There are many, many questions that scientists still have about shark behaviour... The challenges of observing them when they are far out to sea or swimming at great depths, means that many aspects of their lives are mysterious to us. One of the most elusive is the mating ritual. There are only a handful of species of sharks that scientists have observed mating. For the vast majority, we know nothing about where and when they breed or how they choose a partner. So, it is very exciting when researchers witness a shark seduction...
Meet The Whitetip Reefer
Whitetip reef sharks (Triaenodon obesus), are relatively small sharks (only reaching a maximum of 1.6m total length), found throughout a broad tropical region in the Indo-Pacific Ocean. They favour coastal regions, so are especially common around coral reefs, where they prey upon small fish, squid and shellfish. They are often found in large aggregations, and even hunt and rest together in groups. If you would like to learn more about whitetip reef shark group hunting, you can check out Every Shark for Themselves.
It Starts with a Bite
Scientists studying these sharks in Costa Rica, central America have been lucky enough to document their mating behaviour. As they captured video footage of the events, they were able to describe whitetip mating in detail... And they discovered shark sex can be quite violent!
Each mating event began with the male shark grasping one of the female's pectoral fins (the paired fins that stick out the sides) in his mouth. This is known as a 'pectoral grasp'. At this point the male then inserted his 'clasper' into the female's 'cloaca' (At this point the writing will become quite graphic... I will try to deliver the with less cringe than 50 Shades... I apologise in advance). The male then repeatedly thrusted until ejaculation. During this time, the pair sank through the water and often ended up resting on the ocean floor (Whitney et al, 2004).
Ladies' Choice
The scientists noted that not all females were as eager as others. In fact, some were out-right not having it! (That sounds familiar, doesn't it, boys!?) After the male bit her fin, some females actively positioned their body so the male could not mate with her. Some arched away, others blocked the male with their fins and some positioned themselves on the ocean floor where they could not be reached. This is called "avoidance behaviour" and shows that females have a say in who they reproduce with (known as "female mate choice") (Whitney et al, 2004).
It all sounds quite rapey and violent so far, doesn't it!? Well... it gets even more extreme!
Sharks Have Sex in Groups
The scientists also discovered that whitetip reef sharks often mated in groups! In most of the videos, there were more males present at a mating event than females, with other male sharks circling the mating pair. These males were of a similar size to the male mating with the female. Furthermore, the researchers even caught footage of a whole group of whitetip sharks all mating together; with two females and four males all clustered together. This is known as 'group copulation' and has also been documented in other species of sharks (Whitney et al, 2004).
These Behaviours Have Not Evolved by Accident!
We have a little giggle about all this (aren't we mature), but actually, in terms of the science, these findings are fascinating and teach us a lot about whitetip reef shark behaviour.
For example, female mate choice shows us that females are selective in which mate they think is best to reproduce with. This means they are assessing the males for good features and good genes, in order to ensure they produce the healthiest offspring they can. This behaviour leads to 'sexual selection', where the breeding choices made by individual animals, eventually cause the species to adapt and evolve over time. For instance, if females favour large males that are strong swimmers, these males with mate more often, and over time the genes that code for rapid growth and big, strong fins will become more prevalent in the population. Over a very, very long period of time, this can cause the species to evolve so that all the sharks have these features (Darwin, 1859; Jones & Ratterman, 2009).
The group copulation in whitetip reef sharks is also very interesting. When the two males both grasped a female's pectoral fin, they appeared to be actively competing with each other to mate with her. 'Competition' is also a very important process in sexual selection and is common across taxa throughout nature. Male deer butting heads... beetles fighting each other... male birds displaying their incredible plumage all occur for the same purposes: to show a female they are better than any other male around and impress her enough that she will reproduce with him. This is known as 'courtship' and when many males are all competing for the one female, we call it 'group courtship' (Darwin, 1859; Jones & Ratterman, 2009).
This study shows us that whitetip reef sharks do not just mate with any old partner they find, but actively make specific choices about who is best to breed with... Even when the males seem quite pushy and violent, in whitetip reef sharks, it seems that it is the girls that have a lot of the power!
To learn more about shark mating rituals, you can check out Do a Little Dance, Make a Little Love.
References
Darwin C (1859). On the Origin of Species by Means of Natural Selection, or Preservation of Favoured Races in the Struggle for Life. John Murray, London.
Jones AG & Ratterman NL (2009). Mate choice and sexual selection: What have we learned since Darwin? PNAS, 106:1, 10001-10008. Access online.
Tricas TC & Le Feuvre EM (1985). Mating in the reef white-tip shark Triaenodon obesus. Marine Biology, 84, pp.233-237. Access online.
Whitney NM, Pratt HL & Carrier JC (2004). Group courtship, mating behaviour and siphon sac function in the whitetip reef shark, Triaenodon obesus. Animal Behaviour, 68, 1435–1442. Access online.
By Sophie A. Maycock for SharkSpeak
