Suckling behaviour does not measure milk intake in horses, Equus caballus

Studies of parental investment in mammals have frequently used suckling beh aviour to estimate energy transfer from mother to offspring, and consequent ly to measure maternal input. Such studies assume that the more an offsprin g sucks, the more milk it will receive. This assumption has been questioned , and a review of the literature found little support for it. To test if su ckling behaviour provided an accurate index of milk or energy intake we use d a radioactive isotope technique to label the milk of thoroughbred mares a nd to measure milk transfer to foals. We found no significant linear relati onship between usual measures of suckling behaviour and milk or energy inta ke. No behaviours associated with suckling nor with characteristics of mare s and foals improved the relationship; only the number of butts associated with each suck episode even approached significance. If we had used sucklin g behaviour to test theories on differential maternal investment our conclu sions would have been in error. For example, female foals tended to suck fo r longer than males did but there was no difference in the amount of milk t ransferred. Consequently, we show that measures of sucking behaviour do not adequately predict milk intake in the domestic horse and we suggest that c onclusions about differential maternal investment in mammals based on suckl ing behaviour are likely to be in error. (C) 1999 The Association for the S tudy of Animal Behaviour.