diving bouts. Hence, our study illustrates that integration of individual-level and group-level observations can shed new light on the social context of the individual foraging behaviour of animals living in groups.
Foraging decisions of deep-diving cetaceans can provide fundamental insight into food web dynamics of the deep pelagic ocean. Cetacean optimal foraging entails a tight balance between ...oxygen-conserving dive strategies and access to deep-dwelling prey of sufficient energetic reward. Risso's dolphins (
) displayed a thus far unknown dive strategy, which we termed the spin dive. Dives started with intense stroking and right-sided lateral rotation. This remarkable behaviour resulted in a rapid descent. By tracking the fine-scale foraging behaviour of seven tagged individuals, matched with prey layer recordings, we tested the hypothesis that spin dives are foraging dives targeting deep-dwelling prey. Hunting depth traced the diel movement of the deep scattering layer, a dense aggregation of prey, that resides deep during the day and near-surface at night. Individuals shifted their foraging strategy from deep spin dives to shallow non-spin dives around dusk. Spin dives were significantly faster, steeper and deeper than non-spin dives, effectively minimizing transit time to bountiful mesopelagic prey, and were focused on periods when the migratory prey might be easier to catch. Hence, whereas Risso's dolphins were mostly shallow, nocturnal foragers, their spin dives enabled extended and rewarding diurnal foraging on deep-dwelling prey.
Despite their enormous size, whales make their living as voracious predators. To catch their much smaller, more maneuverable prey, they have developed several unique locomotor strategies that require ...high energetic input, high mechanical power output and a surprising degree of agility. To better understand how body size affects maneuverability at the largest scale, we used bio-logging data, aerial photogrammetry and a high-throughput approach to quantify the maneuvering performance of seven species of free-swimming baleen whale. We found that as body size increases, absolute maneuvering performance decreases: larger whales use lower accelerations and perform slower pitch-changes, rolls and turns than smaller species. We also found that baleen whales exhibit positive allometry of maneuvering performance: relative to their body size, larger whales use higher accelerations, and perform faster pitch-changes, rolls and certain types of turns than smaller species. However, not all maneuvers were impacted by body size in the same way, and we found that larger whales behaviorally adjust for their decreased agility by using turns that they can perform more effectively. The positive allometry of maneuvering performance suggests that large whales have compensated for their increased body size by evolving more effective control surfaces and by preferentially selecting maneuvers that play to their strengths.
The detection and use of emitters’ signals by unintended receivers, i.e., eavesdropping, represents an important and often low-cost way for animals to gather information from their environment. ...Acoustic eavesdropping can be a key driver in mediating intra- and interspecific interactions (e.g., cooperation, predator–prey systems), specifically in species such as cetaceans that use sound as a primary sensory modality. While most cetacean species produce context-specific sounds, little is known about the use of those sounds by potential conspecific eavesdroppers. We experimentally tested the hypothesis that a social cetacean, Risso’s dolphin (
Grampus griseus
), is able to gather biologically relevant information by eavesdropping on conspecific sounds. We conducted playback experiments on free-ranging dolphins using three context-specific sounds stimuli and monitored their horizontal movement using visual or airborne focal follow observations. We broadcasted natural sequences of conspecific foraging sounds potentially providing an attractive
dinner bell
signal (
n
= 7), male social sounds simulating a risk of forthcoming agonistic interaction (
n
= 7) and female-calf social sounds representing no particularly threatening context (
n
= 7). We developed a quantitative movement response score and tested whether animals changed their direction of horizontal movement towards or away from the playback source. Dolphins approached the foraging and the social female-calf sounds whereas they avoided the social male sounds. Hence, by acoustically eavesdropping on conspecifics, dolphins can discriminate between social and behavioural contexts and anticipate potential threatening or beneficial situations. Eavesdropping and the ensuing classification of ‘friend or foe’ can thus shape intra-specific social interactions in cetaceans.
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