Wednesday, February 18, 2009

Sexual Dimorphism

A fluffed out male Superb Bird of Paradise courts a female.

Sexual Dimorphism is the term that describes species of animal in which the male and female have a difference in form and/or appearance. Not all species have differences between male and female. Let's have a look at some animals that display this dimorphism, in various modes.

1. Orange Barred Sulphur (Phoebus philea) - Color and Markings

The male Orange barred with bright yellow background and orange markings.

The female has several brown/black spots and no orange marks on the forewing.

A male feeds on a Turk's Cap. The Orange-barred are found from the Southern United States to South America.

2. Yellow & Black Garden Spider (Argiopes aurantia) - Markings, Color, and Size

The female Argiopes is larger and more brightly colored. Animals that lay eggs, where the males do not combat for female or territory, often have much larger females.

The male is smaller and much more drably colored.

Only the females make a web, and the males hang around waiting for chances to steal food and mate with the female. This is wide ranging spider found through out North America and Europe.

* While we associate males with greater size and strength, this is not the case with many animals. Females achieve larger size amongst most varieties of hawks, frogs, turtles, snakes, and spiders.

3. Golden Toads EXTINCT (Bufo periglenes) - Size and Coloration/Markings

The smaller bright orange male mounts/amplexes the yellow black and red mottled female. Species in which the male must attract the female often have more brightly or elaborately colored males.

Golden Toads were found in the cloud forests of Costa Rica, they were last seen in 1989.

4. Eastern Collared Lizard (Crotaphytus collaris) - Color and Markings

The male is more brightly colored, and slightly larger with a more robust head.

A view of the male in breeding season.

The colors also intensify when in direct sun light. Found in the central United States.

6. Scarlet Tanager (Piranga rubra) - Color and Markings

The brightly colored male.

The female has more subdued colors. Scarlets nest in the Eastern U.S. and migrate to Central America for the winter.

7. American Bison (Bison bison) - Size and coloration

A view of the smaller all brown female nuzzling the larger male, who has shaggier black face and head fur. Prairie areas of North America.

Males also have larger horns. Animals whose males fight over females and territory, have larger males. Horned animals usually have males with larger horns, and most animals with antlers have females with no antlers at all.

8. Mandrill (Mandrillus sphinx) - Size and Coloration

The larger and stronger male has more brightly colored facial, rump and genital markings. Males also have considerably larger canine teeth, for fighting and defense.

The smaller and less brightly colored female is on the right.

Here we see a female with 2 young. Mandrills are baboons that live in the rain forests of Western Africa.

Herpetological Breeding Congregations

Let's have a look at some Reptiles and Amphibians that breed in groups.

California Newts (Taricha torosa)

The typical method of mating amongst California Newts involves the male amplexing (grasping the female for mating in a kind of hug). He then rubs his nose on her chin, and if she accepts, he then releases her, and deposits a sperm packet on the bottom of the pond or stream where they have come to reproduce. She takes the sperm packet up into her cloaca, fertilizes the eggs, then shortly after lays them (in small masses) on objects in the water.

In areas where there are more males then females, competition for females can be quite intense.

When there are MANY more males than females, these large breeding congregations occur. These breeding masses are usually seen in small streams that flow into ponds.

Taricha torosa egg mass

The closely related Rough Skinned Newt (T. granulosa) has a similar breeding regime, and occasionally gathers in breeding groups, as well.

African Foam Nest Treefrog (Chiromantis xerampelina)

Also known as African Gray Treefrogs, they mate in congregations in trees with branches overhanging seasonal bodies water. The group whips up a lather of foam made of skin secretions and sperm, into which the eggs are deposited.

These groups can vary in number.

Frogs within the mass may pair off or pile on in groups. With so many eggs and sperm mixing together in the foam nest, the paternity of any individual frog is unsure.

The outside of the foam nest hardens, with the eggs developing in the wet frothy inside.

When the tadpoles have hatched, and then reached a certain size, they wriggle vigorously. This action dissolves the hard shell in spots, and the tadpoles drip out into the pond below.

Red-sided Garter Snakes (Thamnophis sirtalis parietalis)

The Red-sided Garter ranges over the Central U.S. and Central and Western Canada.

In parts of their range, especially in Canada, they hibernate in caves and hollows in large numbers.

When they emerge in Spring, the males come out first, followed by the females days later.

The largest breeding congregations are known from historic sites, which may have been used for hundreds of years, in Manitoba (a province of Canada).

Unlike our previous 2 animals in the post, both amphibians, these reptiles fertilize the eggs internally (the males placing the sperm into the female directly). Therefore, despite the great competition for the larger females, only one male fertilizes a female. She then slithers off, and eventually gives live birth, as garter snakes do.