Alcock, Chapter 6

Behavioral Adaptations for Survival

Hans Kruuk & Black-Headed Gulls (Alcock, p. 177-181)

• Kruuk wanted to know whether the mobbing response of black-headed gulls was an adaptive product of natural selection.

• Kruuk’s hypothesis was that mobbing behavior distracted certain predators, reducing the chance that they would find the mobbers’ offspring which would boost the fitness of mobbing parents gulls.

• Given the variation always exists in populations, some gulls would be better mobbers than others.

• Does the cost to mobbers (time and energy spent in screaming, diving, and flapping about, plus the chance that they, themselves, might be grabbed and eaten) outweigh the benefit of saving their young from predation?

---The question is: do mobbers leave more surviving progeny than non-mobbers? Do they have a higher fitness? And what about the cost and benefits?

• Because distracted crows are probably less likely to find prey eggs or gull hatchlings, Kruuck established

that a probable benefit existed for mobbing.• The benefit of crow mobbing exceeds the cost, given that

rarely are mobbing gulls caught and killed by crows.• Kruuk counted surviving eggs in his research, assuming

that they would become surviving offspring and carry on their parents’ genetic lines (he did not measure full reproductive success by counting the number of surviving offspring produced by individuals over their lifetimes).

Kruuk’s Research)

(Cont’d

Alcock, p. 184, Fig. 6.7Comparing Gull Species

• Members of the same evolutionary gull lineage share a common ancestry, and therefore share many of the same genes, and thus tend to have similar traits, such as mobbing behavior.

• But the effects of shared ancestry can be overridden by a novel selection pressure.

• Reduction in predation pressure has led to divergent evolution by the cliff nesting gull called the kittiwake, which no longer mobs potential enemies.

• The various ground-nesting gulls, including the black-headed gull, mob nest predators as do some colonial swallows, including the bank swallow, even though gulls and swallows are not related, having come from different ancestors long ago.

Comparing Gull Species and Swallows

Kittiwake Bank Swallow --The kittiwake no longer mobs, butthe bank swallow does mob, like black-headed and other ground-nesting colonial gulls.--Even though gulls and swallows are not related, they have

converged on a similar antipredator behavior in response to shared selection pressure from predators that have fairly easy access to nesting colonies.

--Cliff-nesting kittiwakes, although clearly relatives of the black-headed gulls, no longer need to mob as potential predators cannot prey effectively on their precarious nesting areas.

Ground Squirrel Mobbing(Alcock, p. 185-86; Figs. 6.8 & 6.9)

• California ground squirrels mob rattlesnakes.• These squirrels live in groups and dig burrows in the

ground, and react to a hunting rattlesnake by gathering around it and kicking sand in its face, thereby preventing the snake from exploring nest burrows for newborns and infants.

• Given the costs of a rattlesnake bite, researchers predicted that the squirrels would be able to adjust their behavior in relation to the level of risk of snakebite, which could be assessed by listening carefully to the rattling buzzes made by the snakes.

• The rattler’s sounds vary according to size and body temperature—larger snakes and warmer snakes (which can move quickly) present a greater risk.

Ground Squirrels (Cont’d)

• Ground squirrels, as predicted, were less eager to approach speakers playing the rattles of large and warmer snakes.

• The ability to assess the risk would enable squirrels to reduce the cost of their mobbing.

• Also, as their offspring matured and became better able to avoid snakes on their own, mother squirrels were less likely to mix it up with rattlers, as measured by the reduced time they spent close to a tethered snake.

• In other words, as the benefits of mobbing behavior fell, the benefit-to-cost ratio of the activity, declined, and as one would predict, the willingness of adult squirrels to engage in the behavior also decreased.

Costs and Benefits of Anipredator

Behavior (Alcock, p. 187, Fig. 6.10)

• Mud-puddling = Why do butterflies aggregate in large, packed groups around mud puddles on tropical riverbanks where they suck up fluid containing valuable nutrients?

• Possible answer = to dilute the risk of one specific individual being attacked and eaten.

• The risk of death for a member of a group of 1000 butterflies is 1% per day, whereas it is 10x higher for members of a group of 100 butterflies.

• The dilution effect provides a benefit for individual butterflies that tend to mud-puddle with many others.

• Thus the benefit to cost ratio for the tendency to aggregate could well be positive, facilitating the evolution of the behavior.

Synchronized Metamorphosis

• Mayflies (Alcock, p. 187-88, Fig.6.11)

• Some mayflies synchronize their metamorphosis from aquatic nymphs to flying adults so that most individuals emerge from the water during just a few hours on a few days each year.

• Fig. 6.11 presents data indicating that the more female mayflies that emerged together on a June evening the less likely that any individual mayfly could be eaten by a predator

Social Insects and Group Attacks(Alcock, p. 188-89, Fig. 6.12)

• Benefits of group livingthe potential for group attack on a shared enemy (termites, ants, wasps, and bees).

Australian Sawfly Grubs(Alcock, p. 189-90, Fig. 6.13)

• Communal defense by sawfly larvae: these larvae form circular clusters that rest with their heads facing outward during the day.

• When threatened the raise their heads in warning while regurgitation droplets of repellent eucalyptus oils, which the hold in their moths to spew out onto an attacking enemy.

Costs/Benefits of Camouflage: The

Melanic Moths Story (p. 190-94)

• The peppered moth in the U.K. & U.S.A. and the spread of the melanic (black) phenotype as industrial soot darkened tree trunks (Fig. 6.16).

• Although most of these moths rest next to tree limb joints, of those that rest on the trunks, the melanic forms were less often discovered by birds in polluted (darkened) woods, while the typical peppered (largely whitish) forms survived better in unpolluted woods where tree trunks were not darkened (Fig. 6.17).

• Consider here that the benefit vs. costs changes (for each phenotype) when a critical environmental variable changes.

Cryptic Coloration and Body Orientation

(p. 192-193; Figs. 6.18 & 6.19)

• The orientation of the another whitish moth Catocala relicta usually perches head-up, with its forewings over its body, on white birch and other light-barked trees with narrow, intermittent black bands (Fig. 6.18).

• The orientation of the resting C. relicta determines whether the dark lines in its wing pattern match up with the dark lines in birch bark.

• Lab experiments with blue jays indicate that a C. relicta, when properly oriented head-up, with wing patterns aligned horizontally with bark bands, was very likely to be overlooked by blue jays.

• Any C. relicta perched not-head up or on a non-matching substrate were targeted much more (again the cost of not using camouflage properly).

The Assassin Bug’s “Backpack”

• Assassin bugs cover themselves with sand, dust, and/or organic debris such as the carcasses of their own prey to build a “backpack to camouflage themselves.)

• Bugs that retain the dust and debris that they apply to themselves are typically ignored by predatory army ants.

• Those bugs that have lost a portion of all of their camouflage are subject to predation (p. 193, Fig. 6.20).

• Again, an example of the necessity to properly use camouflage or face predation.

Darwinian Puzzle: Why would a Prey Animal become

Conspicuous to its Predators?

• This would seem to be a cost-benefit dilemma, but the monarch butterfly provides a good answer (p.195-96).

• The monarch’s conspicuous coloration warns predators of its chemical toxin (monarch larvae feed on poisonous milkweeds and retain the plants poison in their tissues)

• A naïve predator attempts to eat just one monarch, vomits and avoids this species for life.

• Although a dead animal cannot pass on its genes, a dead monarch educates a predator who will never prey on the monarch’s close relatives (a terrible cost = great benefit).

• So the dead one is a sacrifice that helps genetically similar monarchs survive to pass on shared genes to the next generation.

Why Behave Conspicuously?

• The tephritid fly habitually waves its banded wings as if trying to catch the attention of predators.

• The fly’s wing markings resemble the legs of jumping spiders which prey on them.

• Researchers hypothesized that when the fly waves its wings, it creates a visual effect similar to the aggressive leg-waving displays of the spiders—and this deters an attack by the spiders.

• Experiments confirmed the hypothesis: the fly mimics the spider’s own aggressive signal and this mimicry “releases” escape behavior on the part of the jumping spider.

Stotting in Thomson’s Gazelle and in other African Gazelles

• Leaping into the air, stiff legged, while flaring your white rump patch--another case of why behave conspicuously!?

• Four hypotheses are listed on p. 197, and one by one each is shot down, until only number four remains—the Pursuit Deterrent Hypothesis.

• Gazelles stot to announce to a predator that they will be hard to capture; thus the cheetah will waste energy and time—and this allows the cheetah to make a wise decision.

• Cheetahs get the message, since they are more likely to abandon hunts when gazelles stot than when a potential victim does not stot (p. 198, Fig. 6.26)

Optimality Theory and Antipredator Behavior

• Adaptations have to do more than merely confer a benefit if they are to increase in frequency in a population (p. 201).

• An adaptation, by definition IS better than the alternatives; in other words: the net benefit associated with a true adaptation is greater that that associated with the non-adaptive alternatives.

• If one can measure the fitness costs and benefits associated with four alternative behavior phenotypes in a population, then one can determine which trait confers the greatest net benefit on individuals in that population (see Fig. 6.30).

• This trait would be considered an adaptation—an optimal trait that would replace the alternatives given sufficient evolutionary time.

Optimal Covey Size for Northern Bobwhite Quail

P. 202-02, Fig. 6.31.• Optimal winter covey size for northern bobwhite quail is a

function of the costs and benefits of belonging to groups of different sizes.

• Winter coveys range from 2-22 individuals, but with a strong mean at 10-11 individuals.

• Individuals in smaller groups are less safe from attack; individuals in larger groups are more safe.

• Yet very large groups must suffer from increased competition for food among its members.

• So an effective, optimal cost and benefit ratios are found in mid-sized groups, around 11 individuals.

Game Theory (Costs and Benefits)

• The fundamental competition of life revolves around getting more of one’s genes into the next generation than one’s fellows (p. 202-04).

• Game theory comes into play when the benefit of a behavioral option to one individual depends on what the other members of its population are doing.

• Game theory views evolution as a game in which the players are armed with different strategies that are in competition with one another, with the winners creating a population over time that cannot be invaded by a player with an alternative strategy.

• What’s the flaw with this logic? Mutations? A competitive behavioral arms race spiraling out of hand?

The “Selfish Herd” Notion in the Game Theory Model

• W. D. Hamilton originated the idea that in a “selfish herd” all the individuals were trying to hide behind others to reduce the probability of being selected out by a predator.

• Eventually all the members of this population will be aggregated, with individuals jockeying for the safest position within their groups, actively attempting to improve their odds at some else's expense (p. 204).

• The result would be a selfish herd, whose members would actually be safer if they all could agree to spread out and not try to take advantage of one another.

• Individuals employing a solitary strategy would be vulnerable to a social mutant that uses a hide-behind-another strategy to take fitness from its companions—thus the exploitative tactic could spread through a species.

Are Adelie Penguins a Selfish Herd?

• Adelie penguins often wait on the ice near open water until a group assembles, and only then do they all jump into the water more or less together to minimize the possibility that any one individual will be caught by a leopard seal.

• The seal can only kill and eat a certain small number of penguins in a short time, so by swimming in group, many birds will escape while the seal is engaged in eating one or two of their unfortunate fellows (p. 204-05, Fig. 6.32).

• If you had to run a seal gauntlet, you would do you best not to be the first or the last into the water, so the penguins assembling at the water’s edge qualify as a selfish heard, whose members are engaged in a game whose winners are better than others a judging when to plunge into the sea.

It’s Not Just Leopard Seals!

In Short (p. 206)

• An adaptation is the product of natural selection, which means that it is a hereditary trait that does a better job at promoting individual reproductive or genetic success than any available alternative form of that trait.

• An adaptation has a better ratio of fitness benefits to fitness costs than any alternative characteristic that has happened to appear in a species’ history.

In Short (Cont’d)

• Traits that intrigue adaptationists are those that have substantial fitness costs, which must therefore generate major fitness benefits if they are to spread and persist in a population under natural selection.

• Traits whose costs seem to exceed their benefits constitute Darwinian puzzles; the solution to these puzzles are prized by adaptationists (note: monarch butterflies, tephritid fly wing waving, and gazelle stotting).

In Short (Cont’d)

• The Comparative Method of testing adaptationist hypotheses is based on two key assumptions:

• (1) that related species will exhibit differences in their attributes if they face different selection pressures, despite their having a common ancestor and thus a similar genetic heritage (black-headed gulls vs. kittiwakes), and

• (2) that unrelated species that share similar selection pressures will converge on the same adaptive response, despite their having a different genetic heritage (black-headed gulls & bank swallows).