Female orcas live long than their male counterparts.
CNN  — 

From elephants to lions and orcas to seals, most female mammals outlive males – just like their human counterparts, new research suggests.

Researchers looked at 101 species of wild mammals living in 134 different locations and found that females lived longer than males in 60% of cases, What was particularly surprising to the researchers was just how much longer they lived – on average they had an 18.6% longer life span.

By contrast, in humans, females live 7.8% longer than males, according to the study, which published Monday in the journal Proceedings of the National Academy Of Sciences.

While there were quite a few mammals where the males of the species lived longer – such as horses and some species of bat and rabbit – the magnitude was much lower, said Jean-François Lemaître, a researcher at the French National Centre for Scientific Research and an author of the study.

Lemaître said the study was the widest reaching on the topic to date, although he said they were unable to include many species of small rodents. That’s because there isn’t much data on those animals in the wild.

When it comes to the reasons why there is a longevity gap between sexes, he said there were a number of theories.

Lemaître said that the findings of his study suggested that the differences in male and female longevity were shaped by an animal’s environment and reproductive roles.

“We do not find any consistent sex differences in aging rates. In addition, sex differences in median adult lifespan and aging rates are both highly variable across species,” the study said.

Lemaître said that males in some species grow bigger and have traits like large antlers and that this may have “physiological costs” and affect how they react to environmental factors like pathogens – increasing the differences in lifespans between males and females.

“The substantial allocation of resources in males toward the growth and maintenance of secondary sexual traits might, everything else being equal, make males more vulnerable than females to harsh environmental conditions,” the study said.

A male elephant seal, surrounded by his harem, emits a cry to mark his territory. Elephant seals are one of the species in which females outlive males.

Another theory is that having two copies of the same sex chromosome offers a protective effect. In mammals, females have two X chromosomes while males only have one. In bird species, where males often outlive females, this is reversed.

Similarly, others have theorized something called “the mother’s curse hypothesis.” That’s the possibility that mothers may transmit genes to their children that harm their sons but not their daughters.