Is it always Mum who is left holding the baby? – The evolution of parental care
HSV Public Lecture by Dr Angus Martin, former head of Zoology, University of Melbourne, at Balwyn Library on 25 February 2016
It is interesting to contemplate an entangled bank clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.1
Dr Martin began his lecture with this quotation from Charles Darwin’s Origin of Species. Darwin was curious about the immense diversity of species among plants and animals, but from this vast array was able to distil the principles that favoured survival and selection to a few key concepts. The attributes and strategies that improve the number of surviving offspring are fundamental to evolutionary success.
At first glance it is obvious that the female makes a much greater investment of energy in the reproductive process compared with the male. Her gamete, that is, the egg, is larger and more energy-dense than sperm, and pregnancy and lactation, if that is a requirement, also demand an intensive input. So does it not stand to reason, given this imbalance, that the female will have a greater commitment to the welfare of her offspring?
The answer is, not necessarily. Several examples of breeding and nurturing behaviour are instructive. Firstly the Central American Convict Cichlids which breed as monogamous pairs share parenting duties, such as defending their territory from predators, fanning and cleaning their eggs and guarding the young as they explore. However, if one parent is lost, the remaining parent, whether male or female, has variable success in rearing the young. The male manages very well if there are no predators to contend with, but, counter-intuitively, the female successfully raises her young single-handedly only when there are predators to keep at bay. This rather surprising behavioural pattern is still not well understood.
It is important to distinguish between parental care and parental investment. The former refers to direct interactions between a parent and its young. However, consider a mother turtle who, after laying her eggs and burying them in the sand, returns to the sea before they have hatched. Her actions have been a form of invest-ment to enable them to hatch successfully, but she does not provide any direct parental care after that. Similarly a butterfly must choose carefully where to lay her eggs to ensure that the emerging caterpillar will have an appropriate source of food, but her investment, while critical, stops at that point.
By contrast, the male seahorse carries the fertilised egg in a special pouch and continues after they are born to nurture the young. However, the reason for this unusual behaviour is not well understood.
In regard to reproduction and parental investment, what are the most favourable strategies?
- to have a long life-span and breed several times?
- to have a short life-span and invest everything in one breeding effort?
- to have many, or just a few, offspring each time?
- to have large young, or small ones?
- to care for the newly-born or just cast then adrift?
These questions dovetail to some extent. For example, a long life is a feature of big animals: small animals are more fragile and vulnerable. The larger the litter, the smaller each of the young will be, so that to have larger offspring the number born at any one time should be small.
With the question of care and nurturing the mating system available will have a profound effect. Common mating systems include the following.
Polygyny, where there is one male and several females. Paternal care of the young is very rare in this kind of system.
Monogamy, where one male and one female breed together, with the male frequently sharing the parental care required. This is the familiar pattern in carnivores, apes and man.
Polyandry, with one female and several males interbreeding. This is very rare among mammals. The South African hunting dog or painted wolf provides an unusual example of a social group where there is only one adult female within a pack of 8–15 dogs. She usually has a litter of 16 pups which she suckles for about three weeks. Following this about 90% of the food, usually meat from large animals, is provided by other members of the pack. As the female offspring grow up they leave to join other packs.
Darwin’s analyses have provided valuable and fascinating insights into our evolution and behaviour, but while there is much to celebrate a great deal remains to be understood.
The origin of the human condition is best explained by the natural selection for social interaction – the inherited propensities to communicate, recognize, evaluate, cooperate, compete, and from all these the deep warm pleasure of belonging to your own special group. Social intelligence enhanced by group selection made Homo sapiens the first fully dominant species in Earth’s history.2
- Darwin, Charles, On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life,, John Murray, London, 1859
- Wilson, Edward O., The meaning of human existence, Liveright Publishing Corporation, 2014
Report by Jennie Stuart