In primates, evolution is a ball game

Evolution is fundamentally an exercise in trade-offs – speed versus bulk, running versus tree-climbing, that sort of thing – and now a new cross-species study has identified an intriguing one among male primates.

There is no polite way to say this: males can look amazing or have big balls, but not both.

Research by Stefan Lüpold, an evolutionary biologist at the University of Zurich, Switzerland, and Leigh Simmons and Cyril Grueter from the University of Western Australia, looked at male ornamentation and testicle size across 100 species of monkeys and apes, including humans.

Both traits are fundamentally concerned with reproductive success, but function in different ways.

Ornamentation, such as big manes, long beards, swellings and brightly coloured skin (think of the red bum of a dominant male mandrill) serve to impress females and intimidate other males.

Big testes, relative to body size, roughly correlate with high levels of sperm production, which increases the chances of successful fertilisation during copulation.

Either trait is appealing, but it seems evolution forces males from all species to choose between one or the other.

“Ornament elaboration comes at the expense of testicle size and sperm production,” says Lüpold. “In a nutshell, the showiest males have the smallest testes.”

In many communal primate species – such as baboons – male ornamentation is a mark of social status, and showiness tends to increase when a male becomes dominant and has, thus, more or less exclusive access to all females.

In such circumstances, the researchers suggest, sperm does not need to be overly plentiful or vigorous, because it will not have to compete with that produced by other males.

In species in which multiple mates are a feature of social life, however, extravagant ornamentation is less valuable because every male is presented with opportunities to fertilise, and exclusivity is not a factor.

In these cases, reproductive fitness is determined by sperm volume and motility, so large testes – in as much as these are regarded as proxies for fecundity – are an advantage.

The research, published in the journal Proceedings of the Royal Society B, is the first time the relationship between primate testes, ornamentation and reproductive strategy has been examined.

The relationship between testes size and another important determinant, however, has been the subject of several prior studies, and this did not escape the notice of Lüpold and colleagues. 

The determinant in question is teeth, specifically canine teeth, which serve as weapons during male-on-male mating competitions.

The researchers confirmed earlier findings that big teeth and big balls are positively correlated, although quite why is unclear.

Lüpold and colleagues advance a number of possible explanations. Big teeth, they point out, are, in terms of energy expended on upkeep, much less expensive than ornamental traits (or large testes, for that matter). It is also possible that the genes that control testes growth in some way also influence tooth length.

Whatever the reason, across the entire 100 species tested the central dictum held true: if evolution favours a flashy car, it will be at the expense of what lies under the hood.

Related reading: The evolution of the mammal brain