February 2010

ANIMAL AGGREGATIONS

This month we will discuss the advantages and disadvantages of animal aggregations such as flocks, schools, and herds. The number of individuals and the regularity of their spacing can differ greatly as shown in these first three images of aggregations.

A flock of white ibis at sunset at our Refuge
A school of bait fish with goliath grouper and diver for scale	Part of a migrating herd of wildebeest on the Serengeti

ADVANTAGES AND DISADVANTAGES

The advantages of aggregations more than outweigh the disadvantages. Because aggregations are temporary the disadvantages of spreading disease and competition for food are minimal. A clear disadvantage would seem to be attracting predators. However the presence of many eyes, ears, and noses allow the aggregation to detect a predator before it attacks.

If a predator takes one prey at a time, any individual has a statistical advantage. Think of your chances of being eaten: if you are alone, if you are 1 of 10 prey, if 1 of 100 prey, and if 1 of 1000 prey.

A humongous number of starlings, in separate but adjacent flocks Note how they swirl and bank in complex ways that make it hard for a predator to single out an individual.

Here is a red-tailed hawk as it tries to catch a bat in an aggregation of about 100. Even if it catches one, the rest will be long gone.

	Fish scatter as a shark approaches
Another anti-predator advantage of an aggregation is the confusion effect. Scientists have hypothesized that the apparently random movement of prey around an attacking predator might confuse the predator so much that it might not catch any prey.

I use wiffle ball simulation experiments to test the confusion hypothesis. In each experiment I have five ‘prey’ people in a semicircle around one ‘predator’ person about 4 meters away. Each ‘prey’ person has 4 small plastic balls that are the actual ‘prey.’ On appropriate commands from me, a ‘prey’ person GENTLY tosses a prey ball underhanded toward the ‘predator’ person. With successive trials I have the prey people throw the balls faster and faster. In the final trial all five ‘prey’ people toss all their prey balls as quickly as possible. What do you think happens?

In the first experiment all the prey balls are white. When the balls are tossed slowly and singly the predator easily catches each one. But when they are tossed all at once the predator catches none. This confirms the prediction from the confusion hypothesis.

To add more realism to the simulation I give each ‘prey’ person 1 red and 3 white balls. Can you think what the red balls simulate in a real encounter between a predator and an aggregation of prey?

I give the ‘predator’ person the tactical instruction to concentrate only on trying to catch a red ‘prey’ ball. In fact the ‘predator’ now catches at least one red ‘prey’ ball when all the balls are tossed at once. This is a realistic simulation of nature since a predator usually catches only the old, sick, slow, or oddball individual in an aggregation of prey. These are easiest to separate from the group and then catch.

A cheetah has separated an impala from its herd but the outcome is not clear yet.	In this Larson cartoon the oddball lemming is not the loser!

	Alice and the Red Queen run and run but get nowhere.
Over evolutionary time predators have gotten better but prey have also gotten better. The result of this back and forth co-evolutionary war game is a stalemate in which neither prey nor predatory gains a complete advantage. This has been called the ‘Red Queen’ from Lewis Carroll’s ‘Through the Looking Glass.

GROUP SAFETY FROM OFFENSIVE BEHAVIOR

I have earlier written about how lubber grasshoppers and zebra long-wing butterflies with aposematic warning color roost or move in groups to help teach predators that they taste bad and are poisonous. The next three images are other examples from the Refuge where aggregations of animals advertise that attacking them will be dangerous or eating them will be unpleasant.

A school of bullhead catfish babies may not be toxic but their parents will surely attack you if you wade here.
A crowd of aphids suck sap from a milkweed plant; like other milkweed herbivores (monarch and queen butterflies) this aphid is protected by the toxins it tolerates.
Part of an aggregation of whirligig beetles; they smell like geraniums to me and taste awful!

A variant of group protection is when a group of prey mob a predator. This fits the adage that the best defense is often a good offense. At the Refuge I have seen a flock of crows mob a great-horned owl.

Charley Harper’s OWLTERCATION

When the great horned owl eats crow, it’s not because he’s humbled – he’s hungry. And they’re easy pickin’ in the dark when he can see and they can’t. But sunrise turns the tables, and any sighting of the flying tiger sets off the owlarm that summons a crowliferating posse to pester him. The air is clogged with crowfanities as the raucous ruckus moves from cypress to maple to fig. Finally, tired and partly de-feathered, he is escorted out of town by the most direct route – as the crow flies.’

REGULAR SPACING OF INDIVIDUALS IN AGGREGATiONS

What is the advantage of what seems to be perfectly coordinated movement and spacing in aggregations when no predator is around?

One hypothesis is that individuals that follow a leader spend less energy because they are "drafting". Bikers know about drafting and even semi-trailer trucks draft to save 40-70 percent on energy expenditure. For the group to benefit, individuals must change position as leader and follower so each shares in costs and savings.

	White Ibis echelon flight suggests 'drafting'. And it may be that black wing tips help birds keep the relative positions to draft most efficiently"
	White Ibis Flight Quartet – © Arthur Morris www.birdsasart.com

It is amazing that large herds of mammals and huge schools of fish sometimes maintain what appears to be perfect and very close spacing without bumping into one another.

Part of a perfectly spaced fish school; experiments show that each fish uses vision and lateral line sensing of water pressure to balance attraction and repulsion.

To coordinate speed and alignment to maximize efficiency as a group, individuals do not have to know what all the others are doing. A fun human example of creating a coordinated group movement is ‘The Wave’ in a sports stadium. It depends only on observing the folks immediately next to you.

FEEDING, ROOSTING, & NESTING AGGREGATIONS

In contrast to herds, flocks, and schools, aggregations for feeding, roosting, and nesting are much looser and usually much smaller. The smaller group still gives the advantage of early detection of predators. Often in a small aggregation one individual is a sentinel, as in meerkats (a species of mongoose) and prairie dogs. This is an especially efficient way to detect predators, and individuals take turns as the sentinel while others look for food.

Frequently, different species of wading birds feed, roost, and nest together. Usually these multiple species aggregations include species that are of different sizes and feeding modes so that competition is not so great a problem.

Feeding Mode Extremes: Tactile Detection vs. Visual Detection

Tactile feeders ("cream-skimmers") have opposite tradeoffs from visual feeders ("crumb-pickers"). Ibises and wood storks find their food by touch, and this has tradeoffs. On the plus side the water can be cloudy but on the minus side prey density must be very high. Herons and egrets find their food by sight and this has the opposite tradeoffs. On the plus side they find food when it is scarce but on the minus side they cannot feed easily unless the water is clear. When the two types of feeders are in the same pond the cream skimmers leave first as the prey density declines. The largest crumb-pickers, like great egrets, continue to feed because they do well by feeding on the few ‘crumbs’ of large prey that remain after the cream skimmers have reduced the density of smaller prey.

A small aggregation of wood storks have found a shrinking pool and are groping for the dense fish trapped there

Cream-skimmers and crumb-pickers facilitate each other’s feeding when many prey are concentrated in shrinking pools during the dry season in the Everglades. Each type of feeder stirs up prey which makes it easier for the other type of feeder to catch prey.

Dr. Dale Gawlik has hypothesized that wood storks, white ibis, great egrets, and snowy egrets are white so that they can be seen feeding by flying searchers that join the group. Certainly the numbers of birds increase quickly and dramatically when we experimentally drop the water level in a Refuge impoundment and thus concentrate the fish and crayfish prey.

Cooperative feeding among pelicans herding fish toward shore with white ibis and snowy egrets herding fish away from shore.

Along the shore a white ibis cream-skimmer probes for and catches a baby horseshoe crab and a crumb-picker snowy egret uses its yellow feet to stir up or attract prey. Perhaps the egret scared up the crab and made it easier for the ibis to catch it.
	Ibis Snowy 0361 – © 2004  Ron Reznick www.digital-images.net

African-American Spiritual

Oh ‘dem golden slippers, Oh ‘dem golden slippers
Golden slippers I’se ‘goin to wear ‘cause they are so neat.
Oh ‘dem golden slippers, Oh ‘dem golden slippers
Golden slippers I’se ‘goin to wear to walk God’s golden street.

Possible Advantages of Tree islands with Roosting & Nesting Birds

Last summer, observing a zone of increased plant density and productivity around a Refuge tree island nesting rookery, I hypothesized a chain of positive feedbacks. Birds roost in an aggregation on a tree island → guano deposition → increased plant and animal productivity → birds aggregate on the same island in a nesting rookery because prey density is high locally → increased guano deposition as nestlings grow → still higher plant productivity and animal prey density → better success in raising young.

An extension of these advantages is that alligators in the Refuge, and cottonmouth venomous snakes in other areas, are attracted to rookery islands. They certainly eat a few nestlings but mostly they eat dropped and regurgitated food items. On the plus side the ‘gators and cottonmouths keep raccoons away. This is a big plus since raccoons eat eggs and young in the nests.

A raccoon climbing a tree to prey on eggs or young in wading bird nests.	An alligator eats a raccoon and thus becomes an indirect mutualist to the wading birds: an enemy of my enemy is my friend!

REVIEW QUESTIONS

a. Flocks, schools, and herds have serious disadvantages of easy spread of disease and competition for food.
b.  When a predator attacks an aggregation, the statistical advantage to an individual of not being the one to be eaten and the confusion to the predator of erratic movement of all prey individuals are huge advantages.
c. Super-predators like panthers should be hunted and killed because they will eat almost all of some prey species.
d. Extremely dense aggregations of wood storks, white ibis, great egrets, and snowy egrets that feed together compete severely.