Unlike humans and most other mammals, whose skeletons stop growing once adulthood is reached, kangaroos belong to a rare group of mammals that display indeterminate growth—meaning their bodies continue getting larger throughout most of their lives. For male red kangaroos in particular, this can mean decades of slow, steady growth that produces some genuinely massive individuals by the time old age or a predator finally catches up with them.
Most Mammals Stop Growing—Kangaroos Don’t
The actual size of most mammals, including humans, is largely set from birth, and while environmental factors like nutrition can influence growth rate, our skeletons stop growing entirely once we reach adulthood. Kangaroos are a genuine exception to this rule. The skeletons of kangaroos and larger wallabies continue to grow, slowly, throughout their lives, placing them among a small handful of mammals—alongside elephants and a few whale species—that never fully “finish” growing the way most mammals do.
Growth Plates That Never Fully Close
The biological mechanism behind this comes down to skeletal structure. Kangaroos display a growth pattern that blurs the line between determinate and indeterminate growth, and their skeletal structure allows for this flexibility through growth plates that don’t completely fuse as early as they do in other mammals. In most mammals, growth plates seal shut once adulthood arrives, locking in a final size. In kangaroos, that fusion process stays incomplete for much longer, leaving the door open for continued growth well past sexual maturity.
How Big Male Kangaroos Can Actually Get
The scale of this growth is genuinely striking. Male red kangaroos can reach heights of over 6 feet (1.8 meters) and weigh up to 200 pounds (90 kg), with growth continuing well into their teens—an unusually long growth window for any mammal. The largest documented red kangaroo on record measured 6 feet 11 inches long and weighed 200 pounds, making red kangaroos the largest living marsupials on the planet.
Growth That Responds to Environmental Conditions
What makes kangaroo growth particularly interesting is how responsive it is to real-world conditions, a trait known as developmental plasticity. During droughts or food shortages, kangaroos can essentially pause their growth, resuming when conditions improve. During periods of abundant food, kangaroos continue adding both muscle mass and skeletal growth, while growth may temporarily halt entirely during periods of scarcity. This adaptive strategy allows kangaroos to maximize their size when resources are plentiful while avoiding the physiological cost of continued growth during Australia’s frequent droughts and unpredictable climate swings.
Why Continued Growth Matters for Male Reproduction
This continued growth isn’t just a biological curiosity—it carries real reproductive stakes, particularly for males. Research on kangaroo populations has found that reproduction and growth exist in a genuine trade-off for animals with indeterminate growth, since resources devoted to reproduction can’t simultaneously go toward continued body growth. For males specifically, larger body size generally translates into greater competitive advantage when competing for mates, meaning a male kangaroo that keeps growing longer, or grows faster during good years, may gain a real edge in the competition for breeding opportunities within a mob.
An Adaptation Suited to a Harsh, Unpredictable Environment
Ultimately, this flexible growth pattern reflects a broader evolutionary strategy well-suited to Australia’s demanding climate. Rather than locking in a fixed size early in life, kangaroos hedge their bets—growing opportunistically when conditions allow and conserving energy when they don’t, a strategy that likely helps explain why they’ve thrived across such a historically unpredictable landscape of droughts, food scarcity, and seasonal extremes.
Join The Discussion
Did you know kangaroos never fully stop growing, or was this surprising to learn? Do you find it interesting that their growth can pause and resume depending on food availability, compared to how fixed human growth is? Share your thoughts, questions, or anything else you find fascinating about this unusual biological trait below.
