I’m fortunate enough to get on incredibly well with my brother but this is not something that can be said for a lot of siblings in the natural world. When there is a finite resource required by many it often isn’t long before the natural instinct to survive kicks in; this is especially true of competition for food. My brother and I are never more at war than when there is only one piece of cake left! Some animals however have taken this competition for survival, and resulting sibling rivalry, to a whole new level, resorting to intrauterine cannibalism. Yes, that is exactly as it sounds!
Sharks tend get a bad rep. Let’s be honest, generally speaking this is no fault of their own and they are only ever acting in their nature, something that we exploit when doing things such as cage diving where we tempt them towards humans with the promise of tasty treat and which certainly isn’t helped by the medias apparent obsession with shark attacks. The truth is, if you really look at the figures, the number of shark related attacks is surprisingly low! And we do tend to put ourselves in harm’s way don’t we? A shark that really doesn’t help the family’s reputation however, is the sand tiger shark (Carcharias taurus) which can be found in the Atlantic, Pacific and Indian Oceans and in the Mediterranean and Adriatic Seas. It measures up to 3.5m in length and will literally eat its siblings alive to get ahead in life! It is listed as vulnerable on the International Union for Conservation of Nature Red List and can regularly be found in aquariums due to its ‘tolerance’ for captivity. Female tiger sharks often mate with multiple males and produce up to 50 embryo by various fathers which will develop in one of the females two uterine horns and gain nutrients from their yolk sacs and uterine fluids. The embryos grow steadily until one of them reaches about 10cm in length and becomes “the hatchling”, the more dominant of the embryos. At this point its natural sibling rivalry kicks in at it turns on its smaller brothers and sisters, choosing to eat them one by one to gain the nutrients it requires to grow, a process called adelphophagy meaning literally “eating ones brother”. This “hatchling” will grow to over 1m long over the next year until the mother gives birth to it, a fully independent shark. This process reduces the reproductive rate of the female sand shark to less than one pup per year but ensures that the largest, and therefore presumably healthiest, offspring survives.
Unfortunately their dwindling numbers can be put down to the previously mentioned bad rep given to sharks en masse, and our appalling response to it. When the first wave of “shark hysteria” hit, the Sand Tiger Shark suffered terribly due to its fearsome appearance, namely it’s rows of large, permanently visible, long and sharp teeth. We hunted these naturally shy animals ferociously and their numbers are still recovering. Luckily recovering is the word but with a such a slow reproductive rate it is going to take some time for them to come back from all the damage we did.