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ESTIVATION

Estivation is a state of dormancy, typically triggered by arid conditions when food/water availability is poor or absent and often occurs in hot weather. Many estivators live in desert or semi-arid environments. It is not uncommon for estivating species to spend many months of the year in dormancy, emerging only in brief rainy seasons to feed and reproduce. Estivators typically take steps to minimize water loss from their bodies while dormant using physical barriers (e.g. snails seal the opening of their shell, frogs and lungfish can secrete a cocoon made of shed skin or mucus), biochemical methods (increasing the osmotic pressure of body fluids), and storing large amounts of body water before entering dormancy (e.g. frogs maintain a huge reservoir in the bladder and can resorb water from there).

Estivating animals in the Storey lab

In the Storey lab we work on three model animals: land snails, Spadefoot toads and African clawed frogs.

Estivating snails, Otala lactea

Otala lactea is the edible snail of "escargot" fame. It is native to the arid lands around the Mediterranean sea and is a nocturnal grazer. When environmental conditions are dry and food availability is low, the snails enter estivation. Metabolic rate, as assessed by oxygen consumption, drops to only about 20-30 % of the normal resting rate. The snails seal over the shell opening with an epiphragm made of layers of dried mucus; this acts to reduce water loss from the body. Water loss is also reduced by using apnoic breathing; breathes are only taken intermittently (only a handful per hour), each breath triggered when CO₂ in the lung rises to a critical level. When sprayed with water to wake them up, snails can emerge from their shells in as little as 10-15 minutes. 

LINK TO STOREY LAB PAPERS on ESTIVATION in OTALA

Photo 1 and 2: This is Otala lactea.
Photo 3 and4: This is Otala lactea.
Photo 5: This shows the mucus epiphragm that is secreted over the shell opening when the snails are dormant.	Photo 6. This shows the snail size compared with a Canadian penny.
Photo 4 and 5: Lots and lots of estivating snails.

Estivating toads, Scaphiopus couchii

Couch's spadefoot toad, Scaphiopus couchii, is a native of arid regions of the American southwest. Toads spend about 10 months of the year estivating underground and emerge with the first torrential rain of summer to a frenzy of breeding and egg laying that can be over in less than 24 hours. Adults then gorge themselves on insects and quickly restore their body fuel reserves to a level that can sustain them for another year. During estivation, metabolic rate drops to about 20-30% of the resting rate of aroused toads and breathing is intermittent to minimize the loss of water vapour. Metabolism while estivating depends primarily on the catabolism of body lipid reserves but protein catabolism increases when estivation is prolonged. Toads enter estivation with a huge reserve of water in their bladder which they resorb over time to replace water that is lost during breathing and across their skin. When the soil dries out and water loss begins to be severe, protein catabolism increases and toads accumulate high concentrations of the "waste" product, urea, in their blood and tissues. Urea raises the osmotic concentration of their body fluids and helps to retard further water loss.

LINK TO STOREY LAB PAPERS on ESTIVATION in SPADEFOOT TOADS

Photo 6: Couch's spadefoot toad, Scaphiopus couchii.	Photo 7: Spadefoot toad emerging from underground estivation. From: Cornejo, M. (1987) “For the desert toad, rain starts a race to metamorphosis”. Smithsonian March 1987, pp 98-105 photo by T.A. Wiewandt
Photo 8: Couch's spadefoot toad, Scaphiopus couchii	Photo 9: Couch's spadefoot toad, Scaphiopus couchii
Photo 10: Eastern spadefoot toad, Scaphiopus holbrookii	Photo 11: Eastern spadefoot toad, Scaphiopus holbrookii

Estivating frogs, Xenopus laevis

The African clawed frog, Xenopus laevis, is native to seasonally arid regions of southern Africa. The frogs are almost fully aquatic and are not well designed for moving on land, typically only moving from pond to pond during rainstorms. Frogs can find themselves living in ponds that dry out during the dry season and when water levels fall to a critical level, they have two options. Noctural migration is usually the first choice, the frogs moving to seek better ponds. When this is not possible, they can dig into the soft mud of the drying pond using clawed hind feet and enter a resting state. Like spadefoot toads, they also accumulate high levels of urea in their body fluids to provide osmotic resistance to water loss across their skin. Not a lot is known about estivation in Xenopus but this species has another important role in science – Xenopus eggs have been extensively studied as a model for embryology and development and, as a result, the Xenopus genome has been sequenced. In our lab, we have begun studies of this species by examining gene, protein and enzyme responses to the stress of dehydration, an integral component of estivation. Without good dessication resistance, estivating frogs would soon die. Research planned for early 2007 will give us access to natural populations of Xenopus in South Africa where we can sample and analyze animals from their natural summer habitat.

STOREY LAB PAPERS on DEHYDRATION TOLERANCE in AFRICAN CLAWED FROGS

Malik, A.I. and Storey, K.B. 2009. Activation of extracellular signal-regulated kinases during dehydration in the African clawed frog, Xenopus laevis. J. Exp. Biol. 212, 2595-2603. doi:10.1242/jeb.030627 Abstract PDF

Malik, A.I. and Storey, K.B. 2009. Activation of antioxidant defense during dehydration stress in the African clawed frog. Gene 442, 99-107. DOI:10.1016/j.gene.2009.04.007 Abstract PDF

Photo 12 and 13. Adult female Xenopus in the natural coloring which can be quite variable. Black claws can be seen on 2 toes of the hind legs.

Photo 14 and 15. Adult female Xenopus in the albino version that is commonly used in labs.