It’s easy to map our familiar understanding of human behavior onto the natural world, to think that the default pattern for reproduction in other species involves a neat binary of male and female raising offspring in a small family unit. Not even all human cultures fall into this pattern, and it represents but one of a seemingly endless fount of possibility for how to create the next generation. We’ll look here at abortion as part of a larger context for conserving energy in reproduction.
Saving energy on reproductive costs
As with plants, animals over-produce offspring to compensate for the heavy losses that befall offspring at the hands of predators, a harsh environment, parasites, disease, and genetic defects. Over-producing offspring means an high upfront energetic investment. If only 6 out of 3,000 eggs survive to adulthood, there’s a lot of wasted energy in producing those extra 2,994 eggs (link). Time and energy invested in the 2,994 duds is time and energy siphoned away from supporting the 6. Fortunately females have a few options to help minimize the costs of reproduction, including (but not limited to):
- Paussing the pregnancy
- Reabsorbing the fetus
- Inducing an abortion
- Eating the offspring
- Letting the offspring fight it out
I’ve included other options in addition to abortion to highlight that abortion is but one strategy for females to avoid enduring undue burden to birth and raise offspring when the conditions are unfavorable.
In some mammals (particularly marsupials), females will mate shortly after giving birth. They are then pregnant with their next litter while nursing and caring for the young. Playing double duty is expensive and risky, and if the gestation period is short the female hazards the chance of having to care for two sets of young at the same time, a recipe for disaster. If most of offspring in the first litter survive, then the female will pause the development of the embryos. The tammar wallaby can pause a pregnancy for up to 11 months: source. This happens before the embryo attaches to the uterine wall (this takes about 5 days in humans). If for some unfortunate reason the recently born offspring die, the pregnancy will develop as normal without a pause.
The previous pause followed fertilization of the embryo and is responsive to external conditions. But for some species (like crayfish, mudpuppies, and most weasels), the delay in development precedes fertilization. And again, there may be a considerable gap between when mating occurs and when the embryos are fertilized (e.g. 10 months in long-tailed weasels). In these species, females typically mate sometime in the fall, store sperm internally through the winter, and give birth to their young in the spring. By mating in the fall, females have a wider range of males to choose from (because winter has yet to kill of a sizeable chunk of the population). It may also allow the female to be responsive to spring conditions and more precisely time fertilization with suitable habitat conditions.
Resorbing the fetus
In some species, the female may be able to reabsorb the fetus(es) early on in the pregnancy, recovering nutrients and energy, and eliminating the future costs of carrying the pregnancy to term. This is frequently a response to stress (which could be related to environmental conditions, food scarcity, disease, etc.). When pregnant sharks and rays are captured, for example, the stress can result in “stress-induced parturition” of 2-85% of pregnancies (source).
The Bruce Effect
Reabsorbing the fetus may even be in response to a new male arriving in their territory. If a female meadow vole is exposed to a new male within 24 hours of becoming pregnant (remember, our 9 months gestational period is exceptionally long; gestation can be just 2-3 weeks in some mice), she will reabsorb the fetus and become sexually receptive again. Infanticide of unrelated offspring by dominant males is common in polygynous species (one male, many females, as in lions, gelada baboons, and many mice, voles, and lemmings). When a male takes over a new geographic territory, to ensure the young in his territory have all been sired by him, he may set out killing all other young in the care of females. By terminating the pregnancy early on, the female doesn’t waste energy producing offspring that will likely be killed by the new male.
In communally nesting birds, like ostriches, there may not be enough resources in the environment for each female to raise their young. While all ostriches in their small group (2-7 individuals) may lay eggs (they lay these in a single nest tending by the dominant female), the major ostrich will push the other birds’ eggs to the outside of the nest, effectively killing them.
Because sex results in a mixing of genes between the parents, it’s not a guarantee that the offspring will be genetically superior to their parents. To weed out the weak, parents can set up a system of competition between the young. For example, in our humble Virginia opossum, females have more offspring (18-25) than they have nipples (13). This sets up an immediate competition between siblings for a seat at the table. Those who don’t latch on will inevitable die. Those young that develop quickest and are the strongest are most likely to reach a nipple first and be able to defend it from its siblings. In Muridae (which includes mice, rats, gerbils), young develop “tenacious nipple attachment” in response to intense sibling competition (source). Once attached to the nipple, they are incredibly difficult to displace by a hungry, mewling sibling.
Parents may even eat their young in a sort of post-partum abortion. Eating the eggs may simulate a predator preying on the weak. The parent can select the young to eat and may choose the slowest to develop (rather than a predator which might indiscriminately eat the young), effectively putting selective pressure on rapid development of offspring. In Tasmanian devils, females have just four nipples, but can birth up to 30 young. The first four to the pouch latch and are fed while the others are eaten by the mom, which actually stimulates milk production.