SAT Subject Biology Practice Question 336: Answer and Explanation
6. Which of the following indicates that animals have internal biological clocks?
A. A mouse kept in constant darkness shows a daily rhythm of activity.
B. A rooster crows whenever the sun rises in both winter and summer.
C. An owl kept in constant light drifts away from a 24-hour cycle.
D. Some species of birds can sense fluctuations in the Earth's magnetic field.
E. A squirrel whose night and day are artificially reversed soon adapts to its new schedule.
Correct Answer: A
A If an organism's environment remains absolutely constant, and that organism still exhibits regular rhythms of activity, there must be some internal "clock" that keeps it on schedule (C and E are wrong). Roosters vary the time of their crow as the sun varies the time it rises (B is wrong). The magnetic field has nothing to do with internal clocks (D is wrong).