Monday, April 20, 2015

Living to Love or Loving to Death?

Biologically speaking, animals are the most successful when they have the most descendents. Because reproduction is such a major focus of animal life, we invest a lot in it and take a lot of risks for it. During breeding phases, animals often forgo eating or sleeping well, risk getting in fights, expose themselves to predators, and spend lots of energy on finding potential mates and courting them. Because many specific costs and risks an animal must face to reproduce are particular to the species, many reproductive strategies have emerged as a result.

One major division in reproductive strategies is iteroparity versus semelparity. An iteroparous species is one that can have multiple reproductive cycles in its lifetime. They include all birds, almost all mammals, most reptiles, fish and molluscs, and many insects. A semelparous species is one that has a single reproductive period and then dies. Semelparous animal species include many insects (such as cicadas and mayflies), some moluscs (including some octopus), and several fish (including Pacific salmon). Only a handful of species of amphibians, reptiles and mammals are semelparous.

A silvereye mother feeds her clutch of chicks. She will have another one next year.
Photo by Benjamint444 at Wikimedia Commons.

The advantages to being an iteroparous species seem obvious (we are one, after all). For one thing, losing your virginity isn't a death sentence. This means that if we are not very good at finding or courting a mate, sex, or parenting the first time around, we get more opportunities to improve. It means that if the conditions are crappy in one breeding season, another season will come around later. And it means that with every breeding season that you have offspring, your individual "success" improves.

Pacific salmon spawn their one and only time. Photo by Steve Hillebrand at
the U.S. Fish and Wildlife Service, available at Wikimedia Commons.

The advantages to being a semelparous species are less obvious. What possible advantages can there be to dying after your first breeding season? But if we think about the "success" of an animal being how many successfully reproducing offspring it has, and not how long it lives, this strategy starts to make sense. A semelparous animal can put everything it's got into its one reproductive event. There is no point in holding back if you're never going to get another shot. As a result, semelparous species usually produce more offspring in their one reproductive event than iteroparous species do in any of theirs.

Several theoretical models have emerged to predict under which circumstances a species would use an iteroparous strategy versus a semelparous strategy. It would make sense that species that have a greater risk of dying early would benefit more from a semelparous strategy. Species in which each additional offspring is less costly to produce and care for than the previous offspring would seem to benefit from an iteroparous strategy. However, strangely enough, the data we have on animal reproductive strategies do not clearly show these patterns.

We still have a lot to learn about these reproductive strategies and the complexities of what makes a species live to keep on loving or love to their death.

1 comment:

  1. Great story on semelparous strategy. Salmon swim upstream to lay eggs at its birth pond. Why?