Summary: Reliable and stable paternal care is correlated with the evolution of larger brains in their offspring.
Source: University of Zurich
The bigger the brain in relation to body size, the more intelligent a living organism is. This means that mammalian species with large brains are smarter than small-brained mammals. However, developing a large brain comes at a price: An infant expends around two-thirds of its energy alone on supplying nourishment to its brain. That huge amount of energy must be continually available in the form of milk and, later on, through the intake of food. The females of many large-brained animal species cannot bear the energetic costs of rearing offspring on their own – they are reliant on additional help.
Fathers help dependably
Previously, it was generally assumed that it is immaterial whether it is the father or other members of the group who assist the mother in caring for offspring. However, evolutionary biologist Sandra Heldstab and her colleagues Karin Isler, Judith Burkart and Carel van Schaik from the University of Zurich’s Department of Anthropology have now demonstrated for the first time that it very well does matter who helps the mother. Animal species with paternal care of offspring are particularly able to energetically afford bigger brains. Help from other group members is far less essential to the evolution of a large brain. In their study, the researchers compared brain sizes and the extent and frequency of paternal and alloparental care in around 480 mammalian species.
“Fathers help consistently and dependably with the rearing of offspring, whereas assistance from other group members, such as elder siblings, for instance, is far less reliable,” Sandra Heldstab explains. For example, in the case of wolves and African wild dogs – two mammal species with large brains -, elder siblings often help out less, and they look out for themselves first when food is scarce. Sometimes they even steal the prey that parents bring for the infants. The father, in contrast, actually steps up his willingness to help his young offspring when environmental conditions worsen.
Bigger brains or bigger litters
In the case of other species like meerkats and prairie voles, for instance, elder siblings often defect to a different group when they reach puberty and, unlike the father, are no longer available to help the mother. Moreover, the quality of the paternal help is usually superior to help provided by other group members, who are often young and inexperienced. “A female cannot energetically afford offspring with large brains unless she can rely on help, and such dependable help only comes from the father,” Heldstab says.
If the assistance that the female receives for rearing her young is inconsistent, evolution takes an alternative path. In mammalian species that fit this bill – such as lions and red-ruffed lemurs for example -, mothers do not give birth to few offspring with large brains, but to many with small brains. If there is plenty of help in caring for the young, the entire litter survives. If the female receives little support, some of the young die. This is evolution’s way of making certain that at least some of the young survive even in the event of scant help and ensuring that the female doesn’t needlessly invest energy in an offspring with a large brain that will die in the absence of dependable helpers. The study demonstrates once more that only a stable and dependable supply of energy – procured through paternal help, for instance – enables a large brain to develop in the course of evolution.
Humans are the exception
Humans are unique in this respect: Paternal assistance in child rearing is very reliable, but so is childcare help from other relatives and non-relatives. This circumstance enabled humans to develop the largest brain relative to body size in the animal kingdom and nonetheless to considerably shorten the time span between births compared to that for our closest relatives, apes. “In the world of mammals, only help from fathers is dependable. We humans, though, fortunately, can also count on help from others,” Sandra Heldstab says.
University of Zurich
Sandra Andrea Heldstab – University of Zurich
The image is credited to Sandra Heldstab.
Original Research: Closed access
“Allomaternal care, brains and fertility in mammals: who cares matters”. Sandra A. Heldstab, Karin Isler, Judith M. Burkart, Carel P. van Schaik.
Behavioral Ecology and Sociobiology. doi:10.1007/s00265-019-2684-x
Allomaternal care, brains and fertility in mammals: who cares matters
The expensive brain hypothesis predicts that the lowest stable level of energy input sets the upper limit to a species’ brain size. This prediction receives comparative support from the effects of experienced seasonality (including hibernation) and diet quality on mammalian brain size. Here, we test another prediction, which concerns the temporal stability of energy inputs. Allomaternal care in mammals can be provided by breeding males or other helpers (usually earlier offspring). Male care should be stable and reliable since otherwise, no breeding would occur. Care by others, in contrast, should fluctuate, as the availability of helpers often varies. One would, therefore, predict, other things being equal, that the presence of male care in addition to maternal care should show positive correlated evolution with brain size, whereas care by others would not. However, because females can readily respond through litter size adjustments to variable amounts of energy inputs, helper inputs may be used to increase fertility. A detailed comparative analysis of a large sample of mammals (N = 478 species) showed that male help is correlated with the evolution of larger brains, whereas alloparental help is correlated with higher fertility, but only in species where male care is also present (as in cooperative breeders). Humans evolved an unusual form of multi-family cooperative breeding, which involves stable and reliable care by both fathers and alloparents. This combination helps to explain why humans differ from the other apes in having both an extremely large brain and a relatively high reproductive output.
Allomaternal care provides breeding females with energy, directly or indirectly, and so would be expected to affect fertility and/or brain size. Which path evolution actually took remains controversial, partly because previous studies did not separate between care provided by the breeding male (paternal care) and care by non-breeding helpers (alloparental care). We distinguish between them because we expect that selection only favours increased brain size if the increase in energy available to the female is predictable and constant. Using a sample of 478 mammals, we show that paternal care, which is both reliable and stable, shows correlated evolution with brain size, whereas alloparental care, which fluctuates with varying availability of helpers, is correlated with higher fertility. Thus, constraints on brain size, imposed by its high-energy costs, may predict brain size better than the fitness benefits of improved cognitive abilities per se.