Why do most mammals need a scrotum to keep their sperm cool enough to be viable when birds, which are also warm-blooded, do not?

Kirk A. Janowiak, M.S. (x 2) Biology & Ethology, Purdue University

Updated Aug 6, 2015 · Upvoted by Adriana Heguy, Professor of Pathology at NYU Langone Medical Center · Author has 4.9k answers and 7.5m answer views

Originally Answered: Why do most mammals need a scrotum when birds, which are also warm-blooded, do not? (see details)

Before adding some information to Adriana Heguy's excellent answer--and lesson in reproduction, it should be pointed out that the evolution of birds and mammals diverged a LONG time ago and many many adaptations have since been experimented with (evolutionarily speaking) and either tossed or held on to until we reach modern birds and mammals. Differences between these two major vertebrate groups should always be viewed with an eye on how the two groups have EACH individually come to be adapted to the environments in which they live and the niches (the roles they play in an ecosystem) into w...

Originally Answered: Why do most mammals need a scrotum when birds, which are also warm-blooded, do not? (see details)

Before adding some information to Adriana Heguy's excellent answer--and lesson in reproduction, it should be pointed out that the evolution of birds and mammals diverged a LONG time ago and many many adaptations have since been experimented with (evolutionarily speaking) and either tossed or held on to until we reach modern birds and mammals. Differences between these two major vertebrate groups should always be viewed with an eye on how the two groups have EACH individually come to be adapted to the environments in which they live and the niches (the roles they play in an ecosystem) into which they have inserted themselves.

One set of factors that may help in explaining the differences between many mammals and birds is the time involved in actually producing the sperm and the external regulatory factors that often enter into the production process.

Birds tend to have a discrete and often limited mating/breeding time. This "season" of mating and breeding for the vast majority of birds is determined by hormonal changes that are further determined by changes in day length.

A common wild canary, the American Goldfinch, for instance, has a thin skull and light can penetrate a particularly thin place to reach the pineal gland in the brain. [For a comparable adaptation in the reptilian world, look up the anatomy and of the tuatara.] A pattern of increasing day length causes nearly direct stimulation of the pineal through the skull and more indirectly through the eyes. This stimulation results in a cascading production of hormones that causes the growth of a testis (as well as significant changes in behavior and even coloration).

In most songbirds, only one testis of the pair will typically enlarge, rather than both testes as happens in most mammals. The growth and development of this testis can be prodigious, resulting in an internal organ that temporarily takes up perhaps 25% or more of the pelvic and abdominal volume. Production of sperm cells by birds takes place much more rapidly in the seminiferous vesicles than in mammals (in general) and they are soon released to fertilize the ova (eggs) of the female bird (or birds) during a relatively brief interval during breeding season. Birds of a pair do not typically mate only once, but may copulate many times in a short time period.

Once the pattern of day length begins to reverse to shorter and shorter day lengths, the testis of the goldfinch begins to shrink and results in the waning of the flow of hormones that control reproduction, and sperm production (and some male-typical behaviors) cease.

So, let me summarize what this is saying and how it may apply to the presence of scrota in mammals but not in birds.

Birds produce sperm very quickly (when compared to mammals) and release this sperm in a relatively short period of time during a fairly short breeding season. The time that the sperm cells are exposed to "high" internal body temperature (at or above 100°F or 38°C) is brief. With additional adaptations that we are just beginning to explore in actual sperm cell physiology, this time may not be long enough to allow temperature-related sperm cell damages that would probably occur if these cells were to remain in storage for very long inside the body cavity.

While it is true that many mammals do, indeed, present scrota, if you are observant, you will notice that these structures may not always be present most of the time of the year.

In many small mammals, day length also plays a significant role in reproduction. Increasing day length (as in the case of the goldfinch mentioned earlier) can cause a hormonal change in small mammals that causes changes in behavior and the beginning of serious sperm cell production in anticipation of mating & breeding. When this begins, the scrotum--which has been rescinded into the abdomen or hidden within a kind of slot-like "pouch" of skin on the belly--descends and testes slightly enlarge. At this time, the scrotum offers the advantage of slightly lowered temperatures and the ability to store sperm for longer periods of time without temperature-associated deformations or death.

For animals that come into estrus (also called "heat" in more colloquial language) several times each year or for species that have no explicitly observable estrus and can mate nearly any time (such as humans), the males need a way to be ready to breed whenever the females are ready. Since all the females in the population do not come into estrus all at the same time, but may vary by days or weeks, having a way to constantly make and store sperm cells, with the additional ability to resorb the cells and make fresh ones, becomes important. These will be the animals that you see with visible scrota that are present all of the time.

All of the "deeper" reasons that involve physiological pondering may hinge on these simpler, easy to observe behavioral traits we see in birds and mammals--or the behaviors may have been developed over time to accommodate the physiological differences we are finding between birds and mammals and between mammals of different groups.

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Adriana Heguy, Professor of Pathology at NYU Langone Medical Center

Updated Aug 3, 2015 · Featured on HuffPost and Slate · Author has 2.2k answers and 32.2m answer views

Originally Answered: Why do most mammals need a scrotum when birds, which are also warm-blooded, do not? (see details)

The reason why many mammals evolved to require a cooler temperature to produce viable sperm is not precisely known, and it has been a subject to debate for decades, mainly because having the testes exposed creates a vulnerability that seems to be the opposite of an adaptation. And interestingly, not all mammals have dangling testes and scrota, exceptions being not just pinnipeds (seals and sea lions) and cetaceans (whales and dolphins) but also elephants, which keep their testes inside the body.

There are many theories out there as to why sperm needs to be kept cooler, and why the testes o...

Originally Answered: Why do most mammals need a scrotum when birds, which are also warm-blooded, do not? (see details)

The reason why many mammals evolved to require a cooler temperature to produce viable sperm is not precisely known, and it has been a subject to debate for decades, mainly because having the testes exposed creates a vulnerability that seems to be the opposite of an adaptation. And interestingly, not all mammals have dangling testes and scrota, exceptions being not just pinnipeds (seals and sea lions) and cetaceans (whales and dolphins) but also elephants, which keep their testes inside the body.

There are many theories out there as to why sperm needs to be kept cooler, and why the testes of most mammals hang outside their bodies, inside the scrotal sac. In my opinion, the least likely is the "handicap principle". The easiest way to illustrate this principle is the peacock’s flashy feathers; the animal displays his genetic fitness by being able to survive in spite of those cumbersome feathers that attract the predator’s attention. Male mammals with dangly testes signal to the females that they have good genes if they could keep their hanging, exposed, sensitive gonads safe. The reason why this hypothesis is unlikely is because it has not led to increasingly ornate, highly visible scrota, with a few exceptions of mammals with brightly colored scrota (for example the vervet monkey, in the picture below).

Evolutionary psychologist CC Gallup wrote a very detailed paper a few years back, reviewing the current hypothesis, and proposing his own hypothesis, known as activation hypothesis. According to his hypothesis, when the sperm cells are ejaculated into the vagina, they experience a sudden rise in temperature, as the female’s reproductive tract is at body temperature (37 degrees Celsius). This elevation in temperature activates sperm, temporarily enabling them to increase their motility to go through the cervix and reach the fallopian tubes. The lower temperature in the descended testicles serve to prevent the premature activation of sperm by keeping testicular temperatures below body temperature. Indeed, human sperm that are placed at body temperature become more motile for about 1 hour, which is the time it would take them to reach the oocyte, and then they slow down. Gallup also speculates that the reason for the extreme sensitivity of the testes and the extreme pain that ensues if they are hit, are a result of the activation adaptation. Pain compels male mammals to protect their testes from possible damage. The cremaster muscle, which pulls the testes closer to the body, contracts not only when the outside temperature is too cold, when having an erection (possibly warming the sperm cells a bit, in preparation for the big swim of their lives), and also when there is a hint of possible damage to the testes. All these adaptations make sense in light of the activation hypothesis. This could explain why marine mammals do not need to keep the testes in the scrotum, because they are inside the body but close to the skin and the water temperature may keep them cold enough. But it does not explain the elephant case.

The activation hypothesis is discussed by Jesse Bering here: Why do human testicles hang like that?

And here is the original article by Gallup: On the origin of descended scrotal testicles: the activation hypothesis | Evolutionary Psychology

I also came across a recent article that proposes another hypothesis, that I like the most, perhaps because it highlights one aspect of evolution: that something that evolved a long time ago, as an adaptation to an environment or physiological conditions that no longer exist, may have become “fixed”, and that’s why they are still around. It’s called “endothermic pulses hypothesis”, and it’s described in a heavy-duty evolutionary biology paper, authored by the aptly named South African evolutionary biologist B.G. Lovegrove: Cool sperm: why some placental mammals have a scrotum

This hypothesis argues that the evolution of the scrotum was driven by increases in physiological body temperature (endothermic pulses) that occurred in Boreoeutheria (a clade of mammals, supported by genomic data, that comprises the rodents, primates, lagomorphs such as hares and rabbits, carnivores, bats, and ungulates; Boreoeutheria) during the Cenozoic. These pulses occurred as an adaptation to climate changes, and as an adaptation to cursoriality (the ability to run, which increases body temperature). The model proposes that selection maintained an optimum temperature for spermatogenesis and sperm storage throughout the Cenozoic, at the lower levels of body temperature that prevailed in ancestral mammals for at least 163 million years. The lower temperatures also favor a reduced rate of mutation during spermatogenesis, and evolutionary processes ended up stabilizing these lower temperatures for sperm production and storage. Evolutionary stasis may have been driven by reduced rates of germ-cell mutations at lower body temperatures. The fitness advantages of an optimum temperature of spermatogenesis ultimately outweighed the costs of testes externalization and resulted in the evolution of the scrotum. This hypothesis would explain why elephants do not have external testes (they are not Boreoeutheria, but belong to another clade, Afrotheria, comprising tenrecs, hyraxes and elephants.

This figure shows the 3 possible models for evolution of the scrotum in Eutherian mammals.

We may never know why exactly evolution favored cooler sperm and dangling testes in a scrotum, but it is fun and useful to hypothesize about the reasons for this adaptation.

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Chandravadan Trivedi, former Retired Fishery Scientist

Answered Jul 26, 2015 · Author has 7k answers and 4.7m answer views

Originally Answered: Why do mammals need a scrotum when birds, which are also warm-blooded, do not?

Bird's testis are within its body & they can make & store sperm at higher temperature. The case is different in mammals where scrotum is out side the body & the human sperms need a lower temperature to see that they are viable. They cannot withstand human's normal body temperature.
In case of bird as it's body normal temperature is 110 degrees farenheit, it needs that temperature to keep it's self warm as it flies through air which has lower temperatures.
In the case of mammals, as they do not go fly in air & are almost always near the ground, a mechanism has been put in place - an adaptat...

C Stuart Hardwick, Scifi author and science nerd.

Answered Jul 27, 2015 · Author has 9.5k answers and 82.6m answer views

Originally Answered: Why do most mammals need a scrotum when birds, which are also warm-blooded, do not? (see details)

I can only give you my hypothesis, but of course, I'm right.

It seems likely that our furlessness, bipedalism, and big brain all evolved together as part of our transition from tree swinging fruit eaters to ass-kicking plains hunters. These adaptations enables us to do what no other animals can do, hunt actively through the heat of the African day, tracking and running much faster prey to exhausion, just as some African peoples still do to this day.

If true, then this also explains why we have temperature regulating testes.

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The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme

S. J. Gould, R. C. Lewontin
Published 21 September 1979.DOI: 10.1098/rspb.1979.0086

Abstract
An adaptationist programme h...

Originally Answered: Why do most mammals need a scrotum when birds, which are also warm-blooded, do not? (see details)

It is common to assume a feature-specific selection and adaptation to account for a biological feature. Evolution doesn't always work that way. Prominent biological features need not be and may never have been directly selected. Be aware of "Just so stories".

An interesting essay on this point is available free online. Here's the abstract:

The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme

S. J. Gould, R. C. Lewontin
Published 21 September 1979.DOI: 10.1098/rspb.1979.0086

Abstract
An adaptationist programme has dominated evolutionary thought in England and the United States during the past 40 years. It is based on faith in the power of natural selection as an optimizing agent. It proceeds by breaking an organism into unitary 'traits' and proposing an adaptive story for each considered separately. Trade-offs among competing selective demands exert the only brake upon perfection; non-optimality is thereby rendered as a result of adaptation as well. We criticize this approach and attempt to reassert a competing notion (long popular in continental Europe) that organisms must be analysed as integrated wholes, with Bauplane so constrained by phyletic heritage, pathways of development and general architecture that the constraints themselves become more interesting and more important in delimiting pathways of change than the selective force that may mediate change when it occurs. We fault the adaptationist programme for its failure to distinguish current utility from reasons for origin (male tyrannosaurs may have used their diminutive front legs to titillate female partners, but this will not explain why they got so small); for its unwillingness to consider alternatives to adaptive stories; for its reliance upon plausibility alone as a criterion for accepting speculative tales; and for its failure to consider adequately such competing themes as random fixation of alleles, production of non-adaptive structures by developmental correlation with selected features (allometry, pleiotropy, material compensation, mechanically forced correlation), the separability of adaptation and selection, multiple adaptive peaks, and current utility as an epiphenomenon of non-adaptive structures. We support Darwin's own pluralistic approach to identifying the agents of evolutionary change.

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