We analyze the levels of reproductive steroid hormones in blubber to determine life history traits of wild dolphins and whales. For females, both pregnancy and sexual maturity is indicated by elevated levels of progesterone and estradiol, respectively. In males, elevated testosterone levels are associated with sexual maturity. We quantify these hormones in the blubber and use the information to determine how many animals in a population are sexually mature, pregnant, or both; this in turn tells us about the health and status of that population.
By determining these life history traits (pregnancy rates and maturity status) researchers can further the understanding of some basic cetacean biology, plus monitor the growth or decrease of cetacean populations that have been affected by human activities. There are many dolphin and whale species in which the answers to simple questions are poorly understood, such as: how often do animals reproduce; how many years between birth and sexual maturity; and what proportion of the population are young animals versus adults? By quantifying these hormones we can help answer these questions. In addition, for depleted populations such as certain spotted dolphin populations (Stenella attenuatta) in the eastern Pacific Ocean, knowing these life history traits helps determine if and how well a population is recovering. For example, populations whose pregnancy rates are high and have high proportions of mature animals are more likely to recover after being depleted.
The reason why we examine steroids in blubber is because it is the end result of the most tenable method of obtaining life history information from many wild cetaceans. Imagine your bow of a research vessel and just a few hundred feet before you are a thousand spotted dolphins, zigging and zagging, leaping and diving. At any one time only a fraction of the school is visible from the surface. Because they are moving around so much and spend a small proportion of their time at the surface, merely estimating the total number of animals in the school with any precision is extremely difficult and it is next to impossible to figure out how many are young or old, male or female, or pregnant or not pregnant. To help us, we employ a darting technique that takes a small piece of skin and blubber called a biopsy. This is a technique that has been well developed by cetacean field researchers, and one which can obtain samples quite rapidly — up to fifty biopsies in a single day.
With this sampling technique and newly developed laboratory procedures that quantify the blubber hormone levels we are furthering our knowledge of the life history of many wild cetacean species plus learning about how steroids are stored. We are presently analyzing hormone levels for populations of common dolphins (Delphinus delphis), northern right whale dolphins (Lissodelphis borealis), Pacific white sided dolphins (Sagmatias obliquidens), and humpback whales (Megaptera novaeangliae). Soon we will focus on dolphins of the eastern tropical Pacific and bowhead whales (Balaena myticetus). The laboratory procedures have produced useful results for these animals in term of distinguishing life history traits plus they given us insights in steroid blubber physiology such as where in the blubber to expect higher than average steroid content. All in all these techniques combine to form a useful way to study the life history traits of wild cetaceans.
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