Life in the freezer?

Morse Toad

Many Anurans (frogs and toads) flutter their toes ("digital fluttering") when they detect potential food items. The resulting vibrations could agitate insects, setting them moving in a flurry of activity that triggers the amphibian's tendency to strike at moving prey. Cane toads flutter their toes when small prey appears, but this "pedal lure" also draws small cane toads closer to bigger cannibalistic ones.


Deceptive digits: the functional significance of toe waving by cannibalistic cane toads, Chaunus marinus. 2008 Animal Behaviour 75: 123-131.
Many ambush foraging predators possess specialized structures and behaviours that plausibly function to attract prey, but this hypothesis has rarely been subject to direct empirical tests. If luring evolved to attract specific prey types then we predict that it will be manifested only if that prey type is present, and only by predators of the size class that feed on that prey type. Also, luring should induce closer approach by prey; and aspects of the behaviour (e. g. frequency of movement of the lure) should have been tuned by selection to induce maximal response from prey. We describe a novel luring system: small- and medium-sized (but not metamorph and large) cane toads, Chaunus marinus, wave the long middle toe of the hind-foot up and down in an obvious display. In keeping with the functional hypothesis, toe waving is elicited by moving edible-sized objects such as crickets or metamorphic toads. Metamorphic toads are attracted to this stimulus, and trials with a mechanical model show that both the colour and the vibrational frequency of the toe correspond closely with those most effective at attracting smaller conspecifics towards the lure. The independent evolution of visual luring systems in many animal lineages provides a powerful opportunity for robust empirical tests of adaptive hypotheses about signal design.


Waving or tapping? Vibrational stimuli and the general function of toe twitching in frogs and toads (Amphibia: Anura). 2008 Animal Behaviour 76: e1-e4
Many complex signalling behaviours involve multiple sensory modalities. Generally, different sensory components of the signal have different origins. For example, pheromones and visual cues are produced separately, and they act on different senses. However, in some cases, a single behavioural component may stimulate several senses of receivers. Movements provide the clearest example of this: they can be seen (visual), but may also be detected through displacement of air and be felt (tactile) or heard (auditory). In such examples, the effects of the signal on receivers' senses may be difficult to deduce fully, especially if some sensory components are more obvious to the researcher than others. Furthermore, in some cases, such as the effect of predator movement on prey, different prey types can respond via different sensory modalities to the same behaviour.
We here consider one such possible example. Hagman & Shine (2008) have recently provided empirical evidence that ‘toe waving’ by cane toads, Chaunus marinus, acts as a visual lure. They suggest that the primary function of the behaviour, whereby toads move the middle toes of their hindlegs up and down, is to attract smaller metamorphic toads towards the larger cannibalistic predator. Their hypotheses are based on earlier work that deduced that a similar but more elaborate behaviour, pedal luring in Ceratophrys frogs, is a visual lure to smaller prey amphibians and possibly other small visually hunting vertebrates.
As suggested by Hagman & Shine (2008), toe movement in anurans is taxonomically widespread. However, it is not restricted to taxa that are vertebrate predators or strongly cannibalistic, which would be the groups most expected to benefit from toe movement if such behaviour serves primarily as a visual lure. We here suggest an alternative predatory function, operating on prey via a different sensory modality, which provides a more general explanation of toe movement in anurans, in addition to its secondary use as a visual lure.

Oxygen-seeking frog embryos boost survival

Red-eyed treefrog (Agalychnis callidryas) embryos can rotate inside their eggs to get more oxygen if levels start to drop, their prolonging chances for survival. The embryos are ready to hatch just four days after they are laid, but delaying hatching a few days can boost survival rates because tadpoles that are more mature can more easily swim away from predators. But as an embryo continues to develop, more oxygen is recycled from a limited supply inside the egg. Oxygen levels have been found to be as low as 2% in the middle of the egg. In response to the risk of suffocation, the embryos position their external gills in a small high-oxygen area near the egg’s surface. This rotation allows the embryos to maintain high metabolic rates and viability.

External gills and adaptive embryo behavior facilitate synchronous development and hatching plasticity under respiratory constraint. J Exp Biol. 2008 211: 3627-35
Plasticity in hatching timing allows embryos to balance egg- and larval-stage risks, and depends on the ability of hatching-competent embryos to continue developing in the egg. Hypoxia can slow development, kill embryos and induce premature hatching. For terrestrial eggs of red-eyed treefrogs, the embryonic period can extend approximately 50% longer than development to hatching competence, and development is synchronous across perivitelline oxygen levels (P(O(2))) ranging from 0.5-16.5 kPa. Embryos maintain large external gills until hatching, then gills regress rapidly. We assessed the respiratory value of external gills using gill manipulations and closed-system respirometry. Embryos without external gills were oxygen limited in air and hatched at an external P(O(2)) of 17 kPa, whereas embryos with gills regulated their metabolism and remained in the egg at substantially lower P(O(2)). By contrast, tadpoles gained no respiratory benefit from external gills. We videotaped behavior and manipulated embryos to test if they position gills near the air-exposed portion of the egg surface, where P(O(2)) is highest. Active embryos remained stationary for minutes in gills-at-surface positions. After manipulations and spontaneous movements that positioned gills in the O(2)-poor region of the egg, however, they returned their gills to the air-exposed surface within seconds. Even neural tube stage embryos, capable only of ciliary rotation, positioned their developing head in the region of highest P(O(2)). Such behavior may be critical both to delay hatching after hatching competence and to obtain sufficient oxygen for normal, synchronous development at earlier stages.

Aztec legend in danger of extinction

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Vivarium for sale (UK)

All glass vivarium: 92cm high, 31cm wide, 31cm deep, Epiweb background, fully furnished & planted, glass sliding door, top vented, lighting included (twin 36" fluorescent). Ideal for thumbnail dartfrogs.
Reason for sale: Rearranging the frogroom.

£25, buyer collects (Leicester, UK).

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