Born naked and blind, new baby rabbits are completely dependent on their mothers. Unfortunately rabbits are rather relaxed mothers, nursing their young only briefly once a day, so it is essential that the pups make the most of mum's brief visits. They are stimulated to suckle immediately by a pheromone,2-methylbut-2-enal, released with their mother's milk. Fortunately, rabbit pups don't have to rely on their laid-back mothers for long. Developing in a matter of weeks, domesticated rabbit pups are soon able to forage for themselves, and wild youngsters grow up even faster. Knowing this, Gerard Coureaud and his colleagues in France and Germany wondered whether rabbit pups' sensitivity to maternal chemosignals is restricted to the period when they are dependent on milk and their eyes are closed, and whether the wild and domesticated pups show the same sensitivity to suckling pheromone as they begin to rely less on milk.
First the team tested the responsiveness of youngsters to the mammary pheromone during the course of lactation. Purchasing the mammary pheromone,they allowed leverets to sniff the compound for a 10 s period each day of lactation and once after weaning. The team tested a colossal number of domesticated and wild pups, never testing the same pup twice, as they measured the youngsters' sensitivity to the pheromone from birth to weaning.
By observing how often the pups latched on to the glass stick on which the pheromone was presented and started sucking, the researchers were able to classify the domesticated youngsters' responses to the pheromone. They found that while pup responsiveness was highest in the first few days of lactation,with 93% of all tested pups responding to the pheromone by suckling, it then began decreasing from week to week, reducing to about 50% in the third week,and vanished completely after weaning.
Next the team compared the pheromone responses of wild rabbits with those of their domesticated cousins and found that the wild pups responded just as strongly to the mammary pheromone at the beginning of lactation as the domesticated rabbits. However, the wild pups' sensitivity began decreasing during the second week of lactation and there was virtually no response to the pheromone by the final week of lactation. The wild pups ceased chemical communication with their mums about 1 week earlier than the domestic pups.
Coureaud and his colleagues suspect that domesticated rabbits have been artificially selected to rely on nursing longer than wild rabbits and therefore experience decelerated ontogeny. The fact that wild leverets are more motivated to search for solid food early in their development severs the pheromonal bond between pups and their mums much sooner in the wild than in the hutch.
Coureaud's findings nicely show that the mammary pheromone of the genus Oryctolagus is a good example for efficient chemical communication between a mother and her young to induce suckling behaviour in leverets that are dependent on milk.