A tale of two packrat species

This is Neotoma cinerea, the bushy-tailed packrat:

From Oregon Caves via Wikimedia Commons.

This is Neotoma lepida, the desert packrat:

By Jules Jardinier via Wikimedia Commons.

Bushy-tailed packrats and desert packrats differ in some significant ways. Bushy-tailed rats are notably larger than their desert cousins, weighing 200 to 600 grams (about 7 to 21 ounces) as adults, versus 85 to 240 grams (3 to 8 ounces) for the desert rats. Size for packrats is closely related to temperature tolerance: the big bushy-tails don't like temperatures above 25 °C (77 °F), while the smaller desert rats don't like temperatures below -5 °C (23 °F) (Smith et al. 2009).

This is Death Valley:

Well, part of it; it's a big place. This is looking down into the valley in a southwesterly direction from one of the alluvial fans on the east side.

Death Valley is flanked by the Panamint Range on the west and the Amargosa Range on the east. The Panamint Range reaches higher elevations, including 3,368 meters (11,049 feet) at Telescope Peak, but the Amargosa Range is no slouch itself, touching 2,663 meters (8,738 feet) at Grapevine Peak. At the floor of Death Valley, temperatures can reach 57 °C (134 °F). Given the previous information about the two packrat species, you'd probably guess that bushy-tails don't live at the floor. This is true; in fact, they don't even live on the Amargosa Range, but they do inhabit the Panamint Range above 1,900 to 2,000 meters (6,200 to 6,600 feet). On the other hand, desert packrats range across the valley floor and in the mountains. The ranges of the two species do not overlap much, because bushy-tails are not only quite a bit bigger but are also more aggressive (Smith et al. 2009).

Death Valley hasn't always been quite as inhospitable at it is at the present, though. This is a map of Lake Manly, taken from this USGS site:

Lake Manly was a large body of water that occupied the Death Valley basin a couple of times in recent geologic history, including about 186,000 to 120,000 years ago and 35,000 to 10,000 years ago. These lake stages also happen to match glacial stages and periods of increased moisture. Today, there are sometimes localized ponds in Death Valley when there is a lot of precipitation, but nothing quite like this perennial lake. Given this much different Death Valley, was there a different distribution of packrat species in the past? Felisa Smith and colleagues published a study on this question in 2009.

This is the kind of place you might find a packrat:

Robb Hannawacker/Joshua Tree National Park via Wikimedia Commons.

You may remember packrat middens from a couple of earlier posts. The photo above is of a modern nest, which is not quite the same thing but is a decent start. Packrat middens, as discussed before, can contain plant matter, pollen, invertebrate remains, bones, and any other packrat-portable object that struck a packrat's fancy. They also contain packrat droppings. The width of packrat droppings appears to correlate well to the size of the animal that produced them. This is helpful here because we have two species with very little overlap in adult size, e.g. pellets above a certain size can only have been produced by bushy-tails.

Smith et al. (2009) collected samples from 74 middens in the Titus Canyon area on the Amargosa side of the valley (Grapevine Mountains). The samples span a range in elevation from about 200 to 1,700 meters (660 to 5,600 feet), and range in age from about 22,000 years ago (shortly before the last glacial maximum) to the present. What they found was that bushy-tails did indeed inhabit the Amargosa Range in the recent past, with the oldest sample indicating rats weighing more than 550 grams (19 ounces). Following the glacial maximum, the bushy-tails declined in size rapidly, probably in response to warmer climates. By about 17,000 years ago, they were down to a little above 350 grams (12 ounces), which is about where they stayed until finally disappearing from the Titus Canyon midden record a little after 3,000 years ago. Pellets indicating rats smaller than 300 grams (less than 11 ounces), which could have been produced by either bushy-tailed or desert packrats, appear in the midden record about 13,000 years ago. Interestingly, Smith et al.'s results show a gap in pellets of definite bushy-tail size between about 8,000 and 5,000 years ago, which corresponds to an unusually warm and dry stretch in the Holocene called the Altithermal. Perhaps the bushy-tails left Titus Canyon and then moved back after the climate returned to something a bit more to their tastes, or they had become smaller than the 300 gram threshold during that period. In another interesting point, no middens built by rodents smaller than 300 grams contained juniper remains, suggesting that the retreat of junipers and the retreat of bushy-tails took place at close to the same time. Just as species of plants respond to climate change by changing their range, so too did the rodent species, with the twist of size reduction: the bushy-tailed species appears to have decreased in size and followed the optimal temperatures, with the desert species moving up the mountains to inhabit the areas the bushy-tails abandoned, until the bushy-tailed packrats finally disappeared from the Amargosa Range altogether.

References:

Smith, F. A., D. L. Crawford, L. E. Harding, H. M. Lease, I. W. Murray, A. Raniszewski, and K. M. Youberg. 2009. A tale of two species; extirpation and range expansion during the late Quaternary in an extreme environment. Global and Planetary Change 65(3–4):122–133.