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Size and morphology of males: sexual selection ?

The most amazing fact about elephant seals is the huge difference in size and morphology between males and females. Males may weight up to four tons while maximum recorded weight for females is around 700 kgs, and the most of them are notably lighter. Males are provided by proboscis, enlarged canine teeth and a stiff dermal shield in the chest. All there characters may be the result of evolution by sexual selection: larger males may be much more effective in fighting and in achieving control of females, larger canine teeth could be used to give deeper bites, and the dermal shield could be able to protect from them, females may prefer to mate with males with larger proboscis.

Photogrammetric measurement

The most difficult problem to cope with in studying these morphological characters is the huge size of elephant seals: they are so big that direct weighing and measure would not be possible. Moreover, the most puzzling character, the trunk, should be measured while inflated and with the animal behaving naturally to provide meaning full data. Hence, we used indirect photogrammetric methods to measure length of the body and dimension of the proboscis: by taking pictures of the animal with a calibrated surveying pole in view we are able to achieve a good precision in measurement without any direct manipulation of the animal. The area of the side outline as measured from picture is a very good indicator of weight (with this procedure an estimate of weight may be derived also for the largest males, which, on Sea Lion Island, may be over 4 tons). We verify accuracy of these measurements by taking direct measures with tape and by calculating repeatability indices of photographic measures.

Photogrammetric measuremet of size

Photogrammetric measurement of size