Exam 2, 31 March 2005 Biology 276, Behavioral Ecology

Name__KEY______

This exam has four sections, worth 25 points each. In some cases, there is a text describing natural history background for a set of questions. Be sure to read the entire text carefully prior to starting to answer the questions. Note that in most cases, a question can be answered by one or two brief sentences; please write concise and explicit answers.

Much of the exam will be based upon the natural history of the whirligig beetle. These large aquatic beetles spend their entire life in freshwater, except during pupation (which is spent on the shore in pupal cases built of sand and debris). Eggs are laid in batches on aquatic plants. Both larvae and adults are predaceous. The larvae hunt mites, snails and small aquatic insects. The adults hunt aquatic insects and insects that fall into the water. The adults tend to aggregate in groups that move restlessly on the surface of the water. When threatened, a few may dive while the rest move away on the surface. The adult beetles are eaten by fish and birds.

A. Predator Avoidance

1. (6 pts) Whirligig beetles have unique eye structure: their eyes are split in two, with separate upper (dorsal) and lower (ventral) halves. When the beetle is in at the water surface, the ventral half of the eye is below the surface and the dorsal half of the eye is above the surface. A standard explanation for this odd morphology is that is allows the beetle to see clearly and simultaneously in both the water and air. Below is a hypothetical phylogeny of the order to which whirligig beetles belong, with habitat indicated.

Map eye structure (divided eye vs. undivided eye) in a way that would disprove the hypothesis “divided eyes evolved to allow whirligig beetles to see prey and predators clearly in both water and air”. (many possible alternate correct answers, one is below)

____ Gyrinidae (whirligig beetles – water surface) -- divided eyes ___ || || ||____ Haliplidae (crawling water beetles – underwater plant surfaces) --- divided eyes || ====|| _____Dystiscidae (predaceous diving beetle, under-water hunters) --- divided eyes ||____|| ||_____ Carabidae (terrestrial, includes predaceous tiger beetles, and bombardier beetles) – undivided eyes 2. (9 pts) Stotting behavior in gazelles was an example of using observational studies to test multiple hypotheses. a. (4 pts) Below is a table of 2 of the main hypotheses. Fill in the predictions of each model concerning stotting behavior (if the model is ambiguous relative to one of the questions, then write “ambiguous”).

Hypothesis signal recipient (predator or gazelle) solitary gazelle stotts? alarm signal other gazelles no, solitary does not stott

signal of unprofitability predator yes, solitary stotts

b. (5 pts) Why is it better to state that the “signal of unprofitability” hypothesis is “supported” rather than “proven”?

It is better to state that the “signal of unprofitability” hypothesis is supported because scientific hypotheses or models are never proven.

3. (10 pts) We discussed a paper wherein whirligig beetle adults were used to test two main models for the evolution of grouping behavior in prey, the selfish herd and the co-operative group. Each makes distinct predictions about predator efficiency as a function of group size. In this experiment, the researchers presented three predatory fish with groups of beetles of different sizes. The order of presentation of the groups was randomized. a. (5 pts)The data collected by the authors produce the following graph:

(get the graph from your copy of the paper or from the exam)

Do these data support or refute the “co-operative group” model of beetle aggregation? refute

Explain your answer. The “co-operative group” model predicts that as group size increases, predator efficiency will decline, meaning that the graph would show a “humped” curve rather than straight. Note that I did accept answers explaining that the group sizes were possibly too small to allow proper testing of this model.

2 b. (5 pts) We discussed several problems with the experiments on whirligig beetle behavior, most focused on the use of the fish predators. The same fish predators were used repeatedly in the experiments, and there was a significant decline in the frequency of strikes at the beetles over time regardless of the group size being presented.

What hypothesis can you develop to explain this aspect of the predatory behavior of the fish? They have learned that the beetles, behind a clear plastic shield, cannot be eaten and therefore are ignoring them.

What simple change could you make to the design that would control for this source of variation? There are many possible answers; the simplest is to get new fish for every trial.

B. Prey choice and patch departure decisions

1. (9 pts) The whirligig beetle larvae are predators of aquatic mites, insects, and snails. They are chewing beetles, with crushing mandibles. We can make the following assumptions about the relative value of these three prey types: prey type energy/handling abundance, habitat 1 abundance, habitat 2 time snail high low moderate insect medium medium medium mite low high high

We can plot this information in the following way for habitat 1. a. (5 pts) Redraw the graph for habitat 2.

For habitat 2, the curve for “time between encounters” will have a lower y intercept and a flatter slope, so the profitability curve will peak much closer to the y axis, indicating that the predator should take only higher quality prey.

b. (4 pts) What is the lowest quality prey the beetle larvae should attack in habitat 1? _ insects ___

What is the lowest quality prey they should attack in habitat 2?____ snails ______

3 4 2. (7 pts) One of the central assumptions of prey choice models is that when a predator is capturing, handling, and eating one prey item it cannot be searching for another prey item. This is described as the cost of “lost opportunity. Use the assumption of “lost opportunity” to explain why the abundance of low- quality prey does not influence the decision to attack a low-quality prey item.

It is a “lost opportunity” if, while attacking a low quality prey the predator is prevented from attacking a higher quality prey item. The probability of this “lost opportunity” happening is a function of the abundance of the higher quality item – the predator is more likely to “loose” the opportunity to attack a particular prey item if it is more common. The abundance of the low quality prey item has no affect on the frequency with which the predator would encounter a higher quality prey item.

3. (9 pts) Another aquatic insect is the predatory bug, which attacks relatively large insects. After capture, the bug holds onto the prey, injects digestive juices into it, then sucks out the digested innards. They do not always empty the corpse prior to releasing it and resuming hunting. a. (4 pts) Andrew Sih, in a series of classic experiments, used these bugs to test some predictions of the patch departure model, with each prey item being a “patch”. He kept the bugs in tanks without food for a variable amount of time, then offered them either a large or small prey item. Below is a graph of the classic patch model. Annotate the graph below to reflect the variables Andrew Sih was manipulating or measuring with his bugs, and indicate when the bugs should “depart” from patch 1 and patch 2 if the interval between feedings is the same.

b. (5 pts) If the interval between feedings is increased, does the time spent feeding in patch 1 and patch 2 become more similar or more different?

It becomes more similar. Explain your answer (a graphic explanation is fine).

5 Note that the vertical lines dropping to the x axis are necessary for full credit: they are what indicates departure time.

6 C. Habitat selection: choosing a place to live.

1 (8 pts). a. (4 pts) In many herbaceous insects, the larvae are incapable of moving from one plant to another. Does this mean that there is no habitat selection in these organisms? Explain how habitat selection might occur.

The expected answer: There is habitat selection: The female, when laying the eggs, is making the habitat selection for her offspring. Unexpected correct answer: there may be microhabitat selection among plant parts on the host plant by the larvae. b. (4 pts) How does this affect how we might measure the fitness consequences of habitat selection?

The fitness consequences of the female’s habitat selection behavior must be measured by the survival and reproductive output of her offspring, not her own.

(from the “unexpected answer”, there is no affect on how we measure fitness consequences).

For most of the rest of the exam, we will consider dragonfly behavior. Dragonfly males establish breeding territories along appropriate habitat for egg-laying and larval development (different species have different habitat use, for example slow vs. fast-moving vs. still water). Females are attracted to these territories, and after mating the males guard the females while they lay their eggs.

In contrast to the breeding behavior, dragonflies are not at all territorial when foraging. Foraging commonly occurs away from water, usually at times of day when females are not reproductively active. To forage, the male and female dragonflies capture and eat flying insects on the wing. The large, multi- species aggregations of foraging dragonflies over meadows and parks are a relatively common sight in the mid summer.

2. (9 pts) Two species of dragonfly both live along a particular stream, one with territories along rapid currents and one with territories along slow moving areas with aquatic plants. a. (3 pts) Assuming that the difference between the two genera reflects a behavioral choice by the individual male dragonfly, what is one environmental variable that might serve as a cue? possible environmental cues would include: sunlight, vegetation type, vegetation density. Impossible cues include rainfall, which would not correlate with differences in current flow along different sections of the same stream. b. (6 pts) Frame this as a testable hypothesis, and develop a prediction from your hypothesis. Develop an alternative hypothesis and predictions from that hypothesis. The alternative should be both reasonable and make distinct predictions. Use your answers to fill in this table: hypotheses predictions from each hypothesis

1 differences in habitat use between these the species using slow-flow areas will species are cued by differences in vegetation preferentially come to areas with high density vegetation density; (manipulations of vegetation will result in differences in habitat use by each species.)

7 use by each species.)

2 differences in habitat use between these the species using slow-flow areas will species are cued by differences in sunlight preferentially come to areas with high sunlight (manipulations of light will result in differences in habitat use)

8 3. (9 pts) In one dragonfly species, a pond dweller, the dominant males tend to hold territories in areas with relatively thick cattail growth (cattails are a shore-line aquatic plant with stiff vertical leaves and stems upon which the males perch when not chasing females or other males). Subordinate males hold larger territories in areas with less thick vegetation. a. (5 pts) Draw a graph predicting dragonfly abundance in these two habitat types (“thick vegetation” versus “thin vegetation”) in terms of the Ideal free distribution model.

(graph drawn in class)

b. (4 pts) Based upon the ideal free distribution model, what prediction might you make concerning the reproductive success, in terms of numbers of copulations, for males in each of the two habitat types?

The reproductive success should be the same.

D. Territoriality

1. (12 pts) Dragonflies, while highly territorial in breeding areas, are not at all territorial when foraging. We discussed three main questions concerning territoriality: benefits of holding a territory, costs of holding a territory, and size of territory. Considering likely frequency of intruders, resource predictability, and resource density, answer the following questions. a. (3 pts) How do the two types of resources, female oviposition sites and flying insects, differ in costs of holding a territory?

Female oviposition sites are discrete, compact, predictable (in that a good oviposition site is good for many days), and relatively rare. They should be relatively low-cost to defend. Flying insect densities are dispersed and diffuse, unpredictable in time and space, and areas with abundant insects are common. Areas with high insect abundances would be difficult to defend. b. (3 pts) How do the two types of resources differ in benefits of holding a territory?

Female oviposition sites are absolutely required for successful male reproduction, and the benefit of holding the territory exclusively is that no other male can mate with the females that visit. Holding a foraging territory exclusively would not greatly increase the foraging benefits to the dragonfly, as intruders would not deplete the number of flying insects.

c. (6 pts) Based upon your answers to parts (a) and (b), does it seem logical that territoriality is only expressed in breeding areas not in foraging areas? Yes.

Explain your answer Female oviposition sites relatively low cost to defend, and there are strong and predictable benefits of exclusivity. Flying insect areas would be expensive to defend, and the territory holder would not gain much (and indeed might loose) by trying to hold the territory exclusively.

9 2. (13 pts) We discussed two factors that influence ability to defend a territory, and tendency for a fight to escalate: the relative size/physiological state of the resident and intruder, and the residency versus non- residency of the two individuals. Assume that you can use potted cattails to manipulate plant density, and that the dragonfly males respond to the patches as though they were natural. a. (6 pts) Design two experiments to test the importance first of relative size/weight, and second of residency/non-residency using this system.

I size. Variable manipulated: size

Variable measured: holding a territory against intruders of larger/smaller size

Variables controlled for: residency (by removing and reintroducing individuals so that each believes himself to be “owner”)

Predictions: If size determines contest outcome, then the larger individual should always win and competitions between same-sized individuals should always escalate.

II residency. Variable(s) manipulated: residency time

Variable measured: holding a territory against intruders after a variable residency time

Variables controlled for: size

Predictions: If residency determines contest outcome, then the resident individual should always win and competitions individuals who both believe themselves “owner” should always escalate b. (7 pts) Can you design a single experiment that will simultaneously vary both factors? (follow the format I’ve laid out above, or make a table)

Variable(s) manipulated: residency time and size

Variable measured: holding a territory against intruders

Variables controlled for: habitat, etc., but size and residency are both being manipulated

design & predictions, in tabular form: size intruder same size intruder larger smaller residency long escalation resident resident wins wins

10 short intruder escalation resident wins wins

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