Ligers and tigons are two of the most impressive and most ferocious cat species in the world. Even though they are crossbred, these felines are impressive and stand out due to their amazing sizes. In fact, the liger is the biggest living cat in the world, simply dwarfing both tigers and lions in size and weight. However, why are ligers to much bigger than tigons?
Well, the large size of a liger and small size of a tigon are due to a genetic phenomenon called “genomic imprinting”, meaning the unequal expression of genes depending on parent of origin, that is whether certain growth genes are inherited from the male or the female. In simpler words, lions and tigers – in fact, all big felines – have imprinted genes which can be expressed maternally or paternally, with great restrictions to species and gender. This is also linked to the species’ lifestyle and breeding strategy – whether the female mates with only one male while in heat (non-competitive) or with more than one male (competitive). This results in “growth dysplasia”.
Lions live in prides led by several adult males. The lionesses mate with each of those males. Each male desires his offspring to be the ones to survive. However, the female’s genes want multiple offspring to survive. The lion’s genes promote the size of the offspring to make sure that his offspring out-compete any other offspring in the womb at the same time. Genes from the lioness inhibit growth to make sure that as many offspring as possible survive and that all the cubs have an equal chance.
By contrast, tigers are largely solitary and a female on heat normally only mates with a single male. There is no competition for space in the womb, so the male tiger’s genes do not need to promote larger offspring. Therefore, there is no need for the tigress to compensate, so the offspring’s growth goes uninhibited.
When a male lion mates with a female tigress, though, his genes promote large offspring growth because lions are adapted to a competitive breeding strategy. On the other hand, the tigress does not inhibit the growth because she is adapted to a non-competitive strategy. Therefore, the offspring of a male lion and female tigress – the liger – grows larger and stronger than either parent because the effects do not cancel each other out. While it is true that it takes several years for ligers to reach full adult size, the fact that ligers never stop growing is a complete and utter myth.
When a male tiger mates with a female lioness, his genes do not promote large offspring growth because tigers are adapted to a non-competitive breeding strategy. On the other hand, the lioness inhibits the growth because she is adapted to a competitive strategy. Due to this uneven match, the offspring of a male tiger and female lioness – the tigon – is often smaller and less robust than either parent.
Growth dysplasia has other effects, as well. It can affect the size of the placenta, causing miscarriage, or it can cause the embryo to be aborted at an early stage due to abnormal growth. Also, the cub may be stillborn or may only survive a few days after birth.
This is the scientific explanation why ligers are much bigger than tigons. Other hypothesis are also taken into consideration, but this is the most plausible one. Remember that this explanation is greatly simplified, as a number of other genes are contributed unequally by the male and female parents and also affect the general health and longevity of the offspring.