Females may have driven our evolutionary success

Macaques are the second most successful primate species on the planet (after humans). New research reveals the secret to their success is that more females survive to adulthood. Could something similar explain human success?


Humans are the most successful primates on the planet, inhabiting every single continent. The second most successful primate is macaques. Fun fact, they’re the only other wild primate in Europe.

Macaques wave off their armada, which is departing to conquer new land

This led some researchers to wonder what made the macaques so successful. Perhaps they might also help explain human dominance too1.

It turns out a key factor in macaque success is their females. In particular, keeping them alive. Some macaque groups grow faster than others (with one population doubling in size ~10 years!). The difference in growth seems to be driven by the difference in female survival1.

Could our rapid success be the result of something similar?

Meet the monkeys

Before breaking down this research, it’s probably best if we meet the monkeys we’re talking about. Not just because they’re important to this study, macaques are pretty awesome in their own right.

For instance, they appear to be one of the few non-human animals with cumulative culture. This is when younger generations learn from others, but also make improvements. Then, their offspring learn the improved version and tweak it further. It’s how human and chimp cultures increase in complexity. And also, apparently, macaque culture2.

In the 50s, some macaques discovered that washing their food was great. It helped get all the sand, grit, and soil off. Since that initial discovery, subsequent generations have improved the method. It started just as washing in puddles, now they wash in the ocean; using the waves to help clean2.

Plus the sprinkling of sea salt must be nice, making macaques the only monkey known to use condiments.

A macaque with a sweet potato at the beach, ready to wash it in the ocean

Secret to success

But for the present study, the thing that makes macaques interesting is just how many of them there are.

As I already mentioned, they’re found across Southern Eurasia, as well as a few places in Africa and Europe. This earns them a solid silver medal in the category of “most widespread primate”.

Crucially though, they don’t share the wealth. There are more than 20 known macaque species, but only 4 make up the vast majority of populations. This has earned those 4 species the rather judgy classification of “weed species”1.

The range of just two macaque weed species, along with the bit where they meet up to discuss how well world domination is going

But science doesn’t care about hurt macaque feelings. It just wants to know what makes those weed species more successful than their brethren.

Over the years many theories have been postulated. One popular idea is that the weed species just having a faster reproduction. And it’s true that some weeds do, but not all. And the slower reproducing weed populations seem to grow just as fast as their quickly reproducing counterparts1.

Instead, the key unifying factor of the macaque weed species seems to be the survival of females. For many weed species,>60% of females survive long enough to reproduce. Compare this to chimpanzees, where only 40% live that long. A larger population reaching adulthood has obvious implications for population growth.

What about human females?

Some point in the last 3 million years humans also became “weedy”. Could it be explained by a shift in the survival of our female ancestors? Perhaps at some point, we became more macaque-like in this regards.

The same people behind the research into macaque weed success say “yes.”

They speculate that some social change happened in our species that led to more females making it to adulthood. This would have served as a key population boon that helped take us from a local African species to a worldwide one1.

In particular, they suggest the key factor may have been monogamy, which is more common in humans than chimps3. Two parents forming a long-term relationship to have kids could help more of them reach adulthood. Perhaps because the parents could then engage in a division of labour (e.g. one stands guard whilst the other searches for food)1.

There is some evidence humans were shifting towards monogamy around the time we were becoming increasingly weedy. For instance, males lose their large canines, which are typically used to fight others for control over females3.

A human male and chimp male. Note how our canines aren’t as pointy as a chimps’

On the other hand, this evidence for monogamy is circumstantial at best. The fact it also coincides with a shift towards weediness is also circumstantial. And whilst the macaque data is interesting, their study is limited to macaque groups with a lot of data. How well they represent all macaques is debatable, especially given that 12% of studied groups showed high female mortality and population decline.

But it still seems to an obvious conclusion not dying is a key first step for a species.

For humans and macaques, perhaps “females not dying more” was step 2.

References

  1. Meindl, R.S., Chaney, M.E. and Lovejoy, C.O., 2018. Early hominids may have been weed species. Proceedings of the National Academy of Sciences, p.201719669.
  2. Schofield, D.P., McGrew, W.C., Takahashi, A. and Hirata, S., 2017. Cumulative culture in nonhumans: overlooked findings from Japanese monkeys?. Primates, pp.1-10.
  3. Nakahashi W, & Horiuchi S (2012). Evolution of ape and human mating systems. Journal of theoretical biology, 296, 56-64 PMID: 22155135

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Females may have driven our evolutionary success

by Adam Benton
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