Parent investment strategies across the animal kingdom
By Hunter Doughty
It is not uncommon for potential parents to think about the amount of time, money, or attention, they could provide to their future children. We all want the best for our kids, right? But is this desire to give our offspring the greatest chances for success, or even survival, limited to human beings? Most definitely not. In fact, all animals have strategies for reproducing. And each one of them is attempting to optimize the survival rate of their offspring given their particular circumstances. In ecology, we refer to these evolutionarily refined tactics as ‘parent investment strategies’.
In general, all animals can be placed along a spectrum spanning two opposing approaches to reproduction. These two approaches are known as R strategists, and K strategists. In essence, an R strategist values quantity over quality. They are animals that use sheer numbers of offspring to increase their overall chance of having successful progeny. K strategists, on the other hand, value quality over quantity. These animals invest more energy into fewer offspring to increase each individual offspring’s chance of success. The letters R and K are derived from the mathematical formulas that show biological population trends, but we won’t go into all of that today.1
Let’s first start with a true R strategist to understand some of the factors at play. Musca domestica, better known as the common housefly, is an insect found across every inhabited continent. Its life cycle includes the following stages: egg, larva, pupa, and adult. On average, a female will lay between 75 to 150 eggs in a single batch, and will lay multiple batches throughout her adult life, totaling an approximate 500 eggs. Each egg contains a single offspring that can go from egg to adult in 6.5 days, and at most, in 58 days. Once an adult, it will reach sexual maturity in as little as 36 hours, and will survive between 15 to 60 days. Astonishingly, this means that in theory “a pair of flies beginning reproduction in April, may be progenitors, under optimal conditions and if all were to live, of 191,010,000,000,000,000,000 flies by August.” 2, 3, 4
|Musca domestica (Ref 10)|
The common housefly illustrates a number of key traits associated with R strategists: extremely fast maturation time, large number of offspring produced, precocial offspring (meaning they do not need to learn behavior from their parents in order to survive), high mortality rate of offspring, almost no parental care, and a very short life span. This combination of traits makes for a species that is highly adaptable due to their fast generational turnover. And this fact is why R strategists are usually extremely successful in disturbed habitats such as cleared forests, or temporary habitats such as a pile of trash. As shown by the ubiquitous housefly, many invasive pest species fall under the category of R strategists. 1, 5, 6
On the complete opposite end of the spectrum, we find our K strategists. A prime K strategist example is our very own Elephas maximus, a.k.a. the Asian elephant. A female Asian elephant carries her calf for 18 to 22 months before giving birth (that’s almost two years, and its the longest gestation period of any mammal!). The calf then takes an average of three to five years to fully wean off of its mother’s milk, and will not reach sexual maturity until its teens. A male usually reaches sexual maturity around age 14 to 15 and is then known as a bull. At this point he will join a bachelor herd, and will likely not have access to females until he is around age 30, due to the dominance of other older males. A female will reach sexual maturity at about age 14-16, and will usually have her first calf around age 18 to 20. An adult female, known as a cow, can give birth to a single calf every two to four years under optimal conditions, and on average has five to six (and a max of ten) calves throughout her lifetime. A female will stay in her mother’s herd for her entire life, continually learning from the older females. 7, 8
|Boonjan and baby Denra here in the Golden Triangle|
The Asian elephant’s long-term parenting is a clear depiction of what it means to be a K strategist: very slow maturation time, a few number of offspring produced, larger offspring relative to R strategists, altricial offspring (meaning they must rely on their parents for survival when they are born), lower mortality rate of offspring relative to R strategists, immense parental care, and a long potential life span. 1, 9
How this particular parental investment strategy relates to conservation is important. Because of the life history traits associated with K strategists, these species are often the ones most impacted by human induced change such as loss of habitat due to development, or direct population declines due to hunting. And unfortunately, these same life history traits also mean that these species tend to be the most difficult to save once their numbers have been significantly depleted.
When trying to save a K strategist species, like the elephant, a conservationist has to deal with the fact that an individual of this species is only going to reproduce a few select times over many years. Furthermore, that group of progeny will then take another many years in order to reach sexual maturity so that they may produce the subsequent generation. Which means, that if protection measures are to be effective, they have to be successfully implemented throughout the duration of a vastly large temporal scale. They require the investment and cooperation of many groups, governments, and citizens, to maintain efforts long enough for a species to increase its population to a stable, and self-sustaining, level.
|Herd of African elephants (Ref 11)|
Across the animal kingdom there are a variety of species that fall somewhere between our two extreme examples. A leatherback sea turtle for example has a long life span, but still lays many eggs at a time that must hatch unprotected and immediately fend for themselves. While a grey wolf, who has a much shorter life span, produces fewer offspring over the course of its lifetime and instead chooses to invest more energy into each young. With all of these parenting tactics in mind, if you yourself are debating whether to have many children with hopes that at least one of them will make it to the professional big leagues, or to only have a single child so that you can afford the best coaches in the country to optimize their training, then just remember that you are not alone in strategizing how best to reproduce.
|Leatherback sea turtle hatch-lings, Costa Rica|