Most animals' proportions change as they grow. Human babies are born with massive heads relative to their bodies that scale down as they grow and many young animals look as if they need to grow into their feet. But do earth worms' proportions change as they develop, or are the adults simply scaled up versions of youngsters? And, if the proportions of worms change as they grow, how does that affect their movements? Jessica Kurth and William Kier from the University of North Carolina, USA, explain that unlike most other animals, worms are supported by a hydrostatic skeleton – where a muscular body encases a liquid-filled internal cavity that extends when the muscles contract. They add that little was known about how the proportions of animals with hydrostatic skeletons change, or how growth might affect their ability to burrow. So, Kurth and Kier decided to find out more about how earth worms grow as they mature (p. 1860).
Measuring the vital statistics of the front, middle and rear portions of earth worms ranging from tiny (1–3 g) juveniles up to fully grown adults (3–10 g), the duo found that the worms' proportions did change as they grew. They became slimmer and the ratio of the worms' length to their diameter increased by as much as 128% at the front end and up to 163% at the rear. But how would the alteration in their physical proportions affect how far the animals could extend their bodies and the forces that they could exert as they burrowed?
Calculating the animals' stride length – worms stretch to move forward by contracting their circular muscles – Kurth and Kier were impressed to see that the longer, thinner adults could extend themselves 117% more than the stumpier juveniles: the adults were able to extend further than if they were simply youngsters scaled up to adult size. However, when they calculated how much force the largest worms could exert on the walls of their burrows compared with the thicker youngsters, the older worms' length did not improve their pushing power. In fact, the forces exerted on burrow walls by the adult worms were the same as those exerted by scaled-up youngsters.
But why don't larger earth worms take advantage of their altered proportions to amplify their burrowing power relative to that of younger worms? Kurth and Kier suspect that this could be due to differences in the way that soil behaves as small and large worms burrow. They explain that larger worms have to displace more soil radially while burrowing, increasing the stiffness of the surrounding soil, possibly making it advantageous for larger worms to be slender to reduce the effort required to heft soil aside. Alternatively, they suggest that smaller worms might be stockier to help them fracture compacted soils. They explain that small marine worms are known to force cracks through mud by expanding the front section of their bodies and they suggest that small earth worms may benefit from a similar advantage when boring through hard soils.
Having shown that older earthworms are not simply scaled up versions of their younger selves, Kurth and Kier are keen to find out how different soil types have affected the worms' burrowing technique.