What makes good Rubber Boa habitat? That is a difficult question for sure. Itís been something Iíve been wanting to attempt to document for some time, but Iím finding it would almost be easier to list unsuitable habitats. I think Iíll try to do a little of both.
As I am writing this, Dad and I are driving back from a four day trip visiting some of the southern most ranges of Rubber Boa habitat. We visited the Breckenridge Mts, Tehechapie Mts, Mt. Pinos, Mt. Able, Frazier Park, Kern Plateau, Kennedy Meadows, and others. Also in the area are the Greenhorns and Piutes, both of which are known to have boas. These mountain ranges are dry and hot compared to Ďtypicalí Rubber Boa habitat. Only two mountain ranges (San Jacinto, and San Bernardino) that are inhabited by Rubber Boas lie south of where we are. Due to the climate, only the higher elevations of all these mountains make suitable habitat, making ďislandsĒ of each population, similar to the ďislandsĒ that the Southern Rubber Boas live on in the San Jacinto and San Bernardino Mts.
In this southern end of their range, it appears that most Rubber Boas are found above 5,500 ft elevations. North facing slopes, and canyons may carry them down further, and on South Facing slopes, the lowest elevation is likely more in the neighborhood of 5,800 ft. These are very broad generalizations that obviously vary greatly upon the specific location. The key to this lower elevation limit appears to be available moisture. The lowest record I am aware of is a DOR found on the Tehachapies that was at 5,400 ft.
The vegetation below Rubber Boa habitat is characterized by a dominance of Live Oak, and Pinion Pine. At each of the areas we visited, I do not recall areas where we felt there were Rubber Boas, and those species of flora dominated. Not that Rubber Boas were not there. I have learned over time to not underestimate the versatility of these little snakes, and the amazing areas they sometimes show up. As you climb in elevation, or switch from South facing slopes, to North facing slopes, the flora changes to less, or no Live Oak, and adds Jeffrey Pines and deciduous Black Oak to the mix. Hereís where it appears the Rubber Boa habitat begins. Once the Pinion Pine completely peters out, and Jeffrey Pine and Black Oaks dominate, occupation by Rubber Boas is very likely. I also noted, that in the Mt Pinos, Able, Breckenridge, Tehachapie, etc, areas, there was some minimal number of Manzanita present. Not always large amounts, but at least a few.
As always, for every rule, there are the exceptions. Dad just mentioned a find by Ted Papenfuss North of Baldwin Lake in the San Bernardino Mts at approximately 6,000 ft where the habitat is predominately Sage, Pinion Pine, and Juniper. I probably would not have looked there, but the ability of these awesome little snakes to live in less than optimal conditions amazes me.
I tie my observations of Rubber Boa habitat to the local Flora as each species of plant has varied water requirements. The Black oak and Jeffrey Pine need greater moisture and/or cooler temps to exist than do the Live Oak and Pinion Pines. Itís not that there is standing water or running streams in these areas, but the amount of rainfall, and the lower temps allow greater amounts of moisture to be retained underground through the dry months. It is clear that Rubber Boas are less drought tolerant than are their cousin the Rosy Boa, and need moist areas to retreat to in the summer months.
This requirement, and the susceptibility to desiccation precludes the Rubber Boa from living in the lower elevations of S. California, Nevada, Utah, Wyoming, Montana, and perhaps specific spots in Eastern Oregon and Washington. In California, these areas are generally characterized by Sage brush, Rabbit brush, Creosote brush, Joshua trees, etc., or low elevation grasslands. True deserts (less than 10Ē rainfall per year) are almost always devoid of Rubber Boas unless water requirements are met by other means (lake, river, etc).
On the other end of the spectrum, in the northern climates, or high elevations, the limiting factor for Rubber Boa habitat appears to be adequate access to a long enough warm season to carry on reproduction. Example: In Western Oregon, where the Rubber Boa is extremely common (even if seldom seen), the Rubber Boa inhabits open grasslands, or slightly overgrown areas. But once the overgrowth becomes too dense, not enough sunlight hits the ground for proper thermoregulation. The summertime temps may be adequate, but to be able to reproduce, a female needs access to 80 degree plus temps sometime around May in order for the babies to be born before it becomes too cold in September. Those higher temps in the early season are only available by basking in sunlit areas.
It is unknown to me whether high elevations are a limiting factor in the southern end of the Rubber Boas range. Dad and I discussed his study of the SRB published in 2000, and noted that most of his efforts focused in the San Bernardino Mts in the 6,000 to 7,000 ft elevation ranges. San Grogonio Mt in the San Bernardino Range reaches over 10,000 ft. I would assume, that if there is adequate vegetation to support prey base, there are Rubber Boas even that high, or very close to it.
Another facet to Rubber Boa habitat is the ability to find suitable shelter for an snake that is largely fossorial. Whenever I look over potential Rubber Boa habitat, I look for rodent holes. Numerous rodent holes indicate both and adequate prey base, and suitable shelter. In my searches in Utah, I have found Rubber Boa in areas where I cannot locate a single rodent hole, but adequate rock structure is nearby to be able to retreat underground. The ability to access underground areas appears to be linked with the need so stay hydrated, and not overheat.
Iíll finish by mentioning the temperature preferences of Rubber Boas. My captive Boas really seem adverse to temps over 90 degrees. Even a gravid female, who frantically searches the cage for a heat source when it is removed, will seek cooler spots when the temp is 90 or above. Some males will even avoid areas in the mid to low 80ís preferring high 70ís. This is a critical element to understand when trying to determine what habitats the Rubber Boa lives in. Otherwise, a false negative is the likely conclusion from unsuccessful searches.
Many a person has e-mailed me wondering how on earth they to find a Rubber Boa, as they have searched and searched with no luck. Upon further inquiry, I find out that searches were made when the daytime temp was 80 or above. With an ambient air temp that high, a Rubber Boa will only be found under a shaded piece of cover, and not in a sun-lit area. If the ambient air temp is near optimum, and the sun is shining, optimal temps can be achieved by staying underground, in sunlit areas, where there is more moisture. Very rarely is a Rubber Boa found directly on the surface. Almost always they are found under a rock, board, leaf litter, etc. The temperatures under such an object, sitting in the sun, can potentially exceed a Rubber Boas preferred temp while the ambient air temp is only 65-70.
In the Southern areas of it's range, the preferences for temp and moisture limit the Rubber Boa to higher elevations. In the Northern portion of it's range, the limiting factor often becomes access to warmth, and so open areas are preferred, end extreme elevations are unsuitable. See the Photos Page for additional habitat photos and comments on those photos.