The Enemy of My Enemy is…My Alligator? Mutualism and protection rackets in the Florida Everglades.
When you think of alligators the word friendly isn’t usually at the top of the list of descriptors, but for alligators in the Florida Everglades it’s worth it to cozy up to a colony of nesting heron, egret or spoonbill. Actually, perhaps friendly isn’t the best word for the relationship. In many ways the relationship has more in common with a mafia protection racket; the birds pay the alligators for the privilege of protecting them from nest predation.
It is well established that birds will use the aggressiveness of other species, including alligators, to provide extra protection from predators, but until now it was assumed to be a one-way street. A paper published today in PLOS One, however, shows that alligators gain a steady supply of food in the form of abandoned nestlings in exchange for their presence.
Birds gain protection from nest raiders such as raccoon and opossum, alligators gain a significant source of food as parents push excess chicks from the nest. You see, many nesting birds lay more eggs than they can raise and, depending on the food resources available some unlucky chicks are pushed from the nest. Yes, it’s an unsavory tale, but then again, what mafia story isn’t?
The paper published today builds on cleverly designed Masters research from Brittany Burtner at University of Florida which showed that birds preferentially nest above higher alligator populations. To do this Burtner manipulated the apparent numbers of nesting birds and crocodiles using decoys to show that birds preferentially nested in tree islands with higher alligator densities. In fact, she cleverly modified pink yard flamingos to look like real birds in some cases. Further, Burtner characterized the potential benefits to both birds and alligators in this association based on nestling survival and the amount of food available to crocodiles from nesting colonies. The recent paper by Nell at al. in in PLOS is the first to show the direct impacts of this relationship for alligators.
Nesting and fledging of wading birds happens in the dry season, a period when female alligators are beginning to store energy for their own nesting as well as a time when alligators are less able to thermoregulate to save energy according to Nell and Frederick (preprint available here), who also authored the current paper. So, having a ready supply of baby chicks periodically rain down from above would be, clearly, a welcome feast. Nell and Frederick, as lead authors on the paper released today, show that this young and tender caloric addition can be seen in the body condition of alligators when compared to alligators who aren’t running this ecological protection racket.
Nell et al. compared the physical condition of alligators caught below nesting colonies with those of alligators caught in similar areas without nesting colonies. They found that two factors describing the size and weight of alligators were significantly different between the two groups. The alligators that hung out under nesting bird colonies were bigger and more robust than the alligators who didn’t, even when environmental variables were accounted for. Together with the evidence that nestlings could be a significant source of food their conclusion is that alligators do, indeed, gain an advantage by associating with nesting birds.
There is one question left unanswered by this research however; are the alligators bigger because of the food…or are they bigger because mafia enforcers tend to be the biggest guys around? Yes, this sounds silly, but Nell et al. do mention that the scale of the difference could be related to competition for the food resource as well.
“We hypothesize that alligators are attracted to and, given their territorial behavior, may even compete for territories that include wading bird colonies. We predict from this that alligators should display movements towards bird colonies upon their formation, and alligators occupying colonies should be larger and/or occur more densely than in non-colony sites.” – Nell et al., PLOS One
So, just like mafioso, protection might come in the form of the biggest alligators around. This association remains to be proven, however.
Further, it is possible that birds have a second, indirect effect on alligator fitness simply by being gross. Birds and chicks, you see, frequently vomit their food outside of the nests either by accident while regurgitating or on purpose, according to the paper. They also have to poop somewhere. All of this amounts to a large amount of nutrients ejected into the habitat below.
While these nutrients aren’t directly beneficial to alligators they likely spur the production of plants and aquatic primary production which may attract greater numbers of alligator prey. Also, the rain of vomit and excrement may directly attract other scavengers and opportunistic fish which could become prey to alligators.
So, the next time you see a giant alligator in person or on video, look in the trees above and perhaps you’ll see who it is protecting…albeit for a hefty price.
The behavioral patterns is interesting.