© 2006 Paul Crump (1 of 3)
Bufo houstonensis Sanders, 1953
Donald B. Shepard1
1. Historical versus Current Distribution. Houston toads (Bufo houstonensis) are considered a Pleistocene relict left behind in Texas after the widely distributed parental species (precursor to the present American toad [Bufo americanus]) underwent a range expansion to the north following the retreat of the Wisconsin glaciation 10,000 yr ago (Blair, 1958a, 1965; L.E. Brown, 1971b). Increasing temperatures and aridity during the Holocene were probably instrumental in isolating Houston toads into areas of friable sandy soil where they could burrow to avoid desiccation (Brown and Mesrobian, 2003).
Only a few scattered populations of Houston toads are known to be extant in central and southeastern Texas (Seal, 1994). Houston toads previously have been reported from the following counties: Austin, Bastrop, Burleson, Colorado, Fort Bend, Harris, Lavaca, Lee, Leon, Liberty, Milam, and Roberston (Sanders, 1953; Blair, 1956a: L.E. Brown, 1971b; Seal, 1994; Brown and Mesrobian, 2001; Price, 2003). However, they are now extirpated from Harris County (no encounters since 1976) and believed extirpated from Burleson, Fort Bend, and Liberty counties (L.E. Brown et al., 1984; Brown and Mesrobian, 2003).
2. Historical versus Current Abundance. Since the 1940s, Houston toad populations have declined drastically. For example, in Harris County, John Wottring, the collector of the holotype, collected 66 individuals from a single chorus in 1949 and indicated large numbers were still present in 1953 (L.E. Brown et al., 1984). In the same county, L.E. Brown (1967, 1971b) found only three individuals from 1965–'67; three individuals were found from 1974–'76 (L.E. Brown et al., 1984) and no Houston toads have been found since (L.E. Brown et al., 1984). The largest number of toads occurs in Bastrop County, but population estimates have varied considerably among authors and over time (see Seal, 1994; Price, 2003).
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. Breeding may occur from late January to late June, but usually earlier than May (Kennedy, 1961; L.E. Brown, 1971b; Hillis et al., 1984; Jacobson, 1989). Kennedy (1961) observed ephemeral calling and spawning triggered by warm temperatures and heavy rainfall. Hillis et al. (1984) found initiation of breeding was not associated with rainfall and began when minimum air temperatures for the preceding 24 hr did not fall below 14 ˚C. However, males will call below 14 ˚C (L.E. Brown, 1967). Lack of calling when temperature is suitable and breeding asynchrony among populations experiencing similar environmental conditions suggests other factors besides temperature may influence the timing and intensity of breeding (Hillis et al., 1984).
Around sunset, toads move to breeding sites from underground burrows ≤ 40 m away. Males will call from burrows prior to migrating to the breeding site and may return to the same burrow the next day (Hillis et al., 1984).
ii. Breeding habitat. Houston toads use rain pools, flooded fields, roadside ditches, and natural or manmade ponds for breeding (L.E. Brown et al., 1984). Optimum habitats are non-flowing, fishless pools that persist for at least 60 d (L.E. Brown et al., 1984; Hillis et al., 1984; Price, 2003).
i. Egg deposition sites. Eggs are deposited in strings in water (Kennedy, 1961; L.E. Brown, 1971b). Within the jelly tube, eggs are separated from one another in compartments (Sanders, 1953; L.E. Brown et al., 1984).
ii. Clutch size. Kennedy (1961) reported a female laying 728 eggs while Quinn and Mengden (1984) found clutch size of seven females ranged from 513–6,199. On average, eggs are 1.9 mm in diameter (Quinn and Mengden, 1984). Eggs hatch in about 7 d, but can hatch in as fast as 2 d at warm temperatures (Hillis et al., 1984; Quinn and Mengden, 1984). Hatchlings measure 6.1–6.7 mm TL (Hillis et al., 1984).
i. Length of larval stage. About 53–58 d, depending on water temperature (Hillis et al., 1984).
ii. Larval requirements.
a. Food. Houston toads can use algae for food and will also feed on loblolly pine (Pinus taeda) pollen (Hillis et al., 1984). Where two cohorts of tadpoles co-occur, older tadpoles will eat the jelly envelopes of the discarded egg capsules of the younger cohort (Hillis et al., 1984).
b. Cover. Tadpoles form aggregations during the day but disperse at night (Hillis et al., 1984).
iii. Larval polymorphisms. Not known to occur.
iv. Features of metamorphosis. Tadpoles metamorphose at 20–22 mm TL; newly metamorphosed juveniles are 7–9 mm TL (Hillis et al., 1984).
v. Post-metamorphic migrations. Hillis et al. (1984) reported post-metamorphic Houston toads left ponds and moved in large numbers up gullies that fed ponds. These movements occurred during day and night with individuals found ≤ 100 m from the breeding pond.
D. Juvenile Habitat. Same as adult habitat. Juvenile growth has recently been described by Greuter and Forstner (2003).
E. Adult Habitat. Houston toads are poor burrowers in compact soil (Bragg, 1960a) and are restricted to areas with sandy, friable soil (L.E. Brown, 1971b). These sands are deep, have subsurface moisture, and occur in five geological formations that run in a northeast-southwest direction (Seal, 1994; Brown and Mesrobian, 2001). Houston toads are nocturnal, spending daylight hours burrowed in sand (Brown and Mesrobian, 2001).
Many Houston toad populations are located in or near loblolly pine forests (L.E. Brown, 1973a); however, this is due more to both Houston toads and loblolly pines being dependent on sandy soils than the dependence of Houston toads on pine woods (Brown and Thomas, 1982; Brown and Mesrobian, 2001). Other pristine vegetational associations include mixed deciduous forest, post oak (Quercus stellata) savanna, and coastal prairie (Kennedy, 1961; L.E. Brown, 1971b; L.E. Brown et al., 1984; Seal, 1994; Brown and Mesrobian, 2003; Price, 2003).
F. Home Range Size. Individual males have been observed making round-trip movements greater than 1,400 m between breeding sites (Price, 1992, 2003).
G. Territories. Houston toads are explosive breeders and not known to defend territories (Jacobson, 1989).
H. Aestivation/Avoiding Dessication. Occurs underground in sandy soils (Seal, 1994).
I. Seasonal Migrations. Unknown.
J. Torpor (Hibernation). Occurs underground in sandy soils (Seal, 1994).
K. Interspecific Associations/Exclusions. Licht (1967a) reported that Houston toad larval growth was inhibited when tadpoles were raised in water conditioned by Woodhouse’s toad (Bufo woodhousii) tadpoles, an effect not seen when raised in water conditioned by Texas toad (Bufo speciosus) tadpoles. Houston toads naturally hybridize with Woodhouse’s toads and Gulf Coast toads (Bufo valliceps [now considered to be Coastal-Plain toads, B. nebulifer; see Mulcahy and Mendelson, 2000; Mendelson, this volume) in the Lost Pines area of Bastrop County (L.E. Brown, 1967, 1971b; Hillis et al., 1984).
L. Age/Size at Reproductive Maturity. Males are sexually mature at 45–70 mm SVL and females at 52–80 mm (L.E. Brown, 1973a; L.E. Brown et al., 1984). Sexual maturity takes 1 yr for males and apparently 2 yr for females (Quinn and Mengden, 1984). Most males breed during their first and second breeding season and rarely breed more than twice (Hillis et al., 1984).
M. Longevity. Snider and Bowler (1992) report a maximum longevity of 4 yr, 3 mo, and 10 d for a wild-caught individual.
N. Feeding Behavior. Houston toads utilize active searching and sit-and-wait methods to acquire prey (Thomas and Allen, 1997). They have been observed to eat beetles (Coleoptera), flies (Diptera), lacewings (Neuroptera), moths (Lepidoptera), ants (Hymenoptera: Formicidae), and other anurans (Bragg, 1960a; Thomas and Allen, 1997).
O. Predators. Freed and Neitman (1988) reported tadpole predation by blotched water snakes (Nerodia erythrogaster transversa) and Gulf Coast ribbon snakes (Thamnophis proximus orarius), and toadlet predation by fire ants (Solenopsis invicta). Brown et al. (L.E., 1984) reported predation on adults by blotched water snakes and discussed other potential vertebrate predators.
P. Anti-Predator Mechanisms. Tadpoles form aggregations during daytime hours (Hillis et al., 1984). The eggs and tadpoles of many toad species are unpalatable to predators (Voris and Bacon, 1966; Licht, 1968), and adults have enlarged parotoid glands that produce toxins (L.E. Brown, 1973a).
Q. Diseases. Unknown.
R. Parasites. Cosmocercoides dukae, Oswaldocruzia pipiens, Physaloptera ranae, Rhabdias ranae, and Brachycoelium storeriae (Harwood, 1932; Thomas et al., 1984).
4. Conservation. Houston toads are Federally Endangered (Peters, 1968; Gottschalk, 1970). Since the 1940s, Houston toad populations have declined drastically (L.E. Brown et al., 1984), and only a few scattered populations are known to be extant in central and southeastern Texas (Seal, 1994; Price, 2003). Populations are considered relictual and population sizes appear to be small (L.E. Brown, 1973a). Primary causes of the decline have been urbanization, prolonged droughts during the 1950s, and other types of anthropogenic habitat modification. Major threats to the Houston toad include continued urbanization, recreational over-development, road mortality, agriculture, and deforestation. Introduced red fire ants, known predators of recently metamorphosed toads (Freed and Neitman, 1988), and increasing temperatures and aridity associated with recent warming trends may also threaten the future survival of the Houston toad.
Past conservation efforts have included 1970s land acquisitions by the State of Texas within designated critical habitat in Bastrop County adjacent to Buescher and Bastrop state parks (L.E. Brown et al., 1984). Additionally, an effort was started in 1978 by the Houston Zoo to identify remaining Houston toad populations and supplement them or establish new populations in protected areas using wild caught adults, naturally deposited eggs, or captive-reared juveniles and adults. However, in spite of introducing 500,000 individuals (adults, juveniles, tadpoles) since 1982 into sites at the Attwater Prairie Chicken National Wildlife Refuge, new populations have not been established (Dodd and Seigel, 1991). Conflicting interests have plagued the conservation of the Houston toad and created frequent problematic political issues (reviewed by Brown and Mesrobian, 2003).
1Donald B. Shepard
2Lauren E. Brown
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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