Paternal care

(writing in progress)

Certain amphibians provide care for their offspring, which may seem at odds with the primitive nature of amphibians compared to mammals. However, this should be kept in perspective. Although frogs in particular have evolved a remarkable range of parental tactics, the fact remains that parental care occurs in all mammals but only a small minority of species of amphibians. Furthermore, the number of families is greater in frogs than in any order of mammals, helping to explain the extraordinary diversification of various adaptive features including parental care. Furthermore, much of the parental care in amphibians is owing to the precarious transition between aquatic and terrestrial life and the special need to protect offspring from desiccation. In the final analysis the incidence of parental care hardly contradicts the evolutionary relationship of the two classes, Amphibia and Mammalia.

 
Amphibians exceed mammals in parental care in the following important ways.

• The possession of a larval stage, and the associated metamorphosis, are antithetical to parental care in the sense that larvae are by definition free-foraging juveniles extremely modified for self-sufficiency at a small body size. The fact that all amphibians metamorphose and yet certain amphibians have larvae cared for by the parents is as extreme in evolutionary terms as the lactation unique to mammals.
• Paternal care, in which the father directly protects and provisions offspring, has evolved more frequently in one order of amphibians, namely the frogs, than in all the orders of mammals combined. The males of certain frogs1 even protect and provide for free-foraging larvae including those fathered by other males.

• Amphibians show paternal care without the incentives of monogamy and copulation seen in mammals. No species of amphibian is monogamous and only one species2 copulates in the sense of using a phallus.
  In summary: although caring fathers among amphibians enjoy neither the sexual gratification and assured paternity of a pair-bond, nor the level of intimacy associated with copulation, they nonetheless devote themselves to offspring to a degree that arguably exceeds that seen even in humans3.

1 e.g. Pyxicephalus adspersus in southern Africa

2 Ascaphus has a cloacal protruberance in the male, which is inserted into the cloaca of the female for the purpose of insemination. The evolution of this organ, which is unique among amphibians, is explained by the fact that this genus of frogs lives in streams that flow so fast that the ejaculate would otherwise be swept away before fertilising the eggs.

3 Paternal care in Homo sapiens is variable according to the social system of the community in question. For example, some human societies are polygamous, with no more paternal care than occurs in non-human primates such as baboons. Although marriage is close to a ‘human universal’, paternal care is not.

Frogs father further than mammals

Direct care of eggs and larvae has evolved repeatedly in many families of amphibians, with frogs1 using particularly diverse parental tactics. Some frogs even have fathers that look after offspring, in contrast to the lack of direct fatherly care in any polygamous mammal. However, the fact remains that all amphibians remain tied to metamorphosis from larvae, which is a more primitive2 strategy than the suckling of dependent infants. Because mammals3 by definition lactate, amphibians remain inferior to mammals in parental care.”
 
The larvae of many lineages of frogs, including some ancient enough to be ‘living fossils’4, are cared for directly by one or both parents. This combination of metamorphosis and parental care constitutes – in view of the points below – a greater evolutionary achievement than the lactation unique to mammals.

• The nature of larvae is to be self-sufficient as early in life as possible. Parental care of larvae – as practised by many amphibians including some of the most primitive of frogs – is consequently the opposite of what would be expected.
• Although maternal provision5 of yolk by frogs is usually part of the extended care of the eggs and larvae, in certain species6 the defence and aid of larvae extends even to free-foraging tadpoles independent of yolk.
• Although forming only one order of amphibians, frogs have evolved fatherly (paternal) care – in which the male attends7 directly to offspring – more frequently8 than have all the orders of mammals combined.
  
• Fatherly care in mammals is restricted to monogamous species9, whereas in amphibians it includes polygamous species.
• Polygamous frogs with fatherly care include species10 mating promiscuously enough to form leks. In mammals, by contrast, lekking systems and paternal care are mutually exclusive.
• The males of certain species of amphibians and mammals breed only once per lifetime11. Because one-off reproduction and fatherly care are mutually exclusive in mammals, their combination in certain frogs12 represents yet another evolutionary extreme.

These ancient but still unrivalled accomplishments of maternal and fatherly care in amphibians can partly be explained by a hygienic predicament that has always been peculiar to this class. A moist epidermis is particularly vulnerable to fungal pathogens, making it adaptive for amphibian parents – which produce appropriate antimicrobial substances in their skin – to maintain contact13 with eggs and larvae. In conclusion: viewed from a fresh perspective, amphibians exceed mammals in direct care of their offspring.”

1 class Amphibia, order Anura
2 Marsupial mammals bear extremely small infants, partly analogous with larvae in being able to crawl independently from birth canal to teat. Crucial differences between marsupials and amphibians are that no amphibian parent cares for its offspring after metamorphosis, and no marsupial shows fatherly care of offspring.
3 including the egg-laying monotremes
4 e.g. the primitive family Leiopelmatidae in New Zealand, in which males care for eggs and larvae
5 Frogs provide food to offspring in eggs in several ways. In certain species, the larvae consume not only yolk but also the jelly laid down around eggs. In other species the mother feeds the free-swimming tadpoles special, infertile eggs during extended maternal care.
6 As an extreme example, the male Pyxicephalus adspersus guards and defends free-swimming, free-foraging tadpoles despite the fact that this sexually dimorphic species has males much larger than females, with a polygamous system that extends to lekking. The male also provides for offspring (which may not all be his) by digging channels to restore the water supply where tadpoles risk desiccation in seasonal puddles.
7 In various species of frogs, fatherly care of eggs and larvae includes nest-construction, guiding, protection, guarding, defence, carrying, accommodation in special pouches of the male body, provision of water, and provision of food.
8 Fatherly care occurs in at least 17 families of frogs. By contrast, in only two families of mammals do all wild species show fatherly care: Canidae and Callitrichidae.
9 Direct fatherly care is minimal in polygamous primates, and chimpanzees – arguably the closest living relatives to humans – show negligible fatherly care. All societies of Homo sapiens practise marriage but direct fatherly care is minimal in some polygamous societies of humans.
10 e.g. in families Pyxicephalidae and Dendrobatidae
11 The life history strategy of making only one reproductive attempt before senescing is called semelparity.
12 e.g. Hylidae: Hypsiboas

13 as a means of antibiotic prevention and treatment

(writing in progress)

Posted on June 15, 2022 12:19 AM by milewski