Abstract:
Physically weighing large marine mammals sequentially over time has presented researchers
with a logistical challenge and has severely limited sample sizes. Using a well-established photogrammetry
method we developed a simple mathematical method to calculate accurate mass measurements at specific
stages in the life cycle of a top marine predator. Female southern elephant seals (n523) at Marion Island
were sampled sequentially using photogrammetry and three-dimensional models (based on each
photogrammetry project) were built for estimation of body mass. Simple equations were applied to
obtain mass at critical instances in their life cycle. Marion Island elephant seal mass data was compared to
data obtained from physically weighed elephant seals from King George, South Georgia and Macquarie
islands. Females from Marion Island are smaller, but their percentage lactation mass loss is similar to
females from these other populations. The similarity of percentage mass loss during lactation between
different female populations illustrates the accuracy and practicality of the photogrammetric method over a
temporal scale. Photogrammetric mass estimation can be used alongside datasets of physically weighed
animals and can greatly benefit ecology and life history studies.
