Friday, September 17, 2010
As described by a team of scientists led by the Wildlife Conservation Society's John Goodrich in the latest issue of the Journal of Mammalogy, a 14-year study of Amur tigers in eastern Russia's Sikhote-Alin Biosphere Zapovednik has shown that male and female tigers establish home ranges of different sizes for different reasons. After capturing and radio-collaring 32 individual tigers (adults and cubs), the team of Russian and American scientists was able to determine that male tigers maintained very large territories (about 1,385 square kilometers) which encompassed the home ranges of several females (about 390 square kilometers).
A map of Amur tiger home ranges between 1992-1997. Solid areas marked "F" designate females and dashed lines marked "M" designate males. From Goodrich et al, 2010.
The disparity in territory size was not much of a surprise. Among solitary big cats, males often have larger home ranges than females, and the reason for this difference between the sexes has to do with the different life strategies of male and female Amur tigers. Whereas young male tigers typically leave the home territory of their mother in an attempt to find a vacancy and gain access to as many females as possible, females stake out their territories based upon the resources they can provide for them and their cubs (thus their home ranges can be much smaller).
What intrigued the scientists, however, was that the home ranges of female Amur tigers contracted when they had female cubs, with their daughters taking up residence in the vacant areas. This favored the future reproductive success of the young tigresses as they did not have to face the risks usually encountered by individuals which try to establish themselves elsewhere (and often become victims of poachers). As the authors of the paper state, it appears that the adult female tigers in their study defended larger territories than they actually needed to survive, and by passing down a portion of this land to their daughters they enhanced the potential success of their offspring while mitigating competition for the same resources.
Yet, as the scientists saw firsthand, poaching can disrupt the matrilineal inheritance of territory among Amur tigers. During the early years of their study - from 1997 to 2000 - all but two of the radiocollared tigers living within the Sikhote-Alin Biosphere Zapovednik were killed by hunters. The vacancies were filled by a mix of "immigrant" animals from other areas and individuals which were related to those which had been poached, but, even after a new population became established, it took five years before a mother tiger passed down territory to her female offspring.
The spatial patterning of tigers over time detected by Goodrich and colleagues may very well complicate tiger recovery plans. If there is a large area of land in which tigers were nearly eliminated (as was the case in this study), the new tiger population will not quickly rebound to its maximum capacity. Instead female tigers which move into vacancies will defend larger territories than they require until they pass down some of that area to their daughters, and during this time the tiger population might be more susceptible to poaching as a smaller number of animals will be occupying an area which could actually support many more. But this news isn't all bad. If tigers can be successfully protected long enough for adult animals to become established, the population size and density can potentially double when the next generation of female tigers mark out their own territories within those initially carved out by their mothers. Given enough time, the tattered remnants of a tiger population can begin to recover.
Thursday, September 9, 2010
They were even more surprised to see the mother struggling to care for her litter.
But keepers gave the first-time mother her space and watched as the moments after the births unfolded on a TV screen, rather than interfere with Katja.
Although one cub died soon after and the other is fighting for her life in the Calgary Zoo's intensive care, officials wouldn't have done anything differently.
"It's always our desire to have the cubs mother-raised," Sandie Black, chief veterinarian at the zoo, said Wednesday. "The best way of ensuring that is to give her privacy and keep things as calm and normal as possible."
The 10-year-old cat didn't nurse her young and carried one kitten in her mouth, which is likely how it died.
Once keepers noticed the cubs becoming less active and the mother moved away from them, they went in to check on their condition.
The surviving female has put on weight since Tuesday, but will remain in an incubator for another week or two.
An expert on captive tigers said surprise pregnancies, cubs dying at birth and inexperienced mothers are all common issues for the carnivore.
"I've heard from many people who say isn't it too bad a cub died," said Ron Tilson, director of the Species Survival Plan that oversees all tigers held in captivity across North America. "But what's really great is that one survived."
In the wild and in captivity, one in three newborn tigers die at birth, said Tilson from Minnesota. Often, as appears to be the case with Katja, the death results from mishandling by the mother.
"Probably a third of all inexperienced females with their first litter, they mess up in one way or another," said Tilson. "They either don't let the cubs nurse, they don't have milk let down, or they're too fretful and they're moving around too much. They don't lie still and let the cubs really latch on.
"This is absolutely not uncommon."
The Species Survival Plan has been operating since 1987, and in that time, the program has overseen more than 400 tiger births. Tilson said he knows of at least six cases where a tiger pregnancy went undetected until the cubs were born.
"I know of at least half a dozen, if not more, cases that I got calls from very experienced zookeepers who I knew who said, 'I can't believe it, but we just had cubs this morning,' " said Tilson.
Katja weighs about 136 kilograms and each cub weighed less than 909 grams at birth -- the low end of the normal range -- meaning the mother's appearance wouldn't have changed, said Tilson. And zookeepers say the behaviour of pregnant tigers changes little over the gestation period.
It was through the program Tilson oversees that a nine-year-old male named Baikal was moved to Calgary from the Bronx Zoo in New York in January.
It was hoped he would mate with Katja and produce cubs, but with arthritis in his hips, several attempts under the careful watch of zookeepers proved unsuccessful.
Baikal underwent surgery to relieve the pain in his hips in late May -- around the time impregnation would've had to have occurred for Katja to give birth Tuesday. Gestation in tigers is typically 90 to 110 days.
"Keepers observed lots of attempts, but nothing even close to a successful breeding," said Black.
"In many ways, this was a surprise."
The unexpected birth comes less than three months after the release of a scathing report suggesting the zoo has system-wide problems, including poor staff morale and a high number of human-related animal deaths.
Zoocheck, an animal-rights activist organization, criticized the zoo over Katja's unnoticed pregnancy and dead cub, saying more attention should be focused on habitat conservation rather than breeding.
But Tilson said the whole point of the Species Survival Program is to protect the tiger gene pool for potential reintroduction into the wild.
It's believed there are less than 300 Siberian tigers -- actually known as Amur tigers -- living in the far east of Russia along the Amur River. The numbers have been declining in the past two decades due to poaching.
There are 137 Amur tigers in captivity among 262 tigers kept in 206 zoos across North America.
The number of Amur tigers in the wild may have to be bolstered soon, probably by first allowing young tigers into the area with their captive mothers nearby. The young tigers would have to be trained to hunt.
"There's talks about that in Russia right now," said Tilson.
"It hasn't ever been attempted in any way, but there has been a lot of thinking about it in a number of places across Asia because tigers are disappearing everywhere."
The most recent news:
Unfortunately, despite enormous efforts of people in the Calgary zoo, two tigress did not survive, luckily the third tigress is good and chances are it will survive.