Amplexus (Latin "embrace") is a type of mating behavior exhibited by some externally fertilizing species (chiefly amphibians and horseshoe crabs) in which a male grasps a female with his front legs as part of the mating process, and at the same time or with some time delay, he fertilizes the female eggs as they are released from the body. In amphibians, females may be grasped by the head, waist, or armpits, and the type of amplexus is characteristic of some taxonomic groups. Amplexus involves direct contact between male and female, distinguished from other forms of external fertilization such as broadcast spawning, where sperm and eggs are freely shed into water without direct contact by individuals.
Amplexus chiefly occurs aquatically, but some more terrestrial anurans (frogs and toads) like the disc-tongued frogs (Discoglossidae) perform amplexus on land. In crown-group anurans like the true frogs (Ranidae), the tree frogs (Hylidae), and the true toads (Bufonidae), amplexus is axillary (in the armpits). Other anurans (the Archaeobatrachia, Sooglossidae and Myobatrachidae), show the ancestral state which is inguinal or lumbar amplexus (abdominal, in front of the hindlegs). Some species show cephalic amplexus where the head of the female is held while others show complete lack of amplexus.
In most anurans, the males deposit sperm onto the eggs as they are being laid, however males of the genus Ascaphus possess an intromittent organ, unique among anurans, for internal fertilization. Internal fertilization does occur in a few other genera, including Nectophrynoides, Mertensophryne, and Eleutherodactylus.
In the case of newts the process of amplexus is often observed soon after the newts become seasonally active. In the western USA, for example, this time is typically soon after the onset of the winter rainy season, when intermittent streams and vernal pools become available as breeding habitat. The rough-skinned newt is a specific widespread example of a newt in the western USA that can be observed in quiet stream pools and shallow ponds engaging in amplexus.
Amplexus occurs in all four species of horseshoe crab. The first pair of walking legs is used to tightly clasp the female in all species, and the second pair is also employed in all but Limulus polyphemus.
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- C. Michael Hogan (2008) Rough-skinned Newt (Taricha granulosa), Globaltwitcher, ed. N. Stromberg
- Botton, Mark L.; Shuster, Carl N.; Sekiguchi, Koichi; Sugita, Hiroaki (February 1996). "Amplexus and Mating Behavior in the Japanese Horseshoe Crab, Tachypleus tridentatus". Zoological Science. 13 (1): 151–159. doi:10.2108/zsj.13.151.
- Collette, Joseph H.; Gass, Kenneth C.; Hagadorn, James W. (May 2012). "Protichnites eremita Unshelled? Experimental Model-Based Neoichnology and New Evidence for A Euthycarcinoid Affinity for This Ichnospecies". Journal of Paleontology. 86 (3): 442–454. doi:10.1666/11-056.1.
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