Defensive Spines on Tenrecs Could Come at a Cost to Brain Size
The little mammals of Madagascar appear to have undergone an evolutionary tradeoff between brain size and defensive armor
At least 29 million years ago, a small, shrew-like mammal floated across the Mozambique Channel on a mound of vegetation, or perhaps a hollow log, and landed on the island of Madagascar. Whether this castaway was a pregnant female or a small population of adults remains a mystery. All we know is that this unlikely voyager or voyagers would survive, reproduce and transform into scores of different species, each adapted to life in a different habitat on the island.
Today, we call these creatures tenrecs, and scientists recognize 37 living species of the tiny shape-shifters.
Some, like the lesser long-tailed tenrec, have evolved prehensile tails that allow them to cling to shrubs and tree branches. Others, like the web-footed tenrec, sport adaptations for aquatic life and hunt crustaceans and insect larvae under water. Tenrecs have even conquered the underground, such as the mole-like rice tenrec, equipped with powerful digging claws and beady little eyes, the better to keep soil out.
Perhaps the most well-known branch of the tenrec family tree belongs to five species that resemble hedgehogs. Each has specialized hairs that have, over time, morphed into anti-predator spines.
These defensive measures may have come at a cost. A study published recently in the Journal of Mammalogy found evidence that these extreme, spiky adaptations may have developed alongside a decrease in brainpower.
There’s more than one way to make a spiky tenrec. While two of the species—Echinops telfairi and Setifer setosus—sport short, thick and rigid spines most like those of a hedgehog, others have longer, bendable quills mixed in with bristly fur.
Ted Stankowich, an evolutionary behavioral ecologist at California State University, Long Beach, has made a living investigating the ways that weapons and warnings evolve in mammals. From skunk stripes and scent glands to muntjac tusks and armadillo armor, Stankowich and his lab want to know how and why such defenses form and what happens to other parts of an animal’s body when they do.
Thirty-two species of tenrec that have made do without quills, after all. So what was it about life millions of years ago that coaxed these five other species to get their evolutionary hackles up? And what price did the animals have to pay for their natural defenses?
“The thought is that these are expensive things to build,” Stankowich says. “It takes a lot of energy to construct and carry around a big suit of armor, and so you have to pay for that in some way.”
Brain tissue is also very costly, from an energy standpoint, Stankowich says. In 2017 his lab discovered that among 647 species of mammals, the animals that evolved some sort of specialized defense, be it armor, spikes or chemical weapons, tended to suffer a loss of brain mass as well. And while the size of a species’ brain is determined by many factors, the correlation between developing defensive weapons and decreasing brain size suggests the two are related.
Most of the mammals out there with armor, quills and the like are quite a bit larger than tenrecs, which have an average weight of less than two pounds among the largest species. To figure out if the trend of decreasing brain size continues in tiny tenrecs, Stankowich and a former student, Colin Stensrud, used study skins from museums, as well as natural history data from the International Union for Conservation of Nature.
The scientists found that as the tenrecs evolved larger body sizes and moved into more open habitats, they were more likely to evolve spines. The animals may have started venturing into open fields and forests to feast on the bounty of insects in such places, but in doing so put themselves at risk. Predators, such as birds, snakes and fossas—a cat-like mongoose cousin native to Madagascar—all prey on tenrecs.
The more intense predation may have selected for tenrecs that were tougher to gobble up, perhaps because mutations gave them tougher skin or stiffer fur. And if that process continued slowly for millions of years, eventually tenrecs could evolve with necks and backs full of sharp spikes.
But the heavier armor didn’t come cheap. Not only did relative brain mass decrease for the tenrec species that developed spines, but the researchers also found a correlation between the level of spiny-ness and the amount of decline. The species with more robust spikes had lost more brain mass relative to their body size than those with smaller, seemingly less expensive spines.
So does this mean Roger Alan Wade was talking about developmental biology when he sang, “If you’re gonna be dumb, you gotta be tough”?
According to Stankowich, it’s not necessarily that spiny tenrecs are stupid. More likely, they’re only as smart as they need to be. “You don’t need to be paying a ton of attention to predators either in the air or on the ground because you know that your defenses are effective against them,” he says.
More vulnerable prey species have to invest biological resources in good ears, sniffers, eyes and other sensory arrays that can detect predators before they, themselves, are detected. But spiny tenrecs may be able to skimp on such investments because they can roll into an urchin-like ball or swing their spiky heads around if a predator comes calling.
Some unanswered questions remain, however. Link Olson, an evolutionary biologist and curator of mammals at the University of Alaska Museum, says we don’t really know what Madagascar looked like when the tenrec first arrived, so it’s difficult to say that the evolutionary development of spines is related to open areas just because some species favor open habitats now.
“I think we have to just constantly be aware of the uncertainty and how that uncertainty grows the farther back in time we go,” says Olson, who has spent a quarter century studying tenrecs.
It’s also difficult to classify some of these species as inhabiting either closed or open habitats. Two of the spiny species identified in open habitats, known as the streaked tenrecs, can also be found in forests, Olson says. And another, Tenrec ecaudatus, loses much of its spiny-ness as it gets older and bigger, which would seem to run counter to the trend in other species.
“That being said, there are always exceptions to any general rule we see in nature,” Olson says.
In the end, it will always be impossible to know exactly how a species came to be. But with all their wondrous diversity crammed into one habitat-rich island, the tenrecs provide a remarkably clear window into the past.