Final answer:
Lifelong monogamy occurs in certain animal species due to the mate-guarding hypothesis, male-assistance hypothesis, and female-enforcement hypothesis.
Step-by-step explanation:
Lifelong monogamy occurs in certain animal species for a variety of reasons.
One explanation is the mate-guarding hypothesis, where males stay with the female to prevent other males from mating with her. This is advantageous in situations where mates are scarce.
Another explanation is the male-assistance hypothesis, where males that help rear their young will have more and healthier offspring.
Monogamy is observed in many bird populations where both parents contribute to parental care.
Additionally, the female-enforcement hypothesis suggests that females actively interfere with the male's signaling to attract other mates to ensure that their own offspring do not face competition.