Reproductive Activity

Reproductive activity has always been a focus of scientists studying animals because it forms such a prominent part of the behavioral repertoire of all species. Reproduction must occur, if genes are to be passed on. Most acts of most animals are geared, ultimately, toward production of successfully reproducing young. Tinbergen's work with the stickleback, already discussed in this chapter, is only one example of ethological work focusing on the reproductive cycle.

What useful characteristics of odor make it a widely used means of communication?

Odor is a widely used form of communication in the animal world, particularly in combination with reproductive activity. Odor bearing molecules are both durable and highly specific. They convey directional information if carried in the air or water. Therefore they are ideal for marking territory or attracting a potential mate.

Salmon use odor as a homing signal before spawning (laying eggs). Scientists have known for years that salmon return to the stream or pond where they were born during spawning. This drive is so strong that salmon will beat themselves to death against a dam if one is built across a stream between the spawning pond and the sea.

How do salmon find their home stream during spawning?

Scientists wondered how salmon found their way back hundreds of miles to the same exact pond where they were born. Did the fish have some sort of mental map that enabled them to follow geographical features? Did they respond to magnetic fields of the earth? The angle of the sun? As it turned out, odor was the key. Scientists put a few drops of a chemical with a powerful odor in a spawning pond. When the baby salmon were grown up and returned from the sea, the scientists moved to a new, different pond and put the same chemical there. The salmon went back to this new, different pond. A similar experiment showed that frogs tend to return to a pond that has the same odor as the one where they were born.

What is a pheromone?

Odors called pheromones play a role in mating activity. Technically, a pheromone is an intraspecific distance hormone, which means it is a chemical used within a species (intraspecific) to communicate at a distance. A pheromone is not necessarily a sexual attractant, although that is a common function of pheromones. Prince (1975) listed nine responses of insects to odor, including swarming, grooming, exchanging solid foods and exchanging liquids. A few parts per billion of a pheromone floating in the air can trigger the response. For example, male dogs can travel many city blocks to find a female dog that is sexually receptive ("in heat") and emitting pheromones.

Do humans respond to pheromones for sexual attraction? The makers of the fragrance Andron claimed it had pheromone-like effects on the opposite sex, although the chemist who designed Andron said he did not believe it. The vomeronasal organ in the nose of humans may possibly be involved in pheromone-like sexual attraction but so far nobody has proven that a specific substance can trigger an affectionate response in humans.

Social status often has a direct impact on reproductive success. Less dominant animals may be excluded from mating entirely. "Tournaments" are common in many species. The winners get the healthiest, highest ranking mates. That does not prevent lower status animals from trying to reproduce, but they may not be successful. In animals as in humans, it is common for lower status males to "aim too high" and be rejected by desirable females.

What happens if a low-status ram is sent, alone, into a flock of ewes?

...In New Zealand,...researchers have reversed the apparent infertility of some rams by including them in a group of other rams sent into a flock of ewes, instead of sending them into the flock by themselves, as is current practice. Apparently, a class structure is strong among sheep, and mating is related to dominance. A dominant ram mates best with a dominant ewe, a less dominant ram with a less dominant ewe, and so on, down to the least dominant sheep. However, when a lone ram is released among a flock of ewes , he will invariably attempt to mate with the ewe that is most obviously at the height of estrus. But she will probably reject him if he is not at the right dominance level (Watson, 1975).

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