Abstract
In any clinical assessment of a cetacean body scoring is important, but what body mass changes produce a change in body scoring? Most of the effort has been made attempting to understand physiological changes that occur that lead to blubber deposition and depletion, especially in migratory and toporous species of mammals. However, Kershaw et al. state that blubber thickness as a marker of body condition in cetacean has its limitations.1 Derous et al. argue that muscle mass (and even fat within muscle) may be used as fuel during periods of starvation to conserve blubber for thermoregulation and buoyancy as a consequence of living in an aquatic environment.2
Although blubber thickness varies by site on most odontocetes, with beluga whales demonstrating the greatest variability, it is understood that utilization and deposition likely occurs proportionally over the body. In contrast, beluga whales differ from other odontocetes in possessing rails that are sites of supplemental deposition, likely enabling survival during long fasting periods. In most other odontocetes, changes in blubber thickness, especially when diminished lead to uniform changes such as can be seen as prominence of the neck, rib, base of the dorsal fin, and peduncular vertebral wings.
In an attempt to answer the question whether any of these changes in body shape are due to changes in muscle and/or blubber masses, we measured blubber and muscle thickness by ultrasound (using 2–5 MHz/10–5 MHz, Sonosite Edge® I/II/M-Turbo®, Bothell, WA) in individuals experiencing illness or fasting (due to reductions in base or during intense breeding periods). Knowing the important role of blubber for buoyancy and streamlining and muscle for locomotion, we theorized that illness produced more significant changes in muscle mass, while the opposite is observed during periods of protracted fasting, as was proposed by Swartz et al.3
By measuring blubber in three regions (thorax, abdomen, and peduncle) and muscle in three regions (neck, thorax, and peduncle), we observed significant muscle mass reductions in dolphins affected by cancer (squamous cell carcinoma and leukemia) and serious inflammatory diseases (abscess and pneumonia). In one individual, both muscle and blubber masses were depleted prior to death. In contrast, blubber thickness diminished in individuals receiving less diet or having a greater energetic requirement such as what occurs during breeding. Fortunately, no individual lost enough blubber to alter buoyancy or thermoregulation due to changes in habit or food availability.
Although these data do not prove that ill cetaceans only conserve blubber to maintain thermoregulation and buoyancy, once locomotion becomes impaired, we theorized that blubber begins to deplete. If blubber is significantly depleted shivering ensues, buoyancy is lost, and when combined with loss of locomotion the individual is unable to stay afloat. Unfortunately, if the disease cannot be reversed, such as is the case with cancer or untreatable disease, the individual will be unable to remain buoyant. I have also observed these changes with two irreversible cancer patients where both were unable to swim normally, began shivering, and required assistance to stay afloat. Conversely, if the illness is successfully treated, muscle and blubber should return to normal. I have observed this in three cases of serious systemic illnesses that required aggressive long-term antimicrobial therapy.
Being able to adequately assess the health of marine mammals is vital to early detection of illness. We propose that blubber and muscle measurements add one more feature to a physical examination that might provide clues to detecting early illness or current energetic needs not being met. Of course, these measurements do not always provide the exact cause, but it certainly leads to making careful early diagnostic decisions.
*Presenting author
+Student presenter
References
1. Kershaw JL, Botting CH, Brownlow A, Hall AJ. Not just fat: investigating the proteome of cetacean blubber tissue. Conserv Physiol. 2018;(1):coy003.
2. Derous D, Doeschate M, Brownlow AC, Davison NJ, Lussear D. Toward new ecologically relevant markers of health for cetaceans. Mar Sci. 2020;7.
3. Schwartz JR, Reidarson TH, Mejia-Fava JC. Richardson JL, Winship KA, Eskelien HC. IAAAM. Using ultrasound to measure muscle and blubber thickness as a metric for body condition in Tursiops truncatus. In: Proceedings from the IAAAM. 2018.