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Book Summary: The Book of Humans: A Brief History of Culture, Sex, War and the Evolution of Us

Author: Adam Rutherford

Substory: Sponging Dolphins

Everyone knows how smart dolphins are. They do tricks, rescue swimmers and are legendarily helpful. In all the neuroscience metrics mentioned above, cetaceans (and particularly dolphins) score very well. But despite their big brains, and complex social behaviours, sophisticated and unpleasant sexual behaviours (which we will come to shortly), and communication skills, your average dolphin merely has flippers.

Of the forty living species of dolphin, all have front flippers in which the bones are brilliantly homologous to the bones in our hands, almost perfectly like-for-like, as we both also share with horses’ front legs, and bats’ wings. This unequivocally shows our shared and relatively recent ancestry as mammals.1 But dolphins don’t have any musculature that allows differential dexterity, and flippers are fused as a flat paddle, even though the equivalent finger bones sit within. They don’t do much more than flap backwards and forwards to propel the owner in water. Admittedly, incredibly skilful though they are at swimming, there are few examples of tool use that don’t require clamping hold of an external object in order to manipulate it. Because of their flippers, dolphins, whales, porpoises and other cetaceans aren’t very good at that.

This, again, is a reminder that big brains are necessary but not sufficient to propel a species towards technological prowess. We have our hands and brains, and chimpanzees use their hands, teeth and lips to fashion sticks. Cetaceans have minimal muscular control of their jaws, and no hands. The only real example so far of tool use in these highly intelligent, large-brained mammals comes from Australia, but it is impressive and important nonetheless.

Bottlenose dolphins there do something unusual: they exploit another animal as a tool. Sponges are basal metazoans, meaning that within the animal kingdom, they are among the least sophisticated, and indeed they have no nervous system, and no brain cells at all. Bottlenose dolphins in Shark Bay nurdle sea sponges onto their beaks. Around three-fifths of dolphins are spongers in this area, and researchers think that they are doing this to protect their beaks – more technically, the rostrum – while foraging for sea urchins, crabs and other spikey bottom-dwellers that hide in the craggy seabed. They specifically select cone-shaped sponges too, which presumably sit more comfortably and securely on their beaks. One animal uses a second to eat a third.

The spongers therefore have a very different diet to non-spongers, even within the same pods. Both forage in the same areas, so we can rule out this difference being due to ecological factors – it is as if they’re going to the same buffet but choosing different food because one is using chopsticks.

How the dolphins handle the sponge and what they eat is only a small bit of this story though. There are fascinating peculiarities to be seen in this practice, and they stem from the fact that the vast majority of spongers are female. They mate with males who are not spongers, and have offspring, the females of which become spongers.

As mentioned, here we see biological transmission, and cultural transmission via learning. Some behaviours are encoded in DNA and others are acquired, yet still built on top of a genetic and physiological frame that allows the development of that trait. The scientists who have been studying this population of dolphins since the 1980s took biopsies from spongers to see if they could spot a genetic basis for their unusual foraging tool, and found none. Sponging in dolphins doesn’t appear to be specifically encoded in DNA. It is entirely learnt. By sampling the DNA of the spongers, the scientists could also establish the relatedness of them all, and this revealed something interesting. Sponging seems to stem from a single female dolphin, around 180 years ago, two or three generations back. We are now referring to this tool innovator as ‘Sponging Eve’. We can see the relatedness in this group, and we can see the passage of the sponging, but also that it is not genetically inherited. What that means is that this is cultural transmission of tool use. This is the first known case in cetaceans. Daughters learn sponging from their mothers.

As sponging is a cultural adaptation, it presents a bit of an evolutionary puzzle, as spongers do not appear to reproduce at a greater rate than non-spongers, which suggests that the behaviour doesn’t confer any great benefit or cost. As it stands though, of all the documented cases of tool use in animals, there has been almost no other assessment of the effect that it has on reproductive fitness, which is the key idea in evolutionary biology – characteristics that increase the numbers and survival of offspring are likely to be selected. Darwin’s theories were formalised in the first half of the twentieth century by applying mathematical scrutiny to observations of nature. It was no longer enough to say ‘the giraffe’s neck is the way it is because increased length has been selected as an advantageous trait for reaching succulent leaves’. We could scrutinise and model a potential advantage by looking at how it was passed down through the generations, and if it went forth and multiplied. As far as I am aware, there is a dearth of this standard of evolutionary testing as it relates to tool use.

Cultural transmission is a hugely important idea in our own evolution. Outside of humans, so far, it’s been seen in dolphins, some birds, and some monkeys. There’s an artificial distinction made between biological evolution, which tends to mean genetically encoded, and cultural evolution, which tends to mean taught or learnt. Instinctual behaviour is knowing that food covered in fungus is likely to be detrimental to one’s health; learnt behaviour is recognising that ageing blue cheese is delicious. These two facets are not independent of each other, because the learnt behaviour has to be built upon a biological framework that is capable of acquiring and processing that knowledge. An animal needs a big brain to receive this kind of instruction and act on it.

Cultural transmission also requires innovation, and that is truly rare.