As a mom to three young primates, I spend a lot of time thinking about the large role that biology plays in my life. After all, nothing could be more important (biologically speaking) than birthing and raising these offspring. It’s easy for me to type that previous statement; but it’s not quite so easy for me to wholeheartedly believe it. Am I supposed to forefit all of my worldly desires and career aspirations for the greatest possible representation of my DNA in the human gene pool? Of course not…despite the fact that we biologists ascribe the 'survive and reproduce' dogma to all the other members of the animal kingdom (including primates) – we are far from following Darwin’s rules when it comes to our own biological destiny.
Perhaps it’s a matter of scale: on an overall level I’m doing what I can for the greater good of my genetic representation in future generations. I make sure that my kids have enough to eat, that they have a warm, comfortable place to sleep at night, clothes to wear and so on. I’m a vigilant parent when we’re out and about - my voice is a strong member of the parental choir of everyday life: 'Hold my hand! This is a parking lot! Stay beside me! Don’t touch that! Don’t climb up there!' you get the picture. I work hard to ensure that at the end of each day my offspring are alive and well.
But sometimes I just want a break.
Juvenile members of any species with a high degree of parental care are quite unaware of the painstaking efforts of their parents. For the most part the children take (take, take, take, take), and parents provide. However, there comes a time when parents (and mothers in particular) need to pause from our efforts of providing, for the sake of our own well being. It’s very unlikely (impossible) for an infant to say ‘you look tired mommy, why don’t you have a rest and I’ll just sit quietly here for a while’. Infants don’t know of our sacrifices, they’re only aware of their own needs. However, parents are aware of BOTH our own needs and those of our children, and frankly, sometimes the former are more important.
Welcome to the world of parent-offspring conflict theory.
According to Trivers (1972), parental investment is defined as "actions taken by the parent that increase the chances of current offspring survival, while decreasing parents' ability to invest in other (future) offspring". Conflict between parent and offspring arises when parental investment switches from current to future offspring – and current offspring are left without the doting care of their parents. In order to include the Homo sapiens in the discussion of parent/offspring conflict theory we need to expand on Trivers’ definition just a little. As I mentioned above, humans think a lot about our individuality and lives outside the realm of child-rearing despite the fact that this does not have any positive impact on our overall biological fitness. To adjust Trivers’ definition of parental investment to include the Homo sapiens we might say something like: "actions taken by the parent that increase the chances of current offspring survival, while decreasing the chances of future offspring survival (or even existence) OR decreasing the overall well-being of said parent". I’m sure Trivers would likely rap my knuckles for such a bastardization of his theory, but here’s my bottom line:
There are times when the level of parental sacrifice (measured as either future reproductive potential of the parent or as future well-being of the parent if you are finished reproducing) is greater than the benefit garnered by the offspring by a certain action (example to follow). Homo sapiens mammas don’t want to just wither up and fade away once our offspring are born – we invest a LOT in our offspring, but we want to reserve the right to invest in ourselves too.
Case Study 1: Get off my boob!
I am 3 for 3 when it comes to having breastfed my children. My first son went for 9 months, my daughter for 14 months and my second son for 15 months. For any breastfeeding mother the choice to stop nursing your offspring is a tough one. After heroically embracing 9 months of gestation followed by traumatic childbirth (zen my ass) and several months of breastfeeding – when is it ok to say 'I want my body back'? After all – despite our technological advances in the area of infant nutrition, there is no question that 'breast is best' for any child. The conflict is clear: in one corner you have the infant, requiring the best possible start in life through the best possible food source. In the other corner you have the mother, who is conflicted over wanting the best for her child and the overwhelming need to return her temple to its normal state of being.
Am I overanalyzing this? Does it really matter to an infant whether it’s still suckling mommy’s breast or not? According to recent research on juvenile Anubis baboons (Papio hamadryas) it most certainly does. Utilizing a ground-breaking technique that involves examination of histological thin sections of teeth, a group of scientists from the UK have made the bold suggestion that the weaning process is extremely stressful for infant baboons. Formation of both enamel and dentine are particularly sensitive to physiological changes – and can therefore show the effects of illness or anxiety during their development. The weaning process is recorded in the tooth enamel through changes to the Sr/Ca ratio, and this can be simultaneously compared to the stress levels during this period through examination of the enamel and dentine. A series of analyses of juvenile baboon teeth suggests that weanlings experience physiological stress at predictable times during the weaning process – which could certainly be evidence for a parent-offspring conflict.
Now, mommy baboons can easily 'rationalize' cessation of breastfeeding because investing so much in offspring that are able to obtain nutrition in other ways does not make biological sense. Once free from their breastfeeding duties, mommy baboons can focus their energy into creating more offspring. The parent-offspring conflict is settled by virtue of the fact that mommy’s biological fitness improves by turfing baby A from her breast and getting to work on baby B. This isn’t immediately good news for baby A, but baby A’s inclusive fitness will also increase (albeit not as much as mommy’s) with the arrival of baby B.
What about human mommies? There is not much biological sense to stop breastfeeding our children if we aren’t planning to have any more children. According to the ‘terminal investment hypothesis’ we should, in fact, increase our reproductive efforts with later children since we have ‘little to gain’ by reserving energy for the future.
Case Study 2: Life after childbirth?
A recent study on rhesus macaques (Macaca mulatta) utilized over 50 years of demographic data to examine whether mommy macaques follow the parameters of the terminal investment hypothesis. Older females were found to spend more time in direct contact with their infants, which supports the notion that they have 'little else to do' with their energy and therefore it is best spent on last-ditch attempts to increase biological fitness. The study also found support for the 'senescence' hypothesis, which suggests that females have a reduced reproductive output towards the end of their lifespan due to age-related declines in body condition.
For our species, the terminal investment hypothesis fits perfectly into the ideas surrounding parent offspring conflict theory. Human females are unique in the animal kingdom in that we reach reproductive senescence in the middle of our lifespans. What does this mean biologically? There is a case to be made for the shift of our reproductive energies to raising our grandchildren, which follows the tenets of the 'grandmother hypothesis' and the senescence hypothesis. However, there are a few major problems with making these assumptions:
1. First, many post-menopausal women in the Western world do NOT devote their lives to assisting in the raising of their grandchildren. Much to the contrary, they are playing shuffleboard in the Caribbean, tennis in Arizona or generally having a great time with their retirement freedom. Sure, they may lend a hand with babysitting – but more than likely they will not go so far as to devote the rest of their lives to the well being of their grandchildren.
2. Second, and perhaps more importantly, many human females are post-reproductive BEFORE they hit menopause. We utilize the technologies of contraception or surgery to ensure that we DON’T maximize our biological fitness by being constantly tied to rearing and caring for offspring.
Bottom line: human females (both pre and post menopausal) experience parent-offspring conflict in a major way. Do we ‘throw in the towel’ after our last offspring has been born, sacrificing ourselves in the process? Hell no! We like to enjoy our lives outside of our roles as moms. We like to take pride in our post-partum appearance, and we like to focus our energy on our careers or hobbies. Basically, we are the poster animals for failure when it comes to biological theories like parent-offspring conflict and terminal investment. Does our selfishness take away from the biological fitness of our children? Probably….but since humans aren’t really into the habit of maximizing our biological fitness anyway, this really doesn’t matter.
Despite sometimes pulling the trump card (selfishness) during conflicts with my offspring, I’m pretty sure they will turn out ok. Perhaps one day they will one day follow in the footsteps of their mom and achieve the right balance between sacrifice and selfishness. As any human moms out there will attest, it’s a balance we, unlike most other members of the animal kingdom, struggle to achieve each and every day.
Dirks W, Humphrey LT, Dean MC, & Jeffries TE (2010). The relationship of accentuated lines in enamel to weaning stress in juvenile baboons (Papio hamadryas anubis). Folia primatologica; international journal of primatology, 81 (4), 207-23 PMID: 21124031
Hoffman, C., Higham, J., Mas-Rivera, A., Ayala, J., & Maestripieri, D. (2010). Terminal investment and senescence in rhesus macaques (Macaca mulatta) on Cayo Santiago Behavioral Ecology, 21 (5), 972-978 DOI: 10.1093/beheco/arq098
TRIVERS, R. (1974). Parent-Offspring Conflict Integrative and Comparative Biology, 14 (1), 249-264 DOI: 10.1093/icb/14.1.249
Book Chapter: Trivers RL (1972). Parental investment and sexual selection. In Sexual Selection and the Descent of Man,1871–1971 (Campbell B, ed.), pp 136–179. Chicago, Aldine-Atherton.
Carin Bondar is a biologist, writer and filmmaker whose work has appeared on Discovery Network, Discovery World HD and online at National Geographic Wild and Scientific American. A former ballerina, she now holds a PhD in community ecology from the University of British Columbia. Her thesis work took her to the temperate rainforests of British Columbia and New Zealand where she researched community interactions of invertebrates in small streams. Dr. Bondar blogs at www.carinbondar.com, tweets as @drbondar, and has just released her first book The Nature of Human Nature. When she’s not absorbed in the wonderful world of biology, she is a busy mom of three young children.
The views expressed are those of the author and are not necessarily those of Scientific American.