Sperm Competition in the Florida Manatee (Trichechus manatus latirostris)

Defense Date


Document Type


Degree Name

M.S. Marine Biology

First Advisor

Edward Keith

Second Advisor

Curtis Burney


Across all taxa, animals exhibit a diversity of mating systems such as a single male mating with one female or multiple males competing to mate with a single female. In the Order Sirenia (the fully aquatic herbivorous mammals known as manatees and dugongs) a polyandrous lifestyle is most common.

The Floridamanatee (Trichechus manatus latirostris), a federally endangered marine mammal, inhabits both coasts of theFlorida peninsula. During the breeding season, this species exhibits a promiscuous lifestyle with multiple males mating with an individual female during her time of estrus. This mating strategy suggests that manatees are sperm competitors.

Sperm competition has been identified as the competition between sperm from multiple males attempting to fertilize one female ovum. The focus of this capstone paper is to provide a comprehensive overview of the various mechanisms of sperm competition across a wide range of taxa and determine whether they occur in theFloridamanatee.

A study conducted over 20 years ago first characterized the relationship between testes size and mating strategies. From this study we have gained an allometric equation that defines the testicular size and body mass for typical mammals, those that are monogamous and thus not sperm competitors. This equation has proven particularly useful in determining typical versus above average testes size in theFloridamanatee compared to other marine mammals.

To date there has been only one study regarding the presence of sperm competition in the Floridamanatee. Although the testes of Floridamanatees (0.12% of their body weight) are larger than one would expect for a non sperm competitor, they are not as large as those of known sperm competitors, such as the harbor porpoise (Phocoena phocoena) (2.3± 1.4% of their body weight).

Data for the Floridamanatee, harbor porpoise and common dolphin (Delphinus delphis) revealed that all three species reproduce seasonally and peak sperm production was during the months of May-September. An increase in testicular activity and size was documented during the mating season. In contrast, seasonality has no effect on mating systems or testes size for animals such as primates. For this reason enlarged testes may be a physiological adaptation for sperm competition instead of a consequence of reproductive seasonality as once thought.

From the fish research examined I found that polygamous fish, such as the African cichlid, had significantly higher testes masses than those of their closest monogamous relatives although there were no differences in body mass between the two. This study suggests that both mating system and testes mass are reliable indices of sperm competition. Based on this information, manatees may be considered sperm competitors because they employ a polygamous mating system.

Comparisons of manatees and birds and insects remain much more difficult because there are different mechanisms of sperm competition among the three groups. However from birds and insects I have learned that sperm morphology may be the next key component to determining whether manatees are sperm competitors.

When examining manatee behavioral evidence (promiscuous mating) and the anatomical observations (testicular mass vs. body mass) I see two selective strategies which compliment one another. Therefore, I must conclude that manatees do exhibit sperm competition, but this will be finally confirmed only with more data.

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