A study of almost 500 mammalian species has revealed those in which both parents care for their offspring help provide the conditions in which bigger brains – and therefore greater intelligence – can evolve.
The larger the brain is in relation to body size, the more intelligent a living organism is, generally speaking. However, it comes at a price – maintaining a larger brain requires greater energy and therefore more 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.
Previously, it was thought immaterial whether it was the father or if it was other members of the group who assisted the mother in caring for offspring.
But a detailed study by 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 demonstrated for the first time that it does matter who helps the mother.
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8/17 Eastern Lowland Gorilla
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1/17 Amur Leopard
Endemic to the far east of Russia, the Amur Leopard has a population of around 84 and is critically endangered. Here follows every species that the WWF lists as critically endangered.
Getty
2/17 Yangtze Finless Porpoise
Endemic to China’s Yangtze River, the Yangtze finless porpoise has an estimated population of 1000-1800
Kenichi Nobusue
3/17 South China Tiger
When discovered in the 1950s, the South China tiger population was estimated to be 4000, by 1996 it was estimated to be only 30-80. Scientists consider the tiger to be “functionally extinct” as one has not been sighted for over 25 years
4/17 Sumatran Orangutan
The Sumatran orangutan was once found across the island of Sumatra and even further south on Java. Today it is found only in the island’s north and its population stands at 14,613
Getty
5/17 Western Lowland Gorilla
Though it is the most populous of all gorilla subspecies, the western lowland gorilla is still critically endangered and its population has declined by 60% in the last quarter century
Getty
6/17 Sumatran Rhinoceros
The Sumatran rhinoceros is the smallest of the surviving rhinoceros species. Only 80 are known to be living today. The last male Sumatran rhino in Malaysia died on 28 May 2019
Willem V Strien
7/17 Sumatran Tiger
There are fewer than 400 Sumatran tigers left today. They are severely threatened by deforestation and poaching
Getty
8/17 Eastern Lowland Gorilla
Half of the rainforest of the Democratic Republic of Congo in which these gorillas live has been destroyed the past 50 years. There were 17,000 Eastern Lowland gorillas in the 1990s but scientists estimate their population has declined by over 50% since then
Getty
9/17 Vaquita
The world’s rarest marine animal has a population of only 30. They were discovered in 1958 and are endemic to Mexico’s Gulf of California
Paula Olson / NOAA
10/17 Javan Rhino
The most threatened of all rhino species, there are only an estimated 58-68 Javan rhino left
Reuters
11/17 Saola
The saola was first sighted in 1992, being the first large mammal to be discovered in over 50 years. Scientists have only sighted saola in the wild four times and it is considered critically endangered
12/17 Sumatran Elephant
The Sumatran elephant population now stands at only 2400-2800
Getty
13/17 Malayan Tiger
The Malayan Tiger population now stands at only 250-300
Getty
14/17 Hawksbill Turtle
The population of the Hawksbill Turtle has declined by more than 80% in the last century. They are threatened by black market poachers who kill them for their shell
Getty
15/17 Black Rhino
The population of the black rhino dropped by 98% in the years 1960-1995 due to poaching, it stands today at around 5000
Getty
16/17 Cross River Gorilla
The population of the Cross River gorilla has been damaged by deforestation and poaching, it now stands at 200-300
Julie Langford
17/17 Bornean Orangutan
The population of the Bornean orangutan has been reduced by over 50% in the past 60 years, now standing at around 104,700. Their habitat has been reduced by at least 50% in the 21st century
Getty
Help from other group members is much less impactful to the evolution of a large brain than help from the father, their results show.
In their study, the researchers compared brain sizes and the extent and frequency of paternal and alloparental (non-direct parent) care in 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,” Dr Heldstab said.
The researchers said 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, has greater willingness to help his young offspring when environmental conditions worsen.
In the case of other species like meerkats and north American prairie voles, the research team found 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,” Dr Heldstab said.
If the assistance that the female receives for rearing her young is inconsistent, evolution takes an alternative path.
In mammalian species where males do not provide support – such as lions and red-ruffed lemurs, mothers do not give birth to few offspring with large brains, but instead 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. The researchers said 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.
But homo sapiens are the exception to the rule.
Paternal assistance in child rearing is very reliable, but so is childcare help from other relatives and non-relatives.
The scientists said this circumstance helped enable 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,” Dr Heldstab said.
