When the blood is pumped from the right ventricle to the pulmonary artery.. the "lubb" sound is heard... while ..when the oxygenated blood is pumped by the left ventricle to the aorta.. a larger "dubb" sound is heard...
Oxpecker birds eat parasitic ticks off the backs of zebras is an example of mutualism.
Mutualism is defined as a relationship between organisms in which both the individuals involved are benefitted from each other.
In the case of oxpecker birds and zebras, the oxpecker birds benefit as they get their food from the parasitic ticks living on the zebra and the zebras benefit as the birds help them to get rid of the parasites.
In this way, both these organisms are involved in a mutualistic relationship.
