Liger: Encyclopedia II - Liger - Large size

Liger - Large size

Ligers grow much larger than tigers or lions and it is believed this is because female lions transmit a growth-inhibiting gene to their descendants to balance the growth-promoting gene transmitted by male lions (this gene is due to competitive mating strategies in lions). A male lion needs to be large to successfully defend the pride from other roaming male lions and pass on his genes; also, in prides with multiple male adult lions, a male's cubs need to be bigger than the competing males for the best chance of survival. Thus, his genes favor larger offspring. A lioness, however, will have up to 5 cubs, and a cub is typically one of many being cared for in a pride with many other lions. As such, it has a relatively high survival rate, and need not be huge as it will not need to look after itself very quickly. Smaller cubs are more easily cared for and fed and are less strain on the pride; hence, the inhibiting gene developed.

Male tigers do not compete for status and mates in the way lions do; a tigress only mates with one tiger when in season, so a tiger does not have the same genetic predisposition to produce large competing offspring. Also, a tigress typically has fewer cubs, and those have a much lower survival rate due to the tiger's solitary nature, so being large and growing quickly are an advantage; there is no need for a growth inhibitor. Being the offspring of a male lion and female tiger, the liger inherits the growth-promoting gene unfettered by a growth-inhibiting gene and typically grows larger than either animal; this is called growth dysplasia. Some male ligers grow sparse manes.

Because of the impossibility of a gene being inherited from only females, there is a competing hypothesis. This untested hypothesis holds that the lion's sperm is damaged somehow during fertilization and that a growth-inhibiting gene is typically destroyed. Female tigons and female ligers both possess a tiger X chromosome and a lion X chromosome, yet only the female ligers will grow large, which suggests that either something happens to alter the genes or the cause of the growth dysplasia lies at least partially outside of genetic.

Another possible hypothesis is that the growth dysplasia results from the interaction between lion genes and tiger womb environment. The tiger produces a hormone that sets the fetal liger on a pattern of growth that does not end throughout its life. The hormonal hypothesis is that the cause of the male liger's growth is its sterility — essentially, the male liger remains in the pre-pubertal growth phase. This is not upheld by behavioural evidence - despite being sterile, many male ligers become sexually mature and mate with females. In addition, female ligers also attain great size but are fertile.

Adapted from the Wikipedia article "Large size", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki