The maternal effects on growth and conformation in shire horse-shetland pony crosses

Abstract The relative importance of heredity and environment in determining the size, development, and ultimate proportions of animals has attracted some scientific study and discussion, but little planned experimentation has been attempted with mammals. In the mammal the early and formative stages of development take place in the uterus, where the environment is remarkably constant, and it is not easy to subject the foetus to varying conditions and at the same time not overstep the normal physiological limits. Previous work on the rabbit (Hammond 1934 a; Wishart and Hammond 1933), however, has shown that the size of young at birth is approximately inversely proportional to the number in the litter, and it appears probable that some internal secretion or metabolic product of the mother forms a limiting factor in the growth of the embryo. In a monotocous species this effect does not appear, although exceptional twinning or polytocous births reveal its existence, the young at birth being regularly smaller. That the absolute amount of this hypothetical growth-promoting substance may be related in some way with the size of the mother is suggested by the figures of Gregory (1930), which show that the average number of ova of rabbits increases with the average size of the strain. In a monotocous species therefore the size of the mother might determine the size of the offspring at birth. This is also suggested by the results of reciprocal crosses between donkey and horse. The mule (donkey ♂ x horse ♀) is larger than the hinny (horse ♂ x donkey ♀) (Plumb 1916). Since, however, in a species cross the objection may be raised that abnormal genetic segregation might affect the result, we selected for our experiment two breeds of the same species, but of markedly different size, namely, the Shire horse and the Shetland pony, and reciprocal crosses were made to see how far the size of the mother would affect the size of the offspring. No genetical analysis of size inheritance in horses has yet been attempted, and therefore it may be said that the genetic potentialities of our material were unknown. In the absence of evidence to the contrary, however, we con­sidered it justifiable to assume, on the basis of much genetical experimenta­tion on size inheritance, that a large number of factors would be involved,that segregation and recombination would be at random, and that the off­ spring of the two breeds would be genetically intermediate, irrespective of which way the cross was made. Any difference between the offspring of the reciprocal crosses would therefore be due not to chromosomal differences but to differences in the environment brought about by difference in the size of the mother. In other words we would have a controlled experiment in which “Mother-size” was the only or predominating variable. The horse is particularly well adapted for studying the effects of maternal influence on the offspring, since the duration of pregnancy is relatively long (11 months) and the foal is born in an advanced stage of development, particularly with regard to the skeleton (see Meek 1901). Consequently, the effects of maternal influence acting over a long and formative period should be well marked.