Often compete for access to females

Males often guard individual females or patches of females

Males also compete directly for access to eggs

Example dameselfly: Will guard females from other males while they lay their eggs

Have adaptive horn on penis to remove rival sperm

Polygyny: Male mates with several females. Females usually provide most parental care. 

Polyandry: Female mates with sveral males. Male usually provides most parental care. 

Why be monogamomous? Shouldn't males be polygynous to try and rear as many young as possible? 

2 Hypothesis to explain monogamy

Mate-guarding: Suggest monogamy might be best if females remain receptive after mating or are hard to find.

Mate assistance: males remain with one female to help rear offspring.

Females attempt to block male polygamy. 

Example: razorbill birds, females attack mates that show non-monogamous tendencies. 

Nicroporus beetles

Male and female bury carcass together; female lays eggs in it

Typically monogamous, but after mating the males sometimes release a sex pheromone to attract new mates.

Female will push male until he stops.

Many monogamous birds are not truly monogamous. 

Example: Some female birds and primates will frequently seek out other mates. Why? 

EPC visits by females might be to ensure fertilization of eggs. But most evidence suggest that EPCs relate to 'good genes' of males that are visited. 

Resource defense polygyny: males defend high-quality territories that attract mates (e.g, polygamous birds or fish)

Group defense polygyny: Males might "guard" groups of females from other males (e.g, bats, amphipods)

Aggregations of males defending small territories (often territories with no resources at all)

Lekking males often advertise using visual, audible, and olfactory displays. 

Females visit several males, but are very selective when choosing a mate. 

A small number of males perform most of the copulations.