Question 10 - Science Practice Test for the ACT

For female fiddler crabs, which shows greater variability in width, the cheliped size or the carapace size?

Male and female gold claw fiddler crabs are invertebrates that inhabit sea beaches, brackish intertidal mud flats, lagoons and swamps. They have a hardened body shell called a carapace and two claws called chelpids. In females, the chelipeds are both small, whereas in males, one cheliped is larger than the other. The larger cheliped in the males is used for territorial displays, fighting other males and attracting females, whereas the smaller cheliped is used for eating and scavenging for food. After a fight between two males, the female usually chooses the victor as her mate. Variability in size may be what allows a female to choose the best mate and may relate to fighting ability and ultimately reproductive success.

In the wild, each male crab has a burrow, which it defends against intruders and which provides shelter for the crab. Males rarely fight females for burrows; instead, wandering males fight resident males in order to acquire their burrows. Claws can sometimes be lost during combat, which may affect male reproductive success. Although they do regenerate lost claws, the males have to allot more time and energy into claw regeneration than into actively courting females, which in turn can result in lower production of offspring.

Samira and Donovan, two behavioural ecologists were interested in examining size differences among males and females and length of combat bouts between males. For the first experiment, they measured claw and body length and width. For the second experiment they staged combats, in experimental terrariums, between residents and intruders. Donovan measured the females and Samira measured the males before allowing the males to fight.

Experiment 1:


The data from Table 1 is plotted in Figure 1 and the data from Table 2 is plotted in Figure 2, below.



For their second experiment, Samira and Donovan found that intruders and not residents initiated more combats, however residents won most of the combats. They found that the relationship between size and combat duration was inverse—the larger the size difference in claw length, the less time the crabs spent fighting with each other. They also found that the larger the male crab, the larger the major cheliped size and thus, the more likely the crab was to win in combat.

Experiment 2:


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