What is Nuptial Pad?
Nuptial pad is a pigmented swelling present the base of the first (inner) finger of each forelimb in male frogs and toads. It is like horny and or thickened pads present underside each thumb of the males.
In males, the base of the first (inner) finger is thickened especially in the breeding season, forming the nuptial pad for clasping the female at the time of mating.
These pads are very sensitive and are triggered by androgen hormones. Androgen hormones are highly responsible for the development of secondary sex characteristics such as the nuptial pads, but it is still not so clear what role androgens play in the initiation of amplexus.
It is actually a type of breeding gland (a type of mucous gland) that appears more active and thickened during the breeding season.
It usually appears as a spiked epithelial swelling on the forearm and prepollex that aids with grip, which is used primarily by males to grasp (or clasp) females during amplexus (mating).
In some species, it has also been confirmed that these pads are also used during the fight between males mostly during the breeding season.
It is seen that the loss of nuptial pads occurs in males at the end of the mating season. And, in some rare scenarios, breeding males often don’t develop any nuptial pads and the actual reason for this is still unclear.
Nuptial pad is also known as thumb pad, or nuptial excrescence, or copulatory pad.
What is the function of the nuptial pad in a frog?
During the annual mating season, male frogs develop spiny and rough nuptial pads on their thumbs and arms of each forelimb. These pads are generally used to improve the male’s grip on the female during amplexus (mating).
It is also reported that in some species, during amplexus, the male sits above the female and grabs her from above and attempts to rub his chin on the female’s nasal region to transfer pheromones.
Not only that, but the male frog grasps the female around the torso (trunk of the body) near her chin with his forelimbs and fertilizes the eggs as they emerge. In doing, so the nuptial pads are also reported to release huge amounts of pheromones.
Pheromones emitted from the male frog enhance his chances of spermatophore transfer and egg release by the female.
Thyroid hormone (TH) is the most important hormone in frog metamorphosis. It has been reported that thyroid hormones play an important role in both tail height and nuptial pad presence.
Thus, there seems to be a good relationship between thyroid hormone and the development of nuptial pad, but the function of the thyroid hormone in the nuptial pad is yet not clear.
Androgen is necessary for the development of secondary sexual characters in the male frogs. The role of androgens in male amplexus behavior has been directly examined with the presence of nuptial pads and other sexual characteristics.
In many species, it has been highly reported that frog’s nuptial pads secrete mating season-specific proteins related to salamander pheromones. These pheromones are non-volatile proteinaceous courtship pheromones that attract the females towards the male in order to regulate courtship behavior and mating receptivity.
How do you know the sex of a frog? (With relation to Nuptial Pads)
The best time to distinguish male frogs from the female frogs is during the mating season.
Just have a look at the base of the first (inner) finger of the frog’s forelimbs, and if you find a thickened pad like appearance then this is the nuptial pad that is only present in males during the mating season.
The best way to know the sex of a frog is to identify if that individual has nuptial pads. If it has, then it’s a male. And, if it doesn’t have, then it’s a female.
The nuptial pads are rough, keratinized, thickened, glandular, puffed-like appearing in male frogs with having a patch of rough, raised flesh on each of their fore-thumbs. If the thumb seems to be larger and thicker than the other digits, especially at its base, the frog is probably a male with the nuptial pad.
Males typically enter the breeding ponds before females arrive and then undergo developmental changes such as the recurrence of secondary sexual structures like larger tail fins, cornified nuptial pads, and enlarged cloacal glands along with the enhancement of male reproductive behaviors.
It is also very much important to note that the sexually mature and gravid females are selected for their red cloaca. And, sexually mature males are identified for their black nuptial pads which are located on the inner surface of their forepaws and forearms.
What is Amplexus in frogs?
When a male frog is on top of a female frog, that position of mating is known as amplexus. This amplexus behavior of mating allows the male frog to place his cloaca near the female’s cloaca in order to fertilize her eggs.
Amplexus is a type of mating behavior of frogs and other amphibians, in which a male holds and grasps the female tightly by getting his whole body over her.
During amplexus, male forelimbs have well-developed nuptial pads that help them to provide a proper grip and friction on the female body around her trunk, underneath the chin.
In frogs, the males can also grasp the females by the head, waist, or armpits by sitting over her. Nuptial pads along with the toe pads help in this action.
And, this position of the male sitting over the female can be seen for about hours, days, or even months in some species.
During amplexus, the male attracts and satisfies the female with pheromones it releases from the nuptial pads and skins. This stimulates the release of eggs and so the male is able to fertilize the female as soon as the eggs are released from the female’s body.
Amplexus is a post-mating call behavior and has no relation to the occurrence of any type of mating call by the males. So, in order for amplexus to be initiated, male frogs must first find a mate by attracting one through calls, typically in the evening.
Once a male has successfully attracted a mate the process of amplexus begins while the unsuccessful males are forced to continue their search for a mate through further calls.
The presence of nuptial pads allows the male frog in grasping and holding onto a female very tightly, so much so that, this can lead to the impairment of the female’s ability to move or to feed.
With regards to the male frog engaged in amplexus, they have been found to not feed at all during amplexus. That’s their type of behavior.
Why do male frogs have enlarged thumb pads?
Male frogs have enlarged thumb pads during breeding season because of the presence of nuptial pads. Sexually mature males are identified because of their black nuptial pads.
Just before the mating season starts, the rapid flow of androgen hormones triggers the development of keratinized and spiny nuptial pads on the thumbs and arms of male frogs.
Nuptial pad in males is with an expanded tip of the base of the first (inner) finger of the forelimb that shows modifications of their ventral epidermis for adhesion and gripping the female, only during the breeding season.
The nuptial pad epidermal cells are organized into 3 to 4 rows, possessing keratin bundles, especially in surface nanostructures that are involved in adhesion and proper grasping the female with a better surface area of contact.
The frog’s nuptial pads are enlarged thumb pad like because it holds glandular tissues that secrete mating season-specific proteins related to pheromones which attract females for mating with the male by developing various courtship behaviors.
Nuptial glands share a variety of structural and functional characteristics with mucous glands of the skin and probably have been modified evolutionarily from that parent gland population. This may be another reason for the thickened, glandular, and puffed-like appearance of the nuptial pads.
What is the need for the development of nuptial pads only in males?
Nuptial pads are actually the secondary sexual characteristics of the male body that is produced due to the rapid flow of androgen in males during the breeding season.
Androgen directly plays a crucial for male sexual behavior like mating calls, development of reproductive organs, etc. and in doing so it develops such pads to better perform the amplexus behavior after the mating call.
This indicates that nuptial pads may also synthesize chemical signals involved in courtship and mating. This better helps the male to attract a female and romance with her, in simple words.
Another interesting part is that females directly get more stimulated to lay eggs when the males grasp her with the help of nuptial pads. The female’s ventral skin surface is seen to be often scraped at the site where the male’s spiny nuptial pads have been holding her.
The highly-thicked size of nuptial glands in males increases the glandular parts of the male’s forelimbs, the reason for which is still unclear but this condition is somewhat beneficial to both the male and female during amplexus.
Micro-CT scans of the nuptial pads showed the presence of two types of acinar glands with having a duct exiting at the epidermal surface of the pad, in between the keratinous cones.
In some species, it has also been reported that the nuptial pads can also be used in male–male combat.