The Tooth, the Whole Tooth and Nothing but the Tooth
August 28, 2017 Georgia French
In late 2014 and early to mid 2015, I was lucky enough to collect data on white sharks with the Dyer Island Conservation Trust (DICT) and Marine Dynamics (MD). I’m working on my PhD with the University of Sussex, focussing on sexual and individual variation in white shark ecology. I’m very pleased that the first chapter of my PhD has just been published as a paper in the Journal of Fish Biology.
This paper is all about the white shark’s famous teeth, and how they change shape as they get older. Kind of like in humans, smaller white sharks are thought to have “baby teeth” – pointy in shape, which is believed to be an adaptation for gripping fish prey. When the sharks hit roughly 3m in length, they’re then thought to get their “adult teeth” which are broad in shape and believed to facilitate catching and eating marine mammals like seals and dolphins. This change in tooth shape is taken as established fact, and is accepted and quoted around the world. However, through reading several books and papers I’d found a few pieces of evidence that seemed to show that this wasn’t always the case, so I decided to look into it further.
The first challenge was to figure out how to get measurements of the teeth. All of the previously published work on white shark teeth was from dead sharks – I needed to come up with a method to get the same measurements from live ones. This is where DICT and MD came in. I realised that cage diving provides the perfect opportunity to get good photos of the shark’s teeth – if you look through the MD blog, you’ll see what I mean! When the sharks interact with the seal decoy or bait lure, they often either treat them like prey, or they try to check them out with their mouths (as they are lacking in the hand department, they use their mouths to see what things feel and taste like). This means that they open their mouths a lot – perfect.
I designed a method that meant that I could calculate how “pointy” the shark’s teeth were from photographs taken during cage diving trips. Being able to work from Slashfin was a huge bonus, as it’s very stable and comfortable, and has a nice high viewing deck that meant myself and colleagues could get photos from above, as the sharks came out of the water. In addition to the tooth photos, from Slashfin it was possible to estimate each shark’s length, see if it was male or female, and identify each individual so that I would know if I had measured its teeth twice. I then combined these data with published information and measurements taken from the jaw collection of the KwaZulu-Natal Sharks Board and analysed the relationships between male and female shark length and their tooth shape. The findings were pretty astounding.
It turns out that it seems like the males and females are very different when it comes to their teeth. The main results showed that male sharks do change their teeth from pointy to broad, but females don’t. In the females, I identified three different tooth shapes – pointed, intermediate and broad – and a female of any size could fit one of these categories. This suggests that there may be three “types” or morphs of female, which could be specialising on different types. Alternatively, tooth shape could actually have nothing to do with diet, and instead males could be using their broad teeth to help them hold onto females during mating. Both of these scenarios blow the classic concept of tooth shape change through a white shark’s life out of the water.
Experience and Thanks
During data collection, it was great to be able to talk to interested clients about what I was doing, and to spend time in the company of the most awesome fish in the sea was just fantastic. I was really grateful to all of the crew of Slashfin, who skilfully brought the sharks to the boat, helped me to accurately estimate the shark’s lengths and sexes and identified individual sharks that they knew.
It was also an absolute pleasure to give talks to and work with groups from International Marine Volunteers, who got valuable experience in white shark research through MD and DICT. One of these volunteers, Richard Dolan, is actually a co-author on the tooth paper as he volunteered to help individually identify the sharks in my dataset using DARWIN fin ID software – the same technique used previously by DICT scientists to make the first population estimate for white sharks in South Africa using photo ID. In fact, DICT associate David Edwards, who managed the DARWIN database for that paper, is another co-author on this research as he also helped out massively with my DARWIN work.
I’m hugely thankful to Onno Keller and Kelly Baker, MD Marine Biologists past and present, who in addition to providing me with accurate information on the sharks when we were on the boat, gave me some of their tooth photos from the trips. I also have to mention and thank Reservations Manager Aletta van Bosch for getting me onto the shark trips and of course, Wilfred Chivell, owner of Marine Dynamics and Founder of DICT, without whom none of the fieldwork would have possible.