SHORT ANSWER Question 1:  For each answer a-d, state whether it is True or False and then justify you thinking.  Be sure to refer to the appropriate tables in your answers.  (5 pts. per answer; 20 pts. total)

            Metepeira incrassata is a colonial species of spider.  Individuals aggregate in clusters of varying sizes and each builds a web which is connected to adjacent webs.  The end result is a collective, three dimensional web that covers a large area.  Colonies of different sizes capture significantly different amounts of prey, as shown in Table 1:

                                                No. Spiders per             Mean No. Prey Captured

            Table 1                               Colony                              per Half Hour

                                                    250-300                                7.8 + 3.6

                                                    350-400                                8.0 + 2.4

                                                    >1000                                  15.9 + 4.7

Larger colonies catch more prey because they provide more web space for insects to get caught in.  Spiders within a colony do not cooperate to build the web, capture food or repel predators.  Spiders that build solitary webs and live alone have very low rates of prey capture and survival.

            Spiders are not distributed randomly throughout the web, but are assorted by size.  Larger spiders occupy the core of the web, while smaller ones occupy the periphery.  Large spiders use aggression to drive smaller ones out of the core area.  On average, compared to peripheral females core females produce significantly more egg sacs, which experience less parasitism and have a higher hatching success (Table 2).  (A major parasite of the egg sacs is a type of wasp which lays its eggs in the eggs sacs.)

Table 2                         mean egg sacs/female    % hatching success

  Core Female                      1.23 + 0.17                         78.2     

  Peripheral Female              1.01 + 0.0                           62.7

            Position within a web also affects foraging success and risk of predation (Table 3).  Core spiders have significantly lower foraging success.  However, they experience lower attack rates by predaceous insects and are captured significantly less often.

Table 3                                     No. Prey/half hour         Attacks/hour      Captures by predators/hour

                        Core                             5.14                        0.39                                    0.04

                        Periphery                     14.13                       0.71                                    0.15

            Based on this information, discuss the following statements:

a.  Dominant and subordinate spiders experience the same costs and benefits for group living.

b.  Living in the core or on the periphery of the web represent two alternate strategies that convey equal benefits.

c.  Dominant individuals are making a trade-off between foraging success, predation risk, and reproductive success.

d.  Peripheral spiders will be selected to leave the colony and build a solitary web because the costs of being a subordinate exceed the benefits of group living.

ANSWERS for Question 1:

a.  False, dominants and subordinates have different cost/benefit trade offs for colonial living.  Dominant spiders occupy the core area, where they are less exposed to predators, are less likely to be caught by predators (Table 3), and produce significantly more egg sacs with a greater hatching success rate (Table 2).  However, core spiders have a much lower success rate at capturing food (Table 3).  In contrast, peripheral spiders are more susceptible to predation, more likely to be caught and have lower egg sac production with lower hatching success (Tables 2,3).  However, they have a much higher feeding success rate.  Thus, core spiders have higher personal safety and RS, but lower foraging success, while peripheral spiders have higher predation and lower RS, but greater foraging success.

b.  False, they do not convey equal benefits.  Ultimately, core spiders have higher repro. success and thus living on the periphery is a lesser strategy.  Spiders on the periphery were forced there because they were too small to compete for a core area.  Living in the two areas also does not have equal benefits with respect to predation or foraging success (Tables 2,3)

c.  True:  the main cost to living in the core is reduced foraging success, because most prey will get caught on the periphery.  Yet, living in the core provides protection to both the adult spiders and their eggs.  As a result they have higher RS.  So, they trade off foraging success for greater life time repro. success.

d.  False, will not become solitary because it is stated that solitary spiders have very low rates of survival and prey capture.  Even though peripheral spiders have lower RS than core spiders, their RS is still greater than it would be if they were solitary.  So, they will continue to be selected to live in groups.  

SHORT ANSWER Question 2:   In the Mormon cricket (which is actually a type of katydid), the male produces a spermatophore which can be up to 20% of his body weight.  Much of this spermatophore consists of protein, which the female breaks off and eats (the portion containing the sperm stays in her reproductive tract).  This protein can be used to produce eggs and thus the male contributes to egg development.  Males "sing" to attract females.

            In natural populations, mating behavior and mating systems are highly variable.  You do a series of experiments to examine the relationship between food availability and sexual behavior in the Mormon cricket.  You establish in the lab two groups of crickets, each of which contains 50 males and 50 females:

·             In Group 1, food is abundant (the high food group).  Under these conditions females can get most or all of the food they need for producing eggs through their own foraging. 

·             In Group 2. food is scarce (low food group).  Under these conditions, females cannot get enough food through their own foraging to produce their eggs, but need a supplemental source of food for successful egg development. 

You observe the two groups throughout the mating season and record mating and aggressive behavior for both males and females.  The data for males are summarized in Table 1; data for females are given in Table 2.

TABLE 1:  Male Reproductive Behavior

                        Mean No.                                                                                                  Mean No.

                        Spermat.           % Males                % Singers                 Mean Aggressive    Females

                        Produced             Singing                Attracting Mates       Acts per Male         Rejected per male           

High Food         4.2 + 1.1               78                            35                    8.6 + 3.4                   1.5 + 0.2                              

Low Food         1.01 + 0.3*            12*                         100*                       0*                      5.4 + 1.7*                   

                        *p < 0.05

TABLE 2:  Female Reproductive Behavior

                                                            Mean No.         Mean No.

                        Mean Males                  Males               Aggressive

                        Mated With                   Rejected           Acts over Males

High Food         1.03 + 0.6                     6.1 + 4.8                 0

Low Food         4.7 + 2.1*                           0*                 8.2 + 4.3*       

                        *p < 0.05

          You also observe that under the low food conditions, females that eat more spermatophores produce more eggs. 

            The importance of male song in female mate choice also changes with food availability.  In the high food group females preferred males with songs of longer duration, as shown in Table 3.  Other research has indicated that singing bout length is associated with a male's resistance to some parasites.  Males with longer songs are more resistant, and this resistance is heritable.  However, in the low food group female choice was not influenced by the length of male singing (Table 3).

                                                                                   % females preferring

            TABLE 3:        song bout length            high food          low food 

                                                8-10 sec                        83                    49

                                2-6 sec                         17                     51       

                                                                p < 0.05            p > 0.05

            Males prefer larger females, but only in the low food group (Table 4).  There is a positive correlation between female size and the number of eggs laid, such that larger females lay significantly more eggs than smaller females.

TABLE 4:          Mean weight of            Mean weight of            

                        Accepted Females          Rejected Females                      

High Food         3.6 + 1.0 g                    3.4 + 2.1 g        P>0.05            

Low Food         5.2 + 1.7 g                    2.5 + 3.3 g        P<0.05

            Based on this information, assess each of the following statements.  Indicate if each is true or false and justify your thinking.  Be sure to refer to the appropriate tables in your discussion.

a.  (Food availability has no effect on the cost of male parental investment or its importance to females.

b.  (Tables 2 & 3 indicate the following about female mating behavior:

·                under high food conditions, female mate choice is consistent with the Healthy Male Hypothesis

·                under low food conditions, female mate choice is consistent with the Good Genes Hypothesis

·                under both conditions, female mate choice is consistent with the Chase-away Selection Hypothesis

c.  Tables 1 & 4 indicate the following about male mating behavior:

·              under low food, males are monogamous

·              under high food, males are monogamous

d.  There is evidence of sex role reversal, and it occurs because under low food conditions the insects meet both of the criteria for sex role reversal.

ANSWERS for Question 2:

a.  False.  Food availability affects both the cost of male PI and its value to the female.  Male PI is the protein contained in the spermatophore.  When food is low males can produce on average only one spermatophore, but can produce 4 when food is abundant (Table 1).  Thus, when food is low it is more costly for the male to produce his spermatophore, he can produce only one, and thus he becomes more selective and is more likely to reject females (Table 1).  But, in low food the male PI becomes more important to the female, because now this may be one of the few food sources she has.  Thus, she mates with additional males, rarely rejects males, and fights for access to males during low food, while she rarely does these things in high food conditions (Table 2).

b.

• True or False:  Healthy Male hypothesis states that female choose males based on traits that accurately reflect their level of parasites and disease.  The female gains an immediate personal benefit, because she is then less likely to become infected (and she could also reduce the chance of catching a disease that she then passes on to her offspring).  For this hypothesis to be correct, there would need to be evidence that males showing certain traits (e.g., larger spermatophore; longer songs) had fewer ectoparasites, but not such evidence is given.  Thus, based on a strict interpretaion of the hypothesis, this statement is false and the female's behavior would be more consistent with the Good Genes hypothesis.  However, it is stated that males with larger songs have greater resistance to certain diseases, which could mean that they have fewer parasites on and in their bodies, thus making them less likely to infect a female.  Thus, by this interpretation, the statement is true.
   

• False:  The Good Genes hypothesis states that females choose males based on traits that accurately reflect their genetic quality.  The female gains indirectly, because the male quality is heritable and thus she has healthier offspring.  The hypthesis is supported under high food condition, in which females prefer males with longer songs and these males have heritable resistance to parasites (Table 3).  However, the Good Genes hypothesis does NOT apply under low food conditions, because females show no preference for males based on song length and thus his genetic quality.  Rather, under low food female behavior is most consistent with material benefit mate choice, resulting in polyandry.  Table 2 shows that females mate with up to 5 males to get food from the spermatophores, because now she cannot find sufficient food through her own foraging to produce her eggs.  She is mating with additional males, not because of their genetic traits, but to get their food gifts (although it is possible that under low food, only superior males can produce the single spermatophore, and thus they do have "good genes."  But, this isn't the reason the female is mating multiple times.)
   

•    False:  female's behavior is not consistent with Chase-away selection under either condition.  This hypothesis states that males evolve exaggerated traits through an exploitation of an existing female sensory bias.  Males that have a trait through a random mutation that taps into the bias will be preferred by females, even though the trait is unrelated to male quality.  Females will then be selected to resist the trait (because they don't gain anything; it's not an "honest" signal), which in turn will select for males with more pronounced traits to overcome female resistance.  This results in an arms race of ever more exaggerated male traits to overcome ever increasing female discrimination against the trait.  The female does not benefit and may even be harmed by this "manipulation" of her sensory bias.  Under high food conditions the females prefer males with longer songs, which clearly has genetic benefits for the young (Table 3).  Under low conditions, females fight over males to get extra food; the males resist and reject them (Table 2), which is not consistent with Chase-away selection.  Thus, no evidence under either condition.

c. 

• True:  under low food males can produce only one spermatophore and thus can mate with only one female = monogamy (Table 1) .  Under low food, the cost of the spermatophore to the male is very high, and the benefit to the female is also very high.  Thus, males become selective, do not sing to attract females, don't fight over them, and reject many females (Table 1) (and females compete with each other for access to males to get more food; Table 2).  Males become monogamous because of the increased cost of their male PI.
   

• False:  under high food, males can produce 4 spermatophores and can therefore mate with up to 4 females (Table 1).  Thus, males are polygynous.  Now, the cost of each spermatophore is less and the food gift is less important to the female.  Thus, she can get most of the food she needs through her own foraging and isn't choosing the male ONLY for his food gift.  Rather, she becomes selective and chooses the male with the best genetic quality (assessed through his song; Table 3).  Now males try to "convince" females to mate with them by signing and compete with other males for access to females (Table 1).  This is the typical male strategy associated with polygyny.

d.  True: Sex role reversal occurs in low food conditions.  The two conditions for sex role reversal are:  (1) male PI > female PI; and (2) female reproductive output > male reproductive output. 

        In high food, males can mate multiple times, their PI is of lower value (thus, they give less PI than the female),  and they are the aggressive, less selective sex.  They sing more, compete aggressively for females, rarely reject females, and their repro. success depends upon number of females mated with (Table 1).  Females under high food have higher PI, they cannot increase their RS by additional matings, and are thus selective, taking a long time to respond to males, often rejecting males, never competing for mates, and basing their choice on genetic benefits for parasite resistance as indicated by song length (Table 2, 3).  Because under high food males can produce 4 spermatophores whereas females mate with only one male (which suggests they lay only one batch of eggs), this suggests that male reproductive output is greater than that of the females.  Because male PI < female PI and male reproductive output is > female's, you get the typical male and female sex roles.

          In contrast, under low food males can mate on average only one time, the cost of spermatophore production and thus male PI increases and becomes more than that of the female, and males become the less aggressive, more selective sex.  They sing less, all singers attract females, they never compete aggressively and often reject females (Table 1).  Because of the high cost of their PI and limited number of spermats. they can produce (Table 1), they prefer larger females who can lay more eggs and size is assessed by female push-ups (Table 4).  Under low food female RS is dependent upon number of males mated with, because this increases food intake and thus egg production.  Now females become the aggressive, non-selective sex.  They mate with 4-5 males, respond quickly to all singing males, never reject males and fight over mates (Table 2).  Because males can mate only once and are limited in spermatophore production, this suggests that under low food male reproductive output < female output.  Thus, under low food, male PI > female PI and male output < female output, and thus get sex role reversal.