Bullhorn Acacias

Bullhorn Acacia (Acacia cornigera)

On my recent trip to Mexico, we saw lots of bullhorn acacias (Acacia cornigera), particularly in the sand dune areas at Cansaburro. They are attractive shrubs that are named for their prominent swollen thorns that resemble a bull's horns.

Yes, the thorns really do resemble a bull's horns.

Bullhorn acacias are noteworthy for their complex, mutualistic relationship with one particular ant species, Pseudomyrmex ferruginea. Most acacia species contain lots of bitter alkaloid compounds in their leaves and stems, which act to deter browsing by various herbivores. Bullhorn acacias lack these chemical defenses, but have lots of biting, stinging ants living on them. The ants hollow out the large thorns and nest in them.

Pseudomyrmex ferruginea

This hollowed-out thorn contained an active ant nest

The mutualistic relationship between the ants and the acacias was studied by biologist Thomas Belt, who described the interaction in his charming 1923 book The Naturalist in Nicaragua. The ants deter herbivores with their painful bites and stings. They also remove seedlings that sprout around the base of the tree, thus eliminating potential competitors for sunlight. In return the trees provide food for the ants. Protein-rich nodules called Beltian bodies form at the tips of newly-unfolding leaves. These are harvested and eaten by the ants.

Pseudomyrmex harvesting Beltian bodies

The acacia trees also provide carbohydrates from foliar nectaries that form at the base of some of the leaves. The ant/acacia relationship has a long evolutionary history, with both partners showing a considerable dependence on one another. Colonies of this species of ant are only found living on acacia bushes. Conversely, acacias that have been experimentally cleared of their ant colonies suffer considerably more herbivory than bushes that retain their ants.

Foliar Nectary

I was a bit surprised at the lackadaisical nature of the acacia ants that I encountered in Veracruz. Shaking the bushes, or even giving them a stiff whack with the handle of my butterfly net only produced a small amount of rather tepid ant activity. I contrast this with my encounter with another ant-protected tree some years ago in Costa Rica. In that case, the tree involved was a cecropia. When I struck the trunk just once with a stick, thousands of tiny ants boiled out of small holes in the bark.

Why Andrew Sullivan is wrong about debating evolution

Yesterday, I posted a link to a disagreement that I had with Andrew Sullivan. Several people have commented and emailed me regarding the desirability of such debates. I'm sympathetic to the impulse to favor debates. Science is all about transparency- what could be more transparent than a debate of the issue? In this case, things are not as simple as they might first appear.

As he usually does, Andrew lays out his case with clarity:

I'm a firm believer that debate exposes the flimsiness of arguments, and debate is always preferable to mere stigmatizing.

The problem is that this position presupposes that both parties enter into the debate in good faith. This issue resides in the political agenda of the creationist side of the debate. They are looking for the legitimacy that our society accords science while circumventing the normal processes of obtaining it. The creationists have been unable to produce work that stands up to the process of peer review, so they seek out the forum of a public debate instead. Not only is peer review the standard for vetting modern science, it stands in contrast to public debates, which are a notably poor forum for such evaluation. The approach that the creationist camp takes to these debates illustrates why.

In a creationism/evolution debate, there is an asymmetry of information. All of the evidence rests on the side of science- creationism has no evidence of it's own. Indeed, how could it? Being based on faith and on the supernatural, creationism isn't actually testable. In a debate, creationism is therefore relegated to attempting to point out shortcomings in the theory of evolution.

Therein lies the problem. It's very easy to phrase a critique of a scientific theory. Even when the critique is clearly off base and easily refutable, it typically takes a lot more words and time to present the refutation than it does to present the initial critique. The creationist approach also takes advantage of the fact that lines of evidence from many different disciplines converge in pointing to the theory of evolution. In a debate, a critique of some aspect of the fossil record can be coupled with a critique of molecular evidence and followed up with a critique of carbon dating techniques- all from vastly different branches of science and all delivered within a minute or two. At this point, it doesn't matter whether there is any validity to the creationist arguments or not. The debater on the evolution side of the argument can not, within the time he or she has been allotted to speak, get through all of the refutations. It's a common technique that has come to be called the "Gish gallop," named after Duane Gish the founder of the Creation Research Institute.

The payoff for the creationist side? They get to claim peer status with scientists, often of considerable reputation, and can often claim victory- even when their arguments are entirely without merit. That works for them, because in the end, their aims are really political rather than scientific. If public debate of the sort proposed by creationists were truly of value for sorting out scientific controversies, it would be more widely used for exactly that purpose. Andrew Sullivan comes from a background of history, philosophy, and politics. In these fields oral debate is a much more widely-used tool for resolving disputes, mainly because it's more effective. It's a pattern that has been revealed time and again around the issue of evolution. I'd love to find out if Andrew feels it would be mere stigmatizing if Charlie Brown finally told Lucy van Pelt what she could do with her damn football.

What We Are Fighting Against

It's been a rough morning in blogland. A bunch of folks are posting stuff that is elevating my blood pressure. The worst, via Andrew Sullivan

"When we just saw that man, I think it was Mr. Myers [i.e. biologist P.Z. Myers], talking about how great scientists were, I was thinking to myself the last time any of my relatives saw scientists telling them what to do they were telling them to go to the showers to get gassed … that was horrifying beyond words, and that’s where science — in my opinion, this is just an opinion — that’s where science leads you," - Ben Stein.

I do hope that Mr. Stein and his relatives aren't taking any of their doctors' advice about their health care. After all, that's just evil, immoral scientists telling them what to do, and we all know where that leads. This isn't a potitical blog. I have talked about politics very infrequently here. But the assault from the right wing these days on reason and science cannot go unanswered. Mr. Stein, and his recent movie exemplify the lengths that those folks will go in distorting the truth lying in order to further their own agenda. It's easy to make accusations that somebody is lying. In Stein's case, the accusation has quite a bit of evidence supporting it however. Avoid at all costs Mr. Stein's miserable movie Expelled. If you must see it, at least check out the evidence about the profound untruths he is propagating at the webside Expelled Exposed.