Here’s a second installment on the biology of arroyo toads, Bufo californicus. The first post (
viewtopic.php?f=2&t=1274) covered aspects of reproductive biology, so I’ll focus here on juvenile and adult animals.
Arroyo toads are members of the Bufo americanus species group, which is basically limited to regions with reliable summer rainfall – B. californicus in southern California is a relict of wetter conditions that ended 10-12,000 years ago, and it really should not still be present in a region where annual rainfall of 20” or less occurs only in the winter months. It has been able to persist owing to some unusual topographic features. Without getting too technical, here we have mountain ranges parallel to the coast that are being uplifted vey quickly, with deep canyons cut in wetter times. The foothill portions of these canyons are backfilled with sediment that has accumulated because current rainfall is incompetent to transport it away, and the current streams meander across this backfilled surface, as seen here:
These sand deposits store large volumes of winter storm runoff, and meter it out long into the summer, creating sections of semipermanent stream flow in a climatic zone that should no longer have anything but ephemeral streams. All of the riparian herp species in southern California are dependent on these perched aquifers, but about ¾ of them are now submerged under reservoirs.
Within these drainages there are typically a series of old stream terraces, now drained and covered with chaparral, and low terraces bordering the current stream where groundwater is close enough to support willows and cottonwoods, and oaks at the edges, with sandy benches covering old meanders of the flood channel. Those benches get washed over on a cycle of decades, and reworked on a cycle of centuries by infrequent heavy floods.
Arroyo toads don’t venture into chaparral, so their distribution is confined to the youngest terraces and the streambed itself, and often that’s not much. By contrast, Bufo boreas loves chaparral, and may disperse miles horizontally and thousands of feet vertically away from breeding sites. Western toads will also use a far wider range of breeding sites; in a nutshell, that’s why the one is doing OK and the other is endangered.
On average B. boreas breeds earlier in the year and metamorphoses sooner than does B. californicus, but juveniles of both species overlap from late May into early July. Young B. boreas quickly become nocturnal, but find shelter in burrows, under rocks and in dense vegetation along the stream until they reach a body length of ~20 mm, at which point they all disperse upslope. Juvenile arroyo toads dig in at night and are active by day even at very high temperatures, and they will remain as long as damp sand or gravel is available, into September in some places and years. They don’t disperse far from the streambed, but become nocturnal as the substrate goes dry.
I’m often asked to identify juvenile toads suspected of being B. californicus, and nearly always they turn out to be B. boreas. Here’s a set of each, arroyo toads first, followed by a couple of comparative photos.
They’re really pretty different animals. Side by side notice the more distinct “neck”, pale middorsal line, uniform dorsal ground color, rounded parotid gland, and orange-red “warts” on B. boreas, vs. the short, wide head, pale V on the eyelids, light front half of the oval parotid gland, mottled dorsal ground color, and smaller, yellowish “warts" on the juvenile B. californicus.
If you’re really clueless after all that, turn them over. Bufo californicus are always immaculate white ventrally, whereas even the tiniest metamorph B. boreas have dark speckles (that increase in density with age). Note that both have the standard pigmentless ‘diaper’ of water-absorbing skin – this is why you’ll never see a toad drink, they take up all their water through their little bottoms, even from just damp sand.
So how about adult toads outside of the breeding season? Both species are nocturnal, and usually active from dark to about midnight. Western toads can dig in but seldom do, using mammal burrows in the open or logs and woodrat nests in the chaparral. By contrast, arroyo toads are always somewhere near the creek, and dig themselves in every night. Both species return to the creek to soak, arroyo toads about every third night, western toads maybe weekly into midsummer, but both become much less active in late summer and fall. When you encounter them out at night they are either foraging or traveling to and from the creek.
Arroyo toads are doing things a little differently. In addition to soaking
They are foraging, but very specifically. It is not very widely known that arroyo toads are ant specialists, but they are every bit as specific as horned lizards in this preference. What they are after are velvet tree ants, Liometopum occidentale, which make nests in large tree trunks and send out foraging columns after dark, where the ants locate and milk honeydew from aphids. Liometopum trails are in the same place every night, often for months and sometimes between years, and that’s where arroyo toads are.
An arroyo toad seen after dark anywhere else (outside of breeding season) is either traveling to soak, or heading toward one of the ant trails it knows. Another way to show this is to pick up some feces – remember that lizard feces have a white urates cap at the upstream end, whereas toad poop doesn’t
Bust them open, and you’ll find a wide range of insect parts in western toad turds, especially a lot of large beetles, whereas arroyo toad feces are composed almost entirely of ants.
As always, there is more, but enough for now. Appreciate your riparian areas, they are pretty special places.
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jim