Reproductive strategies shaping life on the planet

Article and images by Dr Julie O'Connor, senior conservation partnerships officer, Sunshine Coast Council

With the closure of schools in many countries around the world, Covid-19 has forced many parents to expand their already significant level of parental care. Generally, as a group, mammals provide the highest level of care for their developing young of any species. We will go to any lengths to raise happy, healthy children because we are biologically wired to do so (and we adore them). But across the planet, the reproductive strategies of mammals are just some of the countless other strategies that shape life on Earth.

All organisms on the planet, from the smallest bacteria to the largest mammal, share one driving force, the need to reproduce themselves. The strategies they adopt to do this are as diverse and extraordinary as the species themselves.

The actual mechanisms of reproduction are geared to improve reproductive success for each species. And just as each species is different, so are the strategies it uses to ensure its genes are not only passed to the next generation, but that those offspring will survive long enough to also reproduce.

There is no single best strategy to suit every species. The reproductive strategy a species adopts will be shaped by a range of conditions and competing pressures, including habitat, resources, environmental conditions, predation pressures etc. One way scientists have grouped reproductive strategies is articulated in the theory of r/K-selection. The r/K-selection model can be viewed more as a continuum than two strictly defined categories. Species leaning more to the “r” end of the spectrum generally produce many young as quickly as possible and invest little to nothing into parental care of their young. Corals are good examples of organisms at the extreme “r” end of the spectrum. Species with reproductive strategies leaning towards the “K” end of the spectrum generally have fewer young and invest significant parental care. Humans are an example of a species at the extreme “K” end of the spectrum.

But, as with everything in nature that we try to wrap up in a neat explanation, the reality is a lot more nuanced. While many of the “K” strategists tend to be large animals, and many of the “r” strategists smaller animals, there are notable exceptions among both. For example, the 160 kg loggerhead turtle Caretta caretta that nests on Sunshine Coast beaches is an “r” strategist, laying in excess of 120 eggs multiple times a season, but she will never see her young emerge from the nest.

Conversely, some of the tiniest spiders exhibit excellent parental care. The ubiquitous daddy-long-legs spider carries her sac of around 50 eggs in her pedipalps for the entire 17-24 day incubation period. Then when they hatch, she continues to hold her bundle of spiderlings for another week or so until they disperse.

The parental efforts of these little spiders notwithstanding, the greatest parental investment made by any group of animals belongs to the mammals. With the exception of the egg-laying monotremes, female mammals carry their developing young within their bodies then, additionally, provide them with prolonged care after birth. Around 65 million years ago, the mammals diverted to evolve as either placental or marsupial mammals. In Australian marsupials, the kangaroos and wallabies serve as an excellent example of how the environment can help shape an animal’s reproductive strategy. For example, in the boom and bust cycle that dominates much of arid Australia, a female kangaroo can support three generations of young simultaneously as insurance against harsh environmental conditions. The strategy probably also helps the population bounce back when favourable conditions return.

Regardless of which reproductive strategy a species develops, the goal of all the strategies on the r/K-selection continuum is the successful production of enough offspring to, at least, replace the parental generation.