Clik here to view.
Image Copyright Carin Bondar 'The Nature of Human Nature'
As parents, most of us vow to keep things equal between our offspring. In theory there should be an equal amount of time investment into Suzie’s piano lessons as there is into Bobby’s tennis practice. However, in reality these things are never quite equal are they? Bobby’s tennis practice is across town, making the investment into his achievements larger than when Suzie walks over to the lady down the street for her piano lesson. Next, the inevitable: Suzie hates piano but Bobby looks as though he’s heading straight to the Olympic trials with his perfected backswing. Resource allocation into the two offspring is suddenly a far cry from 50:50. As humans we may say that we make equal investment into each of our offspring, this is an almost impossible task in the animal kingdom. An additional complicating factor is this: what about offspring with different fathers? Perhaps Bobby was destined to be a champion based on the fact that his biological father is an admirable member of society and a physically fit athlete himself, but poor little Suzie was the outcome of a one night stand after a few too many cocktails with a character whose name you’d rather erase from memory. Life history theory predicts that maternal investment into offspring should reflect the likelihood of said offspring to contribute to the fitness of said mother…which could be bad news for Suzie.
Many species in the animal kingdom mate with different partners in different mating seasons. Based on a number of environmental, physical and random factors a female may not always end up with the mate that she most desires. However, offspring are the inevitable result and so an interesting conundrum presents itself: invest equally in all offspring despite the sub-optimal paternity of some? Biologically speaking the answer should be a resounding no! According to the ‘differential allocation hypothesis1’, females of many species do not contribute equally to the health and well being of offspring sired by fathers of different quality. In experimental mating research on mallard ducks (Anas platyrhynchos), researchers paired individual females with both high ranking and low ranking males in order to investigate whether they would invest differently in their offspring based on the identity of the father2. Egg size is entirely determined by the female, and is a critical trait influencing fitness in birds: larger eggs produce larger chicks that have an increased chance of surviving to adulthood. Sure enough, it was demonstrated that eggs are significantly larger when females mate with a high ranking male. Invest equally in all offspring? I think not!
Image may be NSFW.
Clik here to view.
Blue footed booby (Sula nebouxii) males have a clear signal to show females that they are highly fit and ready to sire the next generation: their feet. Female boobies prefer males with bright blue-green feet, and will actively discriminate against males without this characteristic. Once copulation has occurred, the female generally lays two eggs, the second approximately 4 days after the first. Both parents incubate the clutch and raise the young together until they are ready to leave the nest. In an experiment designed to assess differential allocation hypothesis, researchers painted the feet of the male partner to a dull (unattractive) color just after the first egg was laid3. As a result of this change in the males’ foot morphology, the second egg laid by the female partner had a lower volume as well as a lower hormonal content. So perceptive is the female blue footed booby that within a few days of her mate quality being ‘lowered’, she reacts accordingly in her egg investment!
The effects of partner ‘attractiveness’ do not end at the physiological stage of egg-laying. It has been shown for Zebra finches (Taeniopygia guttata) that the amount of post-natal parental care (by both mothers and fathers) varies with partner quality as well. Parental care activities like nest maintenance, watching over young offspring, and time spent brooding, feeding and grooming them are generally performed by both Zebra finch parents; however, levels of all aforementioned activities are lower when a partner has an unattractive mate4. It’s as if parents are reluctant to place all of their reproductive resources into a set of offspring that may have mediocre genes…tough luck for said offspring!
I suppose most human parents would argue against application of the differential allocation hypothesis in our species….and in many cases this would be with just cause. Most Homo sapiens do an admirable job of investing equally in their offspring. There are many parents out there raising children that are not only born of ‘unattractive’ parents, they are being raised by parents that are not even biologically involved (adoption/foster parenting). Despite the fact that at times Bobby’s tennis talents may overshadow the accomplishments of his sister, as a human offspring Suzie is quite likely to be well provided for. Perhaps the animal kingdom could learn a thing or two about the importance of the nurture part of ‘nature vs nurture’ argument.
1 Burley N. 1986. Sexual selection for aesthetic traits in species with biparental care. American Naturalist 127(4):415–445
2Cunningham, E.J.A. and Russell, A.F. 2000. Egg investment is influenced by male attractiveness in the mallard. Nature 404: 74-77.
3Dentressangle, F., Boeck, L. and Torres, R. 2008. Maternal investment in eggs is affected by male feet color and breeding conditions in the blue-footed booby, Sula nebouxii. Behavioral Ecology and Sociobiology 62: 1899-1908.
4Burley N. 1988. The differential allocation hypothesis: an experimental test. American Naturalist 132:611–628