A night or two after the first big monsoon rain a couple of weeks ago, I could hear Mexican spadefoot toads (Spea multiplicata) calling from a retention pond just downhill from our housing development, and within days there were tadpoles in the pond. I’m sure that the same has been happening all over our region, since the biology of spadefoots is tied intimately to the monsoon rains. I captured a few individuals to keep until I could identify them definitively; I didn’t want all of them to dry up or get eaten by predators until I could!
Some cool things about spadefoots:
They’re not actually toads, but instead more closely related to frogs!
They spend only a few weeks above ground during the year, when they breed and feed in and around ephemeral ponds created by monsoon rains. For the rest of the year, they’re hibernating underground, kept from drying out by a “cocoon” of several layers of unshed skin. They can hibernate in this way for more than a year, waiting for the right conditions to come out and breed.
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Because they develop in ponds that only have water for a short time, spadefoots can grow and complete their larval development extremely rapidly – in some cases in just a little over a week. They even develop functional lungs while they’re still tadpoles, and they can survive for a time on the muddy surface of a drying pond for this reason. You can also observe them coming to the surface of a pond frequently to gulp air because the warm water contains little oxygen.
When spadefoot tadpoles are in a pond that’s drying up rapidly, some individuals develop stronger jaws and become cannibalistic! By eating other tadpoles in the pond, they both grow faster and decrease the number of competitors. Incredibly, the cannibalistic form preys more heavily on unrelated tadpoles than on their own genetic relatives. How do they know?
Hibernating adults emerge from their burrows in response to the low-frequency vibrations caused by the falling raindrops and thunder of intense storms. Unfortunately, they can also emerge at inappropriate times of the year because they mistake the vibrations caused by off-road vehicles for raindrops and thunder. Adults that make this mistake invariably die from desiccation.
The common name “spadefoot” derives from a hardened (with keratin – the same protein that makes up our hair and fingernails) structure on their hind foot that helps them burrow into the soil in anticipation of the dry season.
Females of a different species of spadefoot that occurs in the Midwest — Spea bombifrons — can hybridize with male Mexican spadefoots when they’re breeding in a pond that is rapidly drying up. The hybrids will develop faster than pure S. bombifrons, since S. multiplicata inhabits a drier part of the country and develops faster, but the male hybrid offspring will be sterile. Nevertheless the female hybrid offspring will be able to backcross with male S. bombifrons. This behavior is thought to be an adaptation that allows a female to leave at least some offspring instead of none at all when they’re limited to a rapidly-drying pond.
Most of this information was taken from a website maintained by Drs. David and Karin Pfennig at the University of North Carolina, Chapel Hill. They’ve got some great videos and more details at https://www.davidpfenniglab.com/spadefoots/