The point of Holly

I wrote a fair amount about Holly last year (see here) so you can follow that link to read more about the plant and the lore surrounding it. Holly is a native tree to the UK and was a sacred trees to our ancestors.

I thought I knew a lot about Holly. However, a recent post on social media by a group that celebrates trees stated that:

Holly leaves are naturally leathery and have an oval, smooth edge. However, when the plant gets predated by deer or cattle, it switches genes on to make them spiky when they regrow. So on taller Holly trees, the upper leaves (which are out of reach) have smooth edges, while the lower leaves are prickly.

It was accompanied by this photo:

Was this true I wondered? If so, I was unaware of it. And, besides, the term 'switches genes' didn't really make a lot of sense to me. So I did some research and asked a few questions.

The first few responses I got back were from gardeners - both professional and amateur - who assured me that the same effect is created by pruning. As one wrote: 'Pruning is just intentionally doing what nature does by accident. Want a taller plant? Trick it into thinking critters are nibbling down low. Want it be shorter and fatter? Cut the top so it thinks it ran into something. You trick the plant into taking the shape you want.' But that didn't explain the change in leaf shape.  

Another person explained that: 'In South Yorkshire from the 14th to the 18th century, there is documented evidence of Holly being used as fodder for livestock particularly in winter i.e. when snow prevented access to grass. This took advantage of the fact that the upper leaves of Holly are smooth (as they don't need to protect themselves from browsing by deer like the lower leaves). Special plantations of Holly were called Haggs, and is still a lot of modern place name evidence e.g. Hollin, Hagg etc are all linked to this practice - Fox Hagg in Sheffield, Hollin Busk in Deepcar etc.'

This goes some way towards validating the cause (predation) of leaf prickles but still didn't explain how the plant does it. Thankfully a biologist on Reddit then came forward with this explanation (quoting from a paper*): 'All of the leaves on a tree are genetic twins and share exactly the same DNA sequence. By looking in the DNA for traces of a chemical process called methylation, which modifies DNA but doesn't alter the organism's genetic sequence, the team could determine whether leaf variation was a response to environmental or genetic changes. They found a relationship between recent browsing by animals, the growth of prickly leaves, and methylation. "In holly, what we found is that the DNA of prickly leaves was significantly less methylated than prickless leaves, and from this we inferred that methylation changes are ultimately responsible for leaf shape changes," Herrera said. "The novelty of our study is that we show that these well-known changes in leaf type are associated with differences in DNA methylation patterns, that is, epigenetic changes that do not depend on variation in the sequence of DNA."' 

So, now we know. It is apparently true that predation of Holly causes lower branch leaves to get spiky. And not just by deer and cattle. There are other nibblers too such as caterpillars and weevils. And many birds and even foxes eat the berries so may damage the leaves that way. 

But it is another fascinating example of cooperation in the natural world - the leaves are predated and get spiky - this provides birds and small mammals with a protected space in which to nest - they eat the berries and then distribute the seeds in their droppings - more Holly trees grow to feed the deer and other leaf eaters. 

I love those kinds of stories. We're starting to understand that nature isn't all 'survival of the fittest' after all. Firstly, that was a misquote as Darwin actually said 'Survival of the most fit' i.e the best suited to its environment. And, secondly, species that cooperate tend to do better than species that don't (for a great example of this see here).

That applies to humans too. No one person can build an Airbus A380 but thousands of humans working together can. Cooperative behaviour is what made the human race the dominant species on this planet. 

Now we need to learn to cooperate with other species so that we all flourish. 

___________________________________________

*Herrera. C, Bazaga. P, ‘Epigenetic correlates of plant phenotypic plasticity: DNA methylation differs between prickly and nonprickly leaves in heterophyllous Ilex aquifolium (Aquifoliaceae) trees’, Botanical Journal of the Linnean Society, Wiley, December 2012, DOI: 10.1111/boj.12006

See also this article published by the Royal Linnean Society (Royal Society for Animal Biology).